Seismic Performance Of R C Flat-Slab Building Structural Systems

Transcription

1 Research Paper Volume 2 Issue 9 May 2015 International Journal of Informative & Futuristic Research ISSN (Online): Seismic Performance Of R C Flat-Slab Building Paper ID IJIFR/ V2/ E9/ 027 Page No Subject Area Civil Engineering Key Words RC Slab, Infill Wall, Shear Wall, Flab Slab Basavaraj H S 1 Rashmi B A 2 Assistant Professor Department of Civil Engineering Jyothy Institute of Technology, Bangalore -Karnataka Assistant Professor Department of Science And Humanities PES school of Engineering, Bangalore -Karnataka Abstract In order to be successful in mitigation efforts; the expected damage and the associated loss in urban areas caused by severe earthquakes should be properly estimated. It is also appropriate to consider the expected damage as a measure of seismic vulnerability. The determination of such a vulnerability measure requires the assessment of the seismic performances of all types of building structures typically constructed in an urban region when subjected to a variety of potential earthquakes. In the present work the G+4 and G+8 storied building models are considered. The vulnerability of purely frame and purely flat slab models under lateral loads and ground acceleration were studied. Further the flat slab models are strengthened by perimeter beam, infill walls, shear walls and increasing the cross sectional area of columns and the effect of positioning of infill walls and shear walls on performance of flat slab building models were analysed. The infill walls are modeled as equivalent diagonal strut and the seismic analysis has been performed by Equivalent Lateral Force Method, response spectrum method as per code IS 1893:2002 and linear time history using Electro earthquake data. The results in form of fundamental time period, base shear, lateral displacement and inter storey drift results are compared for purely frame, purely flat slab and seismic strengthened flat slab models and the analysis is done with sap2000 software. From the results it is clear that the flat slab building model strengthened by perimeter beams and shears walls shows better seismic performance. Copyright IJIFR

2 1. Introduction 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. 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 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. (i) Slabs without drop and column head (ii) Slabs without drop and column with column head (iii) Slabs with drop and column without column head (iv) Slabs with drop and column head 2. Literature Review Alpa Sheth explains the behaviour of flat slab system under lateral loads which is dependent on numerous parameters such as the height of the building, floor plate size, size and location of the shear wall core, flat slab spans, amongst others. Importantly, it is also dependent on the provision or otherwise of a perimeter frame. The paper studies the effect of perimeter frames for structural systems with flat slab structure and shear wall core for different locations of the shear wall core and for different heights and spans of three concrete towers. For the study he considered the three concrete towers having concrete flat slabs with shear walls have been analysed for their behaviour with and without a perimeter framing beam. One of the models is also analysed with addition of outrigger system. He conclude that the tall buildings of compact size, regular shape and distributed shear wall core, there is a very marked improvement in performance of the structure with flat slab system and shear wall core when a perimeter frame with closely spaced columns is added to the structure. Farther spaced perimeter column frame has a relatively less impact on reducing drift. For shorter towers of non-compact size and with distributed cores, the perimeter frame does not greatly impact the structural behaviour C.S Garg and Yogendra Singh studied the performance of flat slab under lateral loading using push over analysis.in pushover analysis; a predefined lateral load pattern which is distributed along the building height is applied on building. The lateral forces are increased until some members yield.the structural model is modified to account for the reduce stiffness of yielded members and lateral forces are increased until some other members yield. The process is continued until a control displacement at the top of building reaches a certain defined level of deformation or structure becomes unstable. The parabolic lateral loading pattern has been used according to IS 1893(part 1) (2002). Shyh-Jiann Hwang and Jack p.moehle their study is concerned with two analytical models that are commonly used in design-office practice. These are the effective beam width model, in which the slab action is represented by a flexural slab-beam framing directly between columns, and the equivalent frame model, in which the slab action is represented by a combination of flexural and 3070

3 tensional beams. Characteristics of both models are discussed, and recommendations on proper application are made. The recommendations are based on a detailed experimental and analytical evaluation presented elsewhere. The recommended analytical models are tested by comparison with results obtained on lateral load experiments of a multipanel test slab. 3. Methodology According to ACI-318, flat-slabs can be designed by any procedure that satisfies equilibrium and geometric compatibility provided that every section has strength at least equal to the required strength, and that the serviceability conditions are satisfied. Generally, some of the methods employed for the design of flat-slabs are, A- Direct Design Method, B- Equivalent Frame Method, C -The Yield-line method, D-The Finite Element Method Table 1: Design data for all the buildings Structure OMRF No. of storey G+4 and G+8 Storey height 3.50 m Type of building use Office Seismic zone IV Material Properties Young s modulus of M 25 concrete, E x 10 6 kn/m 2 Grade of concrete M 25 Grade of steel Fe 415 Density of reinforced concrete 25 kn/m 3 Modulus of elasticity of brick masonry x 10 3 kn/m 2 Density of brick masonry 20 kn/m 3 Member Properties G+4-Storeyed Building Outer Beam 0.4 x 0.4 m Column 0.4 x 0.4 m G+8-Storeyed Building Outer Beam 0.4 x 0.40 m Columns up to 5-story 0.5 x 0.50 m 5 to 9 story 0.40 x 0.40 m Thickness of wall 0.25 m Assumed Dead Load Intensities Roof finishes 1.0 kn/m 2 Floor finishes 1.5 kn/m 2 Live Load Intensities Roof 1.5 kn/m 2 Floor 4.0 kn/m 2 Earthquake LL on slab as per clause and of IS 1893 (Part 1): 2002 Roof 0 kn/m 2 Floor 0.5 x 4.0 = 2 kn/m

4 Table 2- IS 1893 (Part 1): 2002 Equivalent Static method Zone IV Zone factor, Z (Table 2) 0.24 Importance factor, I (Table 6) 1.00 Response reduction factor, R (Table 7) 3.0 Damping ratio 5% (for RC framed building) Soil Type II (Medium) Figure 1: Plan for all building models Figure 2: 3D view of G+4 storeyed flat slab building model 3072

5 Figure 3: Elevation of the G+4 storeyed building models strengthened by infill walls and perimeter beam Figure 4: Elevation of the G+4 storied building models strengthened by shear walls and perimeter beam (Model 11) Figure 5: Plan showing the position of shear or infill walls at periphery mid 3073

6 Figure 6: Plan showing the position of shear or infill walls at central core Figure 7: Plan of increasing the cross sectional area of intermediate columns (Model 13) Figure 8: Plan showing increased cross sectional area of periphery columns (Model 12) 3074

7 Figure 9: Plan of increasing the cross sectional area of intermediate columns (Model 13) Figure 10: Plan of increasing the cross sectional area of core columns (Model 14) 4. Results And Discussions 4.1: Fundamental time period and frequency for building models. From the results it is very clear that, stiffness of the building is directly proportional to its natural frequency and hence inversely proportional to the natural period. That is, if the stiffness of building is increased the natural period goes on decreasing, which in turn increases the natural frequency. For G+4 storied building the percentages reduction in natural time periods from the analysis results for model 3 is 11%, model 4 is 12%, model 7 is 59%, model 11 is 74% and model 12 is 0.35% when compared to model 2 For G+4 storied building the percentages reduction in natural time periods from the analysis results for model 3 is 11%, model 4 is 12%, model 7 is 59%, model 11 is 74% and model 12 is 0.35% when compared to model

8 4.2 Base Shear For G+4 Storied Building Models Table 3 Model No. Analytical period (sec) Frequency Codal period(sec) G+4 G+8 G+4 G+8 G+4 G+8 Purely frame Purely flat slab Flat slab (250 mm slab) Strengthened by perimeter wall Strengthened by infill wall Strengthened by shear wall Strengthened by perimeter wall+shear wall Strengthened by increased column wall The base shear is a function of mass, stiffness, height, and the natural period of the building structure. In the equivalent static method design horizontal acceleration value obtained by codal natural period is adopted, and the basic assumption in the equivalent static method is that only first mode of vibration of building governs the dynamics and the effect of higher modes are not significant therefore, higher modes are not considered in this method. Hence base shears obtained from the equivalent static method are larger than the dynamic response spectrum method. From Table 3 & 4 results it is clear that base shear for purely frame model is greater when compared to purely flat slab model. In cases of seismic strengthened building models the model 11 has got maximum base shear. 3076

9 Table 4 Model no Longitudinal direction Transverse direction (kn) V B (kn) SF (kn) (kn) V B (kn) Purely frame SF(kN) Purely Flat Slab Flat slab(250 mm slab) Strengthened by perimeter beam Strengthened( by perimeter beam Infill wall) Strengthened by shear wall Strengthened by( perimeter beam +shear wall) Strengthened by(increase column cross section perimeter beam) : Transverse Displacement Of G+4 Storey Building For Model 1, 2, 3 And Model 4 Figure 11(a), (b) Equivalent static method Response spectrum method 3077

10 4.4 Transverse displacement of G+4 storey flat-slab building models strengthened by perimeter beam and infill walls Figure 12 (a), (b): Equivalent static method Response spectrum method 4.5 Transverse Displacement Of G+4 Storey Flat-Slab Building Models Strengthened By Perimeter Beam And Shear Wall Figure 13(a), (b): Equivalent static method Response spectrum method 4.6 Transverse displacement of G+4 storied flat slab building models Strengthened by increase in column cross section and perimeter beam Figure 14 (a), (b) Equivalent static method Response spectrum method 3078

11 From results it can be observe that the infill which is act as a diagonal strut and the shear walls are responsible to increase the story stiffness. Both for equivalent static force method and response spectrum method the lateral sway is highest for purely flat slab building model and it reduces with increases in story stiffness due to the presence of infill walls, shear walls and perimeter beam. Lateral displacement for flat slab building strengthened by increase in column cross section and perimeter beam were found to be less than the purely flat slab model but more than the models strengthened by infill walls, shear walls and perimeter beam. Among the all seismic strengthened flat slab building models, the model 11 has got least lateral displacement, since the mass and stiffness increases the displacement reduces. 4.7: Inter Story Drift Of G+4 Storied Buildings For Models 1, 2, 3 And Model 4 Figure 15 (a), (b): Equivalent static method Response spectrum method 4.8: Inter story drift of G+4 storied flat-slab building strengthened by perimeter beam and shear walls Figure 16 (a), (b) : Equivalent static method Response spectrum method 3079

12 4.9: Inter story Drift Of G+4 storied flat-slab building Strengthened by increase in column cross section and perimeter beam Figure 17 (a), (b) : Equivalent static method Response spectrum method From the results it can be observe that due to lack of lateral load resisting system i.e. due to absence of interior beams, the inter storey drift was found to be more in purely flat slab model when compared with purely frame model along both longitudinal and transverse directions. Also it can be observe that the inter storey drifts of flat slab building models strengthened by infill walls (infill + perimeter beam) and shear walls (shear wall + perimeter beam) along both longitudinal and transverse directions were found to be within the limits, where as for flat slab building models strengthened by increase in column cross section and perimeter beam along both direction have crossed the limit. 5. Linear Time History Analysis Linear Time History analysis has been carried out using the Imperial Valley Earthquake record of May 18, 1940 also known as the ELCENTRO earthquake for obtaining the various floor responses. The record has 1559 data points with a sampling period of 0.02 seconds. The peak ground acceleration is 0.319g. Figure 18: Response spectrum plot for the ELCENTRO earthquake at 5% damping 3080

13 Figure : Displacement at the top of the structure for purely frame model and purely flat slab model (G+8 storey) Model

14 Model-2 5.2: Top story displacement for G+8 storied building MODELS (Linear time history analysis) Type Of Structure Longitudinal Direction Purely frame Model Purely flat slab Model Flat slab(250 mm slab) Model Strengthened by perimeter beam Model Strengthened by(perimeter beam+infill wall) Model Model Model Strengthened by shear wall Model Model Model Strengthened by( perimeter beam +shear wall) Model Strengthened by(increase column cross section perimeter beam) Model Model Model Transverse Direction From the above results it is observed that the purely flat slab models are more vulnerable to seismic action than the purely frame system. Among all seismic strengthened flat slab buildings, the flat slab model strengthened perimeter beam and shear walls (Model 11) shows the better seismic performance i.e.81% reduction in top story displacement for G+4 story and 85% for G+8 story buildings. 3082

15 The position of shear walls or infill walls at periphery corner is effective in resisting the horizontal forces coming from earthquake. From the above results it is observed that the purely flat slab models are more vulnerable to seismic action than the purely frame system. Among all seismic strengthened flat slab buildings, the flat slab model strengthened perimeter beam and shear walls (Model 11) shows the better seismic performance i.e.81% reduction in top story displacement for G+4 story and 85% for G+8 story buildings. The position of shear walls or infill walls at periphery corner is effective in resisting the horizontal forces coming from earthquake. 6. Conclusions The fundamental natural period of the building decreases with increases in story stiffness due to the presence of infill walls, shear walls and perimeter beam. The empirical expressions provided for period calculations in the code consider only height and width of the structure for infill walls. However from the present study the period obtained from the analysis differs from codal values for regular structure. Base shear increases with the increase in mass and stiffness of building, hence for purely frame and seismic strengthened flat slab buildings base shear is more as compared to purely flat slab building. Both for DBE and MCE levels the lateral sway is highest for purely flat slab building model and it reduces for purely frame and seismic strengthened flat slab building models. Since the mass and stiffness increases the displacement reduces. The inter storey drifts for flat slab building models strengthened by infill walls and shear walls along both longitudinal and transverse directions were found to be within the prescribed limit mentioned in clause No ,IS 1893 (part 1):2002. For equivalent strut model, the models proposed by Smith and Hendry and Holmes can be effectively used. Equivalent strut models are effectively used in building modeling instead of wall modeling. As infill walls behave very well under lateral loads. The presence of infill s can significantly reduce lateral drift and unbalanced moment at slab-column connections in flat-slab buildings. By appropriately adding the infill s, the performance of seismically deficient flat-slab buildings can be significantly improved. High rise flat slab buildings which are vulnerable to lateral loads must need shear walls to reduce lateral deflection and inter storey drift. Most effective location of shear wall is outer periphery of building that are provided in the corner of building and that reduce torsion. Shear wall should be provided in both horizontal directions equally for effective action of shear walls. Shear wall is very effective to resist horizontal forces coming from earthquake and wind forces etc. in multistory structure if it is properly oriented it will reduce torsional effect and storey deflection. The purely flat-slab RC structural system is considerably more flexible for horizontal loads than the traditional RC frame structures which contributes to the increase of its vulnerability to seismic effects. To increase the bearing capacity of the flat-slab structure under horizontal loads, particularly when speaking about seismically prone areas and limitation of deformations, modifications of the system by adding structural elements are necessary. 3083

16 7. References [1] C.S Garg and Yogendra singh: Seismic performances of flat slab shear wall system [2] R. P. Apostolska1, G. S. Necevska-Cvetanovska: The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China Seismic performance of flat-slab building structural systems [3] IS: 456, (2000), Indian Standard Code for Plain and Reinforced Concrete, Bureau of Indian Standards, New Delhi. [4] IS: 1893 (Part 1), (2002), Indian Standard Criteria for Earthquake Resistant Design of Structures, General provision and Buildings, Bureau of Indian Standards, New Delhi. [5] Pankaj Agarwal and Manish Shrikande (2007), Earthquake Resistant Design of Structures, Prentice Hall of India Private Limited, New Delhi, India. [6] S N Sinha (2005), Reinforced Concrete Design, Tata McGraw-hill publishing company Limited, New Delhi, India. 3084