Live-Load Performance Evaluation of Historic Covered Timber Bridges in the United States

The National Historic Covered Bridge Preservation Program (NHCBP), sponsored by the Federal Highway Administration (FHWA), was established to preserve the covered timber bridge structures that were constructed in the early 1800s. Today, many of the approximately 880 covered timber bridges still in existence in the United States are closed to vehicular traffic; furthermore, a large percentage of the remaining bridges open to traffic are restricted by load postings. Unfortunately, there are no current load-rating standards for covered timber bridges, so engineers do not have many resources at their disposal to reliably understand the behavior of these complex structures. As a result, the estimated load postings and/or ratings are often too conservative. To better understand the live-load performance of covered timber bridges and to develop improved criteria for their load ratings, a series of live-load tests were performed on 11 single-span, historic covered timber bridges. This paper explains the field testing conducted on all of the bridges, and makes recommendations for the conduct of other similar tests. The tests consisted of installing a network of multiple displacement and strain sensors on the structures, and monitoring global displacements and member strains at various cross sections during passage of a known test. The vehicle used in the testing met the load restrictions in place at the time of testing. The results of this work serve as a basis for instrumentation and field testing, to investigate the live-load performance on such bridges. This paper outlines the field-testing methods and summarizes the results from the testing of all 11 bridges. Based on the field-testing methods, a field-testing protocol is recommended for the live-load testing of historic covered timber bridges. Feasibility of the recommended protocol is also evaluated using a finite-element model analysis for a selected bridge among the tested bridges.