Fragility functions are determined for low- and mid-rise ordinary concrete buildings, which constitute the most vulnerable construction type in Turkey as well as several other countries prone to earthquakes. A hybrid approach is employed where building capacities are obtained from field data and their dynamic responses are calculated by response history analyses. Field data consists of 32 sample buildings representing the general characteristics of two- to five-story substandard reinforced concrete buildings in Turkey. Lateral stiffness, strength, and deformation capacities of the sample buildings are determined by pushover analyses conducted in two principal directions. Uncertainties in lateral stiffness, strength, and damage limit states are expressed by using statistical distributions. The inelastic dynamic structural characteristics of the buildings investigated are represented by a family of equivalent single-degree-of-freedom systems and their seismic deformation demands are calculated under 82 ground-motion records. Peak ground velocity (PGV) is selected as the measure of seismic intensity since maximum inelastic displacements are better correlated with PGV than peak ground acceleration (PGA). Fragility functions are derived separately for different number of stories, which is a prominent parameter influencing the vulnerability of existing substandard concrete buildings.
An effective step for seismic risk mitigation in large urban areas under high seismic risk is to identify the most vulnerable buildings that may sustain significant damage during a future earthquake. Once they are identified properly, existing seismic risks may be reduced either by retrofitting such buildings, or by replacing them with new buildings in view of a particular risk-mitigation planning strategy. A fast and simple seismic risk-assessment procedure for vulnerable urban building stocks is proposed in this study. It is basically a sidewalk survey procedure based on observing selected building parameters from the street side, and calculating a performance score for determining the risk priorities for buildings. Statistical correlations have been obtained for measuring the sensitivity of damage to the assigned performance score by employing a database consisting of 454 damaged buildings surveyed after the 1999 Düzce earthquake in Turkey. The results revealed that the proposed screening procedure provides a simple but effective tool for selecting those buildings that have significant damage risk. These buildings have to be subjected to a more detailed assessment for a final decision on their seismic risk level.
SUMMARYA generalized pushover analysis (GPA) procedure is developed for estimating the inelastic seismic response of structures under earthquake ground excitations. The procedure comprises applying different generalized force vectors separately to the structure in an incremental form with increasing amplitude until a prescribed seismic demand is attained for each generalized force vector. A generalized force vector is expressed as a combination of modal forces, and simulates the instantaneous force distribution acting on the system when a given response parameter reaches its maximum value during dynamic response to a seismic excitation. While any response parameter can be selected arbitrarily, generalized force vectors in the presented study are derived for maximum interstory drift parameters. The maximum value of any other response parameter is then obtained from the envelope of GPAs results. Each nonlinear static analysis under a generalized force vector activates the entire multi-degree of freedom effects simultaneously. Accordingly, inelastic actions develop in members with the contribution of all 'instantaneous modes' in the nonlinear response range. Target seismic demands for interstory drifts at the selected stories are calculated from the associated drift expressions. The implementation of the proposed GPA is simpler compared with nonlinear response history analysis, whereas it is less demanding in computational effort when compared with several multi-mode adaptive nonlinear static procedures. Moreover, it does not suffer from the statistical combination of inelastic modal responses obtained separately. The results obtained from building frames have demonstrated that GPA is successful in estimating maximum member deformations and member forces with reference to the response history analysis. When the response is linear elastic, GPA and response spectrum analysis produce identical results.
SUMMARYEnergy dissipation characteristics of structural members which exhibit both strength and sti ness deterioration under imposed displacement reversals are investigated. In the experimental part, 17 reinforced concrete beam specimens were tested under constant and variable amplitude inelastic displacement cycles. The constant-amplitude tests were employed to determine the low-cycle fatigue behaviour of specimens where the imposed displacement amplitude was the major variable. A two-parameter fatigue model was developed in order to express the variation of dissipated energy with the number of displacement cycles. This model was then used to predict the energy dissipation of test specimens subjected to variable-amplitude displacement cycles simulating severe seismic excitations. It has been demonstrated that the remaining energy dissipation capacity in a forthcoming displacement cycle is dependent on the energy dissipated along the completed displacement path. Moreover, it is observed that total energy dissipation is dependent on the length of the displacement path.
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