Research Project:

Development of Fatigue Design Procedures for Slender, Tapered Support Structures for Highway Signs, Luminaries, and Traffic Signals Subjected to Wind-Induced Excitation from Vortex Shedding and Buffeting

Principal Investigator | Project Objective | Project Abstract | Task Descriptions, Milestones, and Dates | Student Involvement | Relationship to Other Projects | Technology Transfer Activities | Potential Benefits of the Project | Budget | TRB Keywords

Final Report

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Principal Investigator

Brent Phares
Iowa State University
(515) 294-6979
bphares@iastate.edubphares@iastate.edu

Project Objective

To further understand the phenomenon of vortex shedding and buffeting in slender-tapered high-mast luminary support structures. Computer-based analytical modeling will be conducted and compared with the data collected from a field monitoring site near Mason City, Iowa. To further validate the analytical modeling procedures, specific project information provided by other investigators will be used to generate several other models. This extrapolation of the modeling procedures to other known conditions will help ensure that the developed model is widely applicable. After a sound modeling methodology has been developed and verified with the Iowa data and the data from others, a general, broad-based extrapolation of the modeling will be completed to study the interaction between ranges of wind speeds and ranges of structure properties.

Project Abstract

Cantilevered signal, sign, and light support structures are used nationwide on major interstates, national highways, local highways, and at local intersections for traffic control purposes. Recently, there have been a number of failures of these structures that can likely be attributed to fatigue. In light of the fact that there is considerable uncertainty in the calculation of vortex shedding loads in both the American Association of State Highway and Transportation (AASHTO) and the Canadian Highway Bridge Design Code (CAN/CSA) code provisions, the current equations used for vortex shedding fatigue design need to be reevaluated and likely re-formulated or modified.

A luminary support structure or High Mast Light Pole (HMLP) is generally susceptible to two primary types of wind loading induced by natural wind gusts or buffeting and vortex shedding, both of which excite the structure dynamically and can cause fatigue damage. Vortex shedding is a unique type of wind load that alternatively creates areas of negative pressures on either side of a structure normal to the wind direction. This causes the structure to oscillate transverse to the wind direction.

The primary objective of this study was to develop a procedure for predicting wind loads in the time domain for the fatigue design of slender, tapered luminary support structures. To accomplish this, monitoring of long-term response behavior of a HMLP subjected to wind-induced vibration was needed. This was accomplished by full-scale measurement of the response of a HMLP located near Mason City next to I-35 in Iowa. Wind tunnel testing was also conducted to determine the required aerodynamic parameters of the pole cross section. Further, these aerodynamic parameters were cast into a coupled dynamic model for predicting the response of any HMLP in the time domain. Finally, the model was validated by comparing its results with the data collected from field monitoring.

Task Descriptions, Milestone, and Dates

• Literature Review and Survey (June 2005)
• General Behavior Interpretation (October 2005)
• Pole Modeling and Initial Wind-Structure Modeling (June 2005)
• Modeling and Validation of the Mason City Pole (December 2005)
• Model Extrapolation (March 2006)
• General, Broad-based Modeling (June 2006)
• Final report (August 2006)

Student Involvement (e.g., Thesis, Assistantships, Paid Employment)

None

Relationship to Other Projects

None

Technology Transfer Activities

The Iowa State University Bridge Engineering Center has an on-going, formal relationship with the Iowa DOT Office of Bridges and Structures to provide bridge and other structural engineering expertise. The project will also integrate the Wind Simulation and Testing Laboratory operated by Iowa State University’s Department of Aerospace and Civil Engineering. It is anticipated that the findings from this research will be useful nationally for the design of support structures, and will be distributed and presented accordingly.

Potential Benefits of the Project

The overall research proposed herein will provide information crucial to the revision of the 2001 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals. This will include consideration of vortex-shedding induced fatigue in structures observed to be sensitive to wind loads. This is extremely important as failures of support structures have been observed and attributed to a lack of adequate design provisions. Additionally, the behavior of support structures and load information due to wind excitation will be useful in the design of new support structures and for the proper repair or retrofit methods for known problematic existing structures.

Budget

$35,079 MTC/ $59,519 Cost Share (18-month project)

TRB Keywords

Vortex Shedding, Buffeting, High-Mast, Luminary Support, Wind Gust, Wind Load, Vibration, Oscillation, Excitation, Fatigue Failure, Lock-In Phenomenon