Aseismic Redesign of the Thirty Story S-K Building

This report presents the results of a detailed study to evaluate the seismic performance of a thirty story,
reinforced concrete moment resistant space frame which was redesigned to satisfy the lateral force requirements of
current building codes. Two structural systems are considered; one which uses normal strength concrete (4 ksi)
and the other which uses high strength concrete {10 ksi). Both systems are redesigned using an iterative procedure with a three dimensional computer code which calculates member forces and checks all members for compliance with the given design criteria.

Linear and nonlinear analyses are used to evaluate the performance of the redesigned structures. Ultimate resistance to lateral load is evaluated using a nonlinear static analysis on a two dimensional model of a critical frame under proportional loading. Performance under ground motions representative of the service condition is evaluated using linear and nonlinear dynamic analyses. Behavior of the critical frame under severe ground motions representative of the safety condition is evaluated using two dimensional nonlinear dynamic analyses.

Evaluation of the results leads to the following conclusions and recommendations: Current code requirements for design for safety are in general adequate. Inelastic behavior is confined primarily to the girders while column behavior is predominantly linear elastic. Seismic demands on critical members should be sustainable if the members conform to current design and detailing practice. However, provisions need to be specified for design for service conditions in order to avoid excessive damage due to earthquakes which may be experienced several times during the life of the structure. Such criteria would most likely control the design of this building.

The use of high strength concrete appears to be a viable alternative for the design of high rise structures. The performance of the HSC design was comparable to that of the NSC design. Seismic demands on members of the structure should be met if HSC member behavior is comparable to NSC members. Use of the higher strength material resulted in a 12.2% reduction in the overall dead weight of the system with most of this reduction occurring in the reduced size of the columns.