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International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Investigation on Performance Levels of 8-Storey RC Building with Pushover Analysis Aung Pyae Phyo, Dr. San Yu Khaing Abstract— This paper presents eight-storied regular shaped reinforced concrete building. The structure is located in high seismic risk zone, Mandalay. Its height is 106 ft above ground level. The total length and width are 80 ft and 68 ft respectively. High wind speed 80 mph is used. This structure is composed of special moment resisting frame and it is designed out by using SAP 2000 software. The structural elements are designed base on UBC-97. For earthquake and wind forces, loading data were referenced from UBC-97. Dead loads and live loads are used according to ACI code. The load combinations required for the whole structure is used according to UBC-97. In this study, the performance level of structure are determined by using Push-over analysis. The static pushover analysis is becoming a popular method of predicting seismic force and deformation demands for the purpose of performance evaluation of existing and new structures. It will provide much useful information that cannot be obtained from elastic static or dynamic analysis procedures. The displacement control analysis is used and it value is considered as Case I (2% of total structure height) and Case II (4% of total structure height). The lateral displacement of 25 in and 50.88 in is subjected to roof level joint 65 respectively. The performance level of Case I and Case II are compared and also give out the values of base shear and roof displacement for both cases. Index Terms— SAP 2000, special moment resisting frame, performance level, push-over analysis, lateral displacement. I. INTRODUCTION Myanmar is still a developing country and there is a tendency to raise population in future. So, the high-rise building is the only answer to solve the problem of population dense. In establishing the new modern developed country, the grand and modern high-rise buildings are reflect the scientific and technological achievements of the age. A high-rise building is a building in which structural system is modified to make it sufficiently safe and economical. But, from a structural engineer's point of view the multi-storeyed building or tall building can be defined as one that, by virtue of its height, is affected by lateral forces due to wind or earthquake or both to an extent that they play an important role in the structural design. It is necessary to investigate the earthquake effect by using high technological analysis. Seismic Manuscript received Oct 15, 2011. Aung Pyae Phyo, Department of Civil Engineering, Mandalay Technological University, (e-mail: [email protected]). Mandalay, Myanmar, 09-43207740 Dr. San Yu Khaing, (Associate Professor), Department of Civil Engineering, Mandalay Technological University, Mandalay, Myanmar, 09-2132780 (e-mail: [email protected]). performance of a structure is dependent upon the performance characteristics of its critical components.[1] A performance objective is a goal that a building achieves a certain level of performance for a special level of seismic ground shaking hazard.[1] The static pushover analysis is a partial and relatively simple intermediate solution to the complex problem of predicting force and deformation demands imposed on structures and their elements by severe ground motion. II. PUSHOVER ANALYSIS A. Introduction Nonlinear static analysis, or pushover analysis, has been developed over the past twenty years and has become the preferred analysis procedure for design and seismic performance evaluation purposes as the procedure is relatively simple and considers post-elastic behavior. However, the procedure involves certain approximations and simplifications that some amount of variation is always expected to exist in seismic demand prediction of pushover analysis. Although, pushover analysis has been shown to capture essential structural response characteristics under seismic action, the accuracy and the reliability of pushover analysis in predicting global and local seismic demands for all structures have been a subject of discussion and improved pushover procedures have been proposed to overcome the certain limitations of traditional pushover procedures. Traditional pushover analysis is widely used for design and seismic performance evaluation purposes, its limitations, weaknesses and the accuracy of its predictions in routine application should be identified by studying the factors affecting the pushover predictions. In other words, the applicability of pushover analysis in predicting seismic demands should be investigated for low, mid and high-rise structures by identifying certain issues such as modeling nonlinear member behavior, computational scheme of the procedure, variations in the predictions of various lateral load patterns utilized in traditional pushover analysis, efficiency of invariant lateral load patterns in representing higher mode effects and accurate estimation of target displacement at which seismic demand prediction of pushover procedure is performed. The static pushover analysis is becoming a popular tool for seismic performance evaluation of existing and new structures. The expectation is that the pushover analysis will provide adequate information on seismic demands imposed by the design ground motion on the structural system and its components. 1 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 B. Description Of Pushover Analysis Pushover analysis is an approximate analysis method in which the structure is subjected to monotonically increasing lateral forces with an invariant height-wise distribution until a target displacement is reached. Pushover analysis consists of a series of sequential elastic analyses, superimposed to approximate a force-displacement curve of the overall structure. A two or three dimensional model which includes bilinear or trilinear load-deformation diagrams of all lateral force resisting elements is first created and gravity loads are applied initially. A predefined lateral load pattern which is distributed along the building height is then applied. The lateral forces are increased until some members yield. The structural model is modified to account for the reduced stiffness of yielded members and lateral forces are again increased until additional members yield. The process is continued until a control displacement at the top of building reaches a certain level of deformation or structure becomes unstable. The roof displacement is plotted with base shear to get the global capacity curve. C. Performance-Based Plastic Design Pushover analysis is performed by displacement coefficient method or capacity spectrum method. The Capacity Spectrum Method (CSM), a performance-based seismic analysis technique, can be used for a variety of purposes such as rapid evaluation of a large inventory of buildings, design verification for new construction of individual buildings, evaluation of an existing structure to identify damage states, and correlation of damage states of buildings to various amplitudes of ground motion. The procedure correlation of damage states of buildings to various amplitudes of ground motion. The procedure compares the capacity of the structure (in the form of a pushover curve) with the demands on the structure. Objective of Displacement Coefficient Method (DCM) is to find target displacement which is the maximum displacement that the structure is likely to be experienced during the design earthquake. It provides a numerical process for estimating the displacement demand on the structure, by using a bilinear representation of capacity curve and a series of modification factors, or coefficients, to calculate a target displacement. D. Necessity Of Non-Linear Static Pushover Analysis (NLSA) The existing building can become seismically deficient since seismic design code requirements are constantly upgraded and advancement in engineering knowledge. Further, Myanmar buildings built over past two decades are seismically deficient because of lack of awareness regarding seismic behavior of structures. The widespread damage especially to RC buildings during earthquakes exposed the construction practices being adopted around the world, and generated a great demand for seismic evaluation and retrofitting of existing building stocks. Figure 1: Global Capacity (Pushover) Curve of a Structure Pushover analysis can be performed as force-controlled or displacement-controlled. In force-controlled pushover procedure, full load combination is applied as specified, i.e.; force-controlled procedure should be used when the load is known (such as gravity loading). Also, in force-controlled pushover procedure some numerical problems that affect the accuracy of results occur since target displacement may be associated with a very small positive or even a negative lateral stiffness because of the development of mechanisms and P-delta effects. In displacement-controlled procedure, specified drifts are sought (as in seismic loading) where the magnitude of applied load is not known in advance. The magnitude of load combination is increased or decreased as necessary until the control displacement reaches a specified value. Generally, roof displacement at the centre of mass of structure is chosen as the control displacement. The internal forces and deformations computed at the target displacement are used as estimates of inelastic strength and deformation demands that have to be compared with available capacities for a performance check. E. Advantages Of Pushover Analysis The advantage of the pushover analysis is that it applies equally to the evaluation and retrofit of existing structures as to the design of new ones. A comprehensive evaluation of a lateral system would require the execution of a series of nonlinear time history analyses of the structure subjected to a representative suite of earthquake ground motions. This should be the emphasis for evaluation procedures of the future. The pushover is expected to provide information on many response characteristics that cannot be obtained from a linear elastic static or dynamic analysis. The following are examples of such response characteristics: - Force demands on potentially brittle elements, such as axial force demands in columns, force demands on brace connections, moment demands on beam-to-column connections, shear force demands in deep reinforced concrete spandrel beams and in unreinforced masonry wall piers, etc. - Estimates of deformation demands for elements that have to deform inelastically in order to dissipate the energy imparted to the structure by ground motions. - Consequence of strength deterioration of individual elements on the behavior of the structural system. - Identification of critical regions in which the deformation demands are expected to be high and that have to become the focus of thorough detailing. 2 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 - Identification of strength discontinuities in plan or elevation that will lead to changes in dynamic characteristics in the inelastic range. - Estimates of interstory drifts, which account for strength or stiffness discontinuities and may be used to control damage and evaluate P-delta effects. - Estimates of global drift, which may be used to assess the potential for pounding. - Verification of completeness and adequacy of load path, considering all elements of the structural system, all connections, stiff nonstructural elements of significant strength, and the foundation system. (1) (2) (3) (4) (5) Extensive (near complete) structural and nonstructural damage Significant potential for injury but not wide scale loss of life Extended loss of use Repair may not be practical Losses greater than 30% [1] The last item is perhaps the most important one, provided the pushover analysis incorporates all elements, whether structural or nonstructural, that contribute significantly to lateral load distribution. The pushover is most useful for the evaluation at performance levels that are associated with large inelastic deformations (e.g., collapse prevention level). The method is applicable and useful, however, for evaluation at any performance level at which inelastic deformations will occur. F. Performance Levels Of Structure There are five levels of global structural response depending on the permissible amount of damage suffered by the structure when push-over analysis is performed. These are (1) Operational (O) level, (2) Immediate Occupancy (IO) level, (3) Live Safety (LS) level, (4) Collapse Prevention (CP) level, (5) Collapsed (C) level. [1] Operational (O) level In the Operational level, the following facts can occur in the structure; (1) Negligible structural and nonstructural damage (2) Occupants are safe during event (3) Utilities are available (4) Facility is available for immediate re-use (5) Losses less than 5% of replacement value. [1] Immediate Occupancy (IO) level In the Immediate Occupancy level, the following facts can occur in the structure; (1) Negligible structural damage (2) Occupants are safe during event (3) Minor nonstructural damage (4) Building is safe to occupy but may not function (5) Limited interruption of operations (6) Losses less than 15% [1] Live Safety (LS) level In this level, the following fact can occur in the structure; (1) Significant structural damage (2) Some injuries may occur (3) Extensive nonstructural damage (4) Building not safe for re-occupancy until repaired (5) Losses less than 30% [1] Collapse Prevention (CP) level In this level, the following fact can occur in the structure; Figure 2: Performance Level of Structure [2] where, IO = Intermediate Occupancy LS = Life Safety CP = Collapse Prevention D = Damage Level E = Emergency Level III. DESIGN DATA AND CREATING MODEL The proposed building for study is eight-storied reinforced concrete building at Mandalay. It is considered high seismic hazard near Sagaing fault is situated in the seismic zone 4. The case study of proposed building is shown in Table I. Type Table I Case Study of Proposed Building Eight-storey regular shaped RC building Occupancy Residential Location Mandalay Size Length = 80 ft Width = 68 ft Height Typical storey height = 11 ft Bottom storey height = 12 ft Total height = 106 ft Elevator 1 number The material properties of structure are the strength of concrete is 2500 psi and yield strength of concrete is 40000 psi. Dead loads and live loads are used according to ACI code. The values of dead load and live load are shown in Table II. Exposure type is Type B and wind speed 80 mph is used. 3 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 The structure is situated in seismic zone 4, soil profile type is SD and seismic source type is A. [5] Table II Value of Dead Load and Live Load ( UBC-97 ) Dead Load Unit weight of concrete 150 pcf 9″ thick brick barrier wall 100 pcf 4.5″ thick brick barrier wall 50 pcf Unit weight of water 62.5 lb Weight of lift 3 tons Weight of ceiling and finishing Live Load Live load on residential area 25 psf Live load on lobby 60 psf Live load on stair case 100 psf Live load o roof 20 psf Case I For lateral load, lateral displacement of the building is considered only in X-direction. The method of displacement control analysis is used. The 2% of total structure heights (25in) is used for displacement control value.[2] Built-in default hinges are used and only moment hinges are considered. Hinges location are 5% from the column faces.[4] The control option is Push to Displacement Magnitude which is target displacement for this building. This start after the gravity load case. Monitored storey and direction is the same with the gravity load. Minimum number of saved steps is 10, maximum number of null steps is 50, maximum number of total steps is 200, iteration tolerance is 1.000E-04 and event tolerance is 0.01.[3] The figures 4 to 8 show the results for performance levels of the structure. 40 psf Figure 4 .Iteration step=1, Performance Level = O, Base shear = 9.045 K , Displacement = 0.101 in Figure 3: 3D View of Proposed Building IV. PUSHOVER ANALYSIS RESULTS In this study, to determined performance levels of structure, the following data are used. Nonlinear Static (Push-over) Analysis method is used to check performance levels of structure.[3] To perform pushover analysis, there need to define at least two types of static nonlinear case and the first case must be for gravity load and the other for lateral load. [3] For gravity load, appropriate option for control is load to level defined by pattern. It is zero initial condition and therefore the structure has zero displacement and velocity. And all elements are unstressed and there is no history of nonlinear deformation. Monitored storey is roof level and in Ux direction. Loading pattern are dead load and some portion (mostly 25%) of live load. Minimum number of saved steps is 1, maximum number of null steps is 50, maximum number of total steps is 200, maximum iteration/steps are 10, iteration tolerance is 1.000E-04 and event tolerance is 0.01. [3] Figure 5. Iteration step = 5, Performance Level = IO, Base shear = 294.357 K , Displacement = 8.842 in 4 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Figure 9.shows the relationship between Base shear and monitored displacement. Figure 10, 11, 12 and 13 show Capacity Spectrum Curves for zone 2A, 2B, 3 and 4 respectively. Figure 6. Iteration step = 8, Performance Level = LS, Base shear = 347.66 K , Displacement = 16.91 in Figure 9.Push-over Curve for Displacement and Base Shear Figure 7. Iteration step = 9, Performance Level = CP, Base shear = 359.897 K , Displacement = 19.534 in Figure 8. Iteration step = 12, Performance Level = C, Base shear = 373.701 K , Displacement = 24.997 in Figure 10.ATC-40 Capacity Spectrum Curve for Zone 2A Figure 11.ATC-40 Capacity Spectrum Curve for Zone 2B 5 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Figure 12.ATC-40 Capacity Spectrum Curve for Zone 3 Figure 13.ATC-40 Capacity Spectrum Curve for Zone 4 Table III PUSH-OVER VALUES FOR DISPLACEMENT AND BASE SHEAR Step 0 1 2 3 4 5 6 7 8 9 10 11 12 Displacement in 0.000551 0.100544 1.985903 4.563420 6.054389 8.842094 11.674602 14.204222 16.909797 19.538913 22.228202 24.724109 24.996551 BaseForce Kip 0.000 9.045 152.801 242.890 270.132 294.357 315.451 332.829 347.660 359.897 368.952 373.392 373.701 AtoB BtoIO IOtoLS LStoCP CPtoC CtoD 1754 1750 1546 1358 1298 1244 1234 1204 1184 1168 1152 1150 1147 4 8 212 400 460 440 282 220 181 128 102 72 74 0 0 0 0 0 74 242 334 297 256 215 202 199 0 0 0 0 0 0 0 0 96 200 185 122 106 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 104 212 232 Although the structure is analyzed with seismic zone 2A, it is considered to resist high seismic zone 4. Figure 10. shows that the spectrum curve is reached at Immediate Occupancy Level in zone 2A with performance point of base shear (277.614K) and roof displacement(6.915in) and it is situated in the elastic limit state. In Figure 11.the spectrum curve is also reached at Live Safety Level in zone 2B with base shear (294.666K) and roof displacement (8.884in). In Figure 12.the spectrum curve is reached at Live Safety Level in zone 3 and performance point at base shear (327.491K) and roof displacement (13.427in). Both of Graph is situated in the plastic limit state. In Figure 13.the spectrum curve is reached at Collapse Prevention Level in zone 4 and it is situated beyond the plastic limit state. Base shear and roof displacement of figure 13 is 349.412K and 17.286in respectively. Even small amount of load is added to this structure, it will be reached at Collapse Level. Case II For lateral load, lateral displacement of the building is considered only in X-direction. The method of displacement control analysis is used. The 4% of total structure heights (50.88in) is used for displacement control value.[2] Built-in default hinges are used and only moment hinges are considered. Hinges location are 5% from the column faces.[4] The control option is Push to Displacement Magnitude which is target displacement for this building. This start after the gravity load case. Monitored storey and direction is the same with the gravity load. Minimum number of saved steps is 10, maximum number of null steps is 50, maximum number of total steps is 200, iteration tolerance is 1.000E-04 and event tolerance is 0.01.[3] The figures 14 to 19 show the results for performance levels of the structure. 6 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Figure 14 .Iteration step=1, Performance Level = O, Base shear = 9.045 K , Displacement = 0.101 in Figure 17. Iteration step = 7, Performance Level = CP, Base shear = 367.195 K , Displacement = 21.579 in Figure 15. Iteration step = 5, Performance Level = IO, Base shear = 312..97 K , Displacement = 11.336 in Figure 18. Iteration step = 10, Performance Level = C, Base shear = 375.438 K , Displacement = 28.274 in Figure 16. Iteration step = 6, Performance Level = LS, Base shear = 346.42 K , Displacement = 16.67 in Figure 19. Iteration step = 19, Performance Level = C, Base shear = 374.029 K , Displacement = 36.048 in 7 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Figure 20.shows the relationship between Base shear and monitored displacement. Figure 21, 22, 23 and 24 show Capacity Spectrum Curves for zone 2A, 2B, 3 and 4 respectively. Figure 23.ATC-40 Capacity Spectrum Curve for Zone 3 Figure 20.Push-over Curve for Displacement and Base Shear Figure 21.ATC-40 Capacity Spectrum Curve for Zone 2A Figure 24.ATC-40 Capacity Spectrum Curve for Zone 4 Figure 22.ATC-40 Capacity Spectrum Curve for Zone 2B Although the structure is analyzed with seismic zone 2A, it is considered to resist high seismic zone 4. Figure 21. shows that the spectrum curve is reached at Immediate Occupancy Level in zone 2A with the performance point of base shear(277.479K) and roof displacement(6.96in) . In Figure 22. the spectrum curve is also reached at Immediate Occupancy Level in zone 2B with base shear(294.552K) and roof displacement(9.065in) . Both of Graph is situated in the elastic limit state. In Figure 23. the spectrum curve is reached at Live Safety Level in zone 3 and the base shear is 326.174K and roof displacement is 13.443in. It is situated in the plastic limit state. In Figure 24. the spectrum curve is reached at Collapse Prevention Level in zone 4 and it is situated beyond the plastic limit state. Base shear and roof displacement of figure 24 is 349.139K and 17.316in respectively. Even small amount of load is added to this structure, it will be reached at Collapse Level. 8 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Table IV PUSH-OVER VALUES FOR DISPLACEMENT AND BASE SHEAR V. CONCLUSIONS The proposed building for study is eight-storied reinforced concrete building at Mandalay. The structure is analyzed and designed according to SAP 2000 software, UBC-97 specifications. Beams, columns, slabs and stairs are designed with concrete compressive strength of fc′= 2500 psi and reinforcing yield strength of fy = 40000 psi. When the Static analysis is complete, the roof displacement is subjected to Roof level. For gravity plus earthquake load is only considered. Lateral displacement of the building was considered X-direction only. In Case I, the displacement of 25in is subjected to Roof level joint 65. The performance level is reached from step 1 to step 12. From step 1 to 4 , plastic hinges are found and the structure is reached in the Operational Level. At step 5, 6 and 7, the structure is reached in the Immediate Occupancy Level. The Life Safety Level is reached at step 8. The structure is reached from step 9 to 11, the performance level is in the Collapse Prevention Level. In Case II, the displacement of 50.88in is subjected to Roof level joint 65. For gravity plus earthquake load is only considered. Lateral displacement of the building was considered X-direction only. The performance level is reached from step 1 to 19. From step 1 to 4, plastic hinges are found and the structure is reached in the Operational Level. At step 5, the structure is reached in the Immediate Occupancy Level. The Life Safety Level is reached at step 6. The structure is reached from step 7 to 9, the performance level is in the Collapse Prevention Level. From step 10 to 19, the structure is reached in Collapse Level. In the study, more plastic hinges are found in beam than in column face. From step 1 to 4, Case I and Case II have the same base shear and roof displacement. From step 5 to the final step, the base shear and roof displacement of Case II is greater than that of Case I. But the Collapse Level of Case I and Case II have the same roof displacement and base shear. So, the displacement control value of 2% and 4% of the total structure heights are not very differ. The performance level of Collapse Prevention Level in Case I and Case II, base shear is nearly 346K and roof displacement is nearly 16.7in in both case. They have the same performance level of final overall results. Based on the analytical results, the performance level of the proposed building can be distinguished clearly with pushover analysis. Therefore it is very effective in building structures in order to know the effects of lateral force when the seismic intensity reach in high level. REFERENCES [1] [2] [3] [4] [5] ASCE/SEI 41-06, Seismic Rehabilitation of Existing Buildings, ASCE, American Society of Civil Engineering, 2007. ATC, 1996. Seismic Evaluation and Retrofit of Concrete Buildings, Volume 1. Pushover Analysis Manual in Structural Analysis Program (2000) Version 14.0.0, CSI, Computer and Structure Institute. FEMA 356, Federal Emergency Management Agency , Washington D.C. (2000) Uniform Building Code, Volume 2. Structural Engineering Design Provisions 1997, 8th International conference of Building officials. (1997) 9 All Rights Reserved © 2012 IJSETR