engleski

An evaluation of the displacement controlled design procedures

In the case of extreme earthquakes, when yielding occurs in a structure, formation of a desired earthquake resistant mechanism does not in itself guarantee that repair cost would be tolerable. It has been proved in many research works that adequate strength does not have a decisive influence on expected structural drift. Therefore, maximum displacements, rather than maximum stresses, represent the proper design criteria. This differs from current force­based design philosophy that is based on acceleration spectra, code performance factors that correlate poorly with damage potential, and displacement checks to ensure that non­structural drift limitations are not exceeded. The quality of the drift estimates for the r/c frames by three different methods suggested for displacement controlled design and inelastic displacement evaluation. All model structures were subjected to inelastic time history analysis using LARZWD (4) with the earthquakes of different characteristics. The first procedure (Priestley, 1998) is based on design displacement spectra, and characterisation of the structure by an equivalent secant stiffness to maximum response, with hysteretic energy dissipation represented by equivalent viscous damping. The second procedure is the N2 method (Fajfar et al., 1996) that uses two separate mathematical models (MDOF and SDOF) and combines response spectrum approach and non-linear static (push-over) analysis method. It is commonly used as a second level procedure. The third procedure (Sozen et al. 1997) is based on the linear elastic analysis and limit state design of the sections. Design process defined by this method assigns a minor role to lateral strength in earthquake-resistant design. Inelastic MDOF (“exact”) analysis and N2 method require much longer time for data preparation and need more uncertain parameters than the DBD and LINEAR method. Among them N2-method is less sensitive and can be applied for evaluation of the existing buildings, accumulated damage or energy demand, or as a second step in the design processes. All three methods are limited to the structures that respond in the first mode. Designers interested only in maximum displacements, or who evaluate the chosen structural system based on its expected damage, can get consistent results by using DBD or LINEAR method. Considering many uncertainties involved in establishing input design motion, material characteristics and member behaviour, DBD and LINEAR method give a good idea of the expected lateral drifts during inelastic response.

nonlinear analysis; performance; displacement; design; methods comparisson

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