Seismic Analysis of regular & Irregular RCC frame structures

1

A Seminar on

Seismic Analysis of Regular & Irregular RCC Framed Building

Submitted By :-

Daanish Zama

Submitted To :-

M/s Padmabati SahooAsst.ProfessorCivil Engineering Dept.

S I L I C O N I N S T I T U T E O F T E C H N O L O G YSambalpur, Odisha

CONTENTS

Introduction

Types of Irregularities

Objectives

Methodology

Problem Statement

Structure Modelling

Analysis of building and their Results

Conclusion

Reference

INTRODUCTION

The word earthquake is used to express any seismic occurrence whether natural or caused by humans that can

produce seismic influence around any particular area. Earthquakes are caused generally by rupture of

geological faults inside the earth, but also by other events such as volcanic movement, landslides, mine

blasts, and atomic tests. Vertical irregularities are characterized by vertical discontinuities in the geometry,

distribution of mass, rigidity and strength. Setback buildings are a subset of vertically irregular buildings

where there are discontinuities with respect to geometry. However, geometric irregularity also introduces

discontinuity in the distribution of mass, stiffness and strength along the vertical direction. Majority of the

studies on setback buildings have focused on the elastic response. The behavior of these types of building is

something different. There is a need of more work to be done in this regard. So this research work is an attempt

to reach on more accurate conclusion to reduce their effect on the structure.

We observe that real structures are frequently irregular as perfect regularity is an idealization that

rarely occurs in the practice. Regarding buildings, for practical purposes, major seismic codes across the

globe differentiate between irregularity in plan and in elevation, but it must be realized that irregularity in the

structure is the consequence of a combination of both types. It is seen that irregular structural configurations

either in plan or in elevation were often recognized as one of the major causes of collapse during precedent

earthquakes.

TYPES OF IRREGULARITIES

The Irregularity in the building structures may be due to irregular distributions in their mass, Strength and

stiffness along the height of building. When such buildings are constructed in high Seismic zones, the

analysis and design becomes more complicated. There are two types of Irregularities :-

1. Plan irregularities

2. Vertical irregularities.

Vertical irregularities are one of the major reasons of failures of structures during earthquakes. Vertical Irregularities are mainly of five types :-

i) Stiffness Irregularity :- Under stiffness irregularity the stiffness of the members in a frame are not equal

and they vary according to the floor height, modulus of elasticity of concrete and moment of inertia

of that member.

Types of irregularities Cont…

ii) Mass Irregularity :- Mass irregularity shall be considered to exist

where the seismic weight of any storey is more than 200

percent of that of its adjacent storeys. In case of roof

irregularity need not be considered.

iii) Vertical Geometric Irregular :- A structure is considered to be

vertical geometric irregular when the horizontal dimension of the

lateral force resisting system in any storey is more than 200 percent

of that in its adjacent storey. In case of roofs irregularity need not be

considered.

OBJECTIVES

The goal of this research is to investigate various seismic responses of RC framed regular and vertical

geometric irregular structure. The comparison between various seismic parameters would allow us to

propose the best suitable building configuration on the existing condition. More specifically, the

salient objectives of this research are:

1) To perform a comparative study of the various seismic parameters of different types of reinforced

concrete moment resisting frames (MRF), configuration, and types of irregularity.

2) Comparison between regular and vertical irregular frame on the basis of shear force, bending

moment, storey drift & node displacement etc.

3) To study the change in different seismic response parameters along the increasing height and

increasing bays.

4) To propose the best suitable building configuration on the existing condition.

METHODOLOGY

The steps undertaken in the present study to accomplish the above-mentioned objectives are

as follows:

a) Selected an exhaustive set of regular, stiffness irregular and setback building frame models

with same heights of 4 story, assuming equal bay width of 5 m in both horizontal direction and

different irregularities.

b) Perform static analysis for each of the 4 building models taken in this study.

c) Analysing and comparison of the result of seismic analysis.

d) Presentation of results in the form of graphs and tables.

e) Detailed discussion on the results with the help of graphs and tables considering all the included

parameters.

PROBLEM STATEMENT

STRUCTURAL MODELLING

Equivalent static analysis was performed on regular and various irregular buildings using STAAD.pro.

The storey shear forces were calculated for each floor and graph was plotted for each structure.

Three types of irregular buildings were considered, regular structure, stiffness irregular

structure (structure with ground storey as the soft storey) and two vertically geometric irregular building

with different setbacks. All the structures are of 4-storeys.

1. Regular Structure (4 storey)

2. Stiffness Irregular Structure (Soft Storey)

3. Vertical Geometric Irregular structure (setback at 3rd floor)

4. Vertical Geometric Irregular structure (setback at 2nd and 3rd floor)

Structural modelling Cont…

1. Regular structure (4 storey):

Plan of regular structure 3D view of regular structure


2. Stiffness Irregular structure (soft

storey):

The structure is same as that of regular

structure but the ground storey has a height of

4.5m and doesn’t have brick infill.

Stiffness of each column = 12EI/l3

Therefore, Stiffness of ground floor/stiffness

of other floors = (3.5/4.5)3 = 0.47

Stiffness Irregular Structure


3. Vertical Geometric Irregular structure (setback at 3rd floor):The structure is 4 storeyed with setbacks

in 3rd floor. The setback is along X direction.

Width of top storey = 10 m

Width of ground storey = 15m

Vertical Geometric Irregular Structure 1. 3D view of Vertical Geometric Irregular Structure 1


4. Vertical Geometric Irregular structure (setback at 2nd and 3rd floor): The structure is also 4 storeyed

with setbacks in 2nd and 3rd floor. The setback is along X direction.

Width of top storey = 10 m

Width of ground storey = 15m

3D view of Vertical Geometric Irregular Structure 2Vertical Geometric Irregular Structure 2.

ANALYSIS OF BUILDING AND

THEIR RESULTS

BENDING MOMENTfor beam no.45 (vertical beam in the frame)

19

20

21

22

23

24

25

26

Regular Structure Stiffness Irregular Geometric Irregular 1 Geometric Irregular 2

Max +ve BM in Z-direction(KN-m)

Max +ve BM in Z-direction(KN-…

21.4

21.6

21.8

22

22.2

22.4

22.6

22.8

23

23.2


Max +ve BM in Y-Direction (KN-m)

Max +ve BM in Y-Direction (KN-m)

Geometric Irregular structure frames have less critical bending

moment than Regular frames. There is not much change for the

bending moment of regular frames and stiffness irregular frames.

For geometric irregular frame with setback at 2nd & 3rd floor ishaving maximum BM in Z-direction.

Same goes with the maximum bending moment in Y direction

also. Geometric Irregular structure has the less maximum as

compared to all the different structures along Y direction.

BENDING MOMENTfor beam no.135 (horizontal beam in the frame)

155.5

156

156.5

157

157.5

158

158.5

159

159.5

160


Max +ve BM in Z-direction (KN-m)

Max +ve BM in Z-direction (KN-m)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5


Max BM +ve in Y-direction(KN-m)

BM +ve in Y-direction(KN-m)

For beam no.135, the nature of bending moment along Z-direction is

very same as it was for the previous case. Geometric Irregular

Structure Frame has the least bending moment in comparison with all

the other three structures.

In this case, bending moment in Y-direction is totally different. It is

observed as Geometric Irregular has maximum bending moment

compared to all the structures. While in the previous cases for BM,

the geometric irregular frame 2 poses the least BM.

SHEAR FORCEfor beam no.45 (vertical beam in the frame)

0

2

4

6

8

10

12

14

16

18

Regular structure Stiffness Irregular Geometric Irregular 1 Geometric Irregular 2

Max +ve SF in Y-direction (KN)


For shear force in Z-direction, there is no any

magnitude of maximum positive shear force

acting in z-direction.

Geometric Irregular structure frames have

less shear force than Regular frames. There

is not much change for the bending

moment of regular frames and stiffness

irregular frames. As this conclusion is similar

with the critical seismic parameters like

bending moment.

SHEAR FORCEfor beam no.135 (horizontal beam in the frame)

119.8

120

120.2

120.4

120.6

120.8

121

121.2

121.4

121.6

121.8

Regular

Structure

Stiffness

Irregular

Geometric

Irregular 1

Geometric

Irregular 2



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7


Max +ve SF in Z-direction (KN)

Max +ve SF in Z-Direction (KN)

Same goes for the horizontal beam (beam no.135) also the shear force goes on decreasing to the geometric irregular

structure. While there is not much difference in the value of maximum shear force in both X and Z-direction.

STOREY DRIFT

-0.01

0

0.01

0.02

0.03

0.04

0.05

Regular Stiffness Irregular Geo.Irregular 1 Geo.Irregular 2

Storey Drift X(cm) direction

storey 1 storey 2 storey 3 storey 4

-30

-25

-20

-15

-10

-5

0

Regular Stiffness Irregular Geo.Irregular 1 Geo.Irregular 2

Storey Drift Z(cm) direction

Storey 1 Storey 2 Storey 3 Storey 4

The inter storey drift are taken in both X and Z directions.

From graph we see that the drift lines of all the 4 storeys along X

direction is almost coinciding for regular and Stiffness Irregular

frames. Regular Building configurations have exactly same value

of drift. The irregular frames Stiffness, Vertical Geometric 1,Vertical Geometric 2 have slightly more drift in X direction

Here it is observed that the regular structure too has different storey

drift in Z-direction which was earlier same. It is observed that the

stiffness irregular frame possess higher values of drift than there

corresponding irregular frames for all the four storey height.

Also for storey 1, drift along Z-direction is almost same for all the

varieties of structure.

JOINT DISPLACEMENT

-0.2

0

0.2

0.4

0.6

0.8

1

Regular Stiffness

Irregular

Geo. Vertical 1 Geo. Vertical 2

Joint Displacement X(cm) direction


-30

-25

-20

-15

-10

-5

0

Regular stiffness Irregular Geo.Vertical 1 Geo. Vertical 2

Joint Displacement Z(cm) direction


It is seen that the storey displacement of 4th storey is maximum among

all the frames. However, for regular and stiffness irregular frames, the

joint displacement of all storeys are same along x-direction. And the

vertical geometric frame 2 with setbacks have maximum joint

displacement in every storey.

In this storey- 4 has maximum displacement among all the varieties

of frames. The stiffness irregular structure frame has maximum joint

displacements for all the floor levels. However, Regular and both

the vertical geometric frames have almost same joint displacement

contributing a common nature of displacement under the effects of

seismic forces.

CONCLUSION

Based on the work presented following conclusions can be drawn :-

1) Amount of setback increases, the shear force also increases. The regular building frames possess very low

shear force compared to setback irregular frames.

2) The critical bending moment of irregular frames is more than the regular frame for all the storey heights. This is

due to decrease in stiffness of building frames due to setbacks.

3) According to results of equivalent static method, the stiffness irregular building experienced larger inter storey

drifts as compared to regular frame and geometric irregular frames.

4) It is seen that the storey displacement of 4th storey is maximum among all the frames and the stiffness irregular

structure frame has maximum joint displacements for all the floor levels. However, regular and both the vertical

geometric frames have almost same joint displacement.

5) The seismic performance of regular frame is found to be better than corresponding irregular frames in nearly all

the cases. Therefore it should be constructed to minimize the seismic effects. Among setback frames, the geometric

irregular frame 1 building having setback at 3rd floor configuration is found superior than others.

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