DUAL-LEVEL ApPROACH FOR SEISMIC DESIGN OF ASYMMETRIC-PLAN BUILDINGS

Buildings should be designed to resist moderate ground motion with­ out structural damage and resist intense ground motion with controlled damage. However, most codes do not consider both these requirements explicitly and specify a single design earthquake that generally corresponds to intense ground motion. Investigated in this study is the response of one-story, asymmetric-plan systems designed according to torsional provisions of seismic codes to the two levels of ground motions with the objective of evaluating whether such systems satisfy these requirements. The presented results demonstrate that such systems may not remain elastic during moderate ground motion resulting in structural damage and may experience ductility demand in excess of the design ductility, causing excessive damage during intense ground motion. Therefore, the dual-design approach, pro­ posed earlier for symmetric-plan systems, is extended to asymmetric-plan systems. In this approach, the design earthquakes and the design eccentricities corresponding to the moderate and intense ground motions are considered to be different; for the latter ground motion, the values of design eccentricity are considered to depend on the design ductility of the system. It is shown in this exploratory investigation that systems designed by this extended dual-design approach would satisfy the design requirements for both levels of ground motion. INTRODUCTION The effects of coupling between lateral and torsional motions on the earthquake response of asymmetric-plan buildings and how well these effects are represented in seismic codes have been the subject of numerous inves­ tigations (Chandler and Hutchinson 1987; Chopra and Goel 1991; Esteva 1987; Goel and Chopra 1990, 1991; Humar 1984; Pekau and Rutenberg 1987; Tso and Meng 1982; Tso and Ying 1990, 1992; Tso and Zhu 1992; Zhu and Tso 1992). These studies have often led to contradictory conclu­ sions. Elastic response studies showed that the torsional response is pro­ nounced in systems with close torsional and lateral vibration frequencies, which has led to suggestions to increase the design eccentricity from 1 to 1.5 times the static eccentricity to between three and six times the static eccentricity (Tso and Meng 1982). In contrast, inelastic response studies showed that the torsional motion is reduced significantly by inelastic action of the system, suggesting that the code values of design eccentricity may require a slight modification, if at all, to be consistent with the dynamic response (Chopra and Goe11991; Tso and Ying 1990; Tso and Zhu 1992). As is well known, buildings should be designed to resist moderate ground motion without structural damage and resist intense ground motion with controlled damage; the former criteria is known as the serviceability limit state and the latter as the ultimate limit state. Therefore, the code design procedures for asymmetric-plan systems should be evaluated by simultaIAsst. Res. Engr., Dept. ofCiv. Engrg., Univ. of Califomia, Berkeley, CA 94720. 2Johnson Prof. of Civ. Engrg., Dept. of Civ. Engrg., Univ. of California, Berkeley, CA. neously investigating their elastic response to moderate ground motion, and their inelastic response to intense ground motion. This investigation is a first step towards filling this need. The response of one-story, asymmetric-plan buildings, designed according to torsional provisions of the U.S. seismic codes (Recommended 1990; Uniform 1991; Tentative 1978) to moderate and intense ground motions is investigated. The response of systems designed for the ultimate limit state or serviceability limit state to both ground motions is investigated. Subsequently, the response of buildings designed by the dual design approach, wherein the building is designed for the larger of the forces due to the two limit states, is investigated. Based on these results, shortcomings of the code provisions are identified. In order to alleviate these shortcomings in seismic codes, an extended dual-design approach is proposed, wherein not only the design earthquake but also the values of design eccentricity are defined differently for the two limit states. It is demonstrated that the extended dual-design approach leads to asymmetric-plan systems that satisfy the design requirements for moderate as well as intense ground motion. EARTHQUAKE-RESISTANT DESIGN APPROACH The commentary to the earthquake force recommendations of the SEAOC (Recommended 1990), which are adopted in the UBC-91 (Uniform 1991), states that "structures designed in conformance with these recommendations should, in general, be able to: 1. Resist minor levels of earthquake ground motion without damage. 2. Resist moderate levels of earthquake ground motion without structural damage, but possibly experience some nonstructural damage. 3. Resist major levels of earthquake ground motion having an intensity equal to the strongest either experienced or forecast at the building site, without collapse, but possibly with some structural as well as nonstructural damage. The first two criteria are commonly referred to as the serviceability limit state. This limit state may be interpreted as requiring the building to remain elastic during the serviceability-design earthquake, to avoid structural damage, and the largest of the interstory drifts to remain within a prescribed value in order to limit or avoid nonstructural damage. The third criterion is referred to as the ultimate limit state. This limit state requires that the building possess enough strength and ductility to avoid collapse and nonrepairable structural damage during the ultimate design earthquake. Although UBC-91 and other seismic codes mention both limit states, most codes do not consider both of the limit states explicitly; in particular, the UBC-91 is primarily intended to safeguard against major failures and loss of life (Recommended 1990). In such codes, the forces specified are associated with the ultimate design earthquake. The design force, V, is generally of the form C V W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1) in which C a seismic coefficient, R a reduction factor, and W the weight of the building, including the dead load, a portion of the live and snow load, and total weight of the permanent equipment. i ended ifor t tive ) ­