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Advisory Committee Chair

Fouad H Fouad

Advisory Committee Members

Ian E Hosch

Robert W Peters

Document Type

Thesis

Date of Award

2014

Degree Name by School

Master of Science in Civil Engineering (MSCE) School of Engineering

Abstract

Cantilevered overhead sign structures (COSS) are widely used across highways in the U.S. Several cases of failures have been reported for such structures due to fatigue wind loads. The structure’s dynamic characteristics such as natural frequency and critical damping are responsible for its behavior to these loads. When the frequency of wind gusts load matches the fundamental frequency of the OSS, resonance occurs, which causes excessive vibrations. Consequently, fatigue stresses increase and may exceed the fatigue critical limit, resulting in failure. Vibrations and fatigue stresses amplitudes are dependent on the structure’s natural frequency. The objective of this study is investigating natural frequencies of COSS in order to mitigate fatigue stresses due to natural and truck–induced wind gusts. Investigating damping and other fatigue wind loads are out of the scope in this study. Alterations in factors such as the members’ configuration, arrangement, sizes, and layout of the structure control its stiffness and mass distribution which controls its natural frequency and consequently, fatigue stresses. A parametric study was considered in order to investigate the effect of these factors and recommend the best layout between 4–chord, 2–chord, and monotube COSS in mitigating fatigue. Structures were designed according to the American Association of State Highway and Transportation Officials (AASHTO) 2013, Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (mentioned hereafter as the AASHTO Support Specifications (2013)) and modeled using commercial finite element analysis software. Wind pressure power spectral density and time history loading functions were applied on these structures to simulate natural and truck–induced wind gusts, respectively. Eventually, on the contrary of the 2–chord structure, slanted monotube COSS with curved end post was found to have least mass, highest frequency and nearly smallest fatigue stresses.

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