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The National Academies

NCHRP 15-67 [Anticipated]

Improved Methodology to Accurately Determine Wind Drag Coefficients for Highway Signs and Their Support Structures

  Project Data
Source: Iowa
Funds: $300,000
Staff Responsibility: Waseem Dekelbab
Fiscal Year: 2018

This project has been tentatively selected and a project statement (request for proposals) is expected in July 2017. The project statement will be available on this world wide web site. The problem statement below will be the starting point for a panel of experts to develop the project statement.

Given that wind is the main load that affects highway signs during their lifetime, accurate estimation of wind loads is essential. Moreover, the cyclic oscillations of the total wind load associated with vortex shedding behind the signs may be a main contributor to premature fatigue failure. This is because these cyclic oscillations, which occur even under steady incoming wind conditions, can create a resonance condition. This resonance causes relatively steady, large-amplitude vibrations of support structure members if the shedding frequency coincides with a natural vibration frequency of the structure supporting the traffic sign. To conduct such structural analyses, one needs data not only on the mean pressure drag force, but also on the dominant frequencies and amplitude/energy of the unsteady pressure force oscillations cause by vortex shedding. Besides the wind loads acting on the signs, the sign support structures are also subject to direct wind loads. Accurate determination of these wind loads requires being able to provide estimates of the drag coefficients for different members of the support structure.

The proposed research will use results from state-of-the-art 3D numerical simulations of airflow around traffic signs and the structures supporting those signs to accurately determine drag coefficients for signs of differing shapes and how these coefficients vary with the main geometrical parameters. It will also develop a new methodology to better estimate wind loads on structures supporting various signs. Finally, it will provide a methodology to estimate the inputs needed to perform structural analyses to investigate if vibrations induced by vortex shedding behind the sign can be a main contributor to fatigue failure. The new design recommendations and methodologies will be written in a form that will be suitable for inclusion in future AASHTO LRFDLTS specifications.

The objectives of this research are to: (1) determine mean (time-averaged) drag coefficients needed to estimate wind pressure loads for typical highway signs placed at a certain distance from the ground; (2) develop new methodology to estimate wind loads on the members of the structures supporting highway signs using data from 3D simulations of flow past signs mounted on support structures; and (3) provide quantitative information needed to perform structural analyses to determine if fatigue induced by vortex shedding may be a concern and, if so, to test mitigation devices for reducing or eliminating wind-induced vibrations of support structures for highway signs.

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