About the research
The new revision of the American Association of State Highway Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (SLTS) has changed the wind loading of the SLTS in such a way that can create confusion to both structural and geotechnical designers.The Wisconsin Department of Transportation (WisDOT) uses the 2013 version of the specifications, which defines wind loads in terms of 3-s gust wind speeds instead of the formerly used fastest-mile wind speeds. The wind importance factor was adjusted to represent the wind pressure associated with 10-, 25-, or 100-year mean recurrence intervals and has been addressed in different wind maps consistent with American Society of Civil Engineers (ASCE) 7-2010 (and now 2016) standards.The maps include essential Extreme I load combinations, and wind loads from those maps are not factored (in other words the traditional load factor is already included in developing these maps).
Both of the 2013 and 2015 AASHTO LRFD SLTS design specifications have the same specifications for foundation design of SLTS. Once the design wind load is developed, the total moment and shear load at the ground line or the top of the foundation can be determined. A safety factor is then used to determine the factored moment, shear, torsion, and axial load effects. However, the issue lies in the fact that the two documents handle the design wind speeds differently and this results in potential confusing assumptions when the foundation is designed.
To address this issue, WisDOT has expressed interest in determining the type of loads and deformations experienced at the foundation of two types of wind-loaded structures (such as sign bridges, cantilevers, butterflies, overhead sign supports and high mast lightings). Following WisDOT’s suggestion, this will take place through the monitoring of (at least) two selected structures for a period of one year, capturing the effects of wind loads and member loads and deflections with a special focus on lateral movements, as well as the axial and torsional resistance required of the foundation. Furthermore, predictive models of the two structures and their foundations, validated with field monitoring data, will be developed. This will pave the way for potential modifications to design practice.
The ultimate goal of this research project is to advance the state-of-the-practice by monitoring, validating, and guiding the use of AASHTO LRFD SLTS design specifications for WisDOT.