Njiru Mwaura

Date of Award

January 2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

First Advisor

Iraj I. Mamaghani

Abstract

Accurate numerical modeling is essential for evaluating the seismic performance and load-bearing behavior of partially concrete-filled thin-walled steel tubular columns, particularly those used as bridge piers under complex seismic conditions. This study investigates the performance of such columns enhanced with embedded energy-dissipating shell plates and/or partial concrete infill, subjected to unidirectional and bidirectional seismic loading. A validated finite element model (FEM), developed in ABAQUS and incorporating geometric and material nonlinearities—including the modified Two-Surface Model (2SM)—was employed to simulate cyclic behavior and predict structural response.The model was benchmarked against experimental results and used to analyze a broad range of parameters, including width-to-thickness ratio, infill height, slenderness ratio, shell plate properties, and axial compression ratio. The simulations captured key seismic response indicators such as damage modes, hysteresis behavior, stiffness and strength degradation, and energy dissipation, while also evaluating post-buckling and interaction effects of local and flexural buckling. Findings reveal that partially filling the steel tube—optimally around 50% of the column height—substantially improves strength, ductility, and post-buckling stability, with diminishing returns beyond this threshold. Embedded shell plates and internal stiffeners further enhance performance, increasing ductility by up to 30% and strength by 12%. A novel rocking mechanism with a central cable was also shown to reduce local buckling and improve energy dissipation capacity. Based on the extensive numerical analysis, practical design equations were proposed for use by structural engineers, with recommended design parameters of 0.20 ≤ Rf ≤ 0.5, 0.2 ≤ λ ≤ 0.5, and P/Py ≤ 0.2. These equations provide a reliable foundation for the seismic design and construction of efficient, ductile, and resilient bridge piers.

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