GENERAL SPECIFICATIONS FOR BRIDGES · 2015-08-03 · design of the bridges. Design data are also available for various other loads expected on the bridge such as the earthquake, wind,

DoR-SB001 : 2015

General Specifications for bridges 0

GENERAL SPECIFICATIONS FOR BRIDGES

DoR-SB001:2015

April 2015

DEPARTMENT OF ROADS, MoWHS, THIMPHU

DoR-S8001 : 2015

PITEFACE

The Department of Roads, Ministry of Works and Human Settlement is pleased topublish the first issue of the General Specifications for Bridges - DoR-58001 .201 5 .

Bridges form vital links on {n} road network system and particularly in a country likeours where bridges have to'ipan across deep river gorges and fast flowing mountainrivers, bridges are indeed the life line of the road system. The bridges are also the mostexpensive parl of the road network system, the unit cost being about 25 times more thanthat of road"

Therefore it is imperative that due diligence and care is exercised in the bridgeconstruction process from planning through to design to the execution of the works.During the planning and design phase of bridge, the engineer has to give due

consideration to the constructability, life cycle cost, durability, use of local materialsand aesthetic appearance of the bridge without compromising on the structural strength.In addition, it must also be ensured that the bridge is not too nar:row to hinder the

smooth flow of traffic on the road or is not unnecessarily too wide. To this end, it ishoped that the "General Specifications for Bridges" will provide a useful guide to theplanners and engineers involved in the design and construction of bridges in Bhutan.

Besides the stipulations on the carriage widths for bridges on different classes of roads

in Bhutan, the document also specifies the vehicular live loads to be considered for the

design of the bridges. Design data are also available for various other loads expected onthe bridge such as the earthquake, wind, snow, pedestrian live load, earth pressure etc.

In light of the progress that we are making currently in terms of the expansion of road

network in our country, such a standard document for bridges is timely and I commendthe engineers of the Design Division of DoR for bringing out this publication. Thedocument will surely go a long way in streamlining the bridge construction process inour country.

A11 of us have to agree that we have a lot to achieve as far as the overall quality of thecivil construction works, including bridges, in our country and i sincerely hope that thisdocument will help in promoting the construction of high quality bridges in our country.

,4 r .---/Jl*#-Dasho (Dr.) Sonam TenzinSecretaryMinistry of Works and Human SettlementThimPhu'

$ecreta'I

Aprit 2015 rrnrnrstrv-ul*;-lltn1lse|[ement

General Specifications for bridges

DoR-SB001 : 2015


FOREWARD

The fast expanding road network in the country spurred by rapid socioeconomic growth in the recent years has led to proportionate increase in the number of bridges in our road network. In addition, the existing bridges on the main highways are also being gradually replaced with wider and stronger bridges to cater to heavier, faster and increased volume of vehicular traffic.

The construction of roads and bridges being highly capital-intensive, the Royal Government, in addition to its owns efforts, continues to seek assistance from various donor countries and international agencies not only in the overall expansion of the road network but also in improving the existing ones to keep pace with the changing socioeconomic scenario in the country. As a result, there are currently many donor agencies involved in the development of bridge infrastructure in the country. However, in absence of common standard guidelines for planning and design of the bridges in the country, DoR as a focal government agency for the development of bridge infrastructure in the country, hasn’t been able to exercise proper control over important aspects such as basic geometrics and the load capacities of the bridges on our roads. The result is we have bridges of different carriageway widths and load capacities on a same stretch of highway.

The main purpose of having the General Specifications for Bridges is therefore to streamline and establish a common procedure for planning, design and construction of road bridges in the country.

The Specification stipulates mainly the following:-

Classification of bridges based on span

Standard carriage widths and load capacities for bridges on various classes of roads

Guidelines on planning and design of bridges and recommended design philosophies

Loads to be considered during design and construction of bridges

Recommended detailing requirements for certain components of the bridge

The General Specifications for Bridges, as the name implies, is applicable to all bridge types irrespective of the structural system, material and geometrics. However, certain provisions may require further consideration for bridges with span in excess of 200 m.

The specifications, particularly the provisions covering the loads applicable in the design and construction of bridges, have been prepared with reference to the Indian, Japanese and American codes and other relevant standard literatures on the subject. Appropriate modifications have been made to suit the conditions and context in Bhutan.

The draft of the document was presented and discussed in the Departmental Coordination Committee (DCC) of DoR and the document was subsequently approved by the DCC.

DoR-SB001 : 2015

The DCC comprises of the following members:-

1. Mr. Karma Galey, Director (Chairman)

2. Mr. Tshering Wangdi (A), Chief Engineer, Construction Division

3. Mr. Tshering Wangdi (B), Chief Engineer, Maintenance Division

4. Mr. Tshering Peljore, Chief Engineer, Planning Division

5. Mr. Jangchuk Yeshi, Chief Engineer, Design Division

The draft was also presented to the 5ft DoR quarterly meeting held in October 2Ol4 atThimphu and the feed backs and comments from the Chief Engineers and the ProjectCoordinators from all the Regional Offices and Projects under DoR were incorporated.

The General Specificartons for Bridges,like any other engineering standard/code shallbe revised and updated from time to time so that it stays relevant and current.

Comments and suggestions for improvement of the document from the users shallalways be welcome. The constructive comments and suggestions shall be incorporatedin the subsequent revisions of the document.

April2015 Karma GalayDirector t...,,.,..DoR, MoWHS . :, i ,, i :;1.

-[''i

Gene ral Spe c ifications .fo r bridg e s

DoR-SB001 : 2015


Contents

1.0 SCOPE ................................................................................................................................................ 6

2.0 GENERAL .......................................................................................................................................... 6

2.1 DEFINITIONS ..................................................................................................................................... 6 2.2 CLASSIFICATION OF BRIDGES BASED ON LENGTH ............................................................................ 7

2.2.1 Small Span Bridge: .................................................................................................................. 7 2.2.2 Medium Span Bridge: .............................................................................................................. 7 2.2.3 Long Span Bridge: ................................................................................................................... 7

2.3 CARRIAGEWAY WIDTH AND FOOTPATH ........................................................................................... 7 2.3.1 Specification of carriageway widths for various classification of roads ............................... 7 2.3.2 Footpath ................................................................................................................................... 8

2.4 CLEARANCES .................................................................................................................................... 8 2.5 SUPER ELEVATION ........................................................................................................................... 9 2.6 APPROACHES TO THE BRIDGE ........................................................................................................... 9

2.6.1 Straight distance ...................................................................................................................... 9 2.6.2 Horizontal Curves ................................................................................................................... 9 2.6.3 Super Elevation ..................................................................................................................... 10 2.6.4 Transition Curve .................................................................................................................... 11 2.6.5 Widening of the approaches at the curves ............................................................................ 13

3.0 PLANNING AND DESIGN ............................................................................................................ 14

3.1 INVESTIGATION .............................................................................................................................. 14 3.2 PLANNING ....................................................................................................................................... 14

3.2.1 Selection of bridge location and bridge type ........................................................................ 14 3.2.2 Involvement of the stake holder agencies ............................................................................. 15

4.0 BASIC PRINCIPLES OF DESIGN .............................................................................................. 15

5.0 LOADS .............................................................................................................................................. 16

5.1 COMBINATION OF LOADS AND FORCES FOR LIMIT STATE METHOD OF DESIGN (IRC-6:2014)..... 17 5.1.1 Combination of loads for the verification of equilibrium and structural strength under ultimate state .......................................................................................................................................... 17 5.1.2 Combination Principles ......................................................................................................... 17 5.1.3 Basic Combination ................................................................................................................ 17

5.2 DEAD LOAD (DL) ....................................................................................................................... 23 5.3 LIVE LOAD (LL) ......................................................................................................................... 23

5.3.1 Vehicular Live Load: ............................................................................................................. 23 5.3.2 Footpath, Kerb, Railings, and Crash Barriers ..................................................................... 28 5.3.3 Impact Load (I) ...................................................................................................................... 29 5.3.4 Span length (L) to be considered for calculating the impact percentages ........................... 30 5.3.5 Impact consideration under special circumstances .............................................................. 30

5.4 WIND LOAD (WL) ...................................................................................................................... 31 5.4.1 General Notes ........................................................................................................................ 31 5.4.2 Wind Speed and Wind Pressure ............................................................................................ 31 5.4.3 Design Wind Force on Superstructure ................................................................................. 32 5.4.4 Wind Effect on Live Load ...................................................................................................... 34 5.4.5 Wind Load Computation on Truss Bridge Superstructure ................................................... 34 5.4.6 Design Wind Forces on Substructure ................................................................................... 36 5.4.7 Wind Tunnel Testing .............................................................................................................. 38

5.5 HORIZONTAL FORCES DUE TO WATER CURRENTS ........................................................ 38 5.5.1 Calculation of Intensity of pressure ...................................................................................... 38

5.6 LONGITUDINAL FORCES ......................................................................................................... 40 5.6.1 Braking Effect ........................................................................................................................ 40 5.6.2 Point of application of braking force: .................................................................................. 40 5.6.3 Calculation of Longitudinal Forces under different support conditions ............................. 40

5.7 CENTRIFUGAL FORCES ........................................................................................................... 43 5.8 BUOYANCY AND UPLFIT ........................................................................................................ 44 5.9 EARTH PRESSURE ..................................................................................................................... 44

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5.9.1 General .................................................................................................................................. 44 5.9.2 Compaction ............................................................................................................................ 45 5.9.3 Presence of water .................................................................................................................. 45 5.9.4 Effect of earthquake ............................................................................................................... 46 5.9.5 Earth Pressures ..................................................................................................................... 46

5.10 TEMPERATURE .......................................................................................................................... 50 5.10.1 Uniform temperature ............................................................................................................. 50 5.10.2 Temperature gradient ............................................................................................................ 51 5.10.3 Material Properties ............................................................................................................... 53

5.11 DEFORMATION STRESSES (FOR STEEL BRIDGES ONLY) ......................................................... 53 5.12 SECONDARY STRESSES ........................................................................................................... 53

5.12.1 Steel structures: ..................................................................................................................... 53 5.12.2 Reinforced Concrete structures: ........................................................................................... 53

5.13 ERECTION STRESSES AND CONSTRUCTION LOADS ....................................................... 53 5.14 SEISMIC FORCE .......................................................................................................................... 54

5.14.1 General .................................................................................................................................. 54 5.14.2 Applicability ........................................................................................................................... 54 5.14.3 Seismic Zones ........................................................................................................................ 55 5.14.4 Components of Seismic Motion ............................................................................................. 55 5.14.5 Combination of Component Motions .................................................................................... 56 5.14.6 Computation of Seismic Response ........................................................................................ 56

5.15 SNOW LOAD ................................................................................................................................ 65 5.16 VEHICLE COLLISION LOADS ON BRIDGE AND FLYOVER SUPPORTS ......................... 65

5.16.1 General .................................................................................................................................. 65 5.16.2 Collision Load ....................................................................................................................... 66

5.17 INDETERMINATE STRUCTURES AND COMPOSITE STRUCTURES ............................... 66

6. DETAILING REQUIRMENTS (NOT EXHAUSTIVE) ................................................................ 66

6.1 Drainage .................................................................................................................................... 66 6.2 Water proofing and wearing surface ........................................................................................ 67 6.2.1 Fully Bonded Waterproofing ................................................................................................. 67 6.2.2 Concrete Wearing Surface .................................................................................................... 67 6.3 Water drop profile/drip nose ..................................................................................................... 67 6.4 Jacking provision of replacement of elastomeric bearings ...................................................... 68 6.5 Abutment top details .................................................................................................................. 68

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1.0 SCOPE 1.1 The main objective of the General Specifications for Bridges is to establish a

common procedure for design and construction of road bridges in Bhutan. These specifications are not intended to supplant qualification, training and the exercise of judgement by the engineer and state only the minimum requirements necessary to provide public safety. The design and construction of road bridges is a specialized field requiring thorough knowledge of science and techniques involved and the task shall be entrusted only to the specially trained and appropriately qualified engineers. While the Standard Specifications provide the guidance and for certain aspects of design, regulations, engineers have to apply their judgement and expertise at all stages during design and execution of the works and compliance to these specifications shall not relieve the engineers of their responsibility for the stability and soundness of the structure designed and built by them.

1.2 This Specification covers mainly the basis for planning, design and construction

of road bridges in Bhutan. 1.3 This Specification also covers the regulations for carriageway widths and the

loads for bridges on various classes of roads in Bhutan. 2.0 GENERAL

2.1 Definitions

1. Bridge ......................Bridge is a structure having a total length above 6 m between the inner faces of the dirt walls for carrying traffic or other moving loads over a depression or obstruction such as channel, road or railway.

2. Culvert .....................Culvert is a cross-drainage structure having a total length

of 6 m or less between the inner faces of the dirt walls or extreme ventway boundaries measured at right angles.

3. Submersible Bridge/Vented causeway ...........A Submersible Bridge/Vented

causeway is a bridge designed to be overtopped during floods.

4. Highest Flood Level (HFL) ..........HFL is the level of highest flood ever recorded or the calculated level for the design discharge.

5. Low Water Level (LWL) ..........LWL is the level of the water surface obtained

generally during dry season and shall be specified in the design drawing of the bridge.

6. Length of the bridge ..........Length of a bridge is the overall length measured

along the centreline of the bridge between inner faces of the dirt wall.

7. Bridge span .........Span of a bridge is the overall length measured along the centreline of the bridge between the centre of the bearings at the two end abutments.

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8. Superstructure ..........The girders and deck on the abutments and piers.

9. Substructure .............The abutments and piers and their foundations, which

form a structure to transfer loads from the superstructure to the foundation ground.

10. Concrete bridge .......The bridges in which the major structural members

forming the superstructure are made of concrete.

11. Steel bridge .............The bridges in which the major structural members forming the superstructure are made of steel.

12. Carriageway width ............The width of the carriageway is the minimum clear

width measured at right angles to the longitudinal centreline of the bridge between the inner faces of the roadway curbs or the wheel guards.

13. Footpath ...................The portion of the bridge width used as

pedestrian/bicycle track.

2.2 Classification of bridges based on Length 2.2.1 Small Span Bridge:

A bridge where the overall length between the inner faces of the dirt walls is up to 30m.

2.2.2 Medium Span Bridge:

A bridge having a total length of up to 60 m but more than 30 m. 2.2.3 Long Span Bridge:

A bridge where the overall length is more than 60 m.

2.3 Carriageway width and Footpath 2.3.1 Specification of carriageway widths for various classification of roads

Sl. No.

Road classification Carriage width (m)

1 Asian Highway 7.50 2 Primary National Highway (PNH) 7.00 3 Secondary National Highway (SNH) 5.50 4 Dzongkhag Road 3.50 5 Farm Road 3.50 Note:

1. The carriageway width for bridges on Thromde roads shall be decided by the respective municipalities as per actual requirement.

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2. For semi-permanent Bailey type portable steel bridges which are generally adopted on the farm roads, the available standard single lane width of 3.27 m may be adopted.

3. For bridges on the horizontal curves, the carriage width shall be suitably increased as per the provisions of Clause 2.6.2.

4. The bridges shall provide for either one lane, two lane or multiples of two lanes and three lane bridges with two directional traffic shall not be constructed.

5. If median/central verge is constructed in a wide bridge thus providing two separate carriageways, the carriage way on each side of the verge shall at least be two lanes and the width thereof shall individually comply with the requirements stipulated above. The width of the median/central verge when provided shall not be less than 1.20 m.

6. For culverts of length 3.0 m and less, the width between the outer most faces of the culvert shall be equal to the full formation width of the road.

2.3.2 Footpath

Foot paths for exclusive use of the pedestrians shall be provided for bridges in or near populated areas such as towns and cities. The footpaths when provided shall have a minimum width of 1.50 m. The footpath may be on one side only or on both sides of the bridge depending on the population size, span of the bridge and other relevant considerations.

Figure 2-1 Illustration of Carriageway, Footpath and Median 2.4 Clearances

The minimum horizontal clearance shall be the clear width and the minimum vertical clearance the clear height available for passage of traffic.

The minimum vertical clearance, irrespective of number of lanes, shall be 5.00 m. However for bridges in urban areas and for bridges on roads leading to existing or proposed hydro power projects, the clearance shall be increased to 5.50 m.

The minimum horizontal clearance between the road way and the obstruction on the side shall be 125 mm.

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Refer Figure 2-2 below for details:

Figure 2-2 Horizontal and Vertical clearance (IRC-5) 2.5 Super Elevation

Super elevation on the deck of a bridge in the horizontal curve shall be provided as per the provisions covered under Clause 2.6.3. Due considerations shall be made for change in the force effects on the bridge due to super elevation during analysis and design of the bridge.

2.6 Approaches to the bridge

2.6.1 Straight distance

The approaches on either side of the straight bridge shall have a minimum straight length of 10 m and shall be suitably increased wherever necessary to provide for the minimum sight distance for the design speed.

In difficult and unavoidable situations and for roads with low design speeds such as the Dzongkhag and Farm Roads, the engineer responsible for design may suitable reduce the straight distance of the approaches based on his/her engineering judgement.

The minimum surfaced width of the straight portion of the approaches shall be same as the carriage width of the bridge.

2.6.2 Horizontal Curves

Where horizontal curves have to be provided on the approaches beyond the straight portion, the minimum radius of such curves shall be as per Table 2-1:-

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Table 2-1 Minimum radius of horizontal curves for different terrain conditions for minimum design speeds (IRC-38).

Design Speed (km/hr)

Plain and rolling terrain (m)

Mountainous and steep terrain

Snow bound area (m) Non-snow bound area (m)

20 15* 15* 14*

25 23* 23* 20*

30 33 33 30

35 45 45 40

40 60 60 50

50 90 90 80

65 155

Speeds not applicable 80 230

100 360

* For roads where trucks are expected to ply, the minimum radius of 26 m has to be provided to accommodate them.

Note:

1. The turning radius is the radius of the outer swept path of the approach curve.

2. The values of radii given in Table 2-1 are the minimum and the engineer should design curves for the largest possible radius.

3. The minimum radius of the curve is governed by the minimum turning circle of

the vehicles. As per IRC-38, turning circles of commercial vehicles range widely from 9 to over 26 m but lie mainly between 12 and 21 m diameter.

4. For the approaches to the bridges on the Dzongkhag Roads, Farm Roads and Secondary National Highways that have no potential to cater traffic to major hydro power projects, minimum turning radius of 15 m may suffice.

2.6.3 Super Elevation

When a vehicle is negotiating a curve, it will have the tendency to skid outwards due to centrifugal force. If the road is laterally levelled, friction alone would have to counter this centrifugal force and if the friction developed is not adequate, the vehicle will skid outwards. To prevent skidding, the road is given an inward tilt known as super elevation. The super elevation is the function of the radius of the curve and the design speed.

The Table 2-2 covered in IRC-38 for super elevation for curves of various radii and design speeds is recommended for approach curves on the bridges.

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Table 2-2 Super elevation for different design speeds and curve radii (IRC-38)

Plane/Rolling terrain, Mountainous/Steep terrain bound by snow Mountainous terrain not bound by snow

Curve Radius

Rc (metre)

Superelevation (metre per metre) for design speed km/hr of Curve Radius

Rc

(metre) 20 25 30 35 40 50 65 80 100 20 25 30 40 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

15 0.070 Notes: (i) Superelevation has been calculated by the formula e=V2/225Rc

0.100

(i) Maximun superelevation is 10% these areas

15

20 0.070 0.089 0.100 20

25 0.070 0.070 (ii) No superelevation be provided if the superelevation value is less than normal pavement camber

0.071 0.100 25

30 0.059 0.070 0.059 0.093 0.100 30

40 0.044 0.069 0.070 (iii) For a given design speed, adopt the largest possible radius below the firm stepped line

0.044 0.069 0.100 40

45 0.040 0.062 0.070 0.040 0.062 0.089 45 50 0.036 0.056 0.070 0.070 (iv) Maximum superelevation = 7% 0.036 0.056 0.080 0.100 50 55 0.032 0.051 0.070 0.070 0.032 0.051 0.073 0.100 55 60 0.030 0.046 0.067 0.070 0.070 (v) Minimum camber = 1.7% 0.030 0.046 0.067 0.100 60 70 0.025 0.040 0.057 0.070 0.070 0.025 0.040 0.057 0.100 70 80 0.022 0.035 0.050 0.068 0.070 0.022 0.035 0.050 0.089 0.100 80 90 0.020 0.031 0.044 0.060 0.070 0.070 0.020 0.031 0.044 0.079 0.100 90 100 0.018 0.028 0.040 0.054 0.070 0.070 0.018 0.028 0.040 0.071 0.100 100 125 0.022 0.032 0.044 0.057 0.070 0.022 0.032 0.057 0.089 125 150 0.019 0.027 0.036 0.047 0.070 0.019 0.027 0.047 0.074 150 170 0.016 0.024 0.032 0.042 0.065 0.070 0.016 0.024 0.042 0.065 170 200 0.020 0.027 0.036 0.056 0.070 0.020 0.036 0.056 200 250 0.016 0.022 0.028 0.044 0.070 0.070 0.016 0.028 0.044 250 300 0.018 0.024 0.037 0.063 0.070 0.025 0.037 300 350 0.016 0.020 0.032 0.054 0.070 0.020 0.032 350 400 0.018 0.028 0.047 0.070 0.070 0.018 0.028 400 500 0.022 0.038 0.057 0.070 0.022 500 600 0.019 0.031 0.047 0.070 0.019 600 700 0.016 0.027 0.041 0.063 0.016 700 800 0.014 0.023 0.036 0.056 0.014 800 900 0.020 0.032 0.049 900

1000 0.019 0.028 0.044 1000 1200 0.016 0.024 0.037 1200 1500 0.013 0.019 0.030 1500

1800 0.016 0.025 1800

2000 0.022 2000 2200 0.020 2200 2500 0.018 2500 3000 0.015 3000

2.6.4 Transition Curve

When a vehicle enters the curve from straight, it experiences centrifugal force which tends to throw it outwards thereby causing discomfort to the drivers and the passengers. Transition curve provides smooth transition from straight to the curve of finite radius so that the discomfort level to the drivers and passengers while entering the approach curve due to centrifugal force is minimized.

Spiral shall be used as a transition curve since it satisfies ideally the requirement of a transition, viz., that the radius of curvature is inversely proportion to the length of travel

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and, therefore the rate of change of acceleration is uniform through the length of transition.

Table 2-3 Minimum transition lengths of different speed and curve radii (IRC-38)

Cu

rve

rad

ius

Rc (m

etre

s)

Transition lengths (metres)

Cu

rve

rad

ius

Rc (m

etre

s)

100km/h 80km/h 65km/h 50km/h 40km/h 35km/h 30km/h 25km/h 20km/h

P H P H P H P H P H P H P H P H P H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

14

S P

E E

D

N O

T

A P

P L

I C

A B

L E

S P

E E

D

N O

T

A P

P L

I C

A B

L E

S P

E E

D

N O

T

A P

P L

I C

A B

L E

80 30 14 15 NA 75 30 15 20 NA 35 55 20 20 23 75 -- -- -- 23

25 NA NA 70 25 45 20 25 30 NA 80 30 60 25 40 15 30 33 -- 75 -- -- -- -- -- 33 40 NA 35 60 25 45 20 30 15 40

45 NA 75 30 55 20 40 15 25 15 45 50 40 70 25 50 20 35 15 25 15 50 55 NA 40 60 25 45 20 30 15 20 15 55 60 75 35 55 25 40 15 30 15 20 15 60

70 NA 65 30 50 20 35 15 30 15 15 15 70 80 NA 55 55 25 45 20 30 15 25 15 15 15 80 90 75 45 50 25 40 15 30 15 20 15 15 NR 90

100 70 45 45 20 35 15 25 15 20 15 NR 100

125 NA 55 35 35 15 30 15 20 15 15 NR -- 125 155 80 -- -- -- -- -- -- -- -- -- 155 150 80 45 30 30 15 25 15 20 15 15 -- 150 170 70 40 25 25 15 20 15 15 NR NR -- 170

200 NA 60 35 20 25 15 20 15 15 200 230 90 -- -- -- -- -- -- -- -- 230 250 90 50 30 15 20 15 NR NR NR 250 300 NA 75 40 25 15 NR NR -- 300

350 130 60 35 20 -- -- -- 350 360 130 -- -- -- -- -- -- -- 360 400 115 55 30 20 15 400 500 95 45 25 NR NR 500

600 80 35 20 600 700 70 35 20 700 800 60 30 NR 800 900 55 30 900

1000 50 30 NOTATIONS:

P = PLAIN AND ROLLING TERRAIN H = MOUNTAINOUS AND STEEP TERRAIN NA = RADIUS NOT APPLICABLE NR = TRANSITION NOT REQUIRED

1000 1200 40 NR 1200 1500 35 1500 1800 30 1800 2000 NR 2000

Note: 1. For bridges on Dzongkhag Roads and Farm Roads, where the design speed of

the road is low, transition curves may not be required to be provided.

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2. The length of the transition curve shown in Table 2-3 may be suitably reduced in areas where the space for the approach to the bridge is tight, typical scenarios being having to cut in to steep mountain slopes to obtain the approach curves or for bridges in the town areas where there are permanent structures such as buildings close to the bridge approaches.

2.6.5 Widening of the approaches at the curves

When a vehicle is negotiating a curve, the rear wheels generally do not follow the same track as that of the front wheels and actual position of the rear wheels in relation to that of the front wheels will depend on the speed of the vehicle and the super elevation. Therefore widening of the pavement is necessary to provide for this change in overall width of the track.

For single and double lane bridges, the widening at the approach curves shall be as per Table 2-4.

Table 2-4 Extra width of pavement at horizontal curves (IRC-38)

Note:

1. For multilane bridges, the widening at the approach curves may be calculated by adding half the widening of the two lane bridges to each lane.

2. The widening should be achieved by increasing the width approximately at the uniform rate over the length of the transition curve and the extra width shall be continued over the full length of the circular curve.

3. On curves having no transition curve, the widening shall be achieved in the same way as the super elevation, i.e. two-third being achieved on the straight portion before start of the curve and one-third on the curve.

4. On the hill roads it is preferable that the entire widening is done only on the inside of the curve while on the plain roads the widening shall be applied equally on both sides of the carriageway.

5. The widening shall be applied only on the inside if the curve is plain circular without transition.

6. The widening shall be obtained by offset radial of the centreline and it has to be ensured that the pavement edge lines are smooth and there are no kinks.

Radius of curve (m)

Extra width in metres

Single lane bridge Double lane bridge

Up to 20 0.9 1.5 21 to 40 0.6 1.5 41 to 60 0.6 1.2 61 to 100 nil 0.9 101 to 300 nil 0.6

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3.0 PLANNING AND DESIGN 3.1 Investigation

All the necessary investigations shall be done prior to start of the bridge construction process and the extent and details of such investigations would depend on the site condition, type and scale of the structure and etc. At the end of the investigation phase, the engineer should be able to make reasonable assessment of the conditions under which the bridge is going to be built and take into consideration all the relevant parameters in the planning, design, construction and service stages of the bridge.

3.2 Planning 3.2.1 Selection of bridge location and bridge type

The most important step in the planning process of a bridge is the selection of a suitable bridge location and bridge type. The task shall therefore be entrusted to senior level engineer (s) with relevant qualification and experience. The bridge location and type shall be selected considering the route alignment, the terrain topography, geology, meteorology, crossing objects, fitness to the purpose of use, achievability of the construction quality, ease of maintenance, compatibility with the environment and economy.

3.2.1.1 Bridge location: As regards identifying the location of the bridge in relation to the alignment of approach roads leading to the bridge, following general guide shall be followed:

For small span bridges (span up to 30 m), bridge location shall generally be governed by approach alignments with minimum shifting for improvement of geometrics, if required, unless there are special design problems.

For medium span bridges (span between 30 m and 60 m), requirement of suitable bridge site and proper approach alignments shall be considered together in selecting suitable site for the bridge. In other words, there shall be trade-off between the criteria favouring good bridge location and those favouring suitable approach alignments.

For long span bridges (span more than 60 m), the requirement of most suitable bridge site shall have over-riding consideration and the site so selected shall regulate the approach alignments.

3.2.1.2 Bridge Type:

The following shall be considered in selecting a bridge type:

1. Span of the bridge 2. Constructability 3. Construction cost 4. Maintenance cost 5. Aesthetics 6. Use of local materials 7. Site geology

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8. Local regulations, if any 3.2.2 Involvement of the stake holder agencies

The location of the bridge, span arrangement, pier position, pier shape, space under the bridge, and the likes shall be decided after due consultation with all the stakeholder agencies and the parties that may be affected by the bridge.

Important aspects such as the river training works, vertical clearance when bridging over a road, relocation of utility lines (power, telephone etc.), objects buried underground, underground structures, encroachment on private land and etc. shall be carefully considered during the planning phase. 4.0 Basic principles of design 1. In designing a bridge, consideration shall be given to serviceability, safety of the

structures, durability, achievability of the construction quality, ease of maintenance, environmental compatibility and economy.

Serviceability – the bridge is available for comfortable use by the vehicular traffic

and the pedestrians during its intended service life. Safety – the bridge is structurally safe against all types of loads that it may be

subjected to during its intended service life - dead, live, seismic, wind and so on. Durability – Even if the deterioration from ageing occurs in the bridge, the required

performance can be secured without significant degradation in the serviceability and safety of the structure and its components.

Achievability of construction quality – the structure and its components shall be so

detailed that it is possible to achieve good construction quality with available expertise, equipment and construction technology.

Ease of maintenance – the routine inspections during service, examination of material conditions, repair work and the likes can be easily done. Environmental compatibility – forming a landscape that blends with the peripheral environment. Economy – has minimum life cycle cost. Life cycle cost is the cost of the bridge considering both the construction and maintenance costs.

2. The structures shall normally be designed using Limit State Method of Design as

per IS-456. However, other theoretically valid and/or experimentally verified methods of design may be adopted if that enhances economy or safety or durability of the structures or eases construction process.

While the basis of design and the design process is important for ensuring safe,

serviceable and durable structure, it is equally important to ensure suitable material, good quality control, adequate detailing and good construction supervision.

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Where Limit State Method cannot be adopted conveniently, Working Stress method of design may be adopted.

The experimental based methods, when adopted shall be verified and approved by the engineer-in-charge.

5.0 LOADS The loads and forces to be considered in designing road bridges and culverts:

1. Dead load DL 2. Live load LL 3. Super imposed dead load (railing, kerb, footpath etc.) SIDL 4. Impact factor on vehicular load I 5. Snow load SW 6. Vehicle collision load VC 7. Braking load BK 8. Wind load W 9. Water current WC 10. Centrifugal force CF 11. Buoyancy BO 12. Horizontal earth pressure EH 13. Vertical earth pressure EV 14. Temperature effects T 15. Seismic effects EQ 16. Deformation effects DF 17. Secondary effects SF 18. Erection effects EL 19. Grade effect GE

All members of the structure shall be designed to safely withstand critical combination of the above loads and forces that can co-exist. Only the loads and forces that the structure being designed is likely to be subjected to shall be considered which in turn depends on the site condition and the type of the structure. The Snow Loads (SL) may be based on actual observation or past records in the particular area. Temperature effects (T) in this context is not the frictional force due to the movement of bearings but the forces that are caused by the restraint effects.

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5.1 Combination of Loads and Forces for Limit State Method of design (IRC-6:2014)

5.1.1 Combination of loads for the verification of equilibrium and structural

strength under ultimate state

Loads are required to be combined to check the equilibrium and the structural strength under ultimate limit state. The equilibrium of the structure shall be checked against overturning, sliding and uplift. It shall be ensured that the disturbing loads (overturning, sliding and uplifting) shall always be less than the stabilizing or restoring actions. The structural strength under ultimate limit state shall be estimated in order to avoid internal failure or excessive deformation. The equilibrium and the structural strength shall be checked under basic, accidental and seismic combinations of loads. 5.1.2 Combination Principles

The following principles shall be followed while using these tables for arriving at the combinations: i) All loads shown under Column 1 of Table 5-1 or Table 5-2 or Table 5-3 or

Table 5-4 shall be combined to carry out the relevant verification. ii) While working out the combinations, only one variable load shall be considered

as the leading load at a time. All other variable loads shall be considered as accompanying loads. In case if the variable loads produce favorable effect (relieving effect) the same shall be ignored.

iii) For accidental combination, the traffic load on the upper deck of a bridge (when collision with the pier due to traffic under the bridge occurs) shall be treated as the leading load. In all other accidental situations the traffic load shall be treated as the accompanying load.

iv) During construction the relevant design situation shall be taken into account. v) These combinations are not valid for verifying the fatigue limit state. 5.1.3 Basic Combination

5.1.3.1 For Checking the Equilibrium For checking the equilibrium of the structure, the partial safety factor for loads shown in Column No. 2 or 3 under Table 5-1 shall be adopted.

5.1.3.2 For Checking the Structural Strength For checking the structural strength, the partial safety factor for loads shown in Column No. 2 under Table 5-2 shall be adopted.

5.1.3.3 Accidental Combination For checking the equilibrium of the structure, the partial safety factor for loads shown in Column No 4 or 5 under Table 5-1 and for checking the structural strength, the partial safety factor for loads shown in Column No. 3 under Table 5-2 shall be adopted. 5.1.3.4 Seismic Combination For checking the equilibrium of the structure, the partial safety factor for loads shown in Column No. 6 or 7 under Table 5-1 and for checking the structural strength, the partial safety factor for loads shown I Column No. 4 under Table 5-2 shall be adopted.

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5.1.3.5 Combination of Loads for the Verification of Serviceability Limit State Loads are required to be combined to satisfy the serviceability requirements. The serviceability limit state check shall be carried out in order to have control on stress, deflection, vibration, crack width, settlement and to estimate shrinkage and creep effects. It shall be ensured that the design value obtained by using the appropriate combination shall be less than the limiting value of serviceability criterion as per the relevant code. The rare combination of loads shall be used for checking the stress limit. The frequent combination of loads shall be used for checking the deflection, vibration and crack width. The quasi-permanent combination of loads shall be used for checking the settlement, shrinkage, creep effects and the permanent stress in concrete.

5.1.3.6 Rare Combination For checking the stress limits, the partial safety factor for loads shown in Column No. 2 under Table 5-3 shall be adopted.

5.1.3.7 Frequent Combination For checking the deflection, vibration and crack width in prestressed concrete structures, partial safety factor for loads shown in column no. 3 under Table 5-3 shall be adopted.

5.1.3.8 Quasi-permanent Combinations For checking the crack width in RCC structures, settlement, creep effects and to estimate the permanent stress in the structure, partial safety factor for loads shown in Column No. 4 under Table 5-3 shall be adopted.

5.1.3.9 Combination for Design of Foundations For checking the base pressure under foundation and to estimate the structural strength which includes the geotechnical loads, the partial safety factor for loads for 3 combinations shown in Table 5-4 shall be used. The material safety factor for the soil parameters, resistance factor and he allowable bearing pressure for these combinations shall be as per relevant Indian code.

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Table 5-1 Partial Safety Factor of Verification of Equilibrium

Loads Basic Combination Accidental Combination Seismic Combinations (1) (2) (3) (4) (5) (6) (7)

Overturning or Sliding or Uplift Effect

Restoring or Resisting Effect





Permanent Loads: DL, SL, SIDL except surfacing, backfill weight, settlement, creep & Shrinkage Surfacing Earth pressure due to backfill

1.05

1.35

1.50

0.95

1.00 -

1.00

1.00

1.00

1.00

1.00 -

1.05

1.35

1.00

0.95

1.00 -

Variable Loads: Carriageway LL, associated loads (braking, tractive and centrifugal forces), pedestrian LL a) As a Leading Load b) As accompanying Load c) Construction Live Load Thermal Loads a) As a Leading Load b) As accompanying Load

Wind a) As leading Load b) As accompanying Load LL surcharge effects ( as accompanying load)

1.50 1.15 1.35

1.50 0.90

1.50 0.90

1.20

0 0 0 0 0 0 0 0

0.75 0.20 1.00

-

0.50 - - -

0 0 0 - 0 - - -

-

0.20 1.00

-

0.50 - - -

- 0 0 - 0 - - -

Accidental Effects: i) Vehicle collision or ii) Barge Impact or iii) Impact due to floating

objects

-

-

1.00

-

-

-

Seismic Effects: a) During Service b) During Construction

- -

- -

- -

- -

1.50 0.75

- -

Construction Conditions: Counter weights- a) When density or self

weight is well defined b) When density or self

weight is not well defined

c) Erection effects Wind a) As leading Load b) As accompanying Load

- -

1.05

1.50 1.20

0.90

0.80

0.95 0 0

- - - - -

1.00

1.00 - - -

- - - - -

1.00

1.00 - - -

Hydraulic Loads: (accompanying load) a) Water current forces b) Wave pressure c) Hydrodynamic effect d) Buoyancy

1.00 1.00

- 1.00

0 0 - -

1.00 1.00

- 1.00

- - - -

1.00 1.00 1.00 1.00

- - - -

Notes:

1. For combination principles, refer Clause 5.1.2 2. Wind and thermal load need not be taken simultaneously 3. Partial safety factor for prestress and secondary effect of prestress shall be as recommended in IRC-112:2011 4. Seismic effect during erection stage is reduced to half when construction phase doesn’t exceed 5 years 5. Where ever Snow Load is applicable, Clause 5.15 shall be referred for combination of SL and LL

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Table 5-2 Partial Safety Factor of Verification of Structural Strength (Ultimate Limit State)

Loads Basic Combination Accidental Combination

Seismic Combinations

(1) (2) (3) (4)

Permanent Loads: DL, SL, SIDL except surfacing a) Adding to the effect of the variable load b) Relieving the effect of variable load Surfacing: a) Adding to the effect of the variable load b) Relieving the effect of variable load

Backfill weight Earth pressure due to backfill: a) Leading load b) Accompanying load

1.35 1.00

1.75 1.00

1.50

1.50 1.00

1.00 1.00

1.00 1.00

1.00

-

1.00

1.35 1.00

1.75 1.00

1.00

1.00 1.00

Variable Loads: Carriageway LL, associated loads (braking, tractive and centrifugal forces), pedestrian LL a) As a Leading Load b) As accompanying Load c) Construction Live Load Wind during service and construction a) As leading Load b) As accompanying Load LL surcharge effects ( as accompanying load) Erection effects

1.50 1.15 1.35

1.50 0.90

1.20

1.00

0.75 0.20 1.00

- -

0.20

1.00

0

0.20 1.00

- -

0.20

1.00 Accidental Effects: i) Vehicle collision or ii) Barge Impact or iii) Impact due to floating objects

-

1.00

-

Seismic Effects: a) During Service b) During Construction

- -

- -

1.50 0.75

Hydraulic Loads: (accompanying load) a) Water current forces b) Wave pressure c) Hydrodynamic effect d) Buoyancy

1.00 1.00

- 0.15

1.00 1.00

- 0.15

1.00 1.00 1.00 0.15

Notes:

1. For combination principles, refer Clause 5.1.2 2. Partial safety factor for prestress and secondary effect of prestress shall be as recommended in IRC-112:2011 3. Where ever Snow Load is applicable, Clause 5.15 shall be referred for combination of SL and LL

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Table 5-3 Partial Safety Factor of Verification of Serviceability Limit State



(1) (2) (3) (4)

Permanent Loads: DL, SL, SIDL including surfacing Backfill weight Shrinkage and Creep effects Earth pressure due to backfill Settlement Effects a) Adding to the permanent loads b) Opposing the permanent loads

1.00

1.00

1.00

1.00

1.00 0

1.00

1.00

1.00

1.00

1.00 0

1.00

1.00

1.00

1.00

1.00 0

Variable Loads: Carriageway LL, associated loads (braking, tractive and centrifugal forces), pedestrian LL a) As a Leading Load b) As accompanying Load Thermal Loads a) As a Leading Load b) As accompanying Load Wind a) As leading Load b) As accompanying Load LL surcharge effects ( as accompanying load)

1.00 0.75

1.00 0.60

1.00 0.60

0.80

0.75 0.20

0.60 0.50

0.60 0.50

0

- 0 -

0.50 - 0 0

Hydraulic Loads (accompanying load): a) Water current forces b) Wave pressure c) Buoyancy

1.00 1.00 0.15

1.00 1.00 0.15

- -

0.15

Notes:

1. For combination principles, refer Clause 5.1.2 2. Partial safety factor for prestress and secondary effect of prestress shall be as recommended in IRC-112:2011 3. Where ever Snow Load is applicable, Clause 5.15 shall be referred for combination of SL and LL 4. Thermal load includes restraints associated with expansion/contraction due to type of construction (portal, arch

& elastomeric bearings), frictional restraint in metallic bearings and thermal gradients. The combination is however not valid for the design of bearing and expansion joints.

5. Wind and thermal loads need not be taken simultaneously

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Table 5-4 Combination for Base Pressure and Design of Foundations



(1) (2) (3) (4)

Permanent Loads: DL, SL, SIDL except surfacing Surfacing Settlement effect Earth pressure due to backfill a) Adding to the permanent loads b) Opposing the permanent loads

1.35

1.75

1.00 or 0

1.50 1.00

1.35

1.00

1.00 or 0

1.30 0.85

1.00

1.00

1.00 or 0

-

1.00 Variable Loads: Carriageway LL, associated loads (braking, tractive and centrifugal forces), pedestrian LL a) As a Leading Load b) As accompanying Load Thermal Loads as accompanying load Wind a) As leading Load b) As accompanying Load LL surcharge as accompanying load (if applicable) Accidental effect or Seismic effect Seismic effect during construction Erection effects

1.50 1.15

0.90

1.00 0.90

1.20 - -

1.00

1.30 1.00

0.80

1.30 0.80

1.00 - -

1.00

0.75(if applicable) or 0 0.2

0.50

- 0

0.20

1.00

0.50

1.00

Hydraulic Loads: a) Water current forces b) Wave pressure c) Hydrodynamic effect d) Buoyancy:

For base Pressure For Structural Design

1.00 or 0 1.00 or 0

-

1.00 0.15

1.00 or 0 1.00 or 0

-

1.00 0.15

1.00 or 0 1.00 or 0 1.00 or 0

1.00 0.15

Notes:

1. For combination principles, refer Clause 5.1.2 2. Where two partial factors are indicated for loads, both these factors shall be considered for arriving at the severe

effects 3. Wind and thermal loads need not be taken simultaneously 4. Partial safety factor for prestress and secondary effect of prestress shall be as recommended in IRC-112:2011 5. Where ever Snow Load is applicable, Clause 5.15 shall be referred for combination of SL and LL 6. Seismic effect during erection is reduced to half when construction phase does not exceed 5 years 7. For repair, rehabilitation and retrofitting, the load combination shall be project specific.

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5.2 DEAD LOAD (DL)

The dead load includes the weight of all bridge components, appurtenances and utilities, wearing surface and future overlays. The following unit weights of the materials shall be used in determining loads, unless the unit weights have been determined by actual weighing of representative samples of the materials in question, in which case the actual weights as thus determined shall be used:- Sl. No. Material Unit Weight

1. Steel (cast or rolled) 77.0 2. Cast iron 71.0 3. Aluminum 27.5 4. Reinforced concrete 24.5 5. Prestressed concrete 24.5 6. Plain cement concrete 23.0 7. Cement mortar 21.0 8. Wood 8.0 9. Asphalt pavement 22.5 10. Brickwork (pressed) in cement mortar 21.6 11. Brickwork (common) in cement mortar 18.6 12. Earth (compacted) 19.6 13. Gravel 17.7 14. Macadam (binder premix) 21.6 15. Macadam (rolled) 25.5 16. Sand (loose) 13.7 17. Sand (well compressed) 18.6 18. Coursed rubble stone masonry (cement mortar) 25.5 19. Water 9.8

Note:

1. Dead load has a large influence in design of a bridge. Therefore it is important to use appropriate value for the unit weight of the materials.

2. The unit weight of wood differs with type and age of the tree and the moisture content. Unit weight of 8.0KN/m3 is somewhat excessive for ordinary wood; however this value includes the weight of nails, clamps, bolts, buts etc.

5.3 LIVE LOAD (LL)

The live load shall consist of vehicle load and the pedestrian load on the footpaths

5.3.1 Vehicular Live Load:

The bridges shall be designed for Indian Road Congress (IRC) vehicular loading.

The combination of the Live Loads for various classifications of roads shall be as per Table 5-5

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Table 5-5 Live load combination Sl. No.

Road classification Carriage width (m)

No. of lanes for design purpose

Load combination

1 Asian Highway 7.50 2 One lane of IRC Class 70R (wheeled) or two lanes of IRC Class A

2 Primary National Highway (PNH)

7.00 2 One lane of IRC Class 70R (wheeled) or two lanes of IRC Class A

3 Secondary National Highway (SNH)

5.50 2 One lane of IRC Class 70R (wheeled) or two lanes of IRC Class A

4 Dzongkhag Road 3.50 1

One lane of IRC Class A considered to occupy 2.3m. The remaining width of carriageway shall be loaded with 500kg/m2 distributed area load.

5 Farm Road 3.50 1

One lane of IRC Class B considered to occupy 2.3 m. The remaining width of carriageway shall be loaded with 500kg/m2 distributed area load.

Note: For bridges with carriageway width other than the ones mentioned above, number of lanes to be considered for design purpose and the load combination shall be as per Table 2, IRC-6:2014. 5.3.1.1 IRC Class A train of vehicles:

Figure 5-1 IRC Class A train vehicle

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Notes:

1. The nose to tail distance between successive trains shall not be less than 18.50 m 2. The ground contact area of the wheels shall be as under:

Axle load (tonne)

Ground contact area

B (mm) W (mm)

11.4 250 500 6.8 200 380 2.7 150 200

3. The minimum clearance, f, between outer edge of the wheel and the roadway

face of the kerb and the minimum clearance, g, between the outer edges of the passing of passing vehicles on the multi-lane bridges shall be as given below:- Clear carriageway width g f

5.5m to 7.5m Uniformly increasing from 0.4m to 1.2m

150mm for all carriageway widths

Above 7.5m 1.2m

5.3.1.2 IRC Class B train of vehicles:

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Figure 5-2 IRC Class B train vehicle Notes:

1. The nose to tail distance between successive trains shall not be less than 18.50 m 2. The ground contact area of the wheels shall be as under:

Axle load (tonne)

Ground contact area

B (mm) W (mm)

6.8 200 380 4.1 150 300 1.6 125 175

3. The minimum clearance, f, between outer edge of the wheel and the roadway

face of the kerb and the minimum clearance, g, between the outer edges of the passing of passing vehicles on the multi-lane bridges shall be as given below:- Clear carriageway width g f

5.5m to 7.5m Uniformly increasing from 0.4m to 1.2m

150mm for all carriageway widths

Above 7.5m 1.2m

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5.3.1.3 IRC 70R (wheeled):

Figure 5-3 Axle spacing and tyre contact area for Class 70R vehicle Notes:

1. The loads are in tonnes and the spacing in millimetres. 2. The possible variations in the wheel spacing and tyre sizes, for the heaviest

single axle – col. (i), the heaviest bogie axle – cols. (ii) and also for the heaviest axles of train vehicle (Figure 5-3) are given in cols. (iii), (iv) and (v). The same pattern of wheel arrangement may be assumed for all axles of the wheel train shown in Figure 5-3.

3. The contact areas of tyres on the deck may be obtained from the corresponding tyre loads, maximum tyre pressure and the width of the tyre treads.

4. The first dimension of the tyre size refers to the overall width of the tyre and second dimension to the rim diameter of the tyre. Tyre tread width may be taken as overall tyre width minus 25mm up to 225mm width, and minus 50mm for tyres over 225mm width.

5. The spacing between the successive vehicles shall not be less than 30m. This spacing shall be measured from the centre of the rear-most axle of the leading vehicle to the centre pf the first axle of the following vehicle for the wheeled vehicles.

6. The minimum clearance, f, between the road face of the kerb and the outer edge of the wheel shall be 0.3m when there is only one lane of Class 70R train vehicle moving on the bridge.

Clear carriageway width Minimum value of ‘f’(m)

Single lane bridges – Up to width of 5.3 m

0.3

Multilane bridges- More than 5.3 m

1.2 m

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5.3.1.4 Live load position on the bridge 1. Within the kerb to kerb width of the roadway, the standard vehicle or train shall be

assumed to travel parallel to the length of the bridge and to occupy any position which will produce maximum stresses

2. Vehicles in adjacent lanes shall the taken as headed in the direction producing maximum stresses.

3. The spaces on the carriageway left uncovered by the standard train of vehicles shall not be assumed as subject to any additional live load unless otherwise specified in

Table 5-5 5.3.1.5 Dispersion of Load through fills of arch bridges The dispersion of loads through the fills above the arch shall be assumed at 45 degrees both along and perpendicular to the span in the case of arch bridges. 5.3.1.6 Reduction in the longitudinal effect on bridges accommodating more than two

traffic lanes For bridges having more than two traffic lanes, the probability that all the lanes would be subjected to the characteristics load simultaneously is low. Therefore, reduction in the longitudinal effect on bridges having more than two traffic lanes shall be in accordance with the table shown below:-

Number of lanes Reduction of longitudinal effect For two lanes No reduction For three lanes 10% reduction For four lanes 20% reduction For five or more lanes 20% reduction

Notes:

1. It should be ensured that the reduced longitudinal effects are not less severe than the longitudinal effect resulting from simultaneous loads on two adjacent lanes. Longitudinal effects mentioned above are Bending Moment, Shear Force and Torsion in longitudinal direction.

2. The above table is applicable for individually supported superstructure of multi-laned carriageway. In case of separate sub-structure and foundations, the number of lanes supported by each of them is to be considered while working out the reduction percentage.

5.3.2 Footpath, Kerb, Railings, and Crash Barriers 5.3.2.1 Footpath Live Loads 1. For design of slab or floor system, the footpath shall be loaded with a uniform load

of 5.0 KN/m2 2. For design of the main girders, trusses, arches or other members supporting the

footways, the uniform load to be considered shall be as under:

Span Length L (m) L ≤ 80 80 < L ≤ 130 130 < L Uniform load (KN/m2) 3.5 4.3-0.01L 3.0

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5.3.2.2 Kerbs 1. Kerbs, 0.6m or more in width, shall be designed for same live load as footpath. 2. Irrespective of the width, the kerbs shall be designed for a local lateral force of 7.5

KN/m. But the horizontal force need not be considered for design of the main structural members of the bridge.

5.3.2.3 Crash Barriers The crash barrier, if required to be provided, shall be category P-2: Low containment (containment for 150KN vehicle at 80km/h and 20° angle of impact) as per IRC-6 for all classification of roads. The shape on the traffic side shall be as per IRC-5 or New Jersey (NJ) type of “F” shape as designated by AASHTO.

5.3.3 Impact Load (I) Provision for impact or dynamic action shall be made by an increment of the live load by an impact allowance expressed as a fraction or a percentage of the applied live load.

No impact allowance shall be added to the footway loading.

5.3.3.1 Class A or B Loading

6.3.3.1.1 For Reinforced Concrete Bridges: Span, L (m) L < 3 3 ≤ L ≤ 45 L > 45

Impact fraction/percentage

50% L6

5.4

8.8%

6.3.3.1.2 For Steel Bridges:

Span, L (m) L < 3 3 ≤ L ≤ 45 L > 45


54.5% L5.13

9

15.4%

Where L in length in meters of span as specified in Clause 5.3.4 5.3.3.2 Class 70R Loading (wheeled vehicles)

For Reinforced Concrete Bridges: Span, L (m) L ≤ 12 12 < L ≤ 45 L > 45


25% L6

5.4

8.8%

For Steel Bridges: Span, L (m) L ≤ 23 23 < L ≤ 45 L > 45


25% L5.13

9

15.4%

Where L in length in meters of span as specified in Clause 6.5.3

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5.3.4 Span length (L) to be considered for calculating the impact percentages

a. For spans simply supported or continuous or for arches …………….the effective span on which the load is placed.

b. For bridges having cantilever arms without suspended spans ………….the effective overhang of the cantilever arms reduced by 25% for loads on the cantilever arms and the effective span between supports for loads on the main span.

c. For bridges having cantilever arms with suspended span ……………….the effective overhang of the cantilever arm plus half the length of the suspended span for loads on the cantilever arm, the effective length of the suspended span for loads on the suspended span and the effective span between supports for load on the main span.

d. For individual members of a bridge, such as, cross girder or deck slab etc..………….the value of L shall be the effective span of the member under consideration.

5.3.5 Impact consideration under special circumstances

1. In any bridge structure where there is a filling of not less than 0.6m including the road crust, the impact percentage to be allowed in design shall be assumed to be one-half of what is specified in Clause 5.3.3

2. In the design of members subjected to among others stresses, direct tension, such as, hangers in a bowstring girder bridge and in the design of member subjected to direct compression, such as, spandrel columns or walls in an open spandrel arch, the impact percentage shall be taken the same as that applicable to the design of the corresponding member or members of the floor system which transfer loads to the tensile or compressive members in question.

3. The clauses of impact covered in this specification do not apply to the design of suspension bridges. In cable suspended bridges and in other bridges where live load to dead load ratio is high, the dynamic effects, such as, vibration and fatigue shall be considered.

4. For calculating pressure on the bearings and on the top surface of the bed blocks, full value of the appropriate impact percentage shall be allowed. But, for the design of piers, abutments and structures, generally below the level of the top of the bed block, the appropriate impact percentage shall be multiplied by the factor given below:-

a) For calculating the pressure at the bottom surface of the bed block ….. 0.5

b) For calculating the pressure on the top 3m of the structure below the bed block ……. 0.5 decreasing uniformly to zero

c) For calculating the pressure on the portion of the structure more than 3m below the bed block ……. Zero

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5.4 WIND LOAD (WL)

5.4.1 General Notes

1. This clause is applicable to normal span bridges with individual span length up to 150 m or for bridges with height of pier up to 100m. For all other bridges including cable stayed, suspension, ribbon and other flexible bridges, specialist literature shall be used for computation of design wind load.

2. The wind pressure acting on a bridge depends on the geographical locations, the terrain or surrounding area, the fetch of terrain upwind of the site location, the local topography, the height of the bridge above the ground, the horizontal dimensions and cross-section of bridge or its element under consideration.

3. The maximum pressure is due to gusts that cause local and transient fluctuations about the mean wind pressure.

4. The forces due to wind shall be considered to act in such direction that the resultant in the members under consideration are maximum.

5. In addition to application of the prescribed loads in the design of bridge elements, stability against overturning, uplift and sliding due to wind shall be considered.

5.4.2 Wind Speed and Wind Pressure

The basic wind speed shall be taken as 47m/s for return period of 100 years. The intensity of wind force shall be based on hourly mean wind speed and pressure as shown in Table 5.4-1. Table 5.4-1 Hourly Mean Wind Speed and Wind Pressure (For a basic wind speed

of 47m/sec)

H (m)

Bridge located in Plain terrain Terrain with obstructions Hilly/Mountainous

terrain Vz (m/s) Pz (N/mm2) Vz (m/s) Pz (N/mm2) Vz (m/s) Pz (N/mm2)

Up to 10m 39.60 940.90 25.35 385.55 27.75 462.65 15 41.60 1038.35 27.90 467.05 30.55 560.45 20 43.15 1117.15 29.90 536.40 32.75 643.70 30 44.70 1198.85 32.45 631.80 35.55 758.15 50 47.10 1331.10 35.45 754.00 38.85 904.80 60 47.85 1373.80 36.45 797.15 39.95 956.60 70 48.40 1405.55 37.30 834.80 40.85 1001.7580 49.00 1440.60 38.30 880.15 41.95 1056.20 90 49.70 1482.05 39.15 919.65 42.90 1103.60

100 50.30 1518.05 40.15 967.20 44.00 1160.65

H = the average height in metres of exposed surface above the mean retarding surface (ground or bed or water level)

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Vz = hourly mean speed or wind in m/s at height H

Pz = horizontal wind pressure in N/m2 at height H

NOTES:

1. Intermediate values may be obtained by interpolation

2. Plain terrain refers to open terrain with no obstruction or with well scattered obstructions having height up to 10m.

3. Terrain with obstructions refers to a terrain with numerous closely spaced structures, forests or trees up to 10m in height with few isolated tall structures or terrain with large number of high closed spaced construction like structures, trees, forests etc.

4. Hilly/Mountainous terrain refers to the topography at the structure site like hill, ridge escarpment or cliff that can cause acceleration or funneling of wind.

5. For construction stages, the hourly mean wind pressure shall be taken s 70% of the values as sated above.

6. For the design of foot over bridges in the urban situations and in plain terrain, a minimum horizontal wind load of 1.50 KN/m2 and 2 KN/m2 respectively shall be considered to be acting on the frontal area of the bridge.

5.4.3 Design Wind Force on Superstructure

The superstructure shall be designed for wind induced horizontal forces (acting in transverse and longitudinal directions) and vertical loads acting simultaneously.

The assumed wind direction shall be perpendicular to longitudinal axis for a straight structure or to an axis chosen to maximize the wind induced effects for a structure curved in plan.

5.4.3.1 Estimation of Transverse Wind Force The transverse wind force FT (in N) shall be taken as acting at the centroids of the appropriate areas and horizontally and shall be estimated from:

FT = PZ x A1 x G x CD Where, PZ = the hourly mean wind pressure in N/m2 (see Table 5.4-1) A1 = the solid area in m2 (see Clause 5.4.3.1.3) G = gust factor CD = drag coefficient depending on the geometric shape of the bridge deck 5.4.3.1.1 Gust Factor (G): For bridges up to a span of 150 m, which are generally not sensitive to dynamic action of wind, the gust factor shall be taken as 2.0.

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5.4.3.1.2 Drag Coefficient (CD): Table 5.4-2 CD for normal deck sections

Bridge Deck Type

Slab bridges with b/d ≤10

Decks supported by single beam or box

Decks supported by two or more beams or box girders

Plate girders

b/d=2 b/d≤6 Single girder

Two or more girders

CD

Value 1.1 1.5 1.3 1.5 x of CD for single beam or box but ≥ 1.3

2.2 2(1+c/20D) but ≤ 4

b = width of the deck slab d = depth of the deck slab c = centre to centre distance of the adjacent girders D = depth of the windward girder

For decks supported by single beam or box, the intermediate values for CD shall be obtained by interpolation

Note: 1. For truss girder superstructure, the CD value shall be determined as per Table

5.4-3. 2. For other type of deck cross-sections CD shall be ascertained either from

wind tunnel tests or specialist literature shall be referred to.

5.4.3.1.3 Net/effective area for transverse wind force (A1):

1. For a deck structure:

The area of the structure as seen in elevation including the floor system and railing, less area of perforations in hand railing or parapet walls shall be considered. For open and solid parapets, crash barriers and railings, the solid area in normal projected elevation of the element shall be considered.

2. For truss structures:

Appropriate area as specified Clause 5.4.5 shall be taken

3. For construction stages:

The area at all stages of construction shall be the appropriate unshielded solid area of the structure.

5.4.3.2 Longitudinal Wind Force The longitudinal force on bridge superstructure FL (in N) shall be taken as 25% of the transverse wind load as calculated as per Clause 5.4.3.1 for beam/box/plate girder bridges and 50% of the transverse load as calculated as per Clause 5.4.5 for truss girder bridges.

5.4.3.3 Vertical Wind Load An upward or downward vertical wind load FV (in N) acting at the centroid of the appropriate areas, for all superstructures shall be derived from:

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FV = PZ x A3 x G x CL

Where, PZ = the hourly mean wind pressure in N/m2 at height H (See Table 5.4-1) A3 = the area in plan in m2

CL = the lift coefficient which shall be taken as 0.75 for normal type slab, box, I-girder and plate girder bridges. For other deck cross-sections CL shall be ascertained ether from wind tunnel tests or specialist literature may be referred to.

G = Gust factor as defined in Clause 5.4.3.1.1

5.4.4 Wind Effect on Live Load

1. The transverse wind load per unit exposed area of the live load FT shall be calculated using the same expression given in Clause 5.4.3.1 except that CD in this case shall be taken as 1.2. The exposed frontal area of the LL shall be the entire length of the superstructure seen in elevation in the direction of wind or any part of the length producing critical response, multiplied by a height of 3.0 m above the road way surface. Areas below the top of the solid barrier shall be neglected.

2. The longitudinal wind load on LL shall be taken as 25% of transverse wind load as calculated above.

3. Both the above loads shall be applied simultaneously acting at 1.5m above the roadway.

4. The bridges shall not be considered to be carrying any LL when the wind speed at deck level exceeds 36 m/s.

5. In case of cantilever construction, an upward wind pressure of PZ x CL x G N/m2

on bottom soffit area shall be assumed on stabilizing cantilever arm in addition to the transverse wind effect calculated as per Clause 5.4.3.1. In addition to the above, the construction loads and the erection stresses shall also be taken in to consideration.

5.4.5 Wind Load Computation on Truss Bridge Superstructure

5.4.5.1 Superstructures without live load: The design transverse wind load FT shall be derived separately for the areas of the windward and leeward truss girder and deck elements. Except that FT need not be derived considering the projected areas of windward parapet shielded by windward truss, or vice versa, deck shielded by the windward truss, or vice versa and leeward truss shielded by the deck.

The area A1 for each truss, parapet etc. shall be the solid area in normal projected elevation. The area A1 for the deck shall be based on the full depth of the deck.

5.4.5.2 Superstructures with live load: The design transverse wind load shall be derived separately for elements as specified in Clause 5.4.5.1 and also for the live load depth. The area A1 for the deck, parapets,

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trusses etc. shall be as for the superstructure without live load. The area A1 for the live load shall be derived using the appropriate live load depth.

5.4.5.3 Drag Coefficient CD for All Truss Girder Superstructures

5.4.5.3.1 Superstructures without live load: The drag coefficient CD for each truss and for the deck shall be derived as follows:

For a windward truss CD shall be taken from Table 5.4-4. For leeward truss of a superstructure with two trusses, drag coefficient shall be taken as ηCD. Values of shielding factor η are given in Table 5.4-4

The solidity ratio of the truss is the ratio of the effective area to the overall area of the truss.

Where a superstructure has more than two trusses, the drag coefficient for the truss adjacent to the windward truss shall be derived as specified above. The coefficient for all other trusses shall be taken as equal to this value.

For Deck Construction, the drag coefficient shall be taken as 1.1.

5.4.5.3.2 Superstructure with live load: The drag coefficient CD for each truss and for the deck shall be as for the superstructure without live load. CD for the unshielded parts of the live load shall be taken as 1.45. Table 5.4-3 Force Coefficients for Single Truss Solidity Ratio ( Ф )

Drag Coefficient CD for

Built-up Sections

Rounded Members of Diameter (d) Subcritical flow (dVz<6m2/S

Supercritical flow (dVz>6m2/S

0.1 1.9 1.2 0.7 0.2 1.8 1.2 0.8 0.3 1.7 1.2 0.8 0.4 1.7 1.1 0.8 0.5 1.6 1.1 0.8

NOTES:

1. Linear interpolation between values is permitted 2. The solidity ratio of the truss is the ratio of the net area to overall area of the

truss Table 5.4-4 Shielding Factor η for Multiple Trusses Truss Spacing Ratio

Value of η for Solidity Ratio 0.1 0.2 0.3 0.4 0.5

<1 1.0 0.90 0.80 0.60 0.45 2 1.0 0.90 0.80 0.65 0.50 3 1.0 0.95 0.80 0.70 0.55 4 1.0 0.95 0.85 0.70 0.60 5 1.0 0.95 0.85 0.75 0.65 6 1.0 0.95 0.90 0.80 0.70

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NOTES;

1. Linear interpolation between values is permitted 2. The truss spacing ratio is the distance between centers of trusses divided by

depth of the windward truss 5.4.6 Design Wind Forces on Substructure

The substructure shall be designed for wind induced loads transmitted to it from the superstructure and wind loads acting directly on the substructure. Loads for wind directions both normal and skewed to the longitudinal centerline of the superstructure shall be considered. FT shall be computed using expression in Clause 5.4.3.1 with A1 taken as the solid area in normal projected elevation of each pier. No allowance shall be made for shielding.

For piers, CD shall be taken from Table 5.4-5. For piers with cross-section dissimilar to those given in Table 5.4-5, CD shall be ascertained either from wind tunnel tests or specialist literature shall be referred to.

CD shall be derived for each pier, without shielding.

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Table 5.4-5 Drag Coefficient CD for Piers (IRC 6: 2010)

NOTES:

1) For rectangular piers with rounded corners with radius r, the value of CD derived from Table 5.4-5 shall be multiplied by (1-1.5 r/b) or 0.5, whichever is greater.

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2) For a pier with triangular nosing, CD shall be derived as for the rectangle encompassing the outer edges of pier

3) For pier tapering with height, CD shall be derived for each of the unit heights into which the support has been subdivided. Mean values of t and b for each unit height shall be used to evaluate t/b. The overall pier height and mean breadth of each unit height shall be used to evaluate height/breadth.

4) After construction of the superstructure CD shall be derived for height to breadth ration of 40.

5.4.7 Wind Tunnel Testing Wind tunnel testing by established procedures shall be conducted for dynamically sensitive structures such as cable stayed, suspension bridges etc., including modeling of appurtenances. 5.5 HORIZONTAL FORCES DUE TO WATER CURRENTS Any part of a road bridge which may be submerged in running water shall be designed to sustain safely the horizontal pressure due to the force of the current.

5.5.1 Calculation of Intensity of pressure 5.5.1.1 Piers parallel to the water current On piers parallel to the direction of the water current, the intensity of pressure shall be calculated from the following equation:

P = 52KV2 Where, P = intensity of pressure due to water current, in kg/m2

V = the velocity of the current at the point where the pressure intensity is being calculated, in meter per second, and K = a constant having the values for different shapes of piers given in Table 5.5-1. Table 5.5-1 Value of Coefficient “K” Sl. No

Pier shape Value of coefficient ‘K’ Description Figure

i. Square ended pier (and for superstructure)

1.5

ii. Circular piers or piers with semi-circular ends

0.66

iii. Piers with triangular cut, the angle included between the faces, α ≤ 30°

α

0.50

iv. Piers with triangular cut, the angle included between the faces, 30°< α < 60°

α

0.50 to 0.70

v. Piers with triangular cut, the angle included between the faces, 60°< α < 90°

α

0.70 to 0.90

vi. Piers with cut of equilateral arcs of circles

0.45

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The value of V2 in the equation given in Clause 5.5.1.1 shall be assumed to vary linearly from zero at the point of deepest scour to the square of the maximum velocity at the free surface of water. The maximum velocity for the purpose of this clause shall be assumed to be √2 times the maximum mean velocity of the current.

Figure 5-4 Variation of Flow velocity (IRC-6:2014) 5.5.1.2 Piers at angle to the water current When the current strikes the pier at an angle, the velocity of the current shall be resolved into two components – one parallel and the other normal to the pier.

a) The pressure parallel to the pier shall be determined as indicated in Clause 5.5.1.1 taking the velocity as the component of the velocity of the current in a direction parallel to the pier.

b) The pressure of the current, normal to the pier and acting on the area of the side elevation of the pier, shall be calculated similarly taking the velocity as the component of the velocity of the current in a direction normal to the pier, and the constant K as 1.5, except in the case of circular piers where the constant shall be taken as 0.66

5.5.1.3 Other considerations

i. To provide against possible variation of the direction of the current from the direction assumed in the design, allowance shall be made in the design of piers for an extra variation in the current direction of 20 degrees that is to say, piers intended to be parallel to the direction of current shall be designed for variation of 20 degrees from the normal direction of current and piers originally intended to be inclined at 0 degrees to the length of the pier.

ii. In case of a bridge having an inerodible bed, the effect of cross-currents shall in no case be taken as less than that of a static force due to a difference of head of 250 mm between the opposite faces of a pier.

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iii. When supports are made with two or more piles or trestle columns, spaced closer than three times the width of piles/columns across the direction of flow, the group shall be treated as a solid rectangle of the same overall length and width and the value of K taken as 1.25 for calculating pressures due to water currents, both parallel and normal to the pier. If such piles/columns are braced, then the group should be considered as a solid pier, irrespective of the spacing of the columns.

5.6 LONGITUDINAL FORCES In all road bridges, provision shall be made for longitudinal forces arising from any one or more of the following causes:

a) Tractive effort caused through acceleration of the driving wheels;

b) Braking effect resulting from the application of the brakes to braked to braded wheels;

c) Frictional resistance offered to the movement of free bearings due to change of temperature or any other cause.

NOTE : Braking effect is invariably greater than the tractive effort.

5.6.1 Braking Effect

The braking effect on a simply supported span or a continuous unit of spans or on any other type of bridge unit shall be assumed to have the following value:

a) In the case of a single lane or a two lane bridge : 20% of the first train load plus 10% of the load of the succeeding trains or part thereof, the train loads in one lane only being considered for the purpose of this sub-clause. Where the entire first train is not on the full span, the braking force shall be taken as equal to 20% of the loads actually on the span or continuous unit of spans.

b) In the case of bridges having more than two-lanes: As in (a) above for the first two lanes plus 5% of the loads on the lanes in excess of two.

NOTE : The loads in this Clause shall not be increased on account of impact.

5.6.2 Point of application of braking force: The force due to braking effect shall be assumed to act along a line parallel to the roadway and 1.2 m above it.

While transferring the force to the bearings, the change in the vertical reaction at the bearings should be taken into account. 5.6.3 Calculation of Longitudinal Forces under different support conditions The distribution of longitudinal horizontal forces among bridge supports is effected by the horizontal deformation of bridges, flexing of the supports and rotation of the foundations.

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5.6.3.1 Simply Supported and Continuous Spans on Unyielding Supports

5.6.3.1.1 Simply supported spans on unyielding supports 1. For a simply supported span with fixed and free bearings (other than elastomeric

type) on stiff supports, horizontal forces at the bearing level in the longitudinal direction shall be greater of the two values given below:

Fixed bearing Free bearing

i) Fh – μ(Rg + Rq) μ (Rq +Rg) Or ii) Fh/2 + μ (Rg +Rq) μ (Rg + Rq) Where, Fh = Applied Horizontal force Rg = Reaction at the free end due to dead load Rq = Reaction at free end due to live load μ = Coefficient of friction at the movable bearing which hall be assumed to have the following values:

i) For steel roller bearings 0.03 ii) For concrete roller bearings 0.05 iii) For sliding bearings:

a) Steel on cast iron or steel o steel 0.4 b) Gray cast iron (Mechanite) 0.3 c) Concrete over concrete with bitumen layer in between 0.5 d) Teflon on stainless steel 0.03 and 0.05 whichever is governing

NOTE:

a) For design of bearings, the corresponding forces may be taken as per relevant codes.

b) Unbalanced dead load shall be accounted for properly. The structure under the fixed bearing shall be designed to withstand the full seismic and design braking/tractive for force.

2. In case of simply supported small spans up to 10 m resting on unyielding supports

and where no bearings are provided, horizontal force in the longitudinal direction at the bearing level shall be

Fh/2 or μRg whichever is greater

3. For a simply supported span sitting on identical elastomeric bearings at each end

resting on unyielding supports. Force at each end

= Fh/2+ Vr ltc Vr = shear rating of the elastomeric bearings ltc = movement of deck above bearing, other than that due to applied forces

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4. The substructure and foundation shall also be designed for 10 percent variation in movement of the span on either side.

5.6.3.1.2 For continuous bridges with one fixed bearing or other free bearings (IRC-6:2000):

Fixed bearing Free bearing

Case-I (μR –μL) +ve Fh acting in +ve direction (a) If Fh > 2 μR Fh – (μR +μL) (b) If Fh < 2μR

R

h

n

F

1+ (μR - μL)

μRx

Case-II (μR - μL) +ve Fh acting in –ve direction (a) If Fh > 2 μL Fh – (μR + μL) (b) If Fh < 2μL

L

h

n

F

1+ (μR + μL)

Whichever is greater

μRx

Where nL or nR = number of free bearings to the left or right of fixed bearings, respectively μL or μR = the total horizontal force developed at the free bearing to the left or right

of the fixed bearing respectively μRx = the net horizontal force developed at any one of the free bearings

considered to the left or right of the fixed bearings NOTE : In seismic areas, the fixed bearing shall also be checked for full seismic force

and braking/tractive force. The structure under the fixed bearing shall be designed to withstand the full seismic and design braking/tractive force.

5.6.3.2 Simply Supported and Continuous Spans on Flexible Supports

Shear rating of a support is the horizontal force required to move the top of the support through a unit distance taking into account horizontal deformation of the bridges, flexibility of the support and rotation of the foundation. The distribution of ‘applied’ longitudinal horizontal forces (e.g., braking, seismic, wind etc.) depends solely on shear

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ratings of the supports and may be estimated in proportion to the ratio of individual shear ratings of a support to the sum of the shear ratings of all the supports. The distribution of self-induced horizontal force caused by deck movement (owing to temperature, shrinkage, creep, elastic shortening, etc.) depends not only on shear ratings of the supports but also on the location of the ‘zero’ movement point in the deck. The shear rating of the supports, the distribution of applied and self-induced horizontal force and the determination of the point of zero movement may be made as per recognized theory for which reference may be made to publications on the subjects. The effects of braking force on bridge structures without bearings, such as, arches, rigid frames, etc., shall be calculated in accordance with approved methods of analysis of indeterminate structures. The effects of the longitudinal forces and all other horizontal forces should be calculated up to a level where the resultant passive earth resistance of the soil below the deepest scour level (floor level in case of a bridge having pucca floor) balances these forces. 5.7 CENTRIFUGAL FORCES Where a road bridge is located on a curve, all portions of the structure affected by the centrifugal action of moving vehicles are to be proportioned to carry safely the stress induced by this action in addition to all other stress to which they may be subjected. The centrifugal force shall be determined from the following equation:

R

WVC

127

2

Where C = Centrifugal force acting normally to the traffic

(1) at the point of action of the wheel Loads or (2) uniformly distributed over every metre length on which a uniformly distributed

load acts, in tonnes.

W = Live load (1) in case of wheel loads, each wheel load being considered as acting over the

ground contact length specified in Clause 5.3, in tonnes, and (2) in case of a uniformly distributed live load, in tones per linear metre.

V = The design speed of the vehicles using the bridge in km per hour, and R = The radius of curvature in metres. The centrifugal force shall be considered to act at a height of 1.2 m above the level of the carriageway. No increase for impact effect shall be made on the stress due to centrifugal action.

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The overturning effect of the centrifugal force on the structure as a whole shall also be duly considered. 5.8 BUOYANCY AND UPLFIT The buoyancy refers to the force brought about by the static water pressure acting upward on the bottom of the structure in which pore water exists in the ground or between the ground and structure. The uplift refers to the upward force brought about by the water level difference between front and rear of the structure or by a temporary water level rise around the structure due to surge. 1. In the design of abutments, especially those of submersible bridges, the effects of

buoyancy or uplift shall also be considered assuming that the fill behind the abutments has been removed by scour.

2. To allow for full buoyancy or uplift a reduction is made in the gross weight of the

member effected, in the following manner:

a) When the member under consideration displaces water only, e.g., a shallow pier or abutment pier founded at or near the bed level, the reduction in weight shall be equal to that of the volume of the displaced water.

b) When the member under consideration displaces water and also silt or sand, e.g., a deep pier or abutment pier passing through strata of sand and silt and founded on similar material, the upward pressure causing the reduction in weight shall be considered as made up of two factors:

i) Full hydrostatic pressure due to a depth of water equal to the difference in levels between the free surface of water and the foundation of the member under consideration, the free surface being taken for the worst condition; and

ii) Upward pressure due to the submerged weight of the silt or sand calculated in accordance with Rankine’s theory for the appropriate angle of internal friction.

3. In the design of submerged masonry or concrete structures, the buoyancy effect

through pore pressure may be limited to 15 percent of full buoyancy.

4. In case of submersible bridges, the full buoyancy effect on the superstructure shall be taken into consideration.

5.9 EARTH PRESSURE

5.9.1 General The earth pressure shall be considered as a function of:

Type and density of earth

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Water content Soil creep characteristics Degree of compaction Location of ground water table Earth - structure interaction Amount of surcharge Earthquake effects Back slope angle and Wall inclination

Silt and lean clay shall not be used for backfill. Appropriate drainage provisions shall be provided to prevent hydrostatic and seepage forces from developing behind the walls. 5.9.2 Compaction Where activity by mechanical compaction equipment is anticipated within a distance of one-half of the height of the wall, taken as difference in elevation between where the finished grade intersects the back of the wall and the back of the wall, the effect of additional earth pressure that may be induced by compaction shall be taken in to account. Compaction induced earth pressures may be estimated using the procedures described by Clough and Duncan (1991). The heavier the equipment used, and closer it operates to the wall, larger are the compaction-induced pressures. The compaction induced pressures can be reduced by using small rollers or hand compactors within a distance of one-half of wall height from the back of the wall. 5.9.3 Presence of water If the retained earth is not allowed to drain, the effect of hydrostatic pressure shall be added to that of earth pressure. Submerged densities of the soil shall be used to determine the lateral earth pressure below the ground level. If the ground water level differs on the opposite sides of the wall, the effect of seepage on wall stability and the potential for piping shall be considered. Pore water pressures shall be added to the effective horizontal stresses in determining total lateral earth pressure on the wall. The development of hydro static water pressure on the wall should be eliminated through use of crushed rock, pipe drains, perforated drains or geosynthetic drains, The pore water pressure may be approximated by using flow net procedures or other acceptable analytical methods.

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5.9.4 Effect of earthquake The effects of wall inertia and probable amplification of active earth pressure and/or mobilization of passive earth masses by earthquake shall be considered. Details are covered in Clause 5.14.

5.9.5 Earth Pressures 5.9.5.1 General The earth pressure shall be distributed load acting on the wall surface. The pressure shall be assumed to be linearly proportional to the depth of the earth. Structures designed to retain earth fills shall be proportioned to withstand pressure calculated in accordance with any rational theory. Coulomb’s theory shall be acceptable. For this specification, the backfill is taken as non-cohesive soil. To the extent possible, cohesive or fine-grained soil should be avoided as back fill. If circumstances make use of cohesive soil as backfill unavoidable, specialist literature shall be referred to for estimating lateral earth pressure under such condition. 5.9.5.2 Active, passive and at-rest lateral earth pressures Walls that can tolerate little or no movement should be designed for at-rest earth pressure. Walls which can move away from the soil mass should be designed for pressures between active and at-rest conditions, depending on the magnitude of the tolerable movements. Movement required to reach the minimum active pressure or the maximum passive pressure is the function of the wall height and the soil type. Some typical values of these mobilizing values relative to the wall height are given in Table 5.9-1 below: Table 5.9-1 Approximate values of relative movements required to reach active or passive earth pressure conditions (Clough and Duncan 1991)

Type of backfill Value of Δ/H

Active Passive Dense sand 0.001 0.01 Medium dense sand 0.002 0.05 Loose sand 0.004 0.04 Compacted silt 0.002 0.02 Compacted lean clay 0.010 0.05 Compacted fat clay 0.010 0.05 Where, Δ - Movement of the top of the wall required to reach minimum active or maximum

passive pressure by tilting or lateral translation (mm)

H - Height of the wall (mm)

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5.9.5.3 Active and Passive earth pressures for wall allowed to move (non-cohesive soil)

i) Active Earth Pressure, PA = KA.γ.x + KA.q

ii) Passive Earth Pressure, PP = KP.γ.x + KP.q Where

2

2

2

)cos()cos()sin()sin(

1coscos

)(cos

AK

2

2

2


1coscos

)(cos

PK

Take .00)sin( if 5.9.5.4 Earth pressure for wall not allowed to move

Earth pressure at-rest, PO = KO.γ.x + KO.q

Where γ: Unit weight of soil (KN/m3) (refer Table 5.9-3 for typical values) PA: Active earth pressure intensity at depth ‘x’ (KN/m2) PP: Passive earth pressure intensity at depth ‘x’ (KN/m2) PO: Earth pressure intensity at rest at depth ‘x’ (KN/m2) KA: Coefficient of active earth pressure according to Coulomb’s earth pressure

theory KP: Coefficient of passive earth pressure according to Coulomb’s earth pressure

theory KO: Coefficient of earth pressure at rest x: Depth at which earth pressures act on the wall surface (m) q: Load imposed on the ground surface (KN/m2) φ: Angle of shearing resistance of soil (degrees) α: Angle formed between the ground surface and horizontal plane (degrees) θ: Angle formed between the wall’s rear surface and vertical plane (degrees) δ: angle of wall friction between the wall’s rear surface and soil (refer Table 5.9-2

for values) The angles are positive in the counterclockwise direction.

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Figure 5-5 Earth Pressure (Japan Road Association) The resultant lateral earth load due to weight of the backfill shall be assumed to act at a height H/3 above the base of the wall, where H is the total height of the wall measured from the surface of the ground at the back of the wall to the bottom of the footing. The value of coefficient of earth pressure at rest is said to vary with soil quality and compaction method from 0.4 to 0.7. For ordinary sandy soil, it should be regarded as approximately 0.5. For highly over consolidated sand, KO can be on the order of 1.0. 5.9.5.5 Plane of action of earth pressure on an abutment

1. For a gravity-type abutment, it lies at the back surface of the main body of concrete.

2. For a reversed T-type abutment, it lies at the back surface of the main body of the concrete for sectional calculation of the wall and at the vertical imaginary line at the end of the rear footing for stability calculation.

Figure 5-6 Plane of action of earth pressure on gravity-type abutment (Japan Road Association 2002)

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Figure 5-7 Plane of action of earth pressure on reversed T-type abutment (Japan Road Association 2002) NOTE: For typical walls over 1500mm high with structural grade backfill, calculations indicate that the horizontal movement of the top of the wall due to combination of structural deformation of the stem and the rotation of the foundation is sufficient to develop active conditions (AASHTO LRFD 2007) Table 5.9-2 Friction Angle for Dissimilar Materials (US Department of Navy 1982a) Interface Materials Friction Angle

(δ)° Coefficient of Friction, tan δ

Mass Concrete on the following foundation materials:-

Clean sound rock Clean gravel, gravel-sand mixtures, coarse sand Clean fine to medium sand, silty medium to coarse

sand, silty of clayey gravel Clean fine sand, silty or clayey fine to medium sand Fine sandy silt, non-plastic silt Very stiff and hard residual or pre-consolidated clay Medium stiff and stiff clay and silty clay

Masonry on above foundation materials has the same friction angles

35 29 to 31

24 to 29 19 to 24 17 to 19 22 to 26 17 to 19

0.70 0.55 to 0.60

0.45 to 0.55 0.34 to 0.45 0.31 to 0.34 0.40 to 0.49 0.31 to 0.34

Masonry on masonry, igneous and metamorphic rocks: o Dressed soft rock on dressed soft rock o Dressed hard rock on dressed soft rock o Dressed hard rock on dressed hard rock

Masonry on wood in the direction of the cross grain Steel on steel at sheet pile interlocks

35 33 29 26 17

0.70 0.65 0.55 0.49 0.31

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Table 5.9-3 Unit weight of soil (Japan Road Association, 2002) Ground Soil type Loose Dense

Natural Ground Sand and gravel 18 20 Sandy soil 17 19 Clayey soil 14 18

Embankment Sand and gravel 20 Sandy soil 19 Clayey soil 18

Note:

1. The unit weight values given above are approximate for preliminary design. The actual values shall be found using soil samples taken at the construction site.

2. The unit weight for a particular ground and soil type below the groundwater level may be taken as the respective value in the tables minus 9.

3. Groundwater level shall be taken as the mean value after construction.

6.0 Live load surcharge A live load surcharge shall be applied where the vehicular load shall be expected to act on the surface of the backfill within a distance equal to one-half of the wall height behind the back face of the wall. All abutments and return walls shall be designed for a live load surcharge equivalent to 1.2 m earth fill. Reinforced concrete approach slab with 12 mm diameter 150 mm c/c each direction both at top and bottom as reinforcement in M30 grade concrete covering the entire width of the roadway, with one end resting on the structure designed to retain earth and extending for a length of not less than 3.5 m into the approach shall be provided. 5.10 TEMPERATURE 5.10.1 Uniform temperature Although temperature changes in a bridge do not occur uniformly, bridges are generally designed for uniform temperature change. The design thermal movement associated with uniform temperature change shall be estimated with appropriate consideration of the construction temperature and the temperature range which in turn will depend on type of structure, environmental conditions at the bridging point, materials and dimensions of structural members. 5.10.1.1 Temperature range The ranges of temperature shall be as specified in Table 5.10-1. The difference between the extended lower or the upper boundary and the base construction temperature assumed in the design shall be used to calculate thermal deformation effects.

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Table 5.10-1 Temperature ranges (AASHTO LRFD 2007) Climate Steel or

Aluminum Concrete Wood

Moderate -18 to 50°C -12 to 27°C -12 to 24°C

Cold -35 to 50°C -18 to 27°C -18 to 24°C

The minimum and maximum temperatures shall be called as TminDesign and TmaxDesign. For this specification, a moderate climate may be determined by number of freezing days per year. If the number of freezing days is less than 14, the climate is considered to be moderate. Freezing days are days when the average temperature is less than 0°C. 5.10.1.2 Design thermal movements The design thermal movement range, ΔT shall depend on the extreme bridge design temperatures defined in Clause 5.10.1.1 and determined as: ΔT = αL(TmaxDesign - TminDesign) Where L = Expansion length (mm)

Α = Coefficient of linear expansion (mm/mm/°C 5.10.2 Temperature gradient Effect of temperature difference within the superstructure shall be derived from positive temperature differences which occur when conditions are such that solar radiation and other effects cause a gain in heat through the top surface of the superstructure. Conversely, reverse temperature differences are such that heat is lost from the top surface of the bridge deck as a result of re-radiation and other effects. 5.10.2.1 Temperature gradient for concrete decks Positive and reverse temperature differences for the purpose of design of concrete bridge decks shall be assumed as shown in Figure 5-8. These design provisions are applicable to concrete bridge decks with about 50 mm wearing surface.

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Figure 5-8 Design Temperature differences for Concrete Bridge Decks (IRC-6: 2014) 5.10.2.2 Temperature gradient for steel and composite decks Figure 5-9 may be referred to for assessing the effect of temperature gradient for steel and composite decks.

Figure 5-9 Temperature differences across steel and composite section (IRC-6: 2014) NOTE: For intermediate slab thickness, T1 may be interpolated.

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5.10.3 Material Properties

For the purpose of calculating temperature effects, the coefficient of thermal expansion for RCC, PSC and steel structures may be taken as 12.0 X 10-6/°C. 5.11 DEFORMATION STRESSES (for steel bridges only)

A deformation stress is defined as the bending stress in any member of an open web-girder caused by the vertical deflection of the girder combined with the rigidity of the joints. No other stresses are included in this definition.

All steel bridges shall be designed, manufactured and erected in a manner such that the deformation stresses are reduced to a minimum. In the absence of calculation, deformation stresses shall be assumed to be not less than 16 percent of the dead and live loads stresses. In pre-stressed girders of steel, deformation stresses may be ignored. 5.12 SECONDARY STRESSES 5.12.1 Steel structures:

Secondary stresses are additional stresses brought into play due to the eccentricity of connections, floor beam loads applied at intermediate points in a panel, cross girders being connected away from panel points, lateral wind loads on the end-posts of through girders etc., and stresses due to the movement of supports.

5.12.2 Reinforced Concrete structures:

Secondary stresses are additional stresses brought into play due either to the movement of supports or to the deformations in the geometrical shape of the structure or its member, resulting from causes, such as, rigidity of end connection or loads applied at intermediate points of trusses or restrictive shrinkage of concrete floor beams. All bridges shall be designated and constructed in a manner such that the secondary stresses are reduced to a minimum and they shall be allowed for in the design. For reinforced concrete members, the shrinkage coefficient for purposes of design may be taken as 2 X 10-4. 5.13 ERECTION STRESSES AND CONSTRUCTION LOADS A detailed construction procedure associated with a method statement shall be drawn up during design and considered in the design to ensure that all aspects of stability and strength of the structure are satisfied. If the actual construction procedure at the site and the support system differs from that recommended by the designer, the construction engineer will be required to carry out the construction stage analysis for all the critical construction stages to ensure that the structure and its temporary support system are safe during all the construction stages.

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Construction loads are those which are incident upon a structure or any of its constituent components during the construction of the structures. Examples of Typical Construction Loadings are given below. However, each individual case shall be investigated in complete detail. Examples:

a) Loads of plant and equipment including the weight handled that might be incident on the structure during construction.

b) Temporary super-imposed loading caused by storage of construction material on a partially completed a bridge deck.

c) Unbalanced effect of a temporary structure, if any, and unbalanced effect of modules that may be required for cantilever segmental construction of a bridge.

d) Loading on individual beams and/or completed deck system due to travelling of a launching truss over such beams/deck system.

e) Thermal effects during construction due to temporary restraints. f) Secondary effects, if any, emanating from the system and procedure of

construction. g) Loading due to any anticipated soil settlement. h) Wind load during construction. For special effects, such as, unequal gust load

and for special type of construction, such as, long span bridges specialist literature may be referred to.

i) Seismic effects on partially constructed structure.

5.14 SEISMIC FORCE 5.14.1 General

Bridges play crucial roles as evacuation and emergency routes for rescue, first aid, fire fighting etc during seismic events and therefore it is essential to ensure the structural safety of the bridges during seismic events. There cannot be an earthquake proof structure, but as far as possible, structures should be so designed and built that they are able to respond, without structural damage to shocks of moderate intensities and without total collapse to shocks of heavy intensities. 5.14.2 Applicability

All bridges supported on piers, pier bents and arches, directly or through bearings, are to be designed for horizontal and vertical forces as given in the following clauses. Culverts and minor bridges up to 10 m span however need not be checked for seismic effects Special investigations should be carried out for the bridges of following description:

a) Bridge more than 150 m span

b) Bridges with piers taller than 30 m

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c) Cable supported bridges, such as extradosed, cable stayed and suspension bridges

d) Arch bridges having more than 50 m span

e) Bridges having any of the special seismic resistant features such as seismic isolators, dampers etc.

f) Bridge using innovative structural arrangements and materials. Masonry and plain concrete arch bridges with span more than 10 m shall be avoided. 5.14.3 Seismic Zones

For the purpose of determining the seismic forces, it is first important to determine the expected seismic acceleration coefficients which are usually approximated from the records of the intensities or known magnitudes and known epicenters of the past earthquakes experienced by the place or region. For Bhutan, since records and the information on the past earthquakes are extremely scanty compounded by lack of research and study on the seismology of the country, it is probably difficult to approximate, for engineering purpose, the intensities or magnitudes of the future expected earthquakes based on the past earthquakes or other methods. Therefore, the Seismic Zonation map for India adopted by the Bureau of Indian Standards shall be adopted for design of bridges in the country. As per the Indian Seismic Zonation Map, most parts of the Indian states surrounding Bhutan on the west, east and south fall in Zone V. Since earthquakes do not recognize political boundaries, it would be fairly reasonable to assume Bhutan as falling is Zone V as per Indian Seismic Zonation Map. Hence for the purpose of approximating seismic forces for the bridge structures, the country shall be assumed to be falling in Zone V as per Indian Seismic Zonation Map. As per Indian standards, a zone factor “Z” is associated with each zone and for Zone V, Z is equal to 0.36. 5.14.4 Components of Seismic Motion

The characteristics of seismic ground motion expected at any location depend upon the magnitude of earthquake, depth of focus, distance of epicenter and characteristics of the path through which the seismic wave travels. The random ground motion can be resolved in three mutually perpendicular directions. The components are considered to act simultaneously, but independently and their method of combination is described in Clause 5.14.5. Two horizontal components are taken as of equal magnitude, and vertical component is taken as two third of horizontal component.

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5.14.5 Combination of Component Motions

1) The seismic forces shall be assumed to come from any horizontal direction. For this purpose two separate analyses shall be performed for design seismic forces acting along two orthogonal horizontal directions. The design seismic force resultants (i.e. axial force, bending moments, shear forces, and torsion) at any cross-section of a bridge component resulting from the analyses in the two orthogonal horizontal directions shall be combined as below:

a) ± r1 ± 0.3r2 b) ± 0.3r1 ± r2

Where

r1 = Force resultant due to full design seismic force along x direction. r2 = Force resultant due to full design seismic force along z direction.

2) When vertical seismic forces are also considered, the de4sign seismic force

resultants at any cross section of a bridge component shall b combined as below:

a) ± r1 ± 0.3 r2 ± 0.3 r3 b) ± 0.3 r1 ± r2 ± 0.3 r3 c) ± 0.3 r1 ± 0.3 r2 ± r3

Where r1 and r2 are as defined above and r3 is the force resultant due to full design seismic force along the vertical direction.

5.14.6 Computation of Seismic Response

Following method are used for computation of seismic response depending upon the complexity of the structure and the input ground motion.

5.14.6.1 Elastic Seismic Acceleration method: This method is adequate for most of the bridges. In this method, the first fundamental mode of vibration is calculated and the corresponding acceleration is read from Figure 5-10. This acceleration is applied to all parts of the bridge for calculation of forces as per Clause 5.14.6.3. 5.14.6.2 Elastic Response Spectrum Method: This is a general method, suitable for more complex structural systems (e. g. continuous bridges, bridges with large difference in pier heights, bridges which are curved in plan, etc), in which dynamic analysis of the structure is performed to obtain the first as well as higher modes of vibration and the forces obtained for each mode by use of response spectrum given in Figure 5-10. These modal forces are combined by following appropriate combinational rules to arrive at the design forces. Reference is made to specialist literature for the same.

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Figure 5-10 Response Spectra Note:

a. Rocky or hard soil sites (Type I) – N > 30 b. Medium soil sites (Type II) – 10 < N < 30 c. Soft soil sites (Type III) – N < 10

5.14.6.3 Horizontal seismic force The horizontal seismic forces acting at the centers of mass, which are to be resisted by the structure as a whole, shall be computed as follows: Feq = Ah (Dead Load + Appropriate Live Load) Where Feq = seismic force to be resisted Ah = horizontal seismic coefficient = (Z/2) x (I) x (Sa/g) Appropriate live load shall be taken as per Clause 5.14.6.4 Z = 0.36 (Zone factor) I = Importance Factor (see Clause 5.14.6.3.1) T = Fundamental period of the bridge (in sec.) for horizontal vibrations Fundamental time period of the bridge member is to be calculated by any rational method of analysis adopting the Modulus of Elasticity of Concrete, and taking gross uncracked section for moment of inertia. The fundamental period T(in seconds) of pier/abutment of the bridge along a horizontal direction may be estimated by the following expression:

F

DT

10000.2

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Where

D = appropriate dead load on the superstructure and live load in KN

F = horizontal force in KN required to be applied at the centre of mass of the superstructure for one mm horizontal deflection at the top of the pier/abutment for the earthquake in transverse direction; and for the force to be applied to the top of the bearings for earthquake in longitudinal direction.

Sa/g = Average response acceleration coefficient for 5 percent damping of load resisting elements depending upon the fundamental period of vibration T as given in Figure 5-10. NOTE: In the absence of calculations of fundamental period for small bridges, the

value of Sa/g may be taken as 2.5 For damping other than 5 percent offered by load resisting elements, the multiplying factors as given below shall be used.

Damping % 2 5 10

Factor 1.4 1.0 0.8

Application Prestressed concrete, Steel and Composite steel elements

Reinforced Concrete elements

Retrofitting of old bridges with RC piers

5.14.6.3.1 Seismic importance factor (I)

Bridges are designed to resist design basis earthquake (DBE) level, or other higher or lower magnitude of forces, depending on the consequences of their partial or complete non-availability, due to damage or failure from seismic events. The level of design force is obtained by multiplying (Z/2) by factor ‘I’, which represents seismic importance of the structure. Combination of factors considered in assessing the consequences of failure and hence choice of factor ‘I’, - include inter alia,

a) Extent of disturbance to traffic and possibility of providing temporary diversion, b) Availability of alternative routes, c) Cost of repairs and time involved, which depend on the extent of damages,-

minor or major, d) Cost of replacement, and time involved in reconstruction in case of failure, e) Indirect economic loss due to its partial or full non-availability, Importance

factors are given in Table 5-6 for different types of bridges.

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Table 5-6 Importance Factor (IRC-6: 2014) Seismic Class Illustrative Examples Importance

Factor ‘I’ Normal bridges All bridges except those mentioned in other

classes 1.0

Important bridges a) River bridges and flyovers inside cities b) Bridges on National and State Highways c) Bridges serving traffic near ports and other centers of economic activities d) Bridges crossing railway lines

1.2

Large critical bridges

a) Long bridges more than 1 km length across perennial rivers and creeks b) Bridges for which alternative routes are not available

1.5

NOTE: While checking for seismic effects during construction, the importance factor

of 1 should be considered for all bridges. 5.14.6.4 Live load components

i) The seismic force due to live load shall not be considered when acting in the direction of traffic, but shall be considered in the direction perpendicular to the traffic.

ii) The horizontal seismic force in the direction perpendicular to the traffic shall be calculated using 20 percent of live load (excluding impact factor).

iii) The vertical seismic force shall be calculated using 20 percent of live load (excluding impact factor).

NOTE: The reduced percentages of live loads are applicable only for calculating the

magnitude of seismic design force and are based on the assumption that only 20 percent of the live load is present over the bridge at the time of earthquake.

5.14.6.5 Water current and depth of scour The depth of scour under seismic condition to be considered for design shall be 0.9 times the maximum scour depth. The flood level for calculating hydrodynamic force and water current force is to be taken as average of yearly maximum design floods. For river bridges, average may preferably be based on consecutive 7 years’ data, or on local enquiry in the absence of such data. 5.14.6.6 Hydrodynamic forces under seismic condition In addition to inertial forces arising from the dead load and live load, hydrodynamic forces act on the submerged part of the structure and are transmitted to the foundations. Such forces shall be considered in the design of the bridge. Refer specialist literature for details.

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5.14.6.7 Additional Earth pressures under seismic condition The most commonly used method for calculation of seismic soil forces acting on a bridge is a static approach developed in 1920s by Mononobe (1929) and Okabe (1926). The mononobe-Okabe analysis is an extension of Coulomb’s sliding-wedge theory taking into account horizontal and vertical inertia forces acting on the soil. The following assumptions are made:

1. The abutment is free to yield sufficiently to enable full soil strength or active earth pressure conditions to be mobilized. If the abutment is rigidly fixed and unable to move, the soil forces will be much higher than those predicted by the Mononobe-Okabe analysis.

2. The backfill is cohesionless with friction angle of φ 3. The backfill is unsaturated so that the liquefaction problem will not arise

Considering the equilibrium of the soil wedge behind the abutment, the active earth pressure during earthquake, when the abutment is at the point of failure, shall be calculated as follows: PEA = KAE.γ.x (1-kv) Where seismic active earth pressure coefficient, KAE is calculated as,

2

0

00

20

02


1coscoscos

)(cos

E

EE

AEK

The equivalent expression for passive earth pressure if the abutment is pushed against the backfill, PEP = KPE.γ.x (1-kv)

2

0

00

20

02


1coscoscos

)(cos

E

EE

PEK

Where γ: Unit weight of soil (KN/m3)

H: Height of soil face (m)

q: Load imposed on the ground surface (KN/m2)

φ: Angle of friction of soil (degrees)

θ0: arc tan (kh/(1‐kv)) (degrees)

δE: angle of wall friction between the wall’s rear surface and soil (degrees)

kh: Horizontal acceleration coefficient

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kv: Vertical acceleration coefficient

α: Backfill slope angle (degrees) – denoted as “i” in Figure 5-11 Earth Pressure during

earthquake (AASHTO LRFD 2007)Figure 5-11 θ: Slope of the wall to the vertical, negative as shown (degrees) – denoted as “β”

in Figure 5-11.

Figure 5-11 Earth Pressure during earthquake (AASHTO LRFD 2007) As the seismic inertial angle θ0 increases, values of KAE and KPE approach each other and, for vertical backfill, becomes equal when θ0 = φ. The value of ha, the height at which the resultant of the soil pressure acts on the abutment, may be taken as H/3 for the static case with no earthquake effects involved. However, with earthquake effects, the height shall be taken as H/2 with a uniformly distributed pressure. 5.14.6.7.1 Non-yielding abutments The Mononobe – Okabe analysis, as mentioned earlier, assumes that the abutment is free to yield laterally by sufficient amount to mobilize peak soil strengths in the backfill. As per AASHTO, the peak strength can be assumed to be mobilized if the deflection at the abutment top is 0.5% of the abutment height. For abutment restrained against lateral movement by tie backs or batter piles, the lateral pressures induced by the inertia forces in the backfill will be greater than that approximated by Mononobe – Okabe analysis. AASHTO recommends use of a factor of 1.5 in conjunction with peak ground accelerations for designs where doubts exists that abutment can yield sufficiently to mobilize soil strengths. 5.14.6.7.2 Monolithic abutments Monolithic abutments generally perform well during earthquakes since there are no problems associated with bearing and back wall damage. However higher longitudinal

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and transverse inertia forces are transmitted directly into the backfill, and provision must be made for adequate passive resistance to avoid excessive relative displacements. Although the soil forces can be estimated with better accuracy for the free standing or seat-type abutment, the added joint introduces a possible collapse mechanism. To avoid this collapse mechanism, monolithic abutments are actually recommended from seismic point of view. During seismic event, although the damage may be more severe in bridges with monolithic abutments due to higher forces transmitted to the backfill soil, with adequate abutment reinforcement and proper detailing, the collapse potential is low.

In estimating monolithic abutment stiffness and associated longitudinal displacement during transfer of peak earthquake forces from the structure, the abutments shall be proportioned to restrict displacements of 91.4mm or less in order to minimize damage (AASHTO LRFD 2007). 5.14.6.8 Design forces for elements of structures and use of response reduction

factor The forces on various members obtained from the elastic analysis of bridge structure are to be divided by Response Reduction Factor (R) given in Table 5-7 before combining with other forces as per load combinations given in Clause 5.1. Table 5-7 Response Reduction Factors (IRC-6:2014) Bridge Component

R with ductile detailing

R without ductile detailing

Superstructure N.A 2.0 Substructure i. Masonry/PCC piers, abutments

ii. RCC short plate piers where plastic hinge cannot develop in direction of length and RCC abutments

iii. RCC long piers where hinges can develop

iv. Column

v. Beams of RCC portal frames supporting bearings

-

3.0

4.0

4.0

1.0

1.0

2.5

3.3

3.3

1.0

Bearings 2.0 2.0

Connectors and Stoppers (Reaction blocks) Those restraining dislodgements or drifting away of bridge elements.

When connectors and stoppers are designed to withstand seismic forces primarily, R value shall be taken as 1.0.

When connectors and stoppers are designed as additional safety measures in the event of failure of bearings, R value specified in this table for appropriate substructure shall be used.

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NOTES:

i. Those parts of the structural elements of foundations which are not in contact with soil and transferring load to it are treated as part of sub-structure element.

ii. Response reduction factor is not to be applied for calculation of displacements of elements of bridge and for bridge as a whole.

iii. When elastomeric bearings are used to transmit horizontal seismic forces, the response reduction factor R shall be taken as 1.5 for RCC substructure and as 1.0 for masonry and PCC substructure.

5.14.6.9 Fully Embedded Portions Parts of structure embedded in soil below scour level need not be considered to produce any seismic forces. 5.14.6.10 Liquefaction In loose sands and poorly graded sands with little or no fines, the vibrations due to earthquake may cause liquefaction, or excessive total and differential settlements. Founding bridges on such sands should be avoided unless appropriate methods of compaction or stabilization are adopted. Alternatively, the foundations should be taken deeper below liquefiable layers, to firm strata. Reference should be made to the specialist literature for analysis of liquefaction potential. 5.14.6.11 Foundation Design For design of foundation, the seismic loads should be taken as 1.25 times the forces transmitted to it by substructure, so as to provide sufficient margin to cover the possible higher forces transmitted by substructure arising out of its over strength. 5.14.6.12 Longitudinal Restrainers Friction shall not be considered to be an effective restrainer. Restrainers shall be designed for the force calculated as acceleration coefficient times the permanent load of the lighter of the two adjoining spans or parts of the structure.

If the restrainer is at a point where the relative displacement of the sections of the superstructure is designed to occur during seismic motions, sufficient slack must be allowed in the restrainer so that restrainer doesn’t start to act until the design displacement is exceeded.

Where a restrainer is to be provided at columns or piers, the restrainer of each span may be attached to the column or the pier, rather than to interconnecting adjacent spans.

A typical longitudinal restrainer arrangement used by New Zealand Ministry of Transport and also covered in AASHTO specifications is shown in Figure 5-12. It may be seen that the linkage bolts are provided to prevent the spans from dropping off the supports. The rubber rings act as buffers to prevent impact damage in the event that lateral displacement clearance provided is inadequate. The knock-off back wall accommodates differential displacement between the abutment and the superstructure, with minimum structural damage.

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Figure 5-12 Possible abutment detail

An alternative to above method for preventing dislodgement of superstructure is to use “reaction blocks” as recommended in IRC-6.

5.14.6.13 Hold down devices Hold down devices shall be provided at the supports and at hinges in continuous structures where vertical seismic forces due longitudinal seismic load opposes and exceeds 50%, but is less than 100%, of the reaction due to permanent loads. In this case the net uplift force for the hold-down device shall be taken as 10% of reaction due to permanent loads that would be exerted if the span were simply supported.

5.14.6.14 Seating length Pier and abutment caps shall be generously dimensioned, to prevent dislodgement during severe ground-shaking. The seating lengths shown in Figure 5-13 below, covered in IRC-6, are suggested but the same shall be treated as only indicative and suitable arrangements will have to be worked out for specific cases.

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Figure 5-13 Seating length at the supports 5.14.6.15 Ductile detailing To improve the performance of bridges during earthquakes, the bridges have to be detailed for ductility for which IS 13920 or any other specialist literature may be referred to.

5.14.6.16 Other recommended provisions i. In order to mitigate the effects of earthquake forces described above, special

seismic devices such as Shock Transmission Units, Base Isolation, Seismic Fuse, Lead Plug, etc, may be provided based on specialized literatures, international practices, satisfactory testing etc.

ii. Continuous superstructure (with fewer number of bearings and expansion joints) or integral bridges (in which the substructure or superstructure are made joint less, i.e. monolithic), if not unsuitable otherwise, can possibly provide high ductility leading to better behavior during earthquake.

iii. Where elastomeric bearings are used, a separate system of arrester control in both directions shall be introduced to cater to seismic forces on the bearing.

5.15 SNOW LOAD The snow load of 900 kg/m3 where applicable on the bridge deck shall be taken in the following two conditions to be checked independently:

a) A snow accumulation of 0.25 m over the deck shall be taken into consideration while designing the structure for wheeled vehicles.

b) In case of snow accumulation exceeding 0.50 m maximum snow accumulation based on actual site observation shall be considered without live load.

5.16 VEHICLE COLLISION LOADS ON BRIDGE AND FLYOVER

SUPPORTS

5.16.1 General

Bridge piers of wall type, columns or the frames built in the median or in the vicinity of the carriageway supporting the superstructure shall be designed to withstand vehicle collision loads. The effect of collision load shall also be considered on the supporting elements, such as, foundations and bearings. For multilevel carriageways, the collision loads shall be considered separately for each level.

The effect of collision load shall not be considered on abutments or on the structures separated from the edge of the carriageway by a minimum distance of 4.5 m and shall also not be combined with principal live loads on the carriageway supported by the structural members subjected to such collision loads, as well as wind or seismic load.

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5.16.2 Collision Load

The nominal loads given in Table 5-8 shall be considered to act horizontally as Vehicle Collision Loads. Supports shall be capable of resisting the main and residual load component acting simultaneously. Loads normal to the carriageway below and loads parallel to the carriageway below shall be considered to act separately and shall not be combined. Table 5-8 Nominal Vehicle Collision Loads on Supports of Bridges (IRC-6) Load normal to

the carriageway below (Ton)

Load parallel to the carriageway below (Ton)

Point of application on bridge support

Main Load Component

50 100 At the most severe point between 0.75 and 1.50 m above carriageway level

Residual load component

25 50 At the most severe point between 1 m and 3 m above carriageway level

Note:

i. The loads shown are assumed for vehicles plying at velocity of about 60 km/hr. In case of vehicles travelling at lesser velocity, the loads may be reduced in proportion to the square of the velocity but not less than 50%.

ii. The bridge supports shall be designed for the residual load component only, if protected with suitably designed fencing system taking into account its flexibility, having a minimum height of 1.5 m above the carriageway level.

5.17 INDETERMINATE STRUCTURES AND COMPOSITE STRUCTURES

Stresses due to creep, shrinkage and temperature, etc. should be considered for statically indeterminate structures or composite members consisting of steel or concrete prefabricated elements and cast-in-situ components for which specialist literature may be referred to. 6. DETAILING REQUIRMENTS (not exhaustive)

6.1 Drainage

The drainage system should be such that it is able to carry water off the bridge deck quickly and completely. The cross-fall of the deck slab should be greater than 2%. Drainage is necessary not only for the safety of the motorists, but also to allow the wearing surface to dry out as fast as possible. To prevent possible damage to objects underneath the bridge and to the bridge itself, particularly in the municipal areas, the water removed from the bridge deck should normally flow into the storm sewer system.

Regardless of the intensity of the rainfall, drainage inlets should be located on both edges of the deck slab and spaced about 15 m apart along the length of the bridge. Drainage inlets and pipes must be made of materials that resist abrasion and the effects

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of chlorides. Cast iron is normally chosen for drainage inlets, while tough plastic such as high0density polyethylene (HDPE), is used for pipes.

The drainage pipes should be inclined at 2% slope at least. The pipe diameter required is based on the intensity of precipitation.

6.2 Water proofing and wearing surface

The covering layer of deck slab, normally consisting of water proofing membrane and an asphalt concrete wearing surface, is very important for the durability of the bridges. The water proofing membrane protects the surface of the deck slab concrete against freeze-thaw action, deicing chemicals penetration of chlorides.

Recommended waterproofing-wearing surface systems:

6.2.1 Fully Bonded Waterproofing

This system consists either of a bonded membrane, 1 to 3 mm thick, which is applied in liquid form to the deck concrete by spraying or screeding, or of a hot-glued elastomeric sheet.

The wearing surface consists of a leveling course, preferably liquid applied bitumen, and an asphaltic-concrete surface course, for a total thickness of between 60 mm and 80mm.

6.2.2 Concrete Wearing Surface

A properly designed and constructed concrete wearing surface is extremely durable and provides reliable protection to the structural concrete underneath. An additional thickness of 80mm is normally used. To secure proper bond between the structural concrete and the wearing course, the deck slab must be covered with a thin layer of epoxy mortar before wearing surface is cast using special machines.

The concrete wearing surface must be sufficiently impermeable to resist free-thaw action in the presence of deicing chemicals and to resist penetration of critical chloride concentration to no more than 50mm below the surface.

6.3 Water drop profile/drip nose

Abutments, expansion joints, and the edges of deck slabs must be especially carefully detailed to prevent unsightly stains due to water and dirt. All the edges of the deck slabs must be provided with drip noses to allow the water to collect and drip away. See Figure 6-1 for typical details of the drip nose.

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Figure 6-1 Recommended details at the slab edge 6.4 Jacking provision of replacement of elastomeric bearings

While a bridge is generally designed for a service life of about 100 years, elastomeric bearings have a service life of about 25 years. Hence provisions have to be kept during detailing of the structure for lifting the bridge deck for replacement of the elastomeric bearings. The jacking locations shall be shown in the drawings and the same shall be clearly marked on the bridge with a permanent marker.

A possible arrangement for the insertion of the jack for lifting of the deck recommended by IRC-83 (Part II) is shown in Figure 6-2 below:

Figure 6-2 Pedestal for elastomeric bearing with provision for lifting 6.5 Abutment top details

The abutment top shall be so detailed that it ensures timely inspection of the bearings and the expansion joints. For the purpose of ease of inspection and maintenance, bearings should be accessible from at least two sides and the expansion joints should be accessible from below. The bearings should be located on raised pedestals to protect them from water and dirt. Sufficient rooms should be available for the jacks required to lift the bridge for bearing replacement.

Proper drainage system has to be provided to prevent the water from stagnating on the abutment top. A possible arrangement is shown in Figure 6-3.

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Figure 6-3 Recommended abutment top detail

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REFERENCES

1. IRC:6 - 2014 Standard Specifications and Code of Practice for Road Bridges, Section : II, Loads and Stresses

2. IRC:5 - 1998

3. IRC:38 - 1988 Guidelines for Design of Horizontal Curves for Highways and Design Tables

4. IRC:112 - 2011 Code of Practice for Concrete Road Bridges

5. AASHTO LRFD Bridge Design Specifications 2007

6. Specifications for Highway Bridges, Japan Road Association, March 2002, Part I, Common

7. Specifications for Highway Bridges, Japan Road Association, March 2002, Part V, Seismic Design

8. Guidelines on use of Standard Works Items for Common Road Works, Survey and Design Division, Ministry of Works and Human Settlement, RGoB

9. Prestressed Concrete Bridges, C. Menn

Documents

GENERAL SPECIFICATIONS FOR BRIDGES · 2015-08-03 · design of the bridges. Design data are also available for various other loads expected on the bridge such as the earthquake, wind,