Report - PEER - University of California, Berkeley

ground motion must be given in a form that has a direct correlation with the behaviourof real structures, particularly in terms of the capacity to cause damage. This leads tothe question of whether current code formats for representing earthquake actions canbe directly adapted to the requirements of PBSD.The paper begins with a critical review of design code specifications of seismicactions, highlighting shortcomings even with regards to current design practice. Thethird section of the paper then briefly summarizes the requirements of PBSD in termsof earthquake actions. The fourth section then discusses possibilities for extendingcurrent code formats to cover the needs of PBSD and also explores alternative formatsthat could be used, both to address weaknesses in current approaches and to meetPBSD requirements.2. EARTHQUAKE ACTIONS IN CURRENT SEISMIC CODESMost earthquake-resistant design of structures is carried out by engineers withoutspecialist training in earthquake engineering by following the specifications andguidelines of design codes. To enable this, the codes must provide, in a simplified andaccessible manner, the nature of the ground shaking to be considered, the way theresponse of the structure to this shaking can be calculated, and the stress and/ordeformation criteria that the structure is required to meet under the specified actions.In the following sections, the way in which ground motions are specified for design incurrent codes — including a few recent innovations — are briefly reviewed.2.1 Design Levels of MotionThe first seismic design regulations to be based on a probabilistic seismic hazard mapappeared in ATC 3-06 (ATC, 1978). The hazard map for the USA, showing PGAvalues with a return period of 475 years, was taken from Algermissen and Perkins(1976). The return period of 475 years was the result of selecting 50 years as theexposure period, although it was acknowledged that “the use of a 50-year interval tocharacterize the probability is a rather arbitrary convenience, and does not imply thatall buildings are thought to have a design life of 50 years” (ATC, 1978). Algermissenand Perkins (1978) stated that “for structures which should remain operable afterlarge, damaging earthquakes, the 10 percent exceedance probability in 50 yearsseems reasonable”, although the choice of 10% was adopted on the rather arbitrarybasis of being a significance level often taken by statisticians “to be meaningful”(Perkins, 2004). A very interesting observation made in ATC 3-06 is that it was notdecided a priori to base the design seismic actions on the selected 475-year returnperiod: a map of effective peak acceleration was drafted for ATC 3-06 — “literallyhaving been drawn by a committee” (ATC, 1978) — and was found to be consistentwith the 475-year PGA map of Algermissen and Perkins (1976). On this basis the10% in 50 year map was adopted in ATC 30-6 and subsequently in the 1988 editionof UBC. The extensive commentary in ATC 3-06 provides a rational and honest470