̒seismic demand of soft storey building and it's strengthening ...

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International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 5, May 2018, pp. 602–610, Article ID: IJCIET_09_05_065

Available online at http://iaeme.com/Home/issue/IJCIET?Volume=9&Issue=5 ISSN Print: 0976-6308 and ISSN Online: 0976-6316

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SEISMIC DEMAND OF SOFT STOREY

BUILDING AND IT’S STRENGTHENING FOR

SEISMIC RESISTANC’

N. A. GHATE

PG Student, M.Tech Structural Engineering,

Shri Ramdeobaba College of Engineering and Management, Katol Road, Nagpur

Dr. S. P. SIDDH

Asst. Professor, Dept. of Civil Engineering, Shri Ramdeobaba College of Engineering and Management, Katol Road, Nagpur

ABSTRACT

Building with open ground storey has become a common feature in multi-story

constructions to facilitate the parking requirements. Even though such buildings were

found to be vulnerable to earthquake shaking during past earthquakes, their construction

is still carried out widely. With rapid urbanization and increasing unbalance of required

area to availability of land, it is important to provide open ground storey in both type of

buildings that residential and commercial. Such buildings exhibit stiffness irregularity due

to absence of infill in the open ground story. This sudden reduction in stiffness causes

higher stresses to be concentrated at the ground story columns leading to its failure. The

present analytical study points out various provisions to soft storey which can reduce the

damage during earthquake. The modelling of the whole building is carried out using the

computer program ETABS. This study deals with provision to soft storey in seismic

analysis of RCC building in three ways, firstly by providing shear wall which increase the

load carrying capacity, secondly by providing bracing which increases the stiffness of

load carrying members and thirdly by proving both shear wall and bracing together. The

behaviour of open ground storied building is compared in terms of various seismic

responses such as storey stiffness, base shear, lateral displacement and story drifts.

Keywords: stiffness, shear wall, storey shear, drift, displacement

Cite this Article: N. A. Ghate and Dr. S. P. Siddh, Seismic Demand of Soft Storey

Building and It’s Strengthening for Seismic Resistance, International Journal of Civil Engineering and Technology, 9(5), 2018, pp. 602–610.

http://iaeme.com/Home/issue/IJCIET?Volume=9&Issue=5

1. INTRODUCTION

The Indian seismic code IS: 1893 defines a soft storey as one whose lateral stiffness is less than 70% of that in story above or less than 80% of average lateral stiffness of three stories

Seismic Demand of Soft Storey Building and It’s Strengthening for Seismic Resistance


above. In case of open ground story buildings, the presence of infill walls in the upper stories

makes them much stiffer than the open ground storeys. Thus during earthquake shaking the

upper stories move almost together as a single block and most of the horizontal displacement of the building occurs in soft ground storey. Upper stories being stiffer have smaller inter-

storey drifts, resulting in large shear forces and bending moments to be concentrated in ground storey columns due to reduced lateral stiffness at ground storey. This leads to

formation of storey mechanism in the soft storey which ultimately leads to failure of these buildings. Many buildings collapsed during the past earthquakes especially during Bhuj

earthquake of 2001 were due to soft story effect. The Shear walls are projected along the structure from its base. Shear walls reduce the lateral storey displacement when seismic forces

counter the building. Bracings are adopted to reduce the wind forces and lateral forces and these are easy to install and retrofitted even for the existing building. The combination of

shear wall and bracings can be adopted for the structure at different locations to improve the performance of the building.

2. AIM AND OBJECTIVES

This study mainly aims at studying the effect of soft story in multi-storey building along with

introduction of shear wall and bracing (cross bracing).

The objectives include carrying out the seismic analysis of following four models of G+11 RC building in ETABS software.

Model-1: Building having a RCC members like slab, beams, and columns. Building has

soft storey at ground floor.

Model 2: Building model similar as 1st model with RC shear wall at corners of the

building.

Model 3: Building model similar as 1st model with concrete bracings (cross bracings) at

same place as in model 2.

Model 4: Building model similar as 1st model with RC shear wall and bracing at corner

of the building.

Various seismic responses such as storey stiffness, storey drifts, storey shear and lateral displacement are computed. Based on these responses, the behaviour of soft storey in all

above different models is compared.

3. BUILDING DESCRIPTION

Two plan layout of typical eleven storey (G+11) RC moment resisting frame as shown in

following figures is considered for the analysis. First building has plan(square) dimensions of

20m x 20m and second building has plan(rectangular) dimensions 28m x 20m. The building is intended for residential use. The columns C1 and C2 represent external and internal columns

of the building. Infill walls are assumed to be made of brick masonry. The building is founded on medium strength soil. The effect of soil structure interaction is not considered in analysis.

First building plan has five bays of 4m span in both X & Y directions and second building plan has seven bays and five bays of 4m span each in X & Y direction respectively shown in

following figure 1 and figure 2.

3.1. Details of multi-storey building are as follows

Storey of building: G + 11 storey

Use of building: Residential

Frame type: SMRF

N. A. Ghate and Dr. S. P. Siddh


Floor to floor height: 3 m

Ground storey height: 3.3 m

Seismic zone: Ш

Soil type: medium

Grade of concrete: M30

Grade of steel: Fe500

Importance factor: 1

Response reduction factor: 5

3.2. Section properties

Preliminary section properties are taken into consideration while modelling the structure,

section properties of beam, column and shear walls are as follows.

Beam size: 230 x 450 mm

Column sizes: 300x600 mm

RCC slab: 125 mm thick

RC shear wall: 150 mm thick

Bracing size: 230x230 mm

3.3. Load considered for analysis

External wall load: 13.8 KN/m

Internal wall load: 8 KN/m

Parapet wall load: 4.5 KN/m

Slab live load: 2 KN/m2

Floor finish: 1 KN/m2

Load combination used as per IS1893 (Part 1):2002 clause 6.3.1.2, the following load

cases are considered for analysis

a. 1.5 (DL + IL) b. 1.2 (DL ± IL ± EL) c. 1.5 (DL ± EL) d. 0.9 DL ± 1.5 EL

Figure 1 Plan for first building (20m x20m) Plan for second building (28m x20m) Figure 2



Figure 3 model 1 for first building Figure 4 model 1 for second building

Figure 5 Position of shear wall for model 2 Figure 6 Position of bracing for model 3

For first building for first building

Figure 7 Position of SW & bracing for model 4 a Position of SW & bracing for model 4 b Figure 8

For first building for first building

Position of shear wall and bracing are same in model 2, model 3, model 4(a) and model

(b) for the second (28m x 20m) building.

4. RESULTS AND DISSCUSION

Equivalent static analysis has been performed as per IS 1893 (Part 1): 2002 for each model using ETABS analysis package. The seismic weight is calculated using full dead load plus

25% of live load.



4.1. Comparison of lateral displacement

Figure 9 Lateral displacement for first building

The displacement profile is as shown in the figure 9. The sudden change in the slope of

the displacement profile for the model 1 can be seen. This is due to the stiffness irregularity at first storey. However displacement profile for other storey levels is a smooth curve due to the

presence of infill. It is observed that model 2 has minimum lateral displacement compare to

other models. The lateral displacement reduced by 50.6%, 36.5%, 46% and 44.5% for model 2, model 3, model 4(a) and model 4(b) respectively compare to model 1.

Figure 10(a) lateral displacement in X- lateral displacement in Y- Figure 10(b)

direction for second building direction for second building

The displacement profile is as shown in the figure 10(a) and 10(b). The building is

significantly less stiff in -direction, compared to the -direction, and the displacement in the Y X

Y direction is more than displacement in X direction. Model 1 has maximum lateral displacement among all the different models. However it is reduced by 43.23%, 29.91%,

34.27% and 37.78% for model 2, model 3, model 4(a) and model 4(b) in X-direction. Similarly in Y-direction displacement is reduced by 54.20%, 41.90%, 46% and 49% for

model 2, model 3, model 4(a) and model 4(b).

4.2. Comparison of storey drifts

Table 1(a) Storey drift for first building

Sr. No. Maximum storey drift

MODEL 1 0.001517

MODEL 2 0.000693 MODEL 3 0.000880

MODEL 4(a) 0.000825 MODEL 4(b) 0.000760



Table 1(b) Storey drift for second building

Sr. No. Maximum storey drift

X-direction Y-direction

MODEL 1 0.001706 0.002509 MODEL 2 0.000893 0.000991

MODEL 3 0.00109 0.001247 MODEL 4(a) 0.00104 0.001174

MODEL 4(b) 0.000991 0.00111

Storey drift is the displacement of one level relative to other level below or above and it

shall not exceed 0.004 times the storey height according to IS:1893. Damage to non-structural components of buildings depends on drift. The maximum inter-storey drift of the frame is as

shown in table 1(a) and 1(b). It is observed that model 1 has maximum storey drift due to open ground storey. However storey drift is reduced for model 2, model 3, model 4(a) and

model 4 (b) due to presence of shear wall and bracing at ground story level.

4.3. Comparison of storey stiffness

Table 2(a) Storey stiffness for first building

Sr. No. Storey stiffness (KN/m)

At plinth level At first storey

MODEL 1 739030 356453 MODEL 2 3334885 1941128

MODEL 3 1758732 956106 MODEL 4(a) 2225752 1314620

MODEL 4(b) 2568526 1259024

Table 2(b) Storey stiffness for second building

Sr. No.

Storey stiffness (KN/m)


At plinth level At first storey At plinth level At first storey

Model 1 1016460 495058 467988 274044

Model 2 3767208 2046996 3015760 1821882

Model 3 2015382 1130448 1438717 825099

Model 4(a) 2524422 1290890 2056233 1064994

Model 4(b) 2863322 1406418 2352854 1174643

The variation of lateral stiffness of the building is represented in table 2(a) and 2(b). Due to presence of open ground storey a sudden change in stiffness from plinth level to first slab

for model 1 can be seen. However storey stiffness for model 2, model 3, model 4(a) and model (b), the abrupt change in stiffness is reduced. This is due presence of shear wall and

bracing at first storey which not only increase load carrying capacity but also increase rigidity of the frame.

4.4. Comparison of base shear

Table 3(a) Base shear for first building

Sr. No. Base shear (KN)

MODEL 1 1687

MODEL 2 1850

MODEL 3 1724.35

MODEL 4(a) 1755.75

MODEL 4(b) 1767.42



Table 3(b) Base shear for second building

Sr. No. Base shear (KN)


MODEL 1 2660 2247 MODEL 2 2850 2407

MODEL 3 2705 2285 MODEL 4(a) 2725 2300

MODEL 4(b) 2740 2325

Shear induced at the base of building during earthquake is called base shear which

depends on the seismic mass and stiffness of building. Variation in base shear is as shown in table 3(a) and 3(b). It is observed that due to consideration of infill base shear has increased.

Among all the different models, the building having shear wall i.e. model 2 for both building has maximum base shear.

From table 3(b), it is observed that base in X- direction is more than that in Y-direction as frame is more stiff in X-direction. Higher the base shear higher is the rigidity of the frame and

more is the rigidity lesser is the displacement which can be seen in displacement graphs.

4.5. Comparison of Column forces Shear force

Figure 11(a) Shear force in external column Figure 11(c) Shear force in external column

for first building for second building

Figure 11(b) Shear force in internal column Shear force in internal column Figure 11(d)


Variation in shear force at first storey is as shown in figure 11(a), figure 11(b), figure

11(c) and figure 11(d). Large increase in shear force was observed for both internal and external columns for model 1 in first building and second building due to open ground storey.

However shear force has reduced for model 2, model 3, model 4(a) and model 4(b) due to

presence of infill wall at ground storey which attracts maximum horizontal forces and hence less horizontal forces shared by columns.



4.6. Bending moment

Figure 12 a Bending moment in external column Bending moment in external column Figure 12 c


Figure 12 b Bending moment in internal column : Bending moment in internal column Figure 12 d


Variation in bending moment at first storey is as shown in figure 12(a), figure 12(b),

figure 12(c) and figure 12(d). It can be seen that bending moment has increased for both internal and external columns for model 1 in first building and second building due to open

ground storey. However bending moment has reduced for model 2, model 3, model 4(a) and

model 4(b) due to presence of infill wall at ground storey which attracts maximum horizontal forces and hence less horizontal forces shared by columns.

5. CONCLUSIONS

1. Introduction of shear wall and bracing at corner of building improved the seismic performance.

2. First storey displacement for model 1 is maximum than any other models and abrupt change is observed in storey stiffness at first storey in model 1 is due to absence of

infill.

3. Lateral displacement is reduced by 50.6%, 36.5%, 46% and 44.5% for first building and for the second building it is reduced by 43.23%, 29.91%, 34.27% and 37.78% in X-direction and 54.20%, 41.90%, 46% and 49% in Y-direction for model 2, model 3,

model 4(a) and model 4(b) compare to model 1 due to introduction of shear wall and bracing.

4. Storey drift in model 2, model 3, model 4(a) and model 4(b) is reduced compared to model 1 due to introduction of shear wall and bracing.

5. Storey stiffness in model 2, model 3, model 4(a) and model 4(b) is increased compared to model 1 due to introduction of shear wall and bracing.



6. Column forces such as shear force and bending moment for both internal and external columns of soft storey in model 2, model 3, model 4(a) and model 4(b) are reduced

compare to model 1.

7. Combination of braces and shear walls in a specific arrangement i.e. model 4(a) and model 4(b) improved the performance but when it compare to model 2, it can be seen

that model 2 is very effective in increasing of storey stiffness and in reducing the

storey drifts and lateral displacement.

REFERENCES

[1] V. K. Sehgal, et al, "Seismic Evaluation with Shear Walls and Braces for Buildings World Academy of Science", Engineering and Technology International Journal of Computer

and Information Engineering, Volume 9, Issue 2, ISSN:1307-6892

[2] Pradnya V. Sambary, Shilpa P. Kewate, "Evaluation of seismic response of a building with soft story", International Journal of Scientific & Engineering Research, Volume 8,

Issue 3, ISSN 2229-5518

[3] Dr. Surekha A. Bhalchandra, "Seismic Analysis of Reinforced Concrete Building with Soft First Storey", International Journal of Scientific & Engineering Research, Volume 5,

Issue 5, ISSN 2229-5518

[4] Mr. Abhishek Arora, "Alternative Approach to Soft Storey in Seismic Analysis of R.C.C Building Structures", SSRG International Journal of Civil Engineering (SSRG-IJCE), Volume 2, Issue 1, ISSN: 2348 – 8352

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[7] DR. H. J. Shah, "reinforced concrete vol. 2 (advanced reinforced concrete)" 7th Edition (2014).

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[9] Software used ETABS 2013. [10] IS 456-2000 “Code of practice for plain and reinforced concrete”. [11] Pouya Azarsa, Mahdi Hosseini, Seyed Amin Ahmadi and Prof. N.V. Ramana Rao,

Enhanced Seismic Resistance of Steel Buildings Using Viscous Fluid Dampers.

International Journal of Civil Engineering and Technology, 7(6), 2016, pp.90 – 105.

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