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ABSTRACT
A popular form of concrete building construction uses a flat concrete slab (without beams) as the floor system. Flat slabs analysis and design of flat slabs are still the active areas of research and there is still no general agreement on the best design procedure. The present day Indian Standard Codes of Practice outline design procedures only for slabs with regular geometry and layout. But in recent times, due to space crunch, height limitations and other factors, deviations from a regular geometry and regular layout are becoming quite common. Also behaviour and response of flat slabs during earthquake is a big question. This paper gives the guidelines for analysis of flat slab. In today’s construction activity the use of flat slab is quite common which enhances the weight reduction, speed up construction, and economical. Similarly from the beginning conventional slab has got place in providing features like more stiffness, higher load carrying capacity, safe and economical also. As the advancement era began practice of flat slab becomes quite common. In the present dissertation work a single storey building having flat slab and conventional slab has been analysed for the parameters like base shear, storey drift, axial force, and displacement... In the present work. The present work provides reasonable information about the suitability of flat slab for various seismic zones without compromising the performance over the conventional slab structures
1. Introduction
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. RC slabs with long spans extended over several bays and only supported by columns, without beams known as flat slab. Flat slab system is very simple to construct and is efficient in that it requires the minimum building height for a given number of stories. Such structure contains large bending moment and vertical forces occur in a zone of supports. This gives a very efficient structure which minimizes material usages and decreases the economic span range when compared to reinforced concrete. Post-tensioning improves the structural behavior of flat slab structure considerably. This is more acceptable concept to many designers. It is adopted in some office buildings. The flat slabs are plates that are stiffened near the column supports by means of ‘droppanels’ and/or ‘column capitals’ (which are generally concealed under ‘drop ceilings’). Compared to the flat plate system, the flat slab system is suitable for higher loads and larger spans, because of enhanced capacity in resisting shear and hogging moments near the supports. The slab thickness varies from 125 mm to 300 mm for spans of 4 to 9m. Among the various floor systems, the flat slab system is the one with the highest dead load per unit area. In general, in this type of system, 100 percent of the slab load has to be transmitted by the floor system in both directions (transverse and longitudinal) towards the columns. In such cases the entire floor system and the columns act integrally in a two- way frame action.
Research Objectives
• To design and optimize the beam slab system using finite element analysis
• To prepare BBS for each model and determine the weight of reinforcement required.
• To compare the amount of reinforcement required in each model and select the best system from construction point of view.
1.2. Scope of this Research
• This research focuses on the case of simple beam slab system and steel connections such as welded connections are not part of this research.
• This research is strictly referred to IS 456, 2000 other designing codes and RCC connections are not a part of this research.
• The economic feasibility is not included in this project.
1.3. Literature Review
• The work of Gentry [29] confirmed that, in general, linear elastic finite element analysis is applicable to the design of regular reinforced concrete flat plate systems. Various methods of modeling columns and column-to-slab connections were evaluated. Designs based on finite element analysis were critically compared with designs based on the direct design and equivalent frame methods. Finite element designs using working stress design and ultimate strength design, based on element nodal forces as well as element stress resultants, were evaluated. Gentry showed that design using element forces produces results in closer correlation with the equivalent frame technique than element stress resultants, and that the ultimate strength approach is preferable to working stress with respect to performance and economy. Only a rectangular column layout and bay spacing were considered in that study.
• The work of Saleh [63] evaluated the applicability of linear elastic finite elements to the analysis and shear design of regular reinforced concrete flat plate systems. In particular, Saleh investigated a finite element determination of moment transfer at slab-column connections, shear stress along the critical column perimeter, and the effect of modeling on shear stress results, showing strong correlation with ACI method results.
• Davies [21] applied the finite element method to evaluate deflections and moments in a corner supported rectangular slab. Both simple supports and fixed supports were considered. Several loading conditions were considered, and analytical results were compared with scale model tests. Davies demonstrated strong correlation between analytical and experimental studies and showed that actual slab-to-column connections were generally bounded by the simple and fixed support models evaluated.
• Mohr [54] demonstrated the successful application of elastic finite element analysis in the determination of approximate plastic design results. In this research, the flat plate was modeled as a weak core sandwich based on the assumptions that the reinforcement had yielded and the concrete core was cracked. Based on these assumptions, the plastic section modulus was computed and implemented instead of the elastic section modulus. Eight slabs with various boundary conditions were considered, and the analyses showed strong correlation between the finite element results and other well-known plastic design techniques.