comparison of reinforced concrete and pre-stressed concrete flat slabs in tall buildings

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I JSRD - I nternational Journal for Scientifi c Research & Development| Vol. 3, I ssue 08, 2015 | ISSN (onli ne): 2321-0613

All rights reserved by www.ijsrd.com 607

Comparison of Reinforced Concrete and Pre-Stressed Concrete Flat

Slabs in Tall Buildings Ruchi R Kadamb1 M.Manjunath2

1P.G. Student 2Associate Professor 1,2Department of Civil Engineering

1,2KLE Dr. M.S.Sheshgiri College of Engineering and Technology, Belgaum 590008, India Abstract

— Flat slab are usually preferred floor system for

tall buildings as they result in reduction in floor heights. The

flat slabs without drop panel and column head are called as

flat plates. The design of tall buildings is usually governed

by the lateral loads imposed on the structure. The main

disadvantage of flat slabs and flat plates is their lack of

resistance to lateral loads, such as those due to wind forces

and earthquakes. In the present study tall building with

reinforced concrete flat slab floor system is compared with

pre-stressed concrete flat slab floor system under lateral

forces. A 30 storied tall building with a flat plate floor of

size of 30m x 24m and having a panel size 6m x 6m is

studied for wind forces. These slabs are design as reinforced

concrete as well as pre-stressed (post tensioning) concrete

flat slabs as per Indian standard codes and analysed in

software SAP2000. The analysis and design results of

reinforced concrete and pre-stressed concrete flat plates are

compared to evaluate the performance of the tall building inresisting the lateral wind forces.

Key words: Reinforced Concrete, Pre-Stressed Concrete Flat

Slabs

I. I NTRODUCTION

Tall buildings are developing in a rapid speed nowadays.

Since last 20 years they have gained a lot of importance in

commercial as well as residential buildings. For the businessactivist it is a mark of prestige to own a high rise building at

city centres. A building is said to be tall building when it has

stories more than 10.

The reason behind the rapid construction of tall

buildings is mainly due to economic growth, human

curiosity in building tall structures, increase in population at

a fast rate, land scarcity in urban areas. Land rates have boomed in last few years to a very high cost. Hence tall

buildings make it economical for the people. There is a lot

of improvement in technology and advancement in studies;

this is another reason for the increase in tall building.

As of today there are lot of software which makes

it easier for a structural engineer to analyse a building andget the values to design building in a short period of time.

The conventional manual analysis consumes a lot

of time to design a tall building, hence it is a difficult task to

analyse the building with conventional methods. Hong Kong

is the world’s largest high rise building centre. They have a

lot of tall buildings with the most promising designs for a

structural engineer.

Tall buildings have to resists large amount of loads

such as gravity loads, wind loads and earthquake loads.

Earthquake loads and wind loads are most dominant in such

multi storied buildings. Therefore structures are built toresist these loads by providing several load resisting

systems. Few methods are by providing bracing to the buildings, by constructing a shear wall or coupled shear

wall. For earthquake loads damper are used to resists thevibrations in the building during an earthquake attack.

II. LITERATURE REVIEW

In one of the previous work, the effects of wind loads on

tall buildings are studied. For this investigation 3D models

are prepared in ETABS software. By changing the aspect

ratio up to 2 of the buildings these studies are carried out.

Design is as per IS-875 part III. Even the height of buildings

are varied, buildings from 3 to 20 stories are used. For this

study parameters such as bending moments and axial forces

are included. A building of width 20 meters with 4 bays is

considered for the design purposes. Graphs are drawn by togive out the conclusion to these investigations. Graphs of

moment v/s aspect ratio, moment v/s height, and axial loadv/s aspect ratio are drawn. From these graphs it is proved

that as the aspect ratio reduces wind loads also reduces.

It is concluded that aspect ratio increases as

moments in the column decreases for wind load cases, but

the moments remain same for all aspect ratio for gravity

loads. As the height increases moments in the column

increases for low rise building and remain constant for

medium height buildings. Axial forces in the column are

almost same for all load cases when the height of building is

less than 15mts. Column moments are considered critical

while designing for the tall buildings.

III. PROBLEM DEFINITION AND METHODOLOGY

Basically in this investigation a flat slab performance on a

tall building is studied. Drop panel and column capital isabsent in these slabs. Building of 30 stories is used in this

project and parametric study is carried out to study the

details of wind loads acting on the buildings along with

gravity loads. For the analysis first design is manually done

using Indian standard codes. IS 875 part III, IS 456 2000, IS

875 part I and II and IS 1343.

These design values calculated are applied on the

high rise building to create 3D models in SAP 2000 and get

the analysis results. The results obtained from this softwareare displacements, axial loads, moment, shear values joint

reactions, joint displacements, element forces and momentsfor both shell and frame. These values are used in this

project to compare the flat slab performance. For both the

cases design is manually done and model is created in the

software.

The analysis model for reinforced concrete tall

building and pre-stressed concrete tall building are asfollows:



Comparison of Reinforced Concrete and Pre-Stressed Concrete Flat Slabs in Tall Buildings

(IJSRD/Vol. 3/Issue 08/2015/156)


Fig. 1: SAP 2000 3D model of Reinforced concrete

Fig. 2: SAP 2000 3D model of pre-stressed concrete

IV. R ESULTS AND DISCUSSION

A. Axial Loads Variations for Reinforced Concrete and

Pre-Stressed Concrete Flat Slab Buildings of Columns on

Lee Ward Side and Wind-Ward for the Combination1.2(DL+LL+WL)

Fig. 3: Variation of axial loads on lee-ward side of building

for combination 1.2(DL+LL+WL)

Fig. 4: Variation of axial loads on wind-ward side of

building for combination 1.2(DL+LL+WL)

B. Axial Loads Of Building For Reinforced Concrete And

Pre-Stressed Concrete Flat Slab Buildings Of Columns With

No Wind Load Acting On The Building That Is For TheCombination 1.5(DL+LL)

Fig. 5: Variation of axial load on external columns with no

wind loads acting on the building

C. Displacement variation of reinforced concrete and pre-

stressed concrete flat slab buildings for load combination

1.2(DL+LL+WL).

Fig. 6: Change in displacement of building with respect to

story height

D.

Column Moments of Reinforced Concrete and Pre-Stressed Concrete Flat Slab Buildings for Load

Combination 1.2(DL+LL+WL)

Fig. 7: Bending moment variation on wind-ward side of

building for load combination 1.2(DL+LL+WL)

Fig. 8: Bending moment variation on lee-ward side of

building for load combination 1.2(DL+LL+WL)



Comparison of Reinforced Concrete and Pre-Stressed Concrete Flat Slabs in Tall Buildings

(IJSRD/Vol. 3/Issue 08/2015/156)


V. CONCLUSION

From the analysis made on reinforced concrete and pre-

stressed concrete flat slab buildings following conclusion

can be arrived. The axial loads and moment in columns of

pre-stressed concrete flat-slab building are 25% lesser thanreinforced concrete flat-slab building for both lee-ward side

columns as well as wind-ward side columns. The

displacement of pre-stressed concrete flat-slab building islower as compared to reinforced concrete flat-slab building.

The reduction in displacement is about 30%. This indicates

that the pre-stressed concrete flat-slab system enhances the

stiffness of the tall structure. All the above points prove that

pre-stressed concrete flat-slab systems are more suitable

than reinforced concrete flat-slab systems for tall buildings.

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comparison of reinforced concrete and pre-stressed concrete flat slabs in tall buildings

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