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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
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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.
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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
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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.
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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
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Seismic Zones in India
Zone-II(Zone-Imerged withZone-II)
Zone-III
Zone-IV
Zone-V
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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
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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)
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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.
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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
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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
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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.
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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 .
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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 .
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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
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Some important clauses of IS:13930-1993(contd)
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
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Some important clauses of IS:13930-1993(contd)
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.
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Ductile Detailing Sketches confirming toIS:13920
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Ductile Detailing Sketches confirming toIS:13920
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IS:13920
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IS:13920
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Ductile Detailing Sketches confirming toIS:13920
o s an on t o re n orcement eta ng o
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o s an on t o re n orcement eta ng oRCC members
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Dos and Dont of reinforcement detailing of RCCmembers
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Dos and Dont of reinforcement detailing ofRCC members
Dos and Dont of reinforcement detailing of
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Do s and Don t of reinforcement detailing ofRCC members
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Dos and Dont of reinforcement detailing ofRCC members
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Dos and Dont of reinforcement detailing of RCCmembers
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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.
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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
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