Force Reduction Factors for the Seismic Evaluation of Nuclear Structures

摘要

There is a strong need to quantify the expected performance of structural systems to obtain realistic estimates ofrnseismic performance. This need is driven by the desire to demonstrate adequate margins against seismic failure; and tornminimize personnel exposure and cost associated with unnecessary structural modifications. These seismic evaluations haverntraditionally been performed using elastic analysis techniques and when compared directly to the allowable stress in designrncodes may be quite conservative. The use of force reduction factors would allow a better picture of the seismic demands onrnsystems, structures, and components.rnThis paper proposes a methodology to develop force reduction factors that are appropriate for the evaluation ofrnnuclear facilities. These force reduction factors are functions of (1) the acceptable limit state; (2) the structural system,rnmaterial, and detailing for each individual element; (3) the structure’s natural frequency; and (4) the influence of higherrnmodes and soft stories. The acceptable limit state, structural system, material and detailing are used to develop allowablernelement ductilities. Individual element ductilities are modified to account for either MDOF or soft story effects. Thesernmodified element ductilities are combined with the structure’s natural frequency and an appropriate SDOF dynamic model torndevelop the force reduction factor. SDOF dynamic models for both bilinear and pinched hysteresis loops are considered,rnwhich address the behavior of structures ranging from moment frames and braced frames to shear controlled shear wallrnstructures.rnThis methodology also provides for the inclusion of the structural overstrength which is a function of the degree ofrnredundancy in the structural system and the conservatism in code member capacities. Overstrength may be determined byrnpushover analysis, similarity with a previous analyses, or be conservatively neglected. Seismic member demands are reducedrnby both the inelastic force reduction factor and overstrength; combined with the non-seismic demands; and compared to thernmember capacity on a member by member basis. The value of the force reduction factor varies depending on the individualrnductility of each element.rnThe inelastic force reduction factor methodology is applicable to a broad range of limit states ranging from barelyrnperceptible inelastic deformation to large amounts of inelastic deformation just short of collapse. Sufficient literature data isrnavailable to apply this methodology to steel, reinforced concrete and masonry structures with both code and non-coderncompliant seismic detailing. The inclusion of non-code compliant detailing is essential for the evaluation of existingrnfacilities. The methodology can also be adapted to develop force reduction factors for equipment supports.