Seismic Design of Composite Moment Frame Buildings—Case Studies and Codes Implications

Abstract

Building code seismic design provisions for steel and composite moment frames and their implications on seismic performance are examined through comparative trial designs of three six-story buildings. Using nonlinear analyses, static and dynamic contributions to inelastic force reduction are identified and compared to code-specified assumptions. The trial design study demonstrates that static overstrength is significantly larger for space frame systems compared to perimeter frames, whereas the force reduction attributable to inelastic dynamic response is fairly constant between the frame types. The trial designs are shown to meet the minimum performance implied by building code provisions, primarily due to the large static overstrength resulting from minimum stiffness requirements imposed through seismic drift limits. The study suggests that more consistent reliability can be achieved by disaggregating the R-factor into its component static and dynamic parts and that the default values of static and dynamic inelastic response factors should be reduced from those implied by current R-factors. INTRODUCTION Over the past fifteen years composite moment frames, consisting of reinforced concrete columns and steel beams (so called RCS systems) have been used in the US and Japan. The recently published Intemational Building Code, IBC (1), and AISC Seismic Provisions (2__) include seismic design requirements for RCS systems. Essentially, these standards treat composite systems as extensions of traditional steel or reinforced concrete systems. For instance, seismic response modification and displacement amplification factors (i.e., R and Cd factors) are based largely on consensus opinions from comparable factors for conventional steel and reinforced concrete systems (3_). Judgments of this sort are necessitated by the lack of information regarding the behavior of composite systems and clear methodologies for calculating seismic design coefficients (R and Cd) from first principles. This paper begins with an overview of IBC seismic design provisions for composite RCS moment frames and then presents data from nonlinear Incremented Dynamic Analyses (IDA) to evaluate the adequacy of the provisions including an assessment of the code-specified R and Cd factors. The overall goal is to help establish the reliability of composite systems designed using current standards and to provide better technical underpinnings to the empirical foundation of current code provisions. IBC EQUIVALENT LATERAL FORCE STATIC PROCEDURE FOR SEISMIC DESIGN The Equivalent Lateral Force procedure of the IBC specifies a design base shear, V~, given by the following equation: