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ABSTRACT: A common type of building construction uses a flat slab as floor system. So, this system is simple to construct and it is also efficient inthat it only requires minimum building height for few number of stories. Therefore it is necessary to analyse the lateral load behavior of building for different earthquake zones, soil types and heights to see what are the changes that are going to happen. In this paper, a four irregular multi-storey moment resisting frames are modelled which will be analysed both statically as well as dynamically using SAP (Structural Analysis Programming) software. Static analysis will be carried out by Equivalent Static Analysis method, whereas Dynamic analysis will be done by Time History Analysis method by taking the parameters of BHUJ earthquake. It is observed that from the results for Displacement and Storey drift Model-3 is more suitable in earthquake prone areas.
1. INTRODUCTION
India at present is fast emerging economy which brings about demands in maximizing of infrastructure requirements with the population growth. The increase in demand of land in urban areas is rising day by day. The land available for farming and agriculture industries is imperative that remains intact.as a result to cater the land demand in these regions, vertical development is the only opinion. This type of development will bring challenges to immediate attack on additional lateral loads due to earthquake and wind. This demand changes in the provided structural system which needs to be analyzed to resist these forces. Many researches has been worked out which shows the suitability of various lateral load resisting system against shear and deformation worked out due to the wind and earthquake forces. As a result earthquake resistant design of R.C. frame structures is an on-going area of research since the earthquake engineering has started not only developed countries but India also. After researching the buildings still fail due to some one or the other reason during earthquakes. Rather than the deficiencies in the structure, either due to code practices or error in analyzing and design, the RC frame structural system has played a vital role in catastrophe. An earthquake is a sudden movement of the earth’s crust, which brings naturally at or below the surface. The word natural is more crucial here, because it avoids shock waves caused by man-made explosions, nuclear test etc. About 80% of all earthquakes result from tectonic events. Due to the natural hazard like an earthquake is the main reason to cause damage or collapse to the man-made structures.
2. FRAMED STRUCTURES
Framed structures can be considered ass an assemblage of one dimensional and two dimensional member. The length of a one dimensional member of a structure is large compared to its other dimensions where as the thickness of a two dimensional member is smaller than its other two dimensions. A structure made of line member joined together is referred to as framed structure. In general, frame structures have three dimensional configurations.
While transferring loads acting on the structure, the members of the structure are subjected to internal forces like axial forces, shearing forces, bending and torsion moments. Structural analysis deals with analyzing these internal forces in the members of the structures. The process of analysis commences with planning of a structure, primarily to meet the functional requirements of the user.
Planning a structure involves the selection of the most suitable type of structure and the choice of its general layout and overall dimension on the basis of economic, aesthetic, functional and other criteria. Designing a structure entails determining the disturbancesto which it is expected to be exposed during its life time and then choosing the dimensions of its members as well as the details of their connections. The structure is then analyzed, i.e., the internal forces and moments in its members and the displacements of some of its cross sections are computed. The member of structure must have sufficient strength and rigidity so that when the structure subjected to the disturbances to which it is expected to be exposed, the components of stress and displacement at any of its point do not exceed the maximum allowable values given in the appropriate design course.
If the results of the analysis show that the members of the structure do not have sufficient strength and rigidity to satisfy the aforementioned requirements the structure is redesigned, i.e., new dimensions of cross section are chosen. The process is repeated until the structure is obtained which satisfies all the aforementioned requirements.
3. FLAT SLABS
Common practice of design and construction is to support the slabs by beams and support the beams by columns. This may be called as beam-slab construction.
Fig-1: Typical Flat Slab
The beams reduce the available net clear ceiling height. Hence in warehouses, offices and public halls sometimes beams are avoided and slabs are directly supported by columns. These types of construction are aesthetically appealing also. These slabs which are directly supported by columns are called Flat Slabs.
The column head is sometimes widened so as to reduce the punching shear in the slab. The widened portions are called column heads. The column heads may be provided with any angle from the consideration of architecture but for the design, concrete in the portion at 45º on either side of vertical only is considered as effective for the design.
Fig-2: Flat Slab with Column Head
Moments in the slabs are more near the column. Hence the slab is thickened near the columns by providing the drops as. Sometimes the drops are called as capital of the column. Thus we have the following types of flat slabs.
Fig-3: Flat Slab with drop panel
Flat-slab building structures possesses major advantages over traditional slab-beam-column structures because of the free design of space, shorter construction time, architectural –functional and economical aspects. Because of the absence of deep beams and shear walls, flat-slab structural system is significantly more flexible for lateral loads then traditional RC frame system and that make the system more vulnerable under seismic events.
The system consists of columns resting directly on floor slabs for which sufficient strength and ductility should be provided to enable sustaining of large inelastic deformations without failure. The absence of beams, i.e., the transferring of their role to the floor RC structure which gains in height and density of reinforcement in the parts of the hidden beams, the bearing capacity of the structural system, the plate-column and plate-wall connection, all the advantages and disadvantages of the system have been tested through long years of analytical and experimental investigations. For the last 20 to 30 years, the investigations have been directed toward definition of the actual bearing capacity, deformability and stability of these structural systems designed and constructed in seismically active regions.
A flat slab is a highly indeterminate structure and its exact analysis is difficult. An approximate analysis can be made by considering an interior panel of slab. IS456-2000 provides an empirical approach Direct Design Method and Equivalent Frame Analysis for the analysis of flat slab. Via Direct Design Method this it’s easy to calculate bending moment and shear force in flat slab without use of computer. But the Equivalent Frame Analysis gives more exact results.
The behaviour and design of flat slabs structures for gravity loads are well established but their seismic behaviour is not well understood and generally found to be unsatisfactory. Flat slab is susceptible to progressive brittle punching shear failure under seismic loading. In flat slab building the most vulnerable part is slab-column joint. Flat slabs with drop panels or column capitals are generally constructed as these extra projections provide safety against punching shear and also reduce the heavy negative moment. If drop panels are not provided then the system is termed as Flat Plate.
Extensive research has been carried out to find out the behaviour of slab-column connection. The failure mode depends upon the type and extent of loading. Punching shear strength of slab-column connection is of importance which very much depends on the gravity shear ratio. The mechanism of transfer of moments from slab to column is very complex when subjected to lateral loading and unbalance moments. These unbalanced moments produce additional shear and torsion at the connections and then get transferred into the column which results in excessive cracking of slab leading to further reduction in the stiffness of the slab.
A flat-plate structure, as shown in Figure 4, consists of a slab with uniform thickness supported on the columns with no beams or drop panels. The economy of flat plate buildings has lead to their wide spread utilization throughout the world. Conventionally flat-plate structure is generally used for lightly loaded structures such as apartments, hotels, and office buildings with relatively short spans, typically less than 6m. For longer spans or heavier loads, flat-slabs system with shear capitals or drop panels would be more feasible.
Fig-4: Flat Plate
Flat-plates have been widely used due to the reduced construction cost associated with the simple formwork and simple arrangement of flexural reinforcement. An additional advantage of a flat-plate is reduced building storey heights that result in more usable space in a building for a given or limited height and reduces lateral loads acting on the system, as well as mechanical, electrical, and cladding costs.
4. ANALYSIS METHODS
Equivalent Static Analysis:Seismic analysis of most structures is still carried out on the assumption that the lateral force is equivalent to the actual loading. This method of finding design lateral forces is also known as the static method or the equivalent lateral force method or the seismic coefficient method.
Response Spectrum Analysis: This method is also known as modal method or mode superposition method. The method is applicable to those structures where modes other than the fundamental one significantly affect the response of the structure. Generally, the method is applicable to analysis of the dynamic response of structures, which are asymmetrical or have areas of discontinuity or irregularity, in their linear range of behaviour.
Time History Analysis: A time history analysis overcomes all the disadvantages of a modal response spectrum analysis. This method requires greater computational efforts for calculating the response at discrete times. One interesting advantage of such a procedure is that the relative signs of response quantities are preserved in the response histories. This is important when interaction effects are considered among stress resultants.
5. CASE STUDY
“Study on Behaviour of R.C. Frames Structures with Different Floor Systems Under the Effect of Lateral Loads” done by MohdMohiburRahman, Banulatha.G.N, Dr. Narayana.G, Manu.J
5.1. Modeling
Modeling is carried out using SAP2000 (Structural Analysis Programming) software of version 14.2.4.
Descriptions of the Models
Model-1 = Beam-Column system.
Model-2 = Two Way Flat Plate.
Model-3 = Two Way Flat Plate with Drop cap.
Model-4 = Waffle Slab.
5.2. Salient Features of the Building
1. Type of the structure is multi-storey plan and vertical offset (Irregular)
2. Seismic zone is taken as V as per IS 1893(Part I):2002
3. Layout is L-shape
4. Numbers of storeys are G+9
5. Total height of the building above ground level is 36m
6. Each floor is 4m high
7. Panel size is 7m x 7m
8. Super dead load (floor load) is taken as 1kN/m2 as per IS 875(Part I)
9. Live load is taken as 3kN/m2 as per IS 875(Part II)
10. External wall load is taken as 20.5kN/m
11. Grade of concrete is M40
12. Grade of steel is HYSD 500
13. Size of beams is 450mm x 600mm (for model-1, 2, 3) and 300mm x 600mm (model-4)
14. Size of columns is 750mm x 750mm
15. Thickness of slab is 275mm (model-1), 300mm (model-2),300mm (model-3) and drop (450mm), 200mm (model-4)
16. Method of analysis is static (Equivalent Static Analysis) anddynamic (Time History Analysis)