RESIDUAL LIFE ESTIMATION OF STRUCTURES

Abstract

The current seismic design codes are based on dual level design philosophy, namely, to withstand frequent, moderate earthquakes without any damage and to prevent structural collapse in the event of a rare, severe earthquake. Thus the earthquake resistant design is distinct from gravity load design in that some amount of damage is allowed and a preferred mode of damage in the event of a strong earthquake is designed in the system. In terms of structural safety, these damages reduce the original safety margin of the system. The damage incurred by the structure results in degradation of strength and sti ness, which adversely a ect the service life of the structure. However, current seismic design codes provide neither any estimate of the expected residual life of the system after the damage nor a method or process to estimate it. This information may be useful for engineering planning and decision making for retro tting and rehabilitation of structures. However, there are several sources of uncertainties, namely the uncertainties associated with the earthquake occurrence, probabilistic structural responses, estimating the parameters of damaged state and the probabilistic methods for estimating the residual life. This study proposes a method to estimate the residual life of a structure after it is damaged by a strong earthquake. Structural design codes are formulated for ensuring a certain level of safety for the most commonly used structural forms. These design codes are constantly evolving with the development of new theories, materials, and practices, and, an assured margin of safety is the common thread running through the process of evolution. Though a quantitative measure of safety margin is the key idea behind performance based seismic design, it is rarely explicitly speci ed in the standard codes of practice. This reliability measure is important for calibration of design provisions and for comparison of the alternative designs arrived at by following standard codes of practice and also for the residual life estimation. The design for earthquake e ects is complicated by the uncertainties associated with the earthquake iii occurrence and possible variations in size (intensity) and shape (distribution of energy with respect to frequencies) of earthquake motions. It is then logical to expect di erent acceptable levels of performance (or, safety margins) for di erent types of earthquakes. In the proposed study, a seismic reliability framework is developed for the design provisions of Indian seismic design code, IS-1893. The seismic reliability depends on the statistics of earthquake occurrence in a region, and the probability of structural response exceeding the corresponding limiting value. The primary idea is to aim at no damage state for design basis earthquake (DBE), the serviceability limit state is implied at this performance level and a quanti cation of the implied reliability at this performance provides an important metric for calibration and comparison purposes. The computation of seismic reliability is signi cantly in uenced by the seismicity of the region and it can also be used to verify the consistency of seismic hazard zoning. From this study, similar reliability indices have been found for di erent limit states. With increase in the exposure period or service life the reliability drops signi cantly even without accounting for the material degradation with age. With ageing e ects, the computed reliability will be lower still. For comparison, the margin of safety given in International Organisation for Standardization (ISO) and European Standard appears to be in line with the margin of safety estimated for the IS 1893 provisions. A structure may undergo inelastic deformations during a strong seismic ground motion which may cause structural degradation and strength reduction. Further these degradations cause the reduction of sti ness and have a bearing on the service life of the structure. A damage identi cation after a strong earthquake is required to evaluate the safety of a damaged structure for decisions regarding enhancing its functionality and services. Several model updating techniques are available to account for changes in the state of structure. In this study, Bayesian statistical inference is used for model updating using the response data. The sti ness degradation in the system for di erent damaged states are estimated by varying the intensity of ground accelerations. Di erent degrees of damage due to di erent intensities of shaking a ect the reliability indices as well as the service life. The updated reliability indices are related to the residual life of these damaged structures. For the spectrum compatible as well as the natural earthquakes, the sti ness degradation in terms of Frobenius norms is found to be 7%-10% for the four storey example building, 8%-10% for the eight storey example building without shear walls and 6%-8% for the same building with shear walls building before reaching its collapse state is observed. Also, in terms of the strain energy capacity, the reduction of 5%-6% for the four iv storey example building, 4%-6% for eight storey example building without shear walls and 5%-9% for the shear wall building before collapse is observed. In collapse state the sti ness norms varies in the range 85%-90% and the reduction in strain energy capacity varies in the range 65%-75% for all the example buildings. This study also provides a decision framework about the structural strengthening or renovation, based on the reduction of reliability indices, residual life, sti ness degradation and strain energy capacity parameters.