Adaptive pushover analysis for the seismic assessment of older reinforced concrete buildings.

Abstract

It has been observed in major recent earthquakes that older reinforced concrete buildings experienced more serious damage than newer buildings. This is because they often present many structural deficiencies. To prevent further losses in future earthquakes, the seismic capacity of these older buildings needs to be assessed so that appropriate measures can be implemented to mitigate their vulnerability. In this research an adaptive pushover analysis is proposed as an analytical tool to carry out the seismic assessment of these older reinforced concrete buildings, where the following new methods and procedures are proposed. 1) A compound-spring member is used to model the critical regions in the members. This model accounts for possible failure in flexure, shear, or combined shear-flexure and makes the member's stiffness strength dependent. 2) The modified Rayleigh method is used to determine the lateral forces distribution and increment. 3) An automatic calculation of the structure's ductility is used for the seismic assessment using a force-based method. 4) Automatic calculations of the structure's lateral displacement capacity and effective period are used for the seismic assessment using a displacement based-method. 5) An automatic determination of the structural critical condition. 6) Step-by-step procedure to carry out a seismic assessment using the proposed adaptive pushover analysis. 7) The development of a pre-processor program to overcome difficulties in preparing the input data to carry out the analyses. The term adaptive is used because in each step of the analysis the following parameters are updated and evaluated as the lateral displacement is increased: 2000 11 1) flexural strength in the flexural spring, 2) shear strength in the shear spring, 3) lateral forces distribution and increment, 4) structural lateral strength, 5) structural period, 6) structural lateral displacement, 7) structural ductility. Using the proposed methods and the pre-processor program, the adaptive pushover analysis can directly give the following seismic capacity parameters using data obtained from the site or structural drawings. 1) Fundamental period, T₁ 2) Base shear capacity, Vbase 3) Effective period, Teff 4) Structure's ductility capacity, μ 5) Structure's lateral displacement capacity, δu Based on these parameters, the seismic assessment can then be carried out using either a forcebased method or a displacement-based method as suggested by NZNSEE (1996), where the seismic demand is determined from response spectra. In the force-based method the building is expected to perform satisfactorily during the seismic event corresponding to the given response spectra if the ductility capacity is greater than the ductility demand. In the displacement-based method the building is expected to perform satisfactorily during the seismic event corresponding to the given response spectra if the lateral displacement capacity is greater than the lateral displacement demand. If the seismic capacity is less than the seismic demand, a further step is then carried out to estimate the reliable earthquake return period in which the structure will perform satisfactorily so that decision on appropriate action can be made.