About the research
The load capacities of U-bolt connections used in Iowa Department of Transportation (DOT) steel overhead sign trusses are not known because they are used in ways that do not match available manufacturer data. Although these U-bolt connections appear to have performed satisfactorily in the past, in recent years there has been a growing safety concern because of the need for overhead sign trusses to support larger signs at greater span lengths.
As indicated by the Office of Bridges and Structures (OBS) at the Iowa DOT, it is imperative to determine if the U-bolt connections have adequate strength to safely perform in the current overhead sign support structures as well as in future ones that will need to resist even greater loads.
In 2015, OBS and the Bridge Engineering Center of the Institute of Transportation at Iowa State University conducted a literature search to determine if any acceptable U-bolt capacity information was available in the form of load test data, computer modeling (i.e., finite-element model) results, or verified engineering formulas. When no useful information was found, OBS distributed a questionnaire pertaining to U-bolt capacities to state DOTs through the AASHTO Subcommittee on Bridges and Structures. Fourteen state DOTs responded to the questionnaire, confirming that little U-bolt load capacity research and theoretical design has been done.
The objective of this research is to develop effective and efficient calculation methodologies to realistically estimate the capacity of the U-bolt connections specified in the actual SOST design standards. To achieve the goal, an experimental program is proposed to load test U-bolt connections of the same dimensional and material properties as used in the actual SOST design standards. A numerical program is also proposed to assist in understanding the actual behaviors and failure modes of the U-bolt connections after the numerical models are validated against the load test data. Based on the experimental and numerical results, simplified empirical formulae will be developed for the purposes of estimating the capacity of U-bolt connections.