comparative study of seismic behaviour of multi - storey

Comparative Study of Seismic Behaviour of

Multi - Storey Buildings with Flat Slab, Waffle

Slab, Ribbed Slab &Slab with Secondary Beam

Shivnarayan Malviya

M. E. Scholar, Department of Civil Engineering

Jabalpur Engineering College, Jabalpur, Madhya Pradesh, India

Mr.Vipin Kumar Tiwari Assistant Professor, Department of Civil Engineering

Jabalpur Engineering College, Jabalpur, Madhya Pradesh, India

Abstract— Recent earthquakes in which many concrete structures have been severely damaged or collapsed, have

indicated the need for evaluating the seismic adequacy of existing buildings. About 60% of the land area of our country is

susceptible to damaging levels of seismic hazard. We can’t avoid future earthquakes, but preparedness and safe building

construction practices can certainly reduce the extent of damage and loss. In order to strengthen and resist the buildings

for future earthquakes, some procedures have to be adopted. The use of different type of slabs is evolving as a new trend

and is becoming a big challenge for structural engineers. Therefore, it is necessary to study about its structural behaviour.

This paper deals with the behaviour of different type of slabs such as flat slab, waffle slab, ribbed slab and slab with

secondary beam. We have modelled a G+5 & G+9 storey building in ETAB software having a plinth area of 1600 m2. The

grid spacing is taken as the 8 m for the consideration of large span in both major directions. Total 10 Models have been

prepared with different type of slabs. The response spectrum analysis has been carried out for the seismic zone III. It has

been found that for large span slabs the structure having secondary beams should be avoided for better seismic

performance. Most preferable long span slab on the basis of this study is Building with Waffle Slab.

Keywords- Flat slab, Waffle slab, ribbed slab, multi-storey building, slab with secondary beam, response spectrum

analysis, seismic zone III.

I. INTRODUCTION

Earthquake is a phenomenon that occurs due to the geotechnical activities in the strata of the Earth and is highly

unpredictable and causes heavy losses to both life and property if it occurs in populated regions. Earthquake does

not kill humans, but the buildings do. Thus, it is the prime responsibility of a structural (design) engineer to draw out

the parameters from previous experiences and consider all the possible hazards that the structure may be subjected

to, in future, for the purpose of safe design of structure. There are many available techniques for the analysis of the

structure and to evaluate their performance under the given loading, the most accurate among them being the Non-

Linear Time History Analysis. For the structures with less importance or seismic hazard, some other conventional

methods have been developed named as Non-Linear Static methods (NSPs). The results obtained from these

techniques may or may not be accurate, but these methods do give an approximate idea about the behaviour of the

structure under seismic loading. Isn general slabs are classified as being one-way or two-way. Slabs that primarily

deflect in one direction are referred to as one-way slabs. When slabs are supported by columns arranged generally in

rows so that the slabs can deflect in two directions, they are usually referred to as two-way slabs. Two-way slabs

may be strengthened by the addition of beams between the columns, by thickening the slabs around the columns

(drop panels), and by flaring the columns under the slabs (column capitals). The horizontal floor system resists the

gravity load (dead load and live load) acting on it and transmits this to the vertical framing systems. In this process,

the floor system is subjected primarily to flexure and transverse shear, whereas the vertical frame elements are

generally subjected to axial compression, often coupled with flexure and shear. The floor also serves as a horizontal

diaphragm connecting together and stiffening the various vertical frame elements. Under the action of lateral loads,

the floor diaphragms behave rigidly (owing to its high in plane flexural stiffness) and effectively distribute the

lateral load to the various vertical frame elements and shear walls. The different types of slabs studied are as

follows:

Journal of Xi'an University of Architecture & Technology

Volume XIII, Issue 3, 2021

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Waffle slab: A waffle slab is made of reinforced concrete with concrete joists spanning in mutually perpendicular

directions on its bottom. Due to the grid arrangement generated by the R.C. ribs are termed as waffle. It is also

known as two-way joist slab. It is mainly used when span is greater. It is stronger than other type of slab. The slab

has two parts. The part one is in top side which is flat surface and second part at bottom consist of joists create a grid

like structure. The grid is appeared when moulds are removed. It is also used when heavy loads are acting on the

structure. Under the effect of rigidity this type of slab is used when buildings require minimal vibration, such as used

for laboratory, manufacturing facilities etc.

Ribbed Slab: These types of slabs are cast completely with a series of closely spaced joist which in turn are

supported by a set of beams. The main benefit of ribbed floors is the lowering in weight achieved by removing part

of concrete below the neutral axis. Which makes these type of floor economical for buildings with a long span with

light or moderate loads. Ribbed slabs are slabs cast integrally with a series of closely spaced joist which in turn are

supported by a set of beams. The main advantage of ribbed floors is the reduction in weight achieved by removing

part of concrete below the neutral axis. This makes this type of floor economical for buildings with a long span with

light or moderate loads.

Flat Slab: A reinforced concrete slab supported directly by concrete columns without the use of beams. This type of

slab consists of different system of elements such drops, column head, perimeter beam etc. along with flat slab.

These types of structures use column heads and column strips as a replacement of beams to provide large spans.

Whole slab rests on these column heads and column strips and acts as diaphragm. These structures are vulnerable to

dynamic earthquake forces so analysis regarding dynamic earthquake behaviour of the structure must be done before

designing these structures in earthquake prone areas.

Secondary Beams: The beams which are constructed to transfer the load of slab on main beams are called secondary

beams. Basically, secondary beams are not directly resting on column, but are supported on main beams which are

supported by columns directly. Beam which rest on column directly are termed as primary beams. Secondary beams

are generally used to provide architectural benefits and for space restrictions. Reinforcement details are calculated

on the basis of the quantity and type of load exerting on every beam.

II. OBJECTIVE OF WORK

Following are the objectives of work:-

1) To model G+5 & G+9 multistorey building with different types of slabs.

2) To analyse G+5 & G+9 multistorey building by RSA (Response Spectrum Analysis).

3) To compare the different parametric results such as storey displacement, base shear, overturning moments,

storey shears etc.

III. MODELING AND ANALYSIS

The modeling part includes theG+5 & G+9 storey building with large span consideration. The different types of

slabs have been introduced in the structure. The plan area of all four buildings is same i.e.1600 square meters (40.00

m x 40.00 m) each. These buildings have been designed in compliance with the Indian Code of Practices for

earthquake resistant design of buildings. Base of the building has been considered as fixed. The square sections are

used for structural elements just to focus on the analysis based on variation in slab and to provide secondary beam

on it. The buildings have been modeled using ETABSvr.2018.

Table 1: Material Properties of Member

S. No. Case No. Model Description No. of Stories

01 Case 1 Building having Flat Slab with Drop Panels G+5 and G+9

02 Case 2 Building having Flat Slab with Drop Panels and Perimeter Beams G+5 and G+9

03 Case 3 Building having Waffle Slab G+5 and G+9

04 Case 4 Building having Ribbed Slab G+5 and G+9

05 Case 5 Building having Slab with Secondary Beams G+5 and G+9



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Table 2: Structural Properties

Structural Properties

S. No. Descriptions of Parameters Dimensions / Comments

A) Common Parameters

1 Structure type Rigid frame with flat slab

2 No of storey /total height G+5/21.00 m.

G+9 /35.00 m

3 Plan area 40.00 m x 40.00 m

4 Column size 600 mm x 600 mm

5 Plinth beam size 200 mm x 500 mm

6 Spacing of grid in x –direction 8.00 m. c/c

7 Spacing of grid in y –direction 8.00 m. c/c

8 Individual storey height 3.50 m.

B) Case 1: Building Having Flat Slab with Drops

1 Beam Size No beams

2 Slab Thickness without Drop 275 mm

3 Slab thickness with Drops 350 mm

4 Drop Size 3.00 m x 3.00 m

5 Thickness of Drops 75 mm

C) Case 2: Building Having Flat Slab with Drops with Perimeter Beams

1 Perimeter Beam Size 300 mm x 600 mm

2 Slab Thickness without Drop 275 mm

3 Slab thickness with Drops 350 mm

4 Drop Size 3.00 m x 3.00 m

5 Thickness of Drops 75 mm

D) Case 3: Building Having Waffle Slab

1 Beam Size 300 mm x 600 mm

2 Slab Thickness 150 mm

3 Overall Slab thickness 450 mm

4 Stem Width 250 mm

5 Spacing of Stems in X-Direction 2000 mm c/c

6 Spacing of Stems in Y-Direction 2000 mm c/c

E) Case 4: Building Having Ribbed Slab



3 Overall Slab thickness 450 mm

4 Stem Width 250 mm


F) Case 5: Building Having Slab with Secondary Beams




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3 Secondary Beam Size 250 mm x 400 mm


Table 3: Material Properties

Fig. 1: Plan & 3D model for Case 1: Building with Flat Slab and Drops

Fig. 2: Plan & 3D model for Case 2: Building with Flat Slab and Drops and Perimeter Beams

S. No. Materials Grade of Materials

1 Concrete (beam & column) M25

2 Concrete (Slab) M25

3 HYSD(R/F)/Rebar Fe 500



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Fig. 3: Plan& 3D Model for Case 3: Building with Waffle Slab

Fig. 4: Plan & 3D model for Case 4: Building with Ribbed Slab



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Fig. 5: Plan & 3D Model for Case 5: Building with Secondary Beams

Seismic Information of Structure:

Table 4: Seismic Information of Structures

S.No. Description Value

1. Earthquake Zone III

2. City Jabalpur

3. Importance Factor 1.20

4. Type of Soil Medium soil

5. Response Reduction Factor 4

6. Time Period 0.273 Sec (G+5 Models)

0.455 Sec (G+9 Models)

7. Damping Ratio 5% i.e. 0.05

IV. RESULTS AND DISCUSSION

Storey Displacement:

Deflection of the stories from the initial position is termed as storey displacements and its maximum value is

obtained at the top storey. The values of maximum storey displacement in X and Z directions obtained from the

analysis has been shown in table 5 while graphical representation is described in fig. 6 and fig. 7 for G+5 and G+9

stories model respectively.

Table 5: Maximum Storey Displacement (mm)

S.No. Model G+5 Stories G+9 Stories

X-Dir Z-Dir X-Dir Z-Dir

1 Case 1 188.47 20.89 492.51 40.20

2 Case 2 191.65 22.22 499.59 42.04

3 Case 3 168.81 15.30 439.68 35.12

4 Case 4 177.31 14.33 463.46 33.17

5 Case 5 237.51 29.84 624.77 49.33



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Fig 6: Maximum Storey Displacement for G+5 Stories Model

Fig 7: Maximum Storey Displacement for G+9 Stories Model

From above representation it is clear that the Case 5 Model i.e. Building with secondary beams shows highest value

of maximum storey displacement whether in X or Z direction or in G+5 and G+9 Stories Models. While the lowest

obtained in the Case 3 Models (i.e. Building having waffle slab) in X-direction, and Case 4 Models (i.e. Building

having ribbed slab) in Z-direction.



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Storey Drift:

The relative displacement between two consecutive stories is termed as Storey Drift. The results obtained from the

analysis have been shown in tabular form in Table 6 and Table 7 for G+5 and G+9 Stories Models respectively.

Their graphical representation has been drawn in fig. 8 and fig. 9 for G+5 and G+9 Storey Models respectively.

Table 6: Storey Drift (mm): G+5 Stories

S. No. Stories Case 1 Case 2 Case 3 Case 4 Case 5

1 G+5 14.93 15.06 13.06 14.00 19.56

2 G+4 23.57 23.91 20.96 22.17 30.16

3 G+3 30.55 31.01 27.21 28.75 38.89

4 G+2 35.02 35.57 31.23 32.96 44.47

5 G+1 36.69 37.32 32.84 34.53 46.21

6 Ground 33.34 34.02 30.21 31.37 41.09

Table 7: Storey Drift (mm): G+9 Stories

S.N. Stories Case 1 Case 2 Case 3 Case 4 Case 5

1 G+9 17.14 17.18 15.02 16.07 22.42

2 G+8 27.73 28.01 24.66 26.08 35.51

3 G+7 37.57 38.02 33.45 35.35 47.93

4 G+6 45.58 46.16 40.56 42.90 58.19

5 G+5 51.76 52.43 46.02 48.71 66.15

6 G+4 56.26 57.01 50.00 52.96 71.96

7 G+3 59.29 60.10 52.69 55.81 75.82

8 G+2 60.86 61.74 54.14 57.29 77.62

9 G+1 60.23 61.20 53.82 56.69 76.12

10 Ground 53.31 54.37 48.25 50.17 65.88



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Fig 8: Storey Drift for G+5 Stories Model

Fig.9: Storey Drift for G+9 Stories Model

The maximum drift has been obtained on G+1 storey of G+5 stories model while for G+9 stories model maximum

storey has been obtained on the G+2 storey. In all the cases highest value of storey drift obtained in Case 5 building

i.e. Building with secondary beams and lowest value has been obtained in Case 3 i.e. Building having waffle slab.

Base Shear:

Maximum shear force at the base of the structure is termed as base shear. It depends on the magnitude of lateral

forces and dead weight of the structure. Based on the analysis results base shear are shown in table 8.



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Table 8: Base Shear (kN)

S.N. Model G+5 Stories G+9 Stories

1 Case 1 13047.06 20535.84

2 Case 2 13677.05 21525.82

3 Case 3 13109.31 20633.66

4 Case 4 12282.68 19334.67

5 Case 5 13542.31 21314.08

Fig.10: Bar chart comparison of Base Shear

Case 2 building models depicts highest base shear in both G+5 and G+9 models, while the lowest value obtained in

the Case 4 Structure i.e. building with ribbed slab. While Case 3 structure shows third lowest value among all five

cases.

Storey Acceleration:

Storey Acceleration is a dynamic perimeter for the seismic analysis of structures, which shows the acceleration of

building under dynamic seismic loading. Table 9 shows the value of acceleration for different cases under

consideration in this study. Fig 11 and Fig 12 depicts the bar chart representation of the structures for X-direction

and Z-direction storey acceleration.



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Table 9: Storey Acceleration (mm/sec2)

S.N. Model G+5 Stories G+9 Stories

Ux Uz Ux Uz

1 Case 1 1270.56 143.40 1491.03 104.67

2 Case 2 1268.38 97.51 1487.43 102.77

3 Case 3 1248.76 26.00 1453.91 44.38

4 Case 4 1235.19 21.55 1480.29 39.43

5 Case 5 1318.19 245.42 1553.62 165.36

.

Fig.11: Storey Acceleration in X-Direction



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Fig.12: Storey Acceleration in Z-Direction

Case 5 structures show the maximum value of storey acceleration in all cases. Lowest value in X and Z –direction

has been observed in Case 4 structure for G+5 Stories model, while for G+9 Structures Case 3 and Case 4 Structures

depicts the lowest value of storey acceleration in X and Z direction respectively.

Most preferable long span slab on the basis of this study is building with waffle slab. Minimum displacement due to

the waffle slab containing grid in both X & Z therefore the lateral force is resisting by the grid.

Waffle slab resist gravity as well as lateral load on structure due to ribs provided in the slab. These ribs act as

secondary beams by distributing the large magnitude load into smaller parts. Hence resulting in the higher load

carrying capacity than other type of slabs. A waffle slab is flat on top, while joists create a grid like surface on the

bottom. These grid patterns are along with major directions which are withstand against the lateral forces. The

buildings with waffle slab are designed to be more solid due to increment in dead weight & base shear and useful for

longer spans and with heavier loads acted on the building. There for the current project having a model with waffle

slab gives least lateral deflection under the earthquake forces.

V. CONCLUSIONS

On The basis of above study in which G+5 and G+9 multistorey buildings have been taken into consideration,

following conclusions have been made.

a) Case 5 (i.e. Building having secondary beams) structure shows highest value of maximum storey displacement

among all the models, which is almost 1.5 to 2.0 times of the lowest storey displacement shown by Case 3

structure i.e. Building having waffle slab.

b) The lowest value of storey drift again observed in Case 3 structure i.e. Building having waffle slab with respect

to others cases of models.

c) Case 5 (i.e. Building having secondary beams) structure shows similar results of storey drift to that of maximum

storey displacement. Since less effective case of secondary beam in models of building.



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d) Highest base shear has been obtained in Case 2 structure (i.e. Building having flat slab with drops and

perimeter beams), while the lowest value obtained in Case 4 structure i.e. Building having ribbed slabs.

e) Storey acceleration is maximum in Case 5 structure (i.e. Building having secondary beams), while lowest value

obtained in Case 3 (i.e. Building having waffle slab) and Case 4 structure i.e. Building having ribbed slabs).

f) From here it can be concluded that for large span slabs the structure having secondary beams should be avoided

for better seismic performance.

g) Most preferable long span slab on the basis of this study is Building with Waffle Slab.

VI. FUTURE SCOPE

The following work should be taken in future for research purpose.

a) Use of different types of structural form such as outrigger, core, tube in tube etc with slabs.

b) Assessment of dynamic wind analysis as per CFD & Wind Tunnel data.

c) Study based on Slabs with dampers.

d) Study with Slabs with composite structures.

e) Use of flat slab, waffle slab & ribbed slab in twin towers.

f) Use of recycled materials in concrete to form different slabs and analysis on the software.

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