ISSN Vol.03,Issue.16 July-2014, Pages:

Transcription

1 ISSN Vol.03,Issue.16 July-2014, Pages: Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey C.SURESH BABU 1, E.ARUNAKANTHI 2 1 PG Scholar, Dept of Structural Engineering, JNTUCEA, Anantapur, AP, India, suri.chanti007@gmail.com. 2 Asst Prof, Dept of Structural Engineering, JNTUCEA, Anantapur, AP, India, earunakanthi@gmail.com. Abstract: Recent building codes for seismic design and evaluation in Europe and American feature performance based criteria that entail the estimation of inelastic response of the building due to seismic. Simplified methods based on nonlinear static analysis, known as pushover analysis method have been developed by several regulations to satisfy the performance based criteria for seismic design and evaluation of buildings. In this study, 3D analytical model of multistory buildings have been generating for different buildings models and analyzing using structural analysis tool ETABS. To study the effect of bare frames, infill, at different positions during earthquake, seismic analysis both linear static, linear dynamic (response spectrum method) as well as nonlinear static (pushover) procedure have to be perform. The analytical model of building includes all important components that influence the mass, strength, stiffness and deformability of the structure. The deflections at each story have to be comparing by performing equivalent static, response spectrum method as well as pushover. Pushover analyses have also be perform to determine capacity, demand and performance level of the considering models. Numerical results for the following seismic demands considering the inelastic behavior of the buildings, natural periods, displacements, inter-story drift ratio, ductility coefficients and response of structures. Keywords: Nonlinear static analysis (pushover analysis), bare frames, infill, stiffness, deformability, inelastic behavior, natural periods, displacements, inter story drift ratio, ductility coefficients and response. I. INTRODUCTION A. General The capacity of structural members to undergo inelastic deformations governs the structural behavior and damageability of multi-storey buildings during earthquake ground motions. From this point of view, the evaluation and design of buildings should be based on the inelastic deformations demanded by earthquakes, besides the stresses induced by the equivalent static forces as specified in several seismic regulations and codes. Although, the current practice for earthquake-resistant design is mainly governed by the principles of force-based seismic design, there have been significant attempts to incorporate the concepts of deformation-based seismic design and evaluation into the earthquake engineering practice. In general, the study of the inelastic seismic responses of buildings is not only useful to improve the guidelines and code provisions for minimizing the potential damage of buildings, but also important to provide economical design by making use of the reserved strength of the building as it experiences inelastic deformations. In recent seismic guidelines and codes in Europe and USA, the inelastic responses of the building are determined using nonlinear static methods of analysis known as the pushover methods. Pushover methods are becoming practical tools of analysis and evaluation of buildings considering the performance-based seismic philosophy. This is evident by the recent implementation of pushover methods in several international seismic guidelines and codes, such as the Federal Emergency Management Agency standard 273 (FEMA-273), Euro-Code 8 (EC-8) and International Building Code (IBC-2003). In these seismic regulations, pushover methods of analysis such as the N2-method and the capacity spectrum method are recommended for determining the inelastic responses of the building due to earthquake ground motions. One main step in these pushover methods of analysis for determining the seismic demands is the construction of the pushover curve of the building by using an adequate lateral load pattern simulating the distribution of inertia forces developed through the building when subjected to an earthquake. This pushover curve represents the lateral capacity of the building by plotting the nonlinear relation between the base shear and roof displacement of the building. The intersection of this pushover curve with the seismic demand curve determined by the design response spectrum represents the deformation state at which the performance of the building is evaluated. Simplified approaches for the seismic evaluation of structures, which account for the inelastic behavior, generally use the results of static collapse analysis to define the global inelastic performance of the structure. Currently, for this purpose, the nonlinear static procedure (NSP) which is described in FEMA-273/356 and ATC-40 (Applied Technology Council, 1996) documents are used. Seismic demands are computed by nonlinear static analysis of the 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.

2 structure subjected to monotonically increasing lateral forces with an invariant height-wise distribution until a predetermined target displacement is reached. Nonlinear static (pushover) analysis can provide an insight into the structural aspects, which control performance during severe earthquakes. The analysis provides data on the strength and ductility of the structure, which cannot be obtained by elastic analysis. By pushover analysis, the base shear versus top displacement curve of the structure, usually called capacity curve, is obtained. To evaluate whether a structure is adequate to sustain a certain level of seismic loads, its capacity has to be compared with the requirements corresponding to a scenario event. Performance Based Engineering (PBE) in association with existing concepts of earthquake resistant design requires nonlinear analysis to obtain estimates of deformations for damage assessment for different levels of earthquakes. In the performance based procedure, the desired levels of seismic performance for a building for specified levels of earthquake ground motion are specified. The performance is checked in terms of post elastic deformations. ATC-40 gives the Capacity Spectrum Method for implementing PBE for buildings. It uses Nonlinear Static Pushover (NSP) analysis to develop the capacity curve (a plot of base shear Vs roof displacement). In this dissertation, hypothetical multi-storied buildings (i.e., twelve storied and nine storied with in fills and with bare frame) located in zone V of medium soil sites has been analyzed and designed for load combinations given in code and evaluated using pushover analysis. B. Dissertation Organization The dissertation is divided into five chapters as follows First chapter is introduction work second chapter RC Frame Buildings with Soft Storeys Third chapter includes Analytical Modeling. Fourth chapter is discussion of results by considering different parameters of the building model. Fifth chapter gives summary, conclusion, and at last reference. II. RC FRAME BUILDINGS WITH SOFT STOREYS Many urban multi-storey buildings in India today have open first storey as an unavoidable feature. This is primarily being adopted to accommodate parking or reception lobbies in the first storeys. The upper storeys have brick in filled wall panels. The draft Indian seismic code classifies a soft storey as one whose lateral stiffness is less than 50% of the storey above or below. Interestingly, this classification renders most Indian buildings, with no masonry infill walls in the first storey, to be buildings with soft first storey. Whereas the total seismic base shear as experienced by a building during an earthquake is dependent on its natural period, the seismic force distribution is dependent on the distribution of stiffness and mass along the height. In buildings with soft first storey, the upper storeys being stiff undergo smaller inter-storey drifts. However, the inter-storey drift in the soft first storey is large. The strength demands on the columns in the first storey are also large, as the shear in the first storey is maximum. For the upper storeys, however, the forces in the columns are effectively reduced due to the presence of the Buildings with C.SURESH BABU, E.ARUNAKANTHI abrupt changes in storey stiff nesses have uneven lateral force distribution along the height, which is likely to locally induce stress concentration. This has adverse effect on the performance of buildings during ground shaking. Such buildings are required to be analyzed by the dynamic analysis and designed carefully. The Jabalpur earthquake of 22 May 1997 also illustrated the handicap of Indian buildings with soft first storey. This earthquake, the first one in an urban neighborhood in India, provided an opportunity to assess the performance of engineered buildings in the country during ground shaking. The damage incurred by Himgiri and Ajanta apartments in the city of Jabalpur are very good examples of the inherent risk involved in the construction of buildings with soft first storey. Himgiri apartments are a RC frame building with open first storey on one side for parking, and brick infill walls on the other side. The infill portion of the building in the first storey is meant for shops or apartments. All the storeys on top have brick infill walls. The first storey columns in the parking area were badly damaged including spalling of concrete cover, snapping of lateral ties, buckling of longitudinal reinforcement bars and crushing of core concrete (Fig.1). The columns on the other side had much lesser level of damage in them. There was only nominal damage in the upper storeys consisting of cracks in the filler walls. This is a clear case of columns damaged as a result of the soft first storey. The Ajanta apartment s buildings are a set of almost identical four storey RC frame building located side-by-side. In each of these buildings, there are two apartments in each storey, excepting the first storey. One building has two apartments in the upper storeys, but only one apartment in the first storey. The open space on the other side is meant for parking, and hence has no in filled wall panels. Whereas, only nominal damages were reported in the building with two apartments the first storey, the first storey columns on the open side in the other building were very badly damaged. The damage consisted of buckling of longitudinal bars, snapping of ties, spalling of cover and crushing of core concrete. Fig.1. Damage to columns in Himgiri apartment.

3 Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey In a two-storey (plus stilt storey) C-shaped RC frame building (Youth hostel building) in Jabalpur, the damage to the columns in the stilt storey consisted of severe X-type cracking due to cyclic lateral shear (Fig.2). Here also, the two storeys above the stilt storey have brick in filled wall panels. This makes the upper storeys very stiff as compared to the storey at the stilt level. There was no damage to the columns in the storeys above. The soft first storey at the stilt level is clearly the primary reason for such a severe damage. A. Description of the Sample Building The plan layout for all the building models are shown in figs.3 to 7. Symmetric Building Models: Model 1: Twelve stuttered Building with full infill masonry wall (230 mm thick) in all storeys. Model 2: Twelve storied Building (bare frame) no walls in the all storeys. Model 3: Nine storied Building with full infill masonry wall (230 mm thick) in all storeys Model 4: Nine storied Building (bare frame) no walls in the all storeys. Fig.2. Damage to columns in the stilt storey of Youth Hostel building. III.ANALYTICAL MODELLING Most building codes prescribe the method of analysis based on whether the building is regular or irregular. Almost all the codes suggest the use of static analysis for symmetric and selected class of regular buildings. For buildings with irregular configurations, the codes suggest the use of dynamic analysis procedures such as response spectrum method or time history analysis. Seismic codes give different methods to carry out lateral load analysis, while carrying out this analysis infill walls present in the structure are normally considered as non structural elements and their presence is usually ignored while analysis and design. However even though they are considered as non-structural elements, they tend to interact with the frame when the structures are subjected to lateral loads. In the present study lateral load analysis as per the seismic code for the following type of structures, bare frame, in fills is carried out and an effort is made to study the effect of seismic loads on them and thus assess their seismic vulnerability by performing pushover analysis. The analysis is carried out using ETABS analysis package. Fig.3. Plan Layout. Fig.4. Elevation of twelve storeyed Building Model 1 (full infill).

4 C.SURESH BABU, E.ARUNAKANTHI Fig.5. Elevation of twelve storeyed Building Model 2 (bare frame). Fig.7. Elevation of nine storeyed Building Model 4 (bare frame). Fig.6. Elevation of nine storeyed Building Model 3 (full infill). B. Example Buildings Studied The plan layout, elevation and 3D view of the reinforced concrete moment resisting frame building of twelve storied building for different models is shown in Figs 4.1,. In this study, the plan layout is deliberately kept similar for all the buildings for the study. The each storey height is kept 3.5 m for all the different buildings models. The building is considered to be located in the seismic zone-v and intended for office use. In the seismic weight calculations only 50% of the floor live load is considered. IV. RESULTS AND DISCUSSIONS Most of the past studies on different buildings and unsymmetrical buildings have adopted idealized structural systems without considering the effect of masonry infill and concrete shear walls. Although these systems are sufficient to understand the general behavior and dynamic characteristics of unsymmetrical buildings, it would be interesting to know how real buildings will respond to earthquake forces. For this reason hypothetical buildings, located on level ground having similar ground floor plan have been taken as structural systems for the study. In this chapter, the results of the twelve selected buildings are presented and discussed in detail. The results are includes of all different building models and the response results are computed using the response spectrum and pushover analysis. The analysis and design of the different building models is performed by using ETABS analysis package. The results of natural period of vibration, base shear, lateral displacements and storey drifts, ductility, reduction factor &

5 Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey overall performance for the different building models for each of the above analysis are presented and compared. An effort has been made to study the effect of in fills, concrete core wall and vertical irregularities and mass irregularities in seismic analysis. in longitudinal direction and 2.41 times more in transverse direction. TABLEI: Displacements of 12 Storey Infill Structure in MM. A. Lateral Displacements The maximum displacements at each floor level with respect to ground are presented in Table 4.1 to 4.4 for Equivalent Static, Response Spectrum, and Pushover Analysis. For better comparability the displacement for each model along the two directions of ground motion are plotted in graphs as shown in fig.4.1 to In the three dimensional model, however, there are six degrees of freedom with the two translational degree of freedom along X, Y-axes and rotation degree of freedom about Z (vertical)-axis playing significant role in the deformation of the structure. Apart from the translation motion in a particular direction, there is always an additional displacement due to the rotation of floor. Due to this the maximum displacement at floor levels obtained by three-dimensional analysis are always greater than the corresponding values obtained by one-dimensional analysis. Moreover, the floor rotation is maximum at the top floor, gradually reducing down the height of the building to an almost negligible rotation at the lowest basement floor. From the graphs, it is observed that displacement profile of model-2 and model-4 changes abruptly; it indicated the stiffness of infill masonry is not present. It is observed that displacement profile has changed; the stiffness irregularity is due to open ground storey and presence of masonry infill wall in the upper story. On the other hand nearly all models show a smooth displacement linear profile, which is due to the presence of full infill brick wall. B. Equivalent Static Method As compared to Model 1, Model 2 has 3.68% of less displacement than Model 1, in longitudinal direction and 3.49% less in transverse direction. As compared to Model 3, Mode 4 has 48.8% of less displacement than Model 3 in longitudinal direction and 52.92% less in transverse direction. TABLE II: Displacements of 12 Storey Bare Frame Structure in MM 1. Response Spectrum Method: As compared to Model 1, Model 2 has 7.33% of less displacement than Model 1, in longitudinal direction and 5.42% less in transverse direction. As compared to Model 3, Mode 4 has 82.52% of less displacement than Model 3 in longitudinal direction and 93.35% less in transverse direction. 2. Push over Analysis: In Pushover Analysis different building Models have pushed to its failure and correspondingly displacement is noted. From the displacement table I to IV and graphs 8-9 and as compared to Model 1, Model 2 have % of more displacement than Model 1, in longitudinal direction and 15.59% more in transverse direction. As compared to Model 3, Model 4 has 2.76 times more displacement than Model 3,

6 TABLE III: Displacements of 9 Storey Infill Structure in MM C.SURESH BABU, E.ARUNAKANTHI TABLE IV: Displacements of 9 Bare Frame Structure In MM Fig.9. Displacement of linear static analysis of 15 th storey buildings in y direction. Fig.10. Displacement of linear dynamic analysis of 15 th storey buildings in x direction. Fig.8. Displacement of linear static analysis of 15 th storey Fig.11. Displacement of linear dynamic analysis of 15 th buildings in x direction. storey buildings in y direction.

7 Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey Fig.12. Displacement of linear non static analysis of 15 th storey buildings in x direction. Fig.15. Displacement of linear static analysis of 9 th storey buildings in y direction. Fig.13. Displacement of linear non static analysis of 15 th storey buildings in y direction. Fig.16. Displacement of linear dynamic analysis of 9 th storey buildings in x direction Fig.14. Displacement of linear static analysis of 9 th storey buildings in x direction. Fig.17. Displacement of linear dynamic analysis of 9 th storey buildings in y direction.

8 C.SURESH BABU, E.ARUNAKANTHI TABLE V: Storey Drifts (MM) Along Longitudinal and Transverse Direction For Model 1 Fig.18. Displacement of linear non static analysis of 9 th storey buildings in x direction. Fig.19. Displacement of linear non static analysis of 9 th storey buildings in y direction. C. Storey Drifts The permissible inter storey drift is limited to times the storey height, so that minimum damage would take place during earthquake and pose less psychological fear in the minds of people. The storey drifts of different models along longitudinal and transverse directions are shown in Tables V to VIII. TABLE VI: Storey Drifts (MM) Along Longitudinal and Transverse Direction for Model 2 Fig.20. Drift of linear static analysis of 15 th storey buildings in x direction.

9 Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey TABLE VII: Storey Drifts (MM) Along Longitudinal and Transverse Direction For Model 3 Fig.22. Drift of linear dynamic analysis of 15 th buildings in x direction. storey TABLE VIII: Storey Drifts (MM) Along Longitudinal and Transverse Direction for Model 4 Fig.23. Drift of linear dynamic analysis of 15 th buildings in y direction. storey Fig.21. Drift of linear static analysis of 15 th buildings in y direction. storey Fig.24. Drift of linear non static analysis of 15 th storey buildings in x direction.

10 C.SURESH BABU, E.ARUNAKANTHI Fig.25. Drift of linear non static analysis of 15 th storey buildings in y direction. Fig.28. Drift of linear dynamic analysis of 9 th buildings in x direction. storey Fig.26. Drift of linear static analysis of 9 th storey buildings in x direction. Fig.29. Drift of linear dynamic analysis of 9 th buildings in y direction. storey Fig.27. Drift of linear static analysis of 9 th Fig.30. Drift of linear non static analysis of 9 th storey buildings buildings in x direction. in y direction. storey

11 Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey primarily steel, bend or deform before they fail. We can rely on ductile materials to absorb energy and prevent collapse when earthquake forces overwhelm a building. In fact, adding steel rods to concrete can reinforce it and give the concrete considerable ductility and strength. Concrete reinforced with steel will help prevent it from failing during an earthquake. TABLE X: Response Reduction Factor along transverse direction Fig.31. Drift of linear non static analysis of 9 th buildings in y direction. storey From the above tables it can be seen that, all storey drifts are within the permissible limit (0.004*h=12mm) except the model-2 and model-4. In model-2 and model-4, the drifts are more than the permissible limit due to bare frame story; this is due to the less stiffness of the structure (because infill walls are not present in the story. The displacement profiles of the various models for the three different analysis performed in this study are shown in figs 23 to 31. In these graphs, the abrupt changes in the bare frame storey of model- 2 and model-4 indicate the stiffness irregularity. Hence the inter-storey drift demand is largest in the model-2 and model- 4. In transverse direction also models with full infill shows good results as compared with bare frame storey model-2 and model-4. TABLE IX: Response Reduction Factor along longitudinal direction The property which enables structure to withstand severe earthquake is ductility. By enhancing ductility in structure the design seismic forces can be reduced, and more economical structure can be obtained. Reinforced concrete structures have less ductility capacity as compared to steel structures. The ductility ratio and response reduction factor for different building models in longitudinal and transverse direction are shown in tables IX and X. From the above tables it can be seen that, response reduction factor and ductility ratio decreases as the stiffness of brick wall decreases in bottom storey in model-2 and model-4 along longitudinal and transverse direction. E. Performance Point The performance point of the building models in longitudinal and transverse directions are shown in figs. 32 to 39 as obtained from ETABS. The values of seismic coefficients Ca and Cv for zone-v are taken from the table XI. TABLE XI: Interpolated values of Seismic Coefficient (CA and CV) for the soil type D. Ductility Ratio ( ) and Response Reduction Factor (R) Ductility: Ductility is another factor that can affect the performance of a building during an earthquake. Ductility is the property of certain materials to fail only after large stresses and strains have occurred. Brittle materials, such as non-reinforced concrete, fail suddenly with minimum tensile stresses, so plain concrete beams are no longer used. Other materials,

12 C.SURESH BABU, E.ARUNAKANTHI Fig.32. Performance point of twelve storied building Model 1 along longitudinal direction. Fig.35. Performance point of twelve storied building Model 2 along transverse direction. Fig.33. Performance point of twelve storied building Model 1 along transverse direction. Fig.36. Performance point of nine storied building Model 3 along longitudinal direction. Fig.34. Performance point of twelve storied building Model 2 along longitudinal direction. Fig.37. Performance point of nine storied building Model 3 along transverse direction.

13 Seismic Evaluation of Multi-Storied Buildings with Infills and Bare Frame Storey Fig.38. Performance point of nine storied building Model 4 along longitudinal direction. levels of the building deformation the results are drastically changed, at the level below which there is no infill (bare frame). The graph associated with the building model-2 and model-4 is less stiff and yields at a lower base shear value than that of the other building models. The capacity curve is intersecting the demand curve of the infill structures which indicates that the performance level of the building is good. The capacity curve and demand curve are intersecting only for infill structures. The performance level of the infill structure is good and whereas the bare frame story structure is worst. Plastic hinges formation for the building mechanism has been obtained at different displacement levels. Plastic hinges formation started with beam ends and base columns of lower stories, then propagates to upper stories and continue with yielding of interior intermediate columns in the upper stories. The formation of first hinge is not early in models with in fills and bare frame, but since yielding occurs at events B, IO, LS, the amount of damages in the buildings are limited. The behavior of the building frames is adequate as indicate by the intersection of the demand and capacity curves and the distribution of hinges in the beam and the columns. The results obtained in terms of demand, capacity and plastic hinges into the real behavior of the structures. Fig.38.Performance point of nine storied building Model 4 along transverse direction. From above figures it can be seen that demand curve is increasing the capacity curve which shows the performance of the all models are good. V.CONCLUSION It is essential to consider the effect of masonry infill for the seismic evaluation of movement resisting RC frames especially for the prediction of its ultimate state. In fills increase the lateral resistance and initial stiffness of the frames they appear to have a significant effect on the reduction of the global lateral displacement. In fills having no irregularity in elevation having beneficial effects on buildings in filled frames with irregularities, such as bare frame, damage was found to concentrate in the level where the discontinuity occurs. The displacements and inter storey drift ratios at edge of the buildings are compared at different VI. REFERENCES [1] Arlekar, N.J., Jain K.S., and Murthy, C.V.R. Seismic Response of RC Frame Buildings, Proceedings of the CBRI Golden Jubilee Conference on Natural Hazards in Urban Habitat, New Delhi, [2] Krawinkler, H., and Seneviratna, G.D.P.K. (1998): Pros & Cons of Pushover Analysis of Seismic Performance Evaluation. [3] Ashraf Habibullah, Stephen Pyle, Practical threedimensional non-linear static pushover analysis Structure Magazsanine, winter, [4] Mohamed Nour El-Din Abd-Alla Application of recent techniques of pushover for evaluating seismic performance of multistory buildings, Faculty of Engineering, Cairo University Giza, Egypt September [5] Kasım Armagan KORKMAZ, Fuat DEM_R and Mustafa S_VR Earthquake assessment of R/C structures with masonry infill walls Suleyman Demirel University, Civil Engineering Department, Cunur, Isparta, TURKIYE armagan@mmf.sdu.edu.tr. (Received: ; Accepted: [6] IS: 1893 (Part-I) 2002 (2002): Criteria for Earthquake Resistant Design of Structures, Part-I General Provisions and Buildings, Fifth Revision, Bureau of Indian Standards, New Delhi [7] Kanitkar, R., and Kanitkar, V., Seismic Performance of Conventional Multi-storey Buildings with Open Ground Storey for Vehicular Parking, Indian Concrete Journal, February 2004.

14 Author s Profile: Name : C.Suresh Babu. Qualification : M.Tech. Specialization : Structural Engineering. College Name : JNTUCEA, Anantapur. suri.chanti007@gmail.com. C.SURESH BABU, E.ARUNAKANTHI Name : E.Arunakanthi. Qualification : Ph.D. Specialization : Structural Engineer. Designation : Assistant Professor. College Name: JNTUCEA, Anantapur. earunakanthi@gmail.com.