IJSRD - International Journal for Scientific Research & Development| Vol. 3, Issue 08, 2015 | ISSN (online): 2321-0613
Comparison of Reinforced Concrete and Pre-Stressed Concrete Flat Slabs in Tall Buildings Ruchi R Kadamb1 M.Manjunath2 1 P.G. Student 2Associate Professor 1,2 Department of Civil Engineering 1,2 KLE 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 in resisting the lateral wind forces. Key words: Reinforced Concrete, Pre-Stressed Concrete Flat Slabs I. INTRODUCTION 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 business activist 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 and get 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 to resist 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 the vibrations 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 to give out the conclusion to these investigations. Graphs of moment v/s aspect ratio, moment v/s height, and axial load v/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 is absent 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 software are displacements, axial loads, moment, shear values joint reactions, joint displacements, element forces and moments for 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 as follows:
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Comparison of Reinforced Concrete and Pre-Stressed Concrete Flat Slabs in Tall Buildings (IJSRD/Vol. 3/Issue 08/2015/156)
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 The Combination 1.5(DL+LL)
Fig. 1: SAP 2000 3D model of Reinforced concrete Fig. 5: Variation of axial load on external columns with no wind loads acting on the building C. Displacement variation of reinforced concrete and prestressed concrete flat slab buildings for load combination 1.2(DL+LL+WL).
Fig. 2: SAP 2000 3D model of pre-stressed concrete IV. RESULTS 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 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 PreStressed Concrete Flat Slab Buildings for Load Combination 1.2(DL+LL+WL)
Fig. 3: Variation of axial loads on lee-ward side of building for 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. 4: Variation of axial loads on wind-ward side of building for 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)
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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 prestressed 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 than reinforced 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 is lower 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. REFERENCE [1] A.K.Kandale, S.S.Patil (2013),”Comparative study of Rectangular Prestressed Concrete Flat Slab and RCC Flat Slab” International Journal of Engineering and Innovative Technology, Volume 2, Issue 11, May 2013. [2] Ramos A.Lúcio V (2013), “Safety on punching of prestressed flat slabs” UNIC- central investigation of construction, vol.51 page 2829. [3] Jaroslav halvonik (2013), “The maximum punching shear resistance of flat slabs” Concrete and concrete structures, volume 65, page 376 - 381 . [4] Dan Vasile Bompa and Traian One (2011), “Failure analysis of symmetric flat-slab column connections with shear reinforcement” , concrete engineering, session 1-4, 10 June [5] Alkarani1, Ravindra. R2(2013) , “Evaluation of punching shear in flat slabs” , International Journal of Research in Engineering and Technology, page 102 november 2013 [6] M. Altug ERBERIK and Amr S. ELNASHAI(2003),“Seismic vulnerability of flat-slab structures”, Civil and Environmental Engineering Department University of Illinois at UrbanaChampaign, December 2003. [7] Miguel Fernández Ruiz, Yaser Mirzaei, and Aurelio Muttoni (2013) “Post-Punching Behavior of Flat Slabs” ACI Structural Journal, Vol 110 page. 801812,october 2013. [8] Vakas K. Rahman1, Prof. A. R. Mundhada (2013) ” Comparative Study of RCC and Prestressed Concrete Flat” International Journal of Modern Engineering Research (IJMER), Vol. 3, Issue. 3, pp-1727-1730 June. 2013. [9] K. Micallef a, J. Sagaseta a,*, M. Fernández Ruiz b, A. Muttoni b, “Assessing punching shear failure in reinforced concrete flat slabs subjected to localised impact loading” International Journal of Impact Engineering, ch 1015, February 2014. [10] Paul N Roschke (1991) “Effects of banded post tentioning concrete flat slabs” Journal of structural engineering, vol. 117, page 563-583, February 1991 @ ASCE [11] Boskey Vishal Bahoria and Dhananjay K. Parbat (2013), “Analysis and Design of RCC and Posttensioned Flat Slabs Considering Seismic Effect”
International Journal of Engineering and Technology, Vol. 5, No. 1, February 2013. [12] Madhavi Prajapati and Vivek Tiwari (2014), “Comparative Study of Flat Slab with Old Traditional Two way slabs” International Journal of Latest Trends in Engineering and Technology, Vol. 4 Issue 2 July 2014. [13] Dan Vasile Bompa, Traina "Failure analysis of symmetric flat slab column connection with shear reinforcement," Concrete engineering journal, pp. 1-4, 10 June 2011. [14] Alkarani, R. Ravindra "Evaluation of punching shear in flat slabs," International journal of reasearch in engineering and technology, p. 102, November 2013. [15] M. Thayapraba, "Cost effectiveness of post tensioned and reinforced concrete flat slab systems.," International journal of innovative technology and exploring engineering, vol. 3, no. 12, pp. 2278-3075, May 2014.
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