Torsional Effects On Irregular Buildings Under Seismic Loads Construction Essay

Torsional Effects On Ir stiff Buildings Under seismal Loads locution EssayThis chapter presents a brief review of literature procurable on the subject tortuousnessal cause on atypical buildings below seismic loads. Efforts were do to collect related investigate material. Review of literature encompass research w on the whole accounts on the topic in general and specific wholey aims at latest trend to control asymmetry, tendency fates, configuration requirements, torsional geometrical impermanentity, performance of insurrectionist buildings, and behaviour of appropriate morphological system. At the end of the chapter, selection of askance compact procedures is like rash draw.2.2 RELATED RESEARCH WORKLatest available research papers atomic number 18 studied related to subject of thesis. a few(prenominal) of research papers be described here under1) Torsional constipation of any grammatical construction arouse be determined by conniving the deflections at th e ends in every grade. autographs and guidelines give the definite minutes or coefficients to limit the excess torsion in irregular structures. In this paper adequacy of code provisions regarding the torsional irregularity coefficient is checked and concerned everywhere limits be expressed. For this particular research sours varied groups of buildings are do with different changes in figures such as position of shear w whollys, name of grids and issuance of report etc. Four groups are make videlicet A, B, C and D with different locations of shear walls in plan.At first, adaptation of torsional irregularity with respect to number of grids is investigated. Analysis has been performed for each variation of gridlines in a particular group and conclusions carried out. Graphs are plotted by changing the number of grids lines in each group A, B, C and D against irregularity coefficients. It is observed from theses graphs that in each particular group A, B, C or D there exist different numbers of grid lines against which uttermost results are obtained in that particular group. Maximum value of irregularity coefficient is determent in group C in which shear walls is away from the dryness message but non at the edges. Irregularity coefficient reach a maximum value for certain number of grid lines then ebb by increasing the number of axis.In guerrilla stage, torsional irregularity coefficient is mensurable by changing the number of stages. General trend which graphs shows that with increasing the number of write up for any particular structures, keeping position of shear walls and number of axis same, torsional irregularity coefficient decreases. Curves for structure group C for 1, 2, 4, 6, 8 and 10 narration shows that lesser number of storey yields much than critical results because as the number of stories increases center of rigidity shifts toward center causing lesser torsion consequently gives less critical results.In the last, position of walls is changed to determine the causes on the torsional irregularity coefficient. Graphs are plotted for each indivi dual structural group against the torsional irregularity coefficient. Curves of different storeys predict the lesser the number of storeys more critical will the results. By changing the location of the shear walls in any particular key plan indicate that critical results are obtain for shear wall placed in between the center and edges of the structures. (Guany Ozmen, 2004)2) Parametric analysis of irregular structures under seismic loading reveals the effect of torsion as per Turkish Earthquake Code. For the purpose center of cogency were changed and torsional irregularity was created. Different number of storeys was con facial expressionred which were analyzed using static line procedure and dynamic great power procedures. Results for both of the methods were compared and conclusion drawn. Effect of non-orthogonality was also studied by changing the orienta tion of the non-orthogonal walls. All these cases were studied for five different directions of quake. From these research results limitations in Turkish earthquake code suggested to be improve. (Semih S. Tezcan and Cenk Alhan, 2000)The earthquake vehemences produced in the irregular buildings are unpredictable and can non be determine with greater accuracy thus such structures are more critically prone to earthquakes. A series of five, framed and walled structures are steern with different irregularity coefficients. This paper shows the behavior of different modules against earthquake forces and results drawn. Paper suggests more elaborative measures deficiency to be taken by codes and standards to take over the issue of torsional irregularity. (Ozmen G and Gulay F.G. 2002)3) Codes and Standards direct that along with the static force procedure non linear analysis are need to be performed to complete the exact behavior of the structure. In this paper investigation is done b y creating two different models. In first model eccentricity made moreover in one direction by shifting mass, whereas in second case eccentricity was produced in both directions. Near-fault zone effects were investigated alongwith far-fault results. Research work shows that displacement gather up of the structures remains the same irrespective of distance from fault. The paper concludes that non linear analysis needs to be performed necessarily linear unstained analysis alone are non sufficient for analysis of torsionally irregular structures. ( Emrah Erduran, February 2008)4) To control seismic response of unsymmetrical building viscous silencer are placed. With help of modal analysis effect of plan wise distribution of damping were investigated and torsional dynamic behavior were examined. For input seismic earthquake adequate performance indexes were represented by mean of norms. These norms help to distribute plan wise distribution of extra dampers with help of parametrica l analysis on corrupt plan. Design formulas are prepared to represent the results for norms which were verified by experimentation, which is example of seismic response of asymmetrical systems. (L. Petti , M. De Iuliis, 2008)5) Accidental eccentricity applications provided in codes are evaluated and compared with selection interpretations. An effect of accidental eccentricity is evaluated on the strength of different components. Flexible side elements behavior is investigated and protection measures are described to limit the forces such a comparison is made using different codes. A proposal is made with respect to codes provisions regarding accidental eccentricity, minimum value is specified laterally responding systems. Evaluation of results based on in elasticised dynamic analyses indicates that all codes satisfactorily fulfill the requirements to control the response of torsionally unbalanced buildings. Similarly ductility want and element deformation demand for all the code s are considered. This response demand has a consistence relationship with succession period and geometric of the buildings. Codes requirement in mark of stiff side elements are verified and lay out to be satisfactory. ( A.M Chandler, J.C Correnza and G.L. Hutchinson, 1995)TORSIONAL IRREGULARITYTorsional irregularity is defined in Building Code of Pakistan 2007 (BCP 2007) and is reproduced in Table No.2.1. and Table No. 2.2Table 2.1 Plan structural IrregularitiesIRREGULARITY fibre AND DEFINITION1.Torsional irregularity to be considered when staysare non flexibleTorsional irregularity shall be considered to exist when the maximum storey art gallery, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.2 times the enumerate of the storey drifts of the two ends of the structure.2. Re-entrant cornersPlan configurations of a structure and its lateral-force-resisting system contain re-entrant corners, where both projections of the structure beyond a re-entrant corner are greater than 15 share of the plan dimension of the structure in the given(p) direction.3. Diaphragm discontinuityDiaphragms with incisive discontinuities or variations in rigour, including those having cutout or open areas greater than 50 share of the gross enclosed area of the diaphragm, or changes in effective diaphragm stiffness of more than 50 share from one storey to the next.4. Out-of-plane offsetsDiscontinuities in a lateral force path, such as out-of-plane offsets of the perpendicular elements.5. nonparallel systemsThe vertical lateral-load-resisting elements are not parallel to or symmetric intimately the major orthogonal axes of the lateral-force-resisting system.Table 2.2 Vertical morphological IrregularitiesIRREGULARITY TYPE AND DEFINITION1. Stiffness irregularity nutty storeyA soft storey is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of the averag e stiffness of the three storeys above.2. Weight (mass) irregularity visual sense irregularity shall be considered to exist where the effective mass of any storey is more than 150 percent of the effective mass of an adjacent storey. A roof that is lighter than the floor below need not be considered.3. Vertical geometric irregularityVertical geometric irregularity shall be considered to exist where the naiant dimension of the lateral-force-resisting system in any storey is more than 130 percent of that in an adjacent storey. One-storey penthouses need not be considered.4. In-plane discontinuity in vertical lateral-force-resisting elementAn in-plane offset of the lateral-load-resisting elements greater than the length ofthose elements.5. Discontinuity in capacity weak storeyA weak storey is one in which the storey strength is less than 80 percent of that in the storey above. The storey strength is the total strength of all seismic-resisting elements sharing the storey shear for the direction under consideration.2.4 CONFIGURATION REQUIREMENTS mending structures acquire no significant physical discontinuities in plan or vertical configuration or in their lateral-force-resisting systems such as the irregular features. Irregular structures have significant physical discontinuities in configuration or in their lateral-force-resisting systems. Irregular features include, but are not limited to, those described in code. All structures in Seismic Zone 1 and occupancy Categories 4 and 5 in Seismic Zone 2 need to be evaluated only for vertical irregularities of sheath 5 (Table 2.2) and horizontal irregularities of caseful 1 (Table 2.1). Structures having any of the features listed in Table 2.2 shall be useated as if having a vertical irregularity. (UBC 1629.5.3)Where no storey drift ratio under design lateral forces is greater than 1.3 times the storey drift ratio of the storey above, the structure whitethorn be deemed to not have the structural irregularities of face 1 or 2 in Table 2.2. The storey drift ratio for the top two storeys need not be considered. (UBC 1629.5.3)The storey drifts for this determination may be calculated neglecting torsional effects. Structures may have irregularity in plan or elevation listed in BCP 2007.2.5 STRUCTURAL SYSTEMSStructural systems shall be classified as one of the types listed BCP-2007 and defined under.Bearing skirt organizationA structural system without a complete vertical load-carrying space frame. Bearing walls or bracing systems provide put up for all or most gravity loads. Resistance to lateral load is provided by shear walls or braced frames.Building Frame SystemA structural system with an implicit in(p)ly complete space frame providing support for gravity loads. Resistance to lateral load is provided by shear walls or braced frames.Moment-Resisting Frame SystemA structural system with an essentially complete space frame providing support for gravity loads. Moment-resisting frames provide subway system to lateral load primarily by flexural action of members.Dual SystemA structural system with the chase features comes in the category of dual system1. Essentially complete space frame that provides support for gravity loads.2. Resistance to lateral load is provided by shear walls or braced frames and moment-resisting frames (SMRF, IMRF, MMRWF or steel OMRF). The moment-resisting frames shall be designed to independently resist at least 25 percent of the design base shear.3. The two systems shall be designed to resist the total design base shear in proportion to their relative rigidities considering the interaction of the dual system at all levels.2.6 DRIFT AND STOREY DRIFT LIMILATIONDriftDrift or horizontal displacements of the structure shall be computed where required. For both Allowable Stress Design and military group Design, the Maximum Inelastic resolution Displacement, M, of the structure ca apply by the Design bum Ground Motion shall be determined in accor dance with this section.The drifts tally to the design seismic forces S, shall be determined. To determine M, these drifts shall be amplified. A static, elastic analysis of the lateral force-resisting system shall be prepared using the design seismic forces. Where Allowable Stress Design is employ and where drift is world computed, the related load combinations shall be used. The resulting deformations, denoted as S, shall be determined at all critical locations in the structure.Calculated drift shall include translational and torsional deflections. The Maximum Inelastic Response Displacement, M, shall be computed as follows (BCP 2007)M = 0.7 R S (2.1)Alternatively, M may be computed by nonlinear time history analysis. The analysis used to determine the Maximum Inelastic Response Displacement M shall consider P- effects. storey Drift LimitationStorey drifts shall be computed using the Maximum Inelastic Response Displacement, M. Calculated storey drift using M shall not exceed 0.0 25 times the storey circus tent for structures having a fundamental period of less than 0.7 second. For structures having a fundamental period of 0.7 second or greater, the calculated storey drift shall not exceed 0.020 times the storey height, with exceptions of1. These drift limits may be exceeded when it is present that greater drift can be tolerated by both structural elements and nonstructural elements that could affect life safety. The drift used in this sound judgment shall be based upon the Maximum Inelastic Response Displacement, M.2. There shall be no drift limit in single-storey steel-framed structures classified as Groups B, F and S Occupancies or Group H, Occupancies. In Groups B, F and S Occupancies, the primary quill use shall be limited to storage, factories or workshops. Structures on which this exception is used shall not have equipment attached to the structural frame or shall have such equipment detailed to accommodate the additional drift. Walls that are la terally support by the steel frame shall be designed to accommodate the drift.The design lateral forces used to determine the calculated drift may cut back the limitations and may be based on the period determined, neglecting the 30 or 40 percent limitations.2.7 SELECTION OF LATERAL-FORCE PROCEDUREAny structure may be, and certain structures defined below shall be, designed using the dynamic lateral-force procedures. (UBC 16.8) modify StaticThe simplified static lateral-force procedure may be used for the following structures of occupation Category 4 or 5 (UBC 1629.8.2)1. Buildings of any occupancy (including single-family dwellings) not more than three storeys excluding basements that use light-frame construction.2. Other buildings not more than two storeys in height excluding basements.The static lateral force procedure may be used for the following structures (UBC 1629.8.3)1. All structures, regular or irregular, in Seismic Zone 1 and in OccupancyCategories 4 and 5 in Seismic Z one 2.2. Regular structures under 73.0 meters (240 feet) in height with lateral force resistance provided by different systems.3. Irregular structures not more than five storeys or 20 meters (65 feet) in their height.4. Structures having a flexible upper portion supported on a rigid lower portion where both portions of the structure considered separately can be classified as being regular, the average storey stiffness of the lower portion is at least 10 times the average storey stiffness of the upper portion and the period of the entire structure is not greater than 1.1 times the period of the upper portion considered as a separate structure fixed at the base.Dynamic squinty Force ProcedureThe dynamic lateral-force procedure shall be used for structures, including the following (UBC 1629.8.4)1. Structures 73 meters (240 feet) or more in height2. Structures having a stiffness, fish or geometric vertical irregularity of Type 1, 2 or 3 or structures having irregular features not desc ribed in code.3. Structures over five storeys or 20 meters (65 feet) in height in Seismic Zones 3 and 4 not having the same structural system throughout their height.4. Structures, regular or irregular, located on Soil Profile Type SF that has a period greater than 0.7 second. The analysis shall include the effects of the soils at the site . Structures with a discontinuity in capacity, vertical irregularity Type 5, shall not be over two storeys or 9 meters (30 feet) in height where the weak storey has a calculated strength of less than 65 percent of the storey above. Where the weak storey is capable of resisting a total lateral seismic force of o times the design force prescribed.Whereo = Seismic force over strength factor given in Table 16-N of UBC 97