With increased traffic, multi-lane highways, and complex highway interchanges, highway signs play an important role in the safe operation of the nation’s transportation network. In the past, it was common to see a single large sign supported by a truss structure. In order to improve highway safety and operations, both static signs and dynamic message sign (DMS) must be used to communicate more frequently with drivers. These signs are typically larger and much heavier than the typical highway signs used in the past.
Given that wind is the main load that affects these structures during their lifetime, accurate calculation of the wind loads is essential. The existing specification needs to be updated to incorporate more current and accurate information on how the wind drag coefficient varies with (1) sign geometrical parameters (e.g., sign aspect ratio, ground clearance, and sign thickness), (2) shape of signs, and (3) type of supporting structures (e.g., signs are attached to a bridge superstructure vs. an overhead sign truss).
The objective of this research is to propose revisions to the wind drag coefficients in the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (LRFDLTS-1). At a minimum, the research shall develop a methodology and design examples to calculate wind drag coefficients for signs and supporting structure members that are included in the LRFDLTS-1.
Accomplishment of the project objective will require at least the following tasks.
Task 1. Conduct a literature review of relevant domestic and international research with consideration of the latest developments in accounting for wind drag coefficients and wind loads on highway signs and support structures. The review shall include research conducted through the NCHRP; FHWA; and other national, state, industry, and pooled-fund sponsored research.
Task 2. Synthesize the literature review to identify the knowledge gaps in calculation of wind drag coefficient for highway signs and support structures. Discuss the data required for the wind load calculation of highway signs and support structures. These gaps should be addressed in the final product as budget permits or in the recommended future research.
Task 3. Propose a methodology and a testing program, to be executed in Phase II, to achieve the project objective. At a minimum, the methodology shall include all the parameters that affect drag coefficient calculations for different highway signs including DMS and the unshielded and shielded members of support structures.
Task 4. Identify areas of the LRFDLTS-1 that will be revised and propose design examples using signs and support structures from DOTs inventories to illustrate the application of the new proposed revisions versus the current specifications.
Task 5. Prepare Interim Report No. 1 that documents the results of Tasks 1 through 4 and provides an updated work plan for the remainder of the project. This report is due no later than 4 months after the contract award. The updated plan must describe the process and rationale for the work proposed for Phases II and III.
PHASE II—Plan Execution
Task 6. Develop the proposed methodology and conduct the testing program according to the approved Interim Report No.1.
Task 7. Develop the draft proposed AASHTO revisions, design examples, and the ballot item to submit with the proposed revisions to the AASHTO Committee on Bridges and Structures based on the research results and according to the approved Interim Report No.1.
Task 8. Prepare Interim Report No. 2 that documents the results of Tasks 6 and 7 and provides an updated work plan for the remainder of the project. This report is due no later than 15 months after approval of Phase I. The updated plan must describe the work proposed for Phase III
PHASE III—Final Products
Task 9. Prepare final deliverables including: (1) the proposed AASHTO revisions, design examples, and ballot item that reflect consideration of the panel’s review comments; (2) a final report that documents the entire research effort; and (3) a stand-alone technical memorandum titled “Implementation of Research Findings and Products.” See Special Note D for additional information.
STATUS: Research contractor has been selected for this project. The contracting process is underway.