Graduation Semester and Year

Fall 2024

Language

English

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering

Department

Civil Engineering

First Advisor

Shih-Ho Chao

Abstract

This thesis presents a comprehensive investigation into the seismic design and performance of Special Truss Moment Frames (STMFs) with long spans, focusing on two configurations: STMFs with Vierendeel panels and those with X-braced diagonals in the special segment. The study assesses two case studies with spans of 45 ft and 65 ft, analyzing their behavior under Design Basis Earthquake (DBE) and Maximum Considered Earthquake (MCE) scenarios. The structures were designed according to the latest proposed 2027 revision of the AISC Seismic Provisions, which includes significant changes compared to the published 2022 version. Nonlinear time-history analysis and nonlinear static (push-over) analysis were used to verify whether the structures, once designed based on these updated provisions, demonstrate satisfactory seismic performance.

The findings validate the adequacy of the new design provisions, including the introduction of the 1.3 amplification factor for non-special segment elements and the 0.17 axial demand limitation for special segment chords. These provisions ensure sufficient strength and ductility while addressing the challenges of long-span designs. Although the axial demand limitation increases member sizes, particularly for 65-ft spans, it significantly enhances seismic performance. The comparison between X-braced and Vierendeel configurations reveals that lateral drift performance is comparable under nonlinear conditions, challenging the assumption of superior stiffness in X-braced systems due to the inelastic buckling of diagonals.

This study confirms that STMFs designed using the proposed provisions in the 2027 AISC Seismic Provisions demonstrated satisfactory seismic performance, generally limiting the average story drift ratios to within 1.5% and 2.5% for the 45-ft span, 4-story STMF under DBE and MCE ground motions, respectively. For the two-story, 65-ft span STMF, the average story drift ratios were within 0.8% and 1.0% under DBE and MCE ground motions, respectively. The very small drift ratios were due to the limiting axial demand, 0.17Pn, applied to the special segment chords, which led to heavier special segment chords as well as non-special members outside of the special segments. The small drifts caused minimal rotation of the chords, preventing rapid deterioration of the rotational ductility due to the large axial force induced by heavy gravity loads. This thesis advances the understanding and application of STMFs in high-seismic regions by validating the significantly revised 2027 AISC provisions. Additionally, it provides a comprehensive design guide for practicing engineers interested in using long-span STMFs as the primary seismic force-resisting systems.

Keywords

STMF, Special Truss Moment Frames, long-span, steel, seismic design, non-linear time-history analysis, earthquake, steel structure, seismic force resisting system

Disciplines

Structural Engineering

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