Record selection for nonlinear seismic analysis of structures

Student: Iunio Iervolino
Supervisor: Prof. A. Cornell

 

Abstract

This thesis presents a determinedly transparent study of the question of selection and scaling of accelerograms for predicting the non-linear dynamic response of a structure at a specific site. The preferred current practice is to select carefully records that reflect the expected magnitude, distance and other characteristics of the source of the events that are in some sense most likely to threaten the structure. The records are then typically scaled to some common representative level. Neither aspect of this process, neither selection nor scaling, has received significant research attention to ascertain their effects on the conclusions.

This work approaches these subjects inversely; it hypothesizes that neither the usual principal seismological characteristics nor scaling of records matters to the nonlinear response of structures. It then investigates under what conditions this hypothesis may not be sustainable. The study deals with ordinary records; softer soil site and specific near-fault effects, such as directivity-induced pulses, both of which may cause narrow-band response spectra are carefully avoided. Nonlinear analysis case studies consider different periods, backbones, ductility levels and structural types. Two classes of records sets are compared in each case: one class carefully chosen to represent a specific magnitude and distance scenario (a "target set"), and another class chosen randomly from a large catalogue (an "arbitrary set") and scaled to match the target set in general amplitude.

Results of time-history analyses are formally compared by a simple statistical hypothesis test to assess the difference, if any, between non-linear demands of the two classes of records. The effect of the degree of scaling (by first-mode spectral acceleration level) is investigated in the same way. Results here show little evidence to support the need for careful site-specific process of record selection and that concern over scenario-to-scenario record scaling, at least within the limits tested, may not be justified. This study does not explain the role of systematic spectral shape deviations, such as those due to: soft soil, directivity, or scenarios calling for non-median ground motions.

You may download a digital version of this MSc dissertation here.