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Research Paper

International Journal of Informative & Futuristic Research ISSN (Online): 2347-1697

Volume 2 Issue 9 May 2015

Abstract

In order to be successful in mitigation efforts; the expected damage and the

associated loss in urban areas caused by severe earthquakes should be properly

estimated. It is also appropriate to consider the expected damage as a measure of

seismic vulnerability. The determination of such a vulnerability measure requires

the assessment of the seismic performances of all types of building structures

typically constructed in an urban region when subjected to a variety of potential

earthquakes. In the present work the G+4 and G+8 storied building models are

considered. The vulnerability of purely frame and purely flat slab models under

lateral loads and ground acceleration were studied. Further the flat slab models

are strengthened by perimeter beam, infill walls, shear walls and increasing the

cross sectional area of columns and the effect of positioning of infill walls and

shear walls on performance of flat slab building models were analysed. The infill

walls are modeled as equivalent diagonal strut and the seismic analysis has been

performed by Equivalent Lateral Force Method, response spectrum method as per

code IS 1893:2002 and linear time history using Electro earthquake data. The

results in form of fundamental time period, base shear, lateral displacement and

inter storey drift results are compared for purely frame, purely flat slab and

seismic strengthened flat slab models and the analysis is done with sap2000

software. From the results it is clear that the flat slab building model strengthened

by perimeter beams and shears walls shows better seismic performance.

Seismic Performance Of R C Flat-Slab

Building Structural Systems Paper ID IJIFR/ V2/ E9/ 027 Page No. 3069-3084 Subject Area Civil Engineering

Key Words RC Slab, Infill Wall , Shear Wall, Flab Slab

Basavaraj H S 1 Assistant Professor Department of Civil Engineering Jyothy Institute of Technology, Bangalore -Karnataka

Rashmi B A 2 Assistant Professor Department of Science And Humanities PES school of Engineering, Bangalore -Karnataka

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ISSN (Online): 2347-1697 International Journal of Informative & Futuristic Research (IJIFR)

Volume - 2, Issue - 9, May 2015 21st Edition, Page No: 3069-3084

Basavaraj H S , Rashmi B A :: Seismic Performance of R C Flat-slab Building Structural Systems

1. Introduction

Common practice of design and construction is to support the slabs by beams and support

the beams by columns. This may be called as beam-slab construction. The beams reduce the

available net clear ceiling height. Hence in warehouses, offices and public halls sometimes beams

are avoided and slabs are directly supported by columns. These types of construction are

aesthetically appealing also. These slabs which are directly supported by columns are called Flat

Slabs.

The column head is sometimes widened so as to reduce the punching shear in the slab. The

widened portions are called column heads. The column heads may be provided with any angle from

the consideration of architecture but for the design, concrete in the portion at 45º on either side of

vertical only is considered as effective for the design Moments in the slabs are more near the

column. Hence the slab is thickened near the columns by providing the drops as. Sometimes the

drops are called as capital of the column. Thus we have the following types of flat slabs.

(i) Slabs without drop and column head

(ii) Slabs without drop and column with column head

(iii) Slabs with drop and column without column head

(iv) Slabs with drop and column head

2. Literature Review Alpa Sheth explains the behaviour of flat slab system under lateral loads which is dependent on

numerous parameters such as the height of the building, floor plate size, size and location of the

shear wall core, flat slab spans, amongst others. Importantly, it is also dependent on the provision or

otherwise of a perimeter frame. The paper studies the effect of perimeter frames for structural

systems with flat slab structure and shear wall core for different locations of the shear wall core and

for different heights and spans of three concrete towers. For the study he considered the three

concrete towers having concrete flat slabs with shear walls have been analysed for their behaviour

with and without a perimeter framing beam. One of the models is also analysed with addition of

outrigger system. He conclude that the tall buildings of compact size, regular shape and distributed

shear wall core, there is a very marked improvement in performance of the structure with flat slab

system and shear wall core when a perimeter frame with closely spaced columns is added to the

structure. Farther spaced perimeter column frame has a relatively less impact on reducing drift. For

shorter towers of non-compact size and with distributed cores, the perimeter frame does not greatly

impact the structural behaviour

C.S Garg and Yogendra Singh studied the performance of flat slab under lateral loading

using push over analysis .In pushover analysis; a predefined lateral load pattern which is distributed

along the building height is applied on building. The lateral forces are increased until some members

yield .the structural model is modified to account for the reduce stiffness of yielded members and

lateral forces are increased until some other members yield. The process is continued until a control

displacement at the top of building reaches a certain defined level of deformation or structure

becomes unstable. The parabolic lateral loading pattern has been used according to IS 1893(part 1)

(2002).

Shyh-Jiann Hwang and Jack p.Moehle their study is concerned with two analytical models

that are commonly used in design-office practice. These are the effective beam width model, in

which the slab action is represented by a flexural slab-beam framing directly between columns, and

the equivalent frame model, in which the slab action is represented by a combination of flexural and

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tensional beams. Characteristics of both models are discussed, and recommendations on proper

application are made. The recommendations are based on a detailed experimental and analytical

evaluation presented elsewhere. The recommended analytical models are tested by comparison with

results obtained on lateral load experiments of a multipanel test slab.

3. Methodology According to ACI-318, flat-slabs can be designed by any procedure that satisfies

equilibrium and geometric compatibility provided that every section has strength at least equal to the

required strength, and that the serviceability conditions are satisfied. Generally, some of the methods

employed for the design of flat-slabs are,

A- Direct Design Method,

B- Equivalent Frame Method,

C -The Yield-line method,

D-The Finite Element Method

Table 1: Design data for all the buildings

Structure OMRF

No. of storey G+4 and G+8

Storey height 3.50 m

Type of building use Office

Seismic zone IV

Material Properties

Young’s modulus of M25 concrete, E 25.00 x 106 kN/m

2

Grade of concrete M25

Grade of steel Fe 415

Density of reinforced concrete 25 kN/m3

Modulus of elasticity of brick masonry 13800 x 103 kN/m

2

Density of brick masonry 20 kN/m3

Member Properties

G+4-Storeyed Building

Outer Beam 0.4 x 0.4 m

Column 0.4 x 0.4 m

G+8-Storeyed Building

Outer Beam 0.4 x 0.40 m

Columns up to 5-story 0.5 x 0.50 m

5 to 9 story 0.40 x 0.40 m

Thickness of wall 0.25 m

Assumed Dead Load Intensities

Roof finishes 1.0 kN/m2

Floor finishes 1.5 kN/m2

Live Load Intensities

Roof 1.5 kN/m2

Floor 4.0 kN/m2

Earthquake LL on slab as per clause 7.3.1 and 7.3.2 of IS 1893 (Part 1): 2002

Roof 0 kN/m2

Floor 0.5 x 4.0 = 2 kN/m2

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Table 2- IS 1893 (Part 1): 2002 Equivalent Static method

Zone IV

Zone factor, Z (Table 2) 0.24

Importance factor, I (Table 6) 1.00

Response reduction factor, R (Table 7) 3.0

Damping ratio 5% (for RC framed building)

Soil Type II (Medium)

Figure 1: Plan for all building models

Figure 2: 3D view of G+4 storeyed flat slab building model

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Figure 3: Elevation of the G+4 storeyed building models strengthened by infill walls and perimeter

beam

Figure 4: Elevation of the G+4 storied building models strengthened by shear walls and perimeter beam

(Model 11)

Figure 5: Plan showing the position of shear or infill walls at periphery mid

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Figure 6: Plan showing the position of shear or infill walls at central core

Figure 7: Plan of increasing the cross sectional area of intermediate columns (Model 13)

Figure 8: Plan showing increased cross sectional area of periphery columns (Model 12)

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Figure 9: Plan of increasing the cross sectional area of intermediate columns (Model 13)

Figure 10: Plan of increasing the cross sectional area of core columns (Model 14)

4. Results And Discussions

4.1: Fundamental time period and frequency for building models.

From the results it is very clear that, stiffness of the building is directly proportional to its

natural frequency and hence inversely proportional to the natural period. That is, if the

stiffness of building is increased the natural period goes on decreasing, which in turn

increases the natural frequency.

For G+4 storied building the percentages reduction in natural time periods from the analysis

results for model 3 is 11%, model 4 is 12%, model 7 is 59%, model 11 is 74% and model 12

is 0.35% when compared to model 2

For G+4 storied building the percentages reduction in natural time periods from the analysis

results for model 3 is 11%, model 4 is 12%, model 7 is 59%, model 11 is 74% and model 12

is 0.35% when compared to model 2.

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4.2 Base Shear For G+4 Storied Building Models

Table 3

• The base shear is a function of mass, stiffness, height, and the natural period of the

building structure.

• In the equivalent static method design horizontal acceleration value obtained by

codal natural period is adopted, and the basic assumption in the equivalent static method is

that only first mode of vibration of building governs the dynamics and the effect of higher

modes are not significant therefore, higher modes are not considered in this method. Hence

base shears obtained from the equivalent static method are larger than the dynamic response

spectrum method.

• From Table 3 & 4 results it is clear that base shear for purely frame model is greater

when compared to purely flat slab model. In cases of seismic strengthened building models

the model 11 has got maximum base shear.

Model No. Analytical period (sec) Frequency Codal period(sec)

G+4 G+8 G+4 G+8 G+4 G+8

Purely frame

1 1.1106 1.7757 0.901 0.563 0.641 0.997

Purely flat slab

2 1.3826 2.3357 0.723 0.428 0.641 0.997

Flat slab (250 mm slab)

3 1.227 1.9864 0.814 0.503 0.641 0.997

Strengthened by perimeter wall

4 1.2115 1.9844 0.825 0.503 0.641 0.997

Strengthened by infill wall

5 0.9337 1.3695 1.362 0.73 0.315 0.567

6 0.8286 1.3656 1.206 0.732 0.315 0.567

7 0.5584 1.1793 1.79 0.847 0.315 0.567

Strengthened by shear wall

8 0.5545 1.2461 1.803 0.802 0.315 0.567

9 0.5541 1.0112 1.804 0.988 0.315 0.567

10 0.3572 0.8514 2.799 1.174 0.315 0.567

Strengthened by perimeter wall+shear wall

11 0.3595 0.8514 2.781 1.199 0.315 0.567

Strengthened by increased column wall

12 0.8945 1.6099 1.117 0.621 0.641 0.997

13 0.9948 1.7513 1.005 0.571 0.641 0.997

14 1.1094 1.8812 0.901 0.531 0.641 0.997

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Table 4

4.3: Transverse Displacement Of G+4 Storey Building For Model 1, 2, 3 And Model 4

Figure 11(a), (b) Equivalent static method Response spectrum method

Model no Longitudinal direction Transverse direction

(kN)

VB(kN) SF (kN) (kN) VB(kN) SF(kN)

Purely frame

1 2884.76 1436.73 1.98 2844.76 1436.73 1.98

Purely Flat Slab

2 2667.06 1078.67 2.472 26667.06 1078.67 2.472

Flat slab(250 mm slab)

3 2998.58 1367.51 2.192 2998.58 1367.51 2.192

Strengthened by perimeter beam

4 2836.8 1305.03 2.173 2836.8 1305.03 2.173

Strengthened( by perimeter beam Infill wall)

5 3605.65 2187.83 1.648 3605.65 2187.83 1.648

6 3645.65 2244.33 1.642 3645.65 2244.33 1.642

7 3868.68 2874.06 1.346 3868.68 2874.06 1.346

Strengthened by shear wall

8 3361.38 2491.93 1.348 3361.38 2491.93 1.348

9 3361.38 2524.81 1.331 3361.38 2524.81 1.331

10 3580.13 2529.86 1.38 3580.13 2529.86 1.38

Strengthened by( perimeter beam +shear wall)

11 3780.13 2750.37 1.374 3780.13 2750.37 1.374

Strengthened by(Increase column cross section perimeter beam)

12 3193.24 1829.36 1.745 3193.24 1829.36 1.745

13 3050.67 1589.97 1.918 3050.67 1589.97 1.918

14 2908.09 1416.11 2.053 2908.09 1416.11 2.053

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4.4 Transverse displacement of G+4 storey flat-slab building models strengthened by

perimeter beam and infill walls

Figure 12 (a), (b): Equivalent static method Response spectrum method

4.5 Transverse Displacement Of G+4 Storey Flat-Slab Building Models Strengthened By

Perimeter Beam And Shear Wall

Figure 13(a), (b): Equivalent static method Response spectrum method

4.6 Transverse displacement of G+4 storied flat slab building models Strengthened by increase

in column cross section and perimeter beam

Figure 14 (a), (b) Equivalent static method Response spectrum method

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From results it can be observe that the infill which is act as a diagonal strut and the shear

walls are responsible to increase the story stiffness. Both for equivalent static force method

and response spectrum method the lateral sway is highest for purely flat slab building model

and it reduces with increases in story stiffness due to the presence of infill walls, shear walls

and perimeter beam.

Lateral displacement for flat slab building strengthened by increase in column cross section

and perimeter beam were found to be less than the purely flat slab model but more than the

models strengthened by infill walls, shear walls and perimeter beam.

Among the all seismic strengthened flat slab building models, the model 11 has got least

lateral displacement, since the mass and stiffness increases the displacement reduces.

4.7: Inter Story Drift Of G+4 Storied Buildings For Models 1, 2, 3 And Model 4

Figure 15 (a), (b): Equivalent static method Response spectrum method

4.8: Inter story drift of G+4 storied flat-slab building strengthened by perimeter beam and

shear walls

Figure 16 (a), (b) : Equivalent static method Response spectrum method

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4.9: Inter story Drift Of G+4 storied flat-slab building Strengthened by increase in column

cross section and perimeter beam

Figure 17 (a), (b) : Equivalent static method Response spectrum method

From the results it can be observe that due to lack of lateral load resisting system i.e. due to

absence of interior beams, the inter storey drift was found to be more in purely flat slab

model when compared with purely frame model along both longitudinal and transverse

directions.

Also it can be observe that the inter storey drifts of flat slab building models strengthened by

infill walls (infill + perimeter beam) and shear walls (shear wall + perimeter beam) along

both longitudinal and transverse directions were found to be within the limits, where as for

flat slab building models strengthened by increase in column cross section and perimeter

beam along both direction have crossed the limit.

5. Linear Time History Analysis Linear Time History analysis has been carried out using the Imperial Valley Earthquake record of

May 18, 1940 also known as the ELCENTRO earthquake for obtaining the various floor responses.

The record has 1559 data points with a sampling period of 0.02 seconds. The peak ground

acceleration is 0.319g.

Figure 18: Response spectrum plot for the ELCENTRO earthquake at 5% damping

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Figure 19

5.1 : Displacement at the top of the structure for purely frame model and purely flat slab

model (G+8 storey)

Model-1

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Model-2

5.2: Top story displacement for G+8 storied building MODELS (Linear time history analysis)

From the above results it is observed that the purely flat slab models are more vulnerable to

seismic action than the purely frame system. Among all seismic strengthened flat slab

buildings, the flat slab model strengthened perimeter beam and shear walls (Model 11)

shows the better seismic performance i.e.81% reduction in top story displacement for G+4

story and 85% for G+8 story buildings.

Type Of Structure Longitudinal

Direction

Transverse

Direction

Purely frame Model 1 167.406 167.406

Purely flat slab Model 2 203.762 203.762

Flat slab(250 mm slab) Model 3 169.024 169.024

Strengthened by perimeter beam Model 4 169.125 168.125

Strengthened by(Perimeter

beam+infill wall)

Model 5 137.732 137.732

Model 6 135.582 135.582

Model 7 95.922 95.922

Strengthened by shear wall Model 8 104.715 104.715

Model 9 64.998 64.998

Model 10 35.822 35.822

Strengthened by( perimeter beam

+shear wall)

Model 11 32.142 32.142

Strengthened by(Increase column

cross section perimeter beam)

Model 12 157.017 157.017

Model 13 170.468 170.468

Model 14 168.754 168.754

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The position of shear walls or infill walls at periphery corner is effective in resisting the

horizontal forces coming from earthquake.

From the above results it is observed that the purely flat slab models are more vulnerable

to seismic action than the purely frame system. Among all seismic strengthened flat slab

buildings, the flat slab model strengthened perimeter beam and shear walls (Model 11)

shows the better seismic performance i.e.81% reduction in top story displacement for G+4

story and 85% for G+8 story buildings.

The position of shear walls or infill walls at periphery corner is effective in resisting the horizontal

forces coming from earthquake.

6. Conclusions

The fundamental natural period of the building decreases with increases in story

stiffness due to the presence of infill walls, shear walls and perimeter beam.

The empirical expressions provided for period calculations in the code consider only

height and width of the structure for infill walls. However from the present study the

period obtained from the analysis differs from codal values for regular structure.

Base shear increases with the increase in mass and stiffness of building, hence for

purely frame and seismic strengthened flat slab buildings base shear is more as

compared to purely flat slab building.

Both for DBE and MCE levels the lateral sway is highest for purely flat slab building

model and it reduces for purely frame and seismic strengthened flat slab building

models. Since the mass and stiffness increases the displacement reduces.

The inter storey drifts for flat slab building models strengthened by infill walls and

shear walls along both longitudinal and transverse directions were found to be within the

prescribed limit mentioned in clause No.7.11.1,IS 1893 (part 1):2002.

For equivalent strut model, the models proposed by Smith and Hendry and Holmes

can be effectively used.

Equivalent strut models are effectively used in building modeling instead of wall

modeling. As infill walls behave very well under lateral loads.

The presence of infill’s can significantly reduce lateral drift and unbalanced moment at

slab-column connections in flat-slab buildings. By appropriately adding the infill’s, the

performance of seismically deficient flat-slab buildings can be significantly improved.

High rise flat slab buildings which are vulnerable to lateral loads must need shear

walls to reduce lateral deflection and inter storey drift.

Most effective location of shear wall is outer periphery of building that are provided in

the corner of building and that reduce torsion. Shear wall should be provided in both

horizontal directions equally for effective action of shear walls.

Shear wall is very effective to resist horizontal forces coming from earthquake and

wind forces etc. in multistory structure if it is properly oriented it will reduce torsional

effect and storey deflection.

The purely flat-slab RC structural system is considerably more flexible for horizontal

loads than the traditional RC frame structures which contributes to the increase of its

vulnerability to seismic effects. To increase the bearing capacity of the flat-slab structure

under horizontal loads, particularly when speaking about seismically prone areas and

limitation of deformations, modifications of the system by adding structural elements

are necessary.

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7. References

[1] C.S Garg and Yogendra singh: Seismic performances of flat slab shear wall system

[2] R. P. Apostolska1, G. S. Necevska-Cvetanovska: The 14th

World Conference on Earthquake

Engineering October 12-17, 2008, Beijing, China “Seismic performance of flat-slab building

structural systems

[3] IS: 456, (2000), “Indian Standard Code for Plain and Reinforced Concrete”, Bureau of Indian

Standards, New Delhi.

[4] IS: 1893 (Part 1), (2002), “Indian Standard Criteria for Earthquake Resistant Design of Structures”,

General provision and Buildings, Bureau of Indian Standards, New Delhi.

[5] Pankaj Agarwal and Manish Shrikande (2007), “Earthquake Resistant Design of Structures”, Prentice

Hall of India Private Limited, New Delhi, India.

[6] S N Sinha (2005), “Reinforced Concrete Design”, Tata McGraw-hill publishing company Limited,

New Delhi, India.

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