Date of Graduation

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Prinz, Gary S.

Committee Member

Hale, W. Micah

Second Committee Member

Zhang, Shengfan

Third Committee Member

Murray, Cameron D.

Keywords

Buckling-restrained braced frames; Dynamic loading analysis; Orthogonal effects in 2D and 3D; Seismic design rules; Seismic loads; Special moment frames

Abstract

Steel buildings located within seismically active regions require special design considerations to ensure public safety and prevent collapse during an extreme seismic event. Two commonly used steel systems are special moment frames (SMFs) and buckling-restrained braced frames (BRBFs). When two seismic systems share a common column in an orthogonal configuration (such as at a building corner), design specifications currently consider a 100+30 rule wherein the shared column is designed for 100% fuse demand in one direction, plus 30% fuse demand from the other direction. While this rule has been shown to be reasonable for elastic building response, a few studies performed on inelastic systems suggest that the 100+30 rule may not be reasonable for systems expected to experience significant inelastic response.

This study investigated nonlinear effects resulting from simultaneous earthquake loading of orthogonally oriented seismic systems. Detailed nonlinear time-history analysis of three-dimensional frame configurations was considered, addressing coupled and non-coupled orthogonal system effects on resulting shared column demands. Various seismic system pairs (sharing a column) are considered, including both moment frames and braced frames.

Results indicate that the current 100+30 rule is non-conservative for some frame-type combinations. Bidirectional seismic effects in coupled steel systems showed increased column axial demands over independent demand additions from un-coupled (unidirectional loading) analyses. Braced-frame-to–moment-frame configurations were more affected by bidirectional lateral forces than braced-frame-to-braced-frame orthogonal configurations. Additionally uncoupled steel systems experienced higher inter-story drift demands than the coupled frame configurations of the same geometry. A new approach to estimating shared column demands in orthogonal seismic systems was proposed herein.

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