Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES A very important value in seismic design is the ductility limit. This limit is not necessarily the largest possible energy dissipation, but a significant changing of structural behaviour must be expected at ductilities larger than this limit. Two ductility limit types can be defined: • available ductility, resulting from the behaviour of structures and taking into account its information, material properties, cross-section type, gravitational loads, degradation in stiffness and strength due to plastic excursions, etc; • required ductility, resulting from earthquake actions, in which all factors influencing these action are considered: magnitude, ground motion type, soil influence, natural period of the structure versus ground motion period, number of important cycles, etc. Ductility of a structure is provided by satisfying the limit state criterion: D γ a m ≥ γ D ( 2.1 ) F r where Da is the available ductility determined from the local plastic deformation and Dr, is the required ductility obtained from the global plastic behaviour of a structure. The partial safety factors γm for available ductility and γF for required ductility must be determined considering the scatter of data with a mean plus one standard variation and the uncertainties in available and required capacities. Values of γm=1.3 and γF=1.2 are proposed (Gioncu, 2000) for this check, if the ductility is obtained by deformation ductility (see Figure 2.2). If the available ductility results from fracture, a greater value for γF must be used (i.e. γF =1.5). This check must be included in a performance based seismic design. Figure 2.2. Deformation and fracture ductilities 13
2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES 2.3 The performance based seismic design During recent earthquakes, including those of California and Japan, structures in conformity with modern seismic codes performed as expected; and, as expected, the loss of lives was minimal. However, the economic loss due to sustained damage was substantial. Earthquakes in urban areas have demonstrated that the economic impact of physical damage, loss of function and business interruption was huge and damage control must become a more explicit design consideration. A promising approach to such needed development is the performance-based engineering (PBE); its application to significant seismic hazards is commonly known as performance-based seismic engineering (PBSE). An important phase of performance-based seismic engineering is the performance-based seismic design (PBSD), that is defined as “identification of seismic hazards, selection of the performance levels and performance design objectives, determination of site suitability, conceptual design, numerical preliminary design, final design, acceptability checks during design, design review, specification of quality assurance during the construction and of monitoring of the maintenance and occupancy (function) during the life of the building." (Bertero and Bertero, 2002). Therefore, a comprehensive performance based design involves several steps: 14 • selection of the performance objectives; • definition of multi-level design criteria; • specification of ground motion levels, corresponding to the different design criteria; • consideration of a conceptual overall seismic design; • options for a suitable structural analysis method; • carrying out comprehensive numerical checking. 2.3.1 Performance objectives (POs) A conceptual framework for the performance base seismic design has been developed encompassing the full range of seismic engineering issues to be referred to design of structures for predictable and controlled seismic performance within established levels of risk. The first step is the selection of the performance design objectives (POs). These objectives are selected and expressed in terms of expected levels of damage resulting from expected levels of earthquake ground motions (SEAOC, 1995). POs will range from code minimum requirements (usually based on fully operational under minor earthquake ground motions and on life
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3. SEISMIC BEHAVIOUR OF BOLTED END
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4. SEISMIC RESPONSE OF PARTIAL-STRE
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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