Seismic Analysis and Behaviour Of Different Types Of Rcc ...

129 International Journal of Advances in Arts, Sciences and Engineering, Volume 4 Issue 9 Sep 2016 2320-6144 (Online)

Seismic Analysis and Behaviour Of Different Types Of Rcc Flat Slabs

In Etabs

1. Kolla V Sathya Surya Gowtham 2. K.Vinod Naidu

1.PG Scholar (Structural Engineering), RISE KRISHNA Sai Gandhi Group of Institutions, ONGOLE ,

Andhra Pradesh, India. E-mail id: [email protected]

2. Asst.Professor (Structural Engineering), RISE KRISHNA Sai Gandhi Group of Institutions,

ONGOLE , Andhra Pradesh, India. E-mail id: [email protected]

ABSTRACT:

The flat slab system is currently widely used in construction. It permits flexibility in architecture, more clear

space, low building height, easier formwork, and shorter construction time. However, Flat slab building

structures are significantly more flexible than traditional concrete structures as beams are not present. They are

becoming more vulnerable to earthquakes. The objective of this paper is to investigate the behavior of flat slab in

4 different cases as I). flat slab structure without drop, II). Flat slab structure with column drop, III). Flat slab

structure with shear wall, IV). Flat slab structure with column drop and shear wall together, through response

spectrum method, by using ETABS software. The behavior of the flat slab is investigated in terms of story

displacements, frequency, base shear, story level accelerations. And also most severe problem in flat slabs is

punching shear failure. During the earthquake, unbalanced moments can produce significant shear stresses that

causes slab column connections to brittle punching shear failure. This paper also investigates on which type of

combination produces less punching shear at slab column joint. In this paper we are providing the Zone(Z) –III

and the type of soil Medium.

Keywords: Flat slab, shear wall, Behavior, Response spectrum method, punching shear,ETABS

1. INTRODUCTION

Common practice of design and construction

is to support the slab by beam and beams by

column. This may be called as beam-column

construction. But the beam reduces the

accessible net clear ceiling height. The

aesthetically this type of construction is poor

however performance of those structures are

great. In recent practice slabs are directly put

on the column for aesthetic and architectural

point of view. The load transmission way

changes as a result of deletion of columns.

Regardless, the security of those building is to

be checked. Seismic codes are moreover quiet

on the framework of Flat slab. But from the

past history it can be understood that the Flat

slab is extremely vulnerable in earthquake

point of view. Keeping failure in mind the

behavior of Flat slab building is analyzed

using Response spectrum method.

1.2 ABOUT FLAT SLABS

Flat slab construction is a developing

technology in India. A slab constructed

without supporting beams resting directly

on columns, such slab is called Flat slab.

DROP

Provision of thickened portion of slab

around column is called drop panel.

Drops are provided,

1.To increase the stiffness of slab


2. To increase shear strength of slab-column joint.

3.To increase negative moment caring capacity in

the slab column connections.

Column Capital

These are flared profile around column as

shown in Fig 1.3 Column heads are provided,

1. To increase the perimeter of critical section

for shear.

2. To provide more supporting area.

Slabs of constant thickness which do not have

drop panels or column capitals arecalled as Flat

plates. The strength of the Flat plate structure is

often limited due to punching shear action

around columns. So they are predominantly

used in low seismic areas.

Edge beam

Flat slabs may also provide with edge beams or

a perimeter beam, which increase the stiffness

of discontinuous edge, and also increases, shear

capacity at exterior column supports.

1.3 TYPES OF FLAT SLABS

1. Flat plate system

2. Flat slab with column capital only

3. Flat slab with column drop only

4. Flat slab with column capital and column

drop together

1.4 FAILURE MODES OF FLAT SLABS:

It is very important to thoroughly check the

safety of Flat plates and Flat slabs in shear. In

past most of failures have been reported due to

improper design for shear transfer, especially

at the exterior columns. This is due to

inadequate appreciation of shear forces in Flat

slabs and bending moment produced in

external columns.

1.5 One Way Shear or Beam Shear:

The critical section for two way shear is at a

distance equal to the effective depth from the

face of column and edge of drop.

1.6 Two Way Shear or Punching Shear

Failure:

When a large concentrated load is applied on a

small area, there is a possibility of a ‘punch

through’ type of shear failure. Similarly when

Flat plates or Flat slabs rests on columns and

subjected to gravity loading then they

consequently undergoes two way bending. At

this time the reaction to the loading on the slab

is concentrated on a relatively small area, and if

thickness of slab is not adequate in this area,

shear failure may occur by punching through of

the reaction area along pyramid or a truncated

cone. This type of failure is called as Punching

shear or Two way shear.

1.7 ADVANTAGES OF FLAT SLABS

Flat slab buildings have the following

advantages over the conventional buildings.

1. The ease of construction of formwork.

2. It provides large clear ceiling height.

3. The plain ceiling gives an attractive and

pleasing appearance.

4. In absence of beams provision of acoustical

treatment is easy.

5. Due to the elimination of beam height in

multi storey structures, there will be a reduction

of one storey height for every six story’s.

6. The free space for air pipes, water, etc

between slab and a possible furred ceiling.

7. Seismic weight of the building reduces due

to absent of beams

8. In order to reduce the thickness of flat slab in

long spans, prestressing gives cost effective

9. Flexibility in wall partitions, and layout of

plan.

1.8 DISADVANTAGES OF FLAT SLABS

1. Stiffness of Flat slab system is less compared

to slab-beam-column system. This result in

significant bending moments due to horizontal

loads cannot be effectively transferred in

structure. For this reason Flat slab is used in low

rise and medium rise buildings, which are not

tall enough to be subjected to large lateral loads.

2. Serviceability problems arise with excessive

long term deflections of such relatively thin

slabs. However they are not so serious if

column capitals or drops are provided.

2.LITERATURE REVIEW

Padma Gome , Shruti Ratnaparkhe ,Dr.

Uttamasha Gupta1(2012)Seismic

Behaviour of Buildings Having Flat Slabs with

Drops(International journal of emerging

technology and advanced engineering)

The object of the present work is to compare the

behaviour of multi-storey buildings having Flat

slabs with drops with that of having two way

slabs with beams and to study the effect of part


shear walls on the performance of these two

types of buildings under seismic forces. Present

work provides a good source of information on

the parameters lateral displacement and storey

drift.

For that large plan and short plans of 7,9,

11storeys are assumed and analyzed in zone III,

zone IV and zoneV. Use of Flat slabs with drop

instead of conventional building results in

increase in drift values in shorter plans and

decrease in larger plans, marginally in a range

of 0.5mm to 3mm. Still all drift values are

within permissible limits even without shear

walls. In zone III and IV use of Flat slabs with

drop in place of beam slab arrangements,

though, alters the maximum lateral

displacement values, however, these all are

well within permissible limits, even without

shear walls. Provision of part shear walls in

zone V is not enough to keep maximum

displacements within permissible limits,

whether it is a beam slab framed structure or

framed structure with flat slabs with drop.

Replacement of beam slab arrangement by Flat

slabs with drop results in increase in column

reinforcement, however, presence of shear

walls compensates the increment resulted, but

in larger plans only in all the zones.

Amit A. Sathawane & R.S. Deotale (2012)

Analysis And Design of Flat Slab And Grid

Slab And Their Cost Comparison

(International journal of engineering

research and applications).

Flat-slab building structure is widely used due

to the many advantages it possesses over

conventional moment-resisting frames. These

are generally used with column head and

column drops. Grid floor systems beams are

spaced at regular intervals in perpendicular

directions, and also monolithic with slab. These

are generally used for architectural reasons for

large rooms such as vestibules, theatre 15 halls,

auditoriums, show rooms of shops where

column free space is often the main

requirement. The objective of this project is to

determine the most economical slab between

Flat slab without drop, Flat slab with drop and

grid slab. The total width is 27.22 m and length

of slab is 31.38m. Total area of slab is 854.16

sqm.It concludes that the flat slab with drop is

more economical than Flat slab without drop

and Grid slabs.

Results show that concrete required in grid

slabs is more when compared to Flat slab with

drop and Flat slab without drop. And also Steel

required in Flat slab without drop is more as

compared to steel required in Flat slab with

drop and grid slab.

3. MODELING AND ANALYSIS OF 6

STOREY OFFICE BUILDING

Grade of concrete = 25

Grade of steel = Fe 415

Slab Thickness = 0.260m

Number of stories = (6) G+5

Number of bays along X-direction =4

Number of bays along Y-direction = 5

Storey Height = 3.2 meters

Bay width along X- direction =8m

Bay width along Y-direction = 8m

Column size = 0.7x0.7m

Edge Beam size = 0.3x0.23m

Drop size = 3.0x 3.0m

Slab Thickness at drop = 0.325m

Shear wall thickness = 0.2 m

Loading Specifications: Wall load for the outer side = 14KN/m

Wall load for the inner side = 9 KN/m

Wall load for the terrace = 4 KN/m

Dead load of slab = 6.5 KN/m2


Live load = 4 KN/m2

Earth quake load for the building has been

calculated as per IS-18983:2002

i. Zone (Z) = III

ii Soil = Medium

iii Response Reduction Factor (RF) =5

iv Importance Factor = 1

v Damping Ratio = 0.05

For Seismic loading only 50% of the

imposed load is considered.

Fig. 1: Working plan

Fig. 2: Model 1(Flat slab structure

without drop)

Fig 3: Model 2(Flat slab structure with

column drop only)

Fig. 4: Model 3(Flat slab structure with

shear wall)

Fig. 5: Model 4(Flat slab structure with drop

and shear wall together)

Response spectrum method:

Response-spectrum analysis is useful for

decision making to select structural type, before

designing a structure. It gives the dynamic

performance of a structure. Structures of shorter

period experience greater acceleration, whereas

those of longer period experience greater

displacement. The number of modes to be

considered in analysis should be such that the

sum of total of model mass of all the modes

considered is not less than 90% of total seismic

mass of structure. By considering 12 modes

mass participation of flat slab building is

achieved up to 94%.Therefore 12modes are

considered for all models. Center of mass &

centre of rigidity coincides, due to regularity in

the plan, mass and stiffness of the building. so

providing shear walls at all corners

symmetrically may not affect center of mass

and center of rigidity.

4. RESULTS

Table1: Comparison of frequencies of mode

shapes in all 4 models Mode

. No

MODEL

1 (Hz)

MODEL

2 (Hz)

MODEL

3 (Hz)

MODEL

4 (Hz)

1 0.558 0.669 1.096 1.189

2 0.562 0.673 1.101 1.193


3 0.616 0.707 1.923 1.985

4 1.956 2.262 4.503 4.656

5 1.969 2.277 4.51 4.662

6 2.153 2.403 5.401 6.233

7 4.126 4.545 5.487 6.358

8 4.144 4.566 5.54 6.404

9 4.532 4.854 5.601 6.442

10 5.386 6.211 5.705 6.568

11 5.476 6.369 5.721 6.588

12 5.513 6.369 5.788 6.666

Graph 1: Graph for fundamental mode of

frequencies

Graph 2: Graph for fundamental time

period

Table 2: comparison of design storey shear

in all 4 models

Heigh

t of

buildi

ng Story

MODE

L1

MODE

L2

MODE

L3

MODE

L4

(m) (KN) (KN) (KN) (KN)

2.1

PLINT

H 1164.53 1412.03 2194.68 2405.98

5.6

STOR

Y1 1163.58 1410.9 2193.15 2404.32

8.8

STOR

Y2 1133.17 1374.32 2135.56 2341.74

12

STOR

Y3 1058.17 1284.14 1993.87 2187.75

15.2

STOR

Y4 918.77 1116.53 1730.55 1901.58

18.4

STOR

Y5 695.25 847.75 1308.3 1442.68

21.6

STOR

Y6 367.7 453.88 689.55 770.21

Graph 3: Graph shown for storey shear in

all 4 models

Table 3: Comparison of storey

displacements in x-direction in 4 models

Story MODEL1 MODEL2 MODEL3 MODEL4

(mm) (mm) (mm) (mm)

STORY1 3.1 3 1.1 1.1

STORY2 5.7 5.2 2.2 2.1

STORY3 8.1 7.1 3.5 3.3

STORY4 10.2 8.7 4.8 4.5

STORY5 11.8 9.9 6 5.6

STORY6 13.1 10.7 7.2 6.6

Graph 4: Graph shown for comparison of

storey displacements in y-direction in all

4models

PUNCHING SHEAR FAILURE IN FLAT

SLAB BUILDINGS

0

0.2

0.4

0.6

0.8

1

1.2

1.4

MODEL 1(Hz)

MODEL 2(Hz)

MODEL 3(Hz)

MODEL 4(Hz)

FUNDAMENTAL MODE OF FREQUENCY

fundamental mode offrequency

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

MODEL 1 MODEL 2 MODEL 3 MODEL 4

FUNDAMENTAL TIME PERIOD

FUNDAMENTAL TIME PERIOD

0

500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7

s

t

o

r

e

y

s

h

e

a

r storey no

storey shears

Storey

MODEL 1

MODEL 2

MODEL 3

MODEL 4

0

2

4

6

8

10

12

14

1 2 3 4 5 6

D

i

s

p

l

a

c

e

m

e

n

t

s

(

m

m)

Storey no

Storey Displacements (mm)

Storey

MODEL 1

MODEL 2

MODEL 3

MODEL 4


Table 4: Comparison of shear stresses in

model 1, corresponding to Mx moments in

column C13 (center column) in all stories

Story DD+LL DD+LL+EQX

τv(N/mm2) τ

v(N/mm2)

6 1.180209 1.1678

5 1.181 1.1852

4 1.1789 1.2166

3 1.1832 1.2413

2 1.1858 1.2689

1 1.1927 1.3293

Graph 5: Comparison of shear stresses in

model1corresponding to Mx moments in

column C13 in all stories

Table 5: Comparison of shear stresses in

model 1, corresponding to Mx moments in

column C11 (exterior column)

Story DD+LL DD+LL+EQX

τv(N/mm2) τ

v(N/mm2)

6 1.331665 1.362915

5 1.269498 1.269238

4 1.28516 1.241355

3 1.254228 1.23

2 1.25 1.251425

1 0.891808 1.073618

Graph 6: Comparison of shear stresses

corresponding to Mx moments in column

C11

Table 6: Comparison of punching shear

stresses in column C13 (center column)

corresponding to 4 models

STORE

Y

MODEL

1

MODEL

2

MODEL

3

MODEL

4

NO

τv(N/mm2

)

τv(N/mm2

)

τv(N/mm2

)

τv(N/mm2

)

6 1.1879 0.858 1.140 0.890

5 1.1868 0.878 1.217 0.893

4 1.224 0.905 1.230 0.904

3 1.254 0.920 1.239 0.909

2 1.287 0.932 1.244 0.901

1 1.359 0.948 1.245 0.893

Graph 7: Comparison of punching shear

stresses in column C13 corresponding to 4

models

Table 7: Comparison of punching shear

stresses in column C11 (exterior column)


STORE

Y MODE1 MODE2 MODE3

MODEL

4

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1 2 3 4 5 6

S

h

e

a

r

s

t

r

e

s

s(

N

/

m

m

2)

storey

Shear Stres (N/mm2)

DD+LL

DD+LL+EQX

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1 2 3 4 5 6

S

h

e

a

r

s

t

r

e

s

s(

N

/

m

m

2)

storey

Shear Stres (N/mm2)

DD+LL

DD+LL+EQX

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1 2 3 4 5 6

s

h

e

a

r

s

t

r

e

s

s

storey

shear stress (N/mm2)

MODEL 1

MODEL 2

MODEL 3

MODEL 4


NO

τv(N/mm2

)

τv(N/mm2

)

τv(N/mm2

)

τv(N/mm2

)

6 1.466 1.179 1.387 1.115

5 1.354 1.068 1.298 1.031

4 1.320 1.042 1.303 1.033

3 1.225 0.973 1.238 0.980

2 1.332 1.096 1.391 1.118

1 1.119 0.801 0.931 0.674

Graph 8: Comparison of punching shear

stresses in column C11 (exterior column)


5. CONCLUSION

1. Fundamental mode of frequencies of a flat

slab structure increase 20% when drops panels

are present, as further increasing of stiffness by

providing shear walls those values increases to

96%.

2. Base Shear values increases from model1

to model 4. As weight of structure increases

from model1 to model4

3. Flat slab attracts more shear value, when

flat slab provided with shear wall rather than

flat slab having column drops.

4. Providing column drops to flat slab, storey

displacements reduces slightly, as stiffness

increases slightly. But when flat slabs combine

with shear walls, these displacements reduces

tremendously as stiffness of shear walls

increases overall lateral stiffness of structure.

5. For inner columns, punching shear

stresses are increasing linearly from top stories

to bottom stories. As earthquake moments are

increasing from top stories to bottom stories.

But the punching shear variation due to the

gravity loads are not much changes from storey

to storey. This shows that earthquake moments

are more effective in producing punching shear

at bottom stories.

6. Due to the effect of exterior panel

moments and earthquake moments, punching

shear stresses varying slightly irregular in

exterior columns. In exterior columns punching

shear stress is more in columns at top stories

than the columns in the bottom stories.

7. Punching shear failure occurs, more in flat

plate. On provision of column drops it’s

punching shear stress decreases unto 25%.

8. Provision of shear walls may not effective in

reducing punching shear on intermediate

storey’s but effective in top and bottom storey’s

as shear wall attracts lateral moments from

columns.

6 .REFERENCES:

10. Paz M, Leigh W. Structural Dynamics,

Fifth Edition.

11. Lelekakis GE, Ioannis A. Tegos

Aristotle University of Thessaloniki,

Department of Civil Engineering, Thessaloniki,

Greece. Applications of flat-slab RCC

structures in seismic regions.

12. Bhunia D. Solution of Shear Wall

Location in Multi-Storey Building:

13. Megally S, Ghali A. Punching shear

design of earthquake resistant slab column

connections. ACI Structural Journal, Title No.

97 – S73.

14. Gupta U, Ratnaparkhe S, Gome P.

Seismic Behaviour of Buildings Having Flat

Slabs with Drops. Journal of IJERT 2014; 3(5).

15. IS456 - Indian standard plain and

reinforced concrete code of practice.

16. Agarwal P, Shrikhande M. Earthquake

Resistant Design of Structures.

17. Pan A. Lateral Displacement Ductility

of Reinforced Concrete Flat plates. ACI

Structural journal, Title No. 86–S27.

Durrani AJ. Seismic Resistance of Nonductile

slab–Column Connections in Existing Flat –

Slab Buildings. ACI structural Journal, Title

No. 92–S46.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1 2 3 4 5 6

s

h

e

a

r

s

t

r

e

s

sstorey

Shear stress (N/mm2)

MODEL 1

MODEL 2

MODEL 3

MODEL 4

Date post:	13-Feb-2022
Category:	Documents
Upload:	others
View:	4 times
Download:	0 times

Seismic Analysis and Behaviour Of Different Types Of Rcc ...

Documents