Seismic & Wind Analysis of Bridges

SEISMIC & WIND ANALYSIS OF SEISMIC & WIND ANALYSIS OF BRIDGESBRIDGES

Based on Recommended LRFD guidelines for Based on Recommended LRFD guidelines for Seismic design on Highway bridges (May 2006 ed.)Seismic design on Highway bridges (May 2006 ed.)

Pressure specified shall be assumed to be caused by a base design wind velocity Pressure specified shall be assumed to be caused by a base design wind velocity VVBB = 160 Km/hr. = 160 Km/hr.

For bridges or parts of bridges more than 10,000 mm above low ground or water For bridges or parts of bridges more than 10,000 mm above low ground or water

level, design wind velocity Vlevel, design wind velocity VDZDZ should be adjusted according to: should be adjusted according to:

WIND ANALYSIS OF BRIDGESWIND ANALYSIS OF BRIDGES

Where VWhere VDZ DZ = Design wind velocity at elevation Z (Km/hr) = Design wind velocity at elevation Z (Km/hr)

VV1010 = Wind velocity at 10 000 mm above low ground or design water level (Km/hr) = Wind velocity at 10 000 mm above low ground or design water level (Km/hr)

VVBB = Base wind velocity of 160 Km/hr at 10 000 mm height = Base wind velocity of 160 Km/hr at 10 000 mm height

Z = Height of structure at which wind loads are being calculated as measured from Z = Height of structure at which wind loads are being calculated as measured from low ground or water level > 10 000 mmlow ground or water level > 10 000 mmVV00 = Friction velocity a meteorological wind characteristic taken as specified in Table = Friction velocity a meteorological wind characteristic taken as specified in Table

below below ZZ00 = Friction length of upstream fetch a meteorological wind characteristic taken as = Friction length of upstream fetch a meteorological wind characteristic taken as

specified in Table below.specified in Table below.VV1010 may be established from basic wind speed charts available for various may be established from basic wind speed charts available for various

recurrence intervals, site specific wind surveys or in absence of better criteria, the recurrence intervals, site specific wind surveys or in absence of better criteria, the assumption that Vassumption that V1010 = V = VBB = 160 Km/hr = 160 Km/hr

WIND PRESSURE ON STRUCTURESWIND PRESSURE ON STRUCTURES


The total wind loading shall not be taken less The total wind loading shall not be taken less

than 4.4 N/mm in the plane of windward chord than 4.4 N/mm in the plane of windward chord

and 2.2 N/mm in the plane of leeward chord of and 2.2 N/mm in the plane of leeward chord of

truss or arch components and not less than 4..4 truss or arch components and not less than 4..4

N/mm on beam and girder spans.N/mm on beam and girder spans.

Where wind is not taken as normal to the structure, the base wind pressures PB Where wind is not taken as normal to the structure, the base wind pressures PB for various angles of wind direction may be taken as specified in table below and for various angles of wind direction may be taken as specified in table below and shall be applied to the single place of exposed area. The skew angle shall be shall be applied to the single place of exposed area. The skew angle shall be taken as measured from perpendicular to the longitudinal axis. The wind direction taken as measured from perpendicular to the longitudinal axis. The wind direction for design shall be that which produces the extreme force effect on the component for design shall be that which produces the extreme force effect on the component under investigation.under investigation.

The transverse and longitudinal pressures shall be applied simultaneously.The transverse and longitudinal pressures shall be applied simultaneously.

LOADS FROM SUPERSTRUCTURESLOADS FROM SUPERSTRUCTURES


FORCES APPLIED DIRECTLY TO THE FORCES APPLIED DIRECTLY TO THE SUBSTRUCTURESUBSTRUCTURE

The transverse and longitudinal forces to be applied directly to the The transverse and longitudinal forces to be applied directly to the substructure shall be calculated from an assumed base wind pressure of substructure shall be calculated from an assumed base wind pressure of 0.0019 MPa0.0019 MPa. For wind direction taken skewed to the substructure, this force . For wind direction taken skewed to the substructure, this force shall be resolved into components perpendicular to the end and front shall be resolved into components perpendicular to the end and front elevations of the substructure. The component perpendicular to the end elevations of the substructure. The component perpendicular to the end elevation shall act on exposed substructure area as seen in end elevation, elevation shall act on exposed substructure area as seen in end elevation, and the component perpendicular to the front elevation shall act on the and the component perpendicular to the front elevation shall act on the exposed areas and shall be applied simultaneously with the wind loads from exposed areas and shall be applied simultaneously with the wind loads from superstructure.superstructure.

WIND PRESSURES ON VEHICLES

When vehicles are present the design pressure should be applied on both structure and vehicles. Wind pressure on vehicles shall be represented by an interruptible moving force of 1.46 N/mm acting normal to and 1800 mm above the roadway and shall be transmitted to the structure. When wind on vehicles is not taken as normal to the structure, the components of normal and parallel force applied to the live load may be taken as specified in the table below



Example:

WIND ANALYSIS

SEISMIC ANALYSIS OF SEISMIC ANALYSIS OF BRIDGES BRIDGES

IMPORTANCE CATEGORIES1. Critical bridges (open to all vehicles immediately after EQ. 2500 year return period)

2. Essential bridges (open to emergency / defense / security vehicles immediately after EQ. 475 year return period event)

3. Other bridges

SEISMIC PERFORMANCE ZONE

SEISMIC ANALYSIS OF BRIDGESSEISMIC ANALYSIS OF BRIDGES SOIL PROFILE TYPE

SOIL PROFILE I: Rock either shale or crystalline in nature.

SOIL PROFILE II: Stiff cohesive or deep cohesion less soil.

SOIL PROFILE III: Soft to medium stiff clay and sand.

SOIL PROFILE IV: Soft clay or silt.

ELASTIC SEISMIC RESPONSE COEFFICIENT

SEISMIC ANALYSIS OF BRIDGESSEISMIC ANALYSIS OF BRIDGES

•For bridges on Soil profile III & IV & in areas where A is not less than 0.3, Csm

need not exceed 2.0 A.

•For soil profiles III & IV and for modes other than fundamental mode that have period less than 0.3 s, Csm shall be taken as

If period of vibration for any mode exceeds 4.0 s the value of Csm for that mode shall be taken as

SEISMIC ANALYSIS OF BRIDGES SEISMIC ANALYSIS OF BRIDGES

As per AASHTO LRFD Bridges in seismic As per AASHTO LRFD Bridges in seismic zone 1 need not be analyzed for seismic zone 1 need not be analyzed for seismic loads regardless of their importance and loads regardless of their importance and geometry. However the minimum geometry. However the minimum requirements in sections 4.7.4.4 & 3.10.9 requirements in sections 4.7.4.4 & 3.10.9 shall be applied. shall be applied.

Seismic analysis for is not required for Seismic analysis for is not required for single span bridges, regardless of the single span bridges, regardless of the seismic zone.seismic zone.


Seismic analysis required / not required

• * = No seismic analysis is required

• UL = Uniform load elastic method

• SM = Single mode elastic method

• MM = Multimode elastic method

• TH = Time history method

Regular & Irregular bridgesRegular & Irregular bridges


Bridges satisfying the above table requirements may be regarded as Bridges satisfying the above table requirements may be regarded as Regular Regular bridgesbridges otherwise otherwise Irregular bridgesIrregular bridges. Curved bridges comprise of multi simple-. Curved bridges comprise of multi simple-span shall be considered as irregular bridge if subtended angle in plan is greater span shall be considered as irregular bridge if subtended angle in plan is greater than 20than 20o. o. Such bridges shall be analyzed by either multimode elastic method or Such bridges shall be analyzed by either multimode elastic method or time history method.time history method.

A curved continuous girder bridge may be analyzed as if it A curved continuous girder bridge may be analyzed as if it were straight, provided all of the following requirements are were straight, provided all of the following requirements are meet.meet.1. The bridge is regular as defined in table above except 1. The bridge is regular as defined in table above except that for a two span bridge the maximum span length ratio that for a two span bridge the maximum span length ratio from span to span must not exceed 2.from span to span must not exceed 2.2. The subtended angle in plan is not greater than 90 deg.2. The subtended angle in plan is not greater than 90 deg.3. The span lengths of the equivalent straight bridge are 3. The span lengths of the equivalent straight bridge are equal to the arc lengths of the curved bridge.equal to the arc lengths of the curved bridge.If the requirements are not satisfied then the curved If the requirements are not satisfied then the curved continuous girder-bridge must be analyzed using the actual continuous girder-bridge must be analyzed using the actual curved geometry.curved geometry.



Where

Po = uniform load arbitrarily set = 1 (N/mm)

Vs(x) = Deformation corresponding to Po (mm)

W(x) nominal unfactored dead load of the bridge superstructure and tributary substructure (N/mm)

Then calculate the time period of the bridge structure

g = Acceleration due to gravity (m/sec2)

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Seismic & Wind Analysis of Bridges