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Design of RCC Flat Slab Structure Under Earthquake Loading Using Etabs

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By
M.RAJAGOPAL REDDY 1, P.RAJESH 2
1- Post Graduate student, Department of civil Engineering,VFSTR university , Vadlamudi.
2-Assistant Professor, Department of civil Engineering, VFSTR university, Vadlamudi.

ABSTRACT
The FLAT SLAB SYSTEM being used in majority of the constructions. It elevates more clear space in architecting the construction design in easy manner and duration of construction would be short due to the flat slabs size. Compare to the traditional concrete construction slab system is more viable due to the fact that it avoids the heavy beams, which are the big vulnerability in case of earthquakes. Objective of this paper is to investigate the behavior of flat slab system in few different use cases.

1. Flat slab structure without drop.
2. Flat slab structure with column drop.
3. Flat slab structure with shear wall.
4. Flat slab structure with column drop and shear wall together through response spectrum method by using ETABS software.

The behavior of flat slab is investigated in terms of the following factors:
1. Frequency
2. Base Shear
3. Storey level accelerations

Also most severe problem in flat slabs as follows:
1. Failure punching shears
2. Shear stresses during ground unbalance
3. Slab column connections to brittle punching shear stresses during earthquakes.

Also this paper investigates about the combinations that can produce less punching shear at slab column joint.

INTRODUCTION
Flat slab based construction is a developing technology in India. A reinforced concrete slab supported directly by concrete columns without the use of beams, such slab is called flat slab. When drop panel have thickened portion of slab around column, that provides negative reinforcement in the slab column connections and it increase shear strength of slab. Column heads are flared profile around column and it also provide to increase the perimeter of critical section for shear.

Slabs of constant thickness, which do not have, drop panels or column capitals are called 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. The performance of flat slab building under seismic loading is poor as compare to frame structure due to lack of frame action, which leads to excessive lateral deformation. This leads to instability in the structure. And also Transfer of lateral displacement induces moment at slab column connection, which is of complex 3-dimensional behaviour. Despite the advantages of flat slab, it fails to gravity loads by punching shear. It can be overcome by providing column drops in low seismic areas. But when these flat slab structures situated in seismic zones, the movements transferring from slab to column through shear increases further more and becoming more tendency to punching shear failure during earthquakes. Due to the flexibility of flat slab buildings, they must be combined with a stiffer lateral force resisting system in high seismic regions like shear walls, braces to reduce lateral loads on structural frame. When flat slab is used in combination with bracings, shear

wall for lateral load resistance, the column in building can be designed for only 25% of the design seismic force. Thus the behavior of a structure for dynamic loads can be determined by model analysis and dynamic behavior can be examined by considering the parameters as Storey drift, base shear, time period and acceleration of model.

PROBLEM FORMULATION

Here, we are mainly depending on design of flat slab RCC structure in four different types. It may be mainly two parts, they are 1) with  drops , 2) without drop and these two models are modeled with shear walls at corners. When a flat slab structure referenced by clear from previous   literature  are unstable for seismic forces,  analytically investigating the behavior of flat slab during the earthquakes and checked for increase of punching shear from gravity loads to earthquake loads by taking one center column and one  exterior column in intermediate frame in model 1 and also checked for tendency of punching shear failure in flat slabs . The design has to be Response spectrum method is considered to analyze the structure by using ETABS software.

Here, FOUR models were created and all are analyzed for seismic loads.

Those are
1. Flat slab structure without drop
2. Flat slab structure with column drops
3. Flat slab structure with shear wall
4. Flat slab structure with column drops and shear wall together.

MODELING AND ANALYSIS OF 6 STOREY COLLEGE BUILDING

Grade of concrete= M25
Grade of steel =Fe 415
Slab thickness = 0.260 m
Number of stories = (6) G+5
Number of bays along X-direction = 4
Number of bays along Y-direction = 5
Storey height = 3.2meters
Bay width along X-direction = 8m
Bay width along Y-direction = 8m
Column =0.7×0.7m
Edge beam  =0.3×0.23m

Loading specifications.
Wall load for the outer side = 14 kN/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

Earthquake load for the building has been calculated as per IS-1893:2002
Zone (Z)       = III
Soil      =medium

Response Reduction Factor ( RF ) = 5
Importance Facto      = 1
Damping Ratio      = 0.05

For Seismic loading only 50% of the imposed load is considered.


FIG1: working plan

FIG2 Model 1(Flat slab structure without drop)
FIG2:Model 1(Flat slab structure without drop)

FIG3 Model 2(Flat slab structure with column drop only)
FIG3:Model 2(Flat slab structure with column drop only)

FIG4 Model 3(Flat slab structure with shear wall)
FIG4:Model 3(Flat slab structure with shear wall)

FIG5 Model 4(Flat slab structure with drop and shear wall together)
FIG5: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.

RESULTS:

Table1: comparison of frequencies of mode shapes in all 4 models.

Mode. No MODEL1

(Hz)

MODEL2

(Hz)

MODEL3

(Hz)

MODEL4

(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

 

Graph1 graph for fundamental mode of frequencies.
Graph1: graph for fundamental mode of frequencies.

Graph 2 graph for fundamental time period
Graph 2: graph for fundamental time period

 
Table 3: comparison of design storey shear.

height of building(m) Story

 

MODEL1

(KN)

MODEL2

(KN)

MODEL3 (KN) MODEL4 (KN)
2.1 Plinth 1164.53 1412.03 2194.68 2405.98
5.6 STORY1 1163.58 1410.9 2193.15 2404.32
8.8 STORY2 1133.17 1374.32 2135.56 2341.74
12 STORY3 1058.17 1284.14 1993.87 2187.75
15.2 STORY4 918.77 1116.53 1730.55 1901.58
18.4 STORY5 695.25 847.75 1308.3 1442.68
21.6 STORY6 367.7 453.88 689.55 770.21

 

Graph 3 graph shown for storey shear in all 4 models.
Graph 3: graph shown for storey shear in all 4 models.

Table4: comparison of storey displacements in x-direction in 4 models

Story MODEL1

(mm)

MODEL2

(mm)

MODEL3

(mm)

MODEL4

(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

 

Graph4 graph shown for comparison of storey displacements in x-direction
Graph4: graph shown for comparison of storey displacements in x-direction.

 

PUNCHING SHEAR FAILURE IN FLAT SLAB BUILDINGS
Through checking punching shear stress( Τv )variation at various places in prescribed 4models

Table 5: Comparison of shear stresses, corresponding to Mx moments in column C13 (center column).

Story DD+LL

Τv(N/mm2)

DD+LL+EQX

Τv(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

 
Graph5 Comparison of shear stresses corresponding to Mx moments in column C13
Graph5: Comparison of shear stresses corresponding to Mx moments in column C13

TABLE6: Comparison of shear stresses corresponding to Mx moments in column C11 (exterior column).

Story DD+LL

Τv(N/mm2)

DD+LL+EQX

Τv(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

 

GRAPH6 Comparison of shear stresses
GRAPH6: Comparison of shear stresses corresponding to Mx moments in column C11

TABLE7: Comparison of punching shear stresses in column C13 (center column) corresponding to 4 models

STOREY NO MODEL1

Τv(N/mm2)

MODEL2

Τv(N/mm2)

MODEL3

Τv(N/mm2)

MODEL4

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

Graph7 Comparison of punching shear stresses in column C13 corresponding to 4
Graph7: Comparison of punching shear stresses in column C13 corresponding to 4 models.

Table8: Comparison of punching shear stresses in column C11 (exterior column) corresponding to 4 models

STOREY NO MODE1

Τv(N/mm2)

MODE2

Τv(N/mm2)

MODE3

Τv(N/mm2)

MODEL4

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

 

Graph8 Comparison of punching shear stresses in column C11 (exterior column) corresponding to 4 models
Graph8: Comparison of punching shear stresses in column C11 (exterior column) corresponding to 4 models

CONCLUSIONS:
Within the scope of present work following conclusions are drown

• 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%.

• Base Shear values increases from model1 to model 4. As weight of structure increases from model1 to model4

• Flat slab attracts more shear value, when flat slab provided with shear wall rather than flat slab having column drops.

• 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.

• 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.

• 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.

• Punching shear failure occurs, more in flat plate. On provision of column drops it’s punching shear stress decreases unto 25%.

• 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.

REFERENCES:
1. Structural Dynamics By Mario Paz & W. Leigh – FIFTH EDITION.
2. “Applications of RCC flat slab structures in seismic regions”. George E. Lelekakis, Ioannis A. Tegos Aristotle University of Thessaloniki, Department of Civil Engineering, Thessaloniki, Greece.
3. “ Solution of Shear Wall Location in Multi-Storey Building”, referenced by Dipendu Bhunia, Assistant Professor, Civil Engineering Group, BITS Pilani, Rajasthan, in India.
4. “Punching shear design of earthquake resistant slab column connections” by Sami Megally and Amin Ghali, ACI Structural Journal, Title No. 97 – S73.
5. “Seismic Behaviour of Buildings Having Flat Slabs with Drops” referenced by Dr. Uttamasha Gupta1, Shruti Ratnaparkhe, Padma Gome Professor, SGSITS, journal of IJERT, Vol 3, Issue 5, May 2014.
6. IS456 – Indian standard plain and reinforced concrete code of practice.
7. “Earthquake Resistant Design Of Structures” by Pankaj agarwal, manish shrikhande.
8. “Lateral Displacement Ductility of Reinforced Concrete Flat plates” by Austin Pan, ACI Structural journal, Title No. 86 – S27.
9. “Seismic Resistance of Nonductile slab – Column Connections in Existing Flat – Slab Buildings” by A. J. Durrani, ACI structural Journal, Title No. 92 – S46.
10. The design of structure ,authors thank Dr R.K.Ingle and Dr. O.R.Jaiswal of VNIT Nagpur and Dr. Bhupinder Singhof NIT Jalandhar for their review and assistancein the development of this example problem.

We at engineeringcivil.com are thankful to Er M.RAJAGOPAL REDDY for submitting this paper to us. We hope this will provide immense help to other civil engineers in understanding the Design of RCC Flat Slab Structure Under Earthquake Loading Using Etabs.

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