Report - PEER - University of California, Berkeley

Current code design procedures succeeded in reducing loss of life during majorseismic events. However, much remains to be done in the area of damage reduction.Performance-based design is a general design philosophy in which the designcriteria are expressed in terms of achieving stated performance objectives when thestructure is subjected to stated levels of seismic hazard. The performance targets maybe a level of stress not to be exceeded, a load, a displacement, a limit state or a targetdamage state (Ghobarah 2001). Specifying structural performance objectives in termsof drift limits has not been extensively studied. A set of performance objectivesdefined in terms of drift was given by several publications such as SEAOC (1995)and FEMA (1997). The definition of comprehensive and realistic drift limits that areassociated with known damage states remains one of the important unresolved issuesin performance-based design procedures.The relationship between performance objectives and damage is best illustratedby the typical performance curve shown in Figure 1. Vision 2000 definedperformance objectives are marked on the capacity curve. In addition, the states ofdamage of the structure are identified on the capacity curve. The structure isconsidered to suffer no damage or sustain very minor damage up to concrete cracking.Between concrete cracking and the first yield of steel, the crack sizes are normally < 2mm and damage is considered to be repairable. Past steel yield, the cracks are widerthan 2 mm and repair becomes difficult, impractical or costly, thus the irreparabledamage classification. The described performance applies to ductile systems.However, nonductile systems may suffer brittle failure at any drift level that isassociated with repairable or irreparable damage states.The structural response in terms of displacement can be related to strain-basedlimit state, which in turn is assumed related to damage. The defined performance of astructure in terms of a state of damage, strain or deformation gives better indicator ofdamage than stresses. However, relating displacement limits and drift of the structureto damage is an oversimplification since the level of damage is influenced by severalother factors such as the structural system, the accumulation and distribution ofstructural damage, failure mode of the elements and components, the number ofcycles and the duration of the earthquake and the acceleration levels in case ofsecondary systems.The objective of this investigation is to develop the approach to quantify the driftlimits associated with different damage levels for some reinforced concrete structuralsystems such as moment resisting frames (MRF) and walls.322