Seismic Analysis of Rcc Framed Structures by Static (1)

7/31/2019 Seismic Analysis of Rcc Framed Structures by Static (1)

1/31

SEISMIC ANALYSIS OF RCC FRAMED

STRUCTURES BY STATIC ANALYSIS METHOD ANDDETAILING PROCEDURE

1Prasanta Kumar Tripathy, MIE,Asst. Engineer(WorksDepartment,GoO)

2Akshay Kumar Sahoo, AIE ,Asst. Engineer(Works


2/31

India witnessed many large earthquakes in the last centuryresulting in heavy loss of life and property. The recent Bhuj

earthquake in January 2001 was a large one and of magnitude7.7 in Richter scale. It is essential to have an idea about basicseismology i.e. internal structure and behavior of earth withregard to earthquake phenomenon. It has become highlyessential for major constructions to be designed forearthquake force on the basis of IS-1893-2002. Earthquake

resistant reinforced concrete buildings should be designed toresist anticipated seismic forces. The structure must satisfysafety and serviceability conditions in order to resist expectedloadings. It is necessary to understand the behavior ofmaterials like concrete and steel under seismic loading. Inorder to resist the earthquake the structure must have

adequate ductility in order to dissipate the energy throughinelastic deformation. It is also essential to provide adequatedetailing of reinforcement in members of a structure as perIS 13920-1993, so that the structure can safely respond tostrong ground motion. An overview on Earthquake Basic ,calculation of design seismic force on RCC framed structure by

Static Analysis Method and ductile detailing as per IS 13920-1993 is narrated in this paper.


3/31

Points ofDiscussions

Design seismic forces

EQ basics,Terminology

Seismic Waves Behaviour of seismic

waves

Seismic zones of India

Analysis of seismicforces as perIS:1893(Staticmethod)

Ductile detailing procedure

Necessity of ductilestructure

Indian Standard codesfor the purpose

General specifications asper IS:13920-

1993,IS:4326 Important

Reinforcement sketchesfrom IS:13920-1993


4/31

EARTHQUAKE: BASIC TERMINOLOGY

Focus is a point on the fault

where slip starts.

Epicenter is a point just abovethe focus on the ground.

Epicentral Distance is thedistance of place of interest fromepicenter on ground.

Most of the damaging

earthquakes have shallowfocus with focal depths less than

about 70km.

The earthquakes (of smallsizes) occurring after the bigearthquake (Main shock) arecalled Aftershocks.


5/31

EARTHQUAKE: SEISMIC WAVES

Strain energy released due tomovement of rocks

in the Fault plane generates seismicwaves which

moves in all directions andundergoes reflections

& refractions at various elastic layerinterfaces.

Seismic Waves: A) Body waves

B) Surface Waves

Body Waves 1) Primary waves (P-waves)

2) Secondary Waves (S-Waves)

Surface Waves: 3) Rayleighwaves

4) Loves Waves


6/31

Seismic Zones in India

Zone-II(Zone-Imerged withZone-II)

Zone-III

Zone-IV

Zone-V


7/31

Calculation of Design Seismic Force byStatic Analysis Method

ProblemStatement:Consider a four-storey reinforcedconcrete office building shown in Fig.1.1. The building is located in seismiczone III . The soil conditions aremedium stiff and the entire building issupported on a raft foundation. TheR. C. frames are infilled with brick-masonry. The lumped weight due todead loads is 9.00 kN/m2 on floors

and 7.00 kN/m2 on the roof. Thefloors are to cater for a live load of 4kN/m2 on floors and 1.5 kN/m2 on theroof. Determine design seismic loadon the structure as per new code.

PLAN

4 @ 5.00 M

3 @ 5.00 M

X

Y

ELEVATION


8/31

Solution:

Design Parameters:

For seismic zone III the zone factor Z is 0.16 (Table 2 of IS: 1893). Being an office building,the importance factor, I, is 1.0

(Table 6 of IS:1893). Building is required to be provided with moment resisting frames detailed as per IS:13920-1993.

Hence, the response reduction factor,R, is 5.(Table 7 of IS: 1893 Part 1)

Seismic Weights:

The floor area is 1520=300 sq. m. Dead load is 9.00 KN/sqm including brick load. Since the live load class is 4kN/sq.m,

only 50% of the live load is lumped at the floors. At roof, no live load is to be lumped. Hence, the total seismic weight on the

floors and the roof is:

Floors:

W1=W2 =W3 =300(9+0.54)= 3,300 kN, Roof:W4 = 3007=2100 kN

(clause7.3.1, Table 8 of IS: 1893 Part 1)

Total Seismic weight of the structure,

W= Wi = 33,300 + 21,00

= 12,000 kN

Fundamental Period:

Lateral load resistance is provided by moment resisting frames in filled with brick masonry panels. Hence, approximate

fundamental natural period:

(Clause 7.6.2. of IS: 1893 Part 1)


9/31

SOLUTION CONTINUED..

EL in X-Direction:

T= 0.09h /d= 0.09(13.8) / 20= 0.28 sec

The building is located on Type II (medium soil).From Fig. 2 of IS: 1893, for T=0.28 sec, Sa/g =2.5

Ah =(Z/2) x (I/R)x( Sa/g) = (0.16x1x2.5)/ (2x5) = 0.04

(Clause 6.4.2 of IS: 1893 Part 1)

Design base shearV B = A h W= 0.04 12,000= 480 kN

(Clause 7.5.3 of IS: 1893 Part 1)

EL in Y-Direction:

T= 0.09h/ d

= 0.09(13.8) /

15= 0.32 secSa/g =2.5Ah = 0.04

Therefore, for this building the design seismic force in Y-direction is same as that in the X direction.

Fig. 1.2(b) shows the design seismic force on the building in the Y-direction.


10/31

Table 1.1Lateral Load Distribution with Height by the Static Method

Storey

Level

Wi(kN) hi(m) Wihi 2

x(1000)

Wihi2

Wihi2

Lateral force in ith level for

EL in direction in (KN)

X direction Y direction

4 2100 13.8 399.92 0.396 191.00 191.00

3 3300 10.6 370.78 0.367 177.00 177.00

2 3300 7.4 180.70 0.178 86.00 86.00

1 3300 4.2 58.21 0.057 26.00 26.00

1009.61 1000 480.00 480.00

SOLUTION CONTINUED


11/31

SOLUTIONCONTINUED

191 KN

177 KN

86 KN

26 KN

191 KN

177 KN

86 KN

26 KN

Fig 1-2(a)Fig 1-2(b)

Force Distribution with Building Height:

The design base shear is to be distributed with height as per clause 7.7.1. Table 1.1 gives the calculations. Fig. 1.2(a) shows

the design seismic force in X-direction for the entire building.

D til D t ili i RCC t t


12/31

Ductile Detailing in RCC structure

Purpose:

The main structural elements and their connection shall be

designed to have a ductile failure. This will enable the structureto absorb energy during earthquakes to avoid sudden collapse ofthe structure. Providing reinforcing steel in masonry at criticalsections, as provided in this standard will not only increasestrength and stability but also ductility.

Relevant Codes :

I.S.: 13920-1993 has taken note of latest developments,experiences gained from the performance of structures whichwere designed and detailed as per I.S. 4326, during the recentearthquakes. It covers provisions for earthquake resistant designand detailing of reinforced concrete structures in particular. (assuch it includes provisions of I.S. 4326 also) Now all ductilitydetailing shall comply I.S.: 13920.

I.S. 4326 , The code of practice for earthquake resistant designand construction of building, while commenting on certain specialfeatures for the design and construction of earthquake resistantbuildings, included some details for achieving ductility in

reinforced concrete buildings.


13/31

Some important clause of IS:13920-1993CLAUSE1.1.1Provisions of this code shall be adopted in all reinforced concrete Structures which satisfy one ofthe following 4 conditions.(i) The structure is located in seismic zone IV or V.(ii) The structure is located in Seismic Zone III and has importance factor (I) greater than 1.0.(iii) The structure is located in Seismic Zone III and is an industrial structure.(iv) The structure is located in Seismic Zone III and is more than 5 storeys.

Clause 3.4 :Hoop- It is closed stirrup having a 135 degree hook with 10 diameter extension (but not less than75 mm ) at each end that is embedded in the confined core of the section .

Clause 5.2 :For all buildings which are more than 3 storeys in height the minimum grade of concrete shall beM20.

Clause 5.3 :Steel reinforcement of grade Fe 415 or less only shall be used .

Clause 6:For flexural members

6.1.1 The factored axial stress on the member under earthquake loading shall not exceed 0.1 fck.6.1.2 The member shall have a width to depth ratio of more than 0.36.1.3 Width of flexural member not less than 200mm.6.1.4 Depth if member not less than 0.25 of the clear span .


14/31

Some important clause of IS:13920-1993

Clause 6.2 Longitudinal reinforcement :6.2.1(a) At least two bars at top and two bars at bottom shall be provided through out the member

length .

(b) The tension steel ratio on any fact at any section shall not be less thanRho (min)= 0.24 [(square root of fck)/fy] .6.2.2 The maximum steel ratio on any face at any section shall be not exceed Rho(max) = 0.025.6.2.3 The positive steel at joint face must be at least equal to half the negative steel at that face.6.2.4 The steel provided at each of the top and bottom face of the member at any section along its

length shall be at least equal to one fourth of the maximum negative moment steel providedat the face of either joint .

6.2.5 In an external joint both the top and bottom bars of the beam shall be provided withanchorage length beyond the inner face of column equal to development length in tension plus10 times the bar diameter minus the allowance for 90 degree bends (s) In an internal joint,both face bars of the beam shall be taken continuously through the column.

6.2.6 The longitudinal bars shall be spliced, only if hoops are provided over the entire splice lengthat a spacing not exceeding 150 mm.The lap length shall not be less than the bar developmentlength in tension.

Lap splices shall not be provided(a) Within joint.

(b) Within a distance of 2d from joint face and(c) Within a quarter length of member where flexural yielding may generally occur under theeffect of earthquake forces . Not more than 50 percent of bars shall be spliced at one section .


15/31

Some important clauses of IS:13930-1993(contd)

6.3.5 The spacing of hoops over a length of 2d at either end of a beam shall not exceed.a)d/4.and

b) 8x dia of smallest bar ,But not less than 100 mm.

The first hoop shall be at a distance not exceeding 50 mm from the joint face. Vertical hoops at thesame spacing as above shall also be provided over a length equal to 2d on either side of asection where flexural yielding may occur under the effect of seismic forces . Elsewhere thebeam shall have vertical hoops at a spacing not exceeding d/2.

Clause 7 Columns subjected to bending and axial load.

7.1.1 These requirement apply to columns which have factored axial force in excess of (0.1 fck)under the effect of earthquake forces.

7.1.2 The minimum dimension of column shall be 200 mm . However where in frames wherebeams have c/c span exceeding 5m, or column having unsupported length exceeds 4m theshortest dimension shall not be less than 300 mm.

7.1.3 The ratio of shortest dimension to the perpendicular dimension shall be preferably NOT lessthan 0.4.

Clause 7.2 Longitudinal Reinforcement7.2.1 Lap splices shall be provided only in the central half of the member length.Itshould be

proportioned as a tension splice .Hoops hall be provided over entire the splice length atspacing not exceeding 150 mm center to center .

Not more than 50 percent of bars shall be spliced at one section.7.2.2 Any area of column that extends more than 100 mm beyond the confined core due to

Architectural requirements shall be detailed in the matter .In case of the contribution of the area to strength has been considered then it will have the

minimum longitudinal and transverse reinforcement asper this code .However if this area has been treated as non structural the minimum reinforcement shall be


16/31


Clause 7.3 Transverse Reinforcement7.3.2 The spacing of rectangular hoops shall not be more than 300 mm c/c .If the length of anyside of stirrup , exceeds 300 mm a cross tie shall be provided or a pair of overlapping hoops

may be provided.Clause 7.4 Special Confining Reinforcement7.4.1 This shall be provided over a length of (lo) from each joint face towards mid span on

either side of any section lo shall not be less than(a) larger lateral dimension of the member .

(b) 1/6 of clear span of member and

(c) 450 mm.7.4.2 When a column terminates in to a footing or mat special confining reinforcement shall

extended at least 300 mm in to the footing or mat.7.4.3 The spacing of hoops used as a special confining reinforcement shall not exceed of

minimum member dimension but need not be less than 75 mm nor more than 100 mm.7.4.4 The minimum area of cross section of bar forming circular hoops or spiral to be used as special confining

reinforcement shall not be less thanAsh = .09 S Dk (fck/fy) [(Ag/Ak) -1.0]WhereAsh = area of the bar cross section .S = Pitch of spiral or spacing of hoops.Dk = diameter of core measured to the outside of spiral or hoop . Fck = characteristic compressive strength of concrete cube .Fy = yield stress of (spiral/ hoop ) steelAg = gross area of column cross section .Ak = area of concrete core should not exceed 300mm


17/31


7.4.8 The area of cross section Ash of the bar forming rectangular hoop to beused as special confining reinforcement shall not be less thanAsh = 0.18 S.h. (fck/fy) [(Ag/Ak) -1.0]WhereH = longer dimension of rectangular hoop.Ak = Area of concrete core in the rectangular hoop measured to its outside

dimensions.Clause 8 Joints of frames8.1 The special confining reinforcement as required at the end of column shall be

provided through the joint is confined as specified by 8.28.2 A joint, which has beams framing in to all vertical faces of it and where each

beam which is at least of the column width, may be provided with half thespecial confining reinforcement required at the end of column. The spacingof hoops shall not exceed 150 mm.


18/31

Ductile Detailing Sketches confirming toIS:13920


19/31



20/31

IS:13920


21/31

IS:13920


22/31


o s an on t o re n orcement eta ng o


23/31

o s an on t o re n orcement eta ng oRCC members


24/31

Dos and Dont of reinforcement detailing of RCCmembers


25/31

Dos and Dont of reinforcement detailing ofRCC members

Dos and Dont of reinforcement detailing of


26/31

Do s and Don t of reinforcement detailing ofRCC members


27/31

Dos and Dont of reinforcement detailing ofRCC members


28/31

Dos and Dont of reinforcement detailing of RCCmembers


29/31

Conclusion

After distribution of base shear on different storeys , the frame is anlysed using

portal frame method and final moment and shear on the vertical and horizontal

members can be found out . The RCC members can be designed for critical load

combination as per IS456. The detailing for the RCC members shall be done as as

per IS:13920-1993.


30/31

References :

Reinforced Concrete , Limit State Design by A.K.Jain

Criteria for Earthquake Resistant Design of Structures IS

1893(part-I) 2002

Ductile Detailing of RCC structures subjected to seismic

forces IS 13920 :1993

Earthquake TIPs by CVR Murty

Reinforcement detailing of RCC members byEr.T.Rangarajan


31/31

Documents

Seismic Analysis of Rcc Framed Structures by Static (1)