Evaluation and retrofit of floorbeam cracking on a tied arch bridge

Abstract

Several tied arch and truss bridges have experienced horizontal floorbeam cracking in the region of the upper web-to-flange weld or in the web gap between the top flange and the connection angles where simple web shear connections have been used. A recent study on a 189 m (620 ft) span tied arch bridge located in Pittsburgh, Pennsylvania over the Monongahela River has evaluated the cause of this type of cracking and resulted in an effective retrofit strategy. Remote monitoring of two retrofitted floorbeam connections was conducted for a period of almost 40 days as random vehicles crossed the bridge. The measurements have confirmed that the primary cause of the observed cracking is the relative longitudinal displacements between the top flange of the floorbeam and the face of the tie girder. This relative movement was focused within the horizontal web gap between the connection angles and the bottom of the top flange. This resulted in the development of horizontal fatigue cracks along the web-to-flange weld in the floorbeam. Several cracks were observed to branch and begin to turn downward into the web. In order to alleviate this problem, a portion of the floorbeam and connection angles were removed at the top of the connection in order to provide sufficient flexibility to allow the required movement and reduce stresses. Instrumentation consisting of strain gages and LVDTs (displacement sensors) was installed at the upstream and downstream side of an interior floorbeam and used to verify the performance and effectiveness of the retrofit. The exact geometry of the retrofit was determined through a parametric finite element study, which was subsequently confirmed with field-measured data. The retrofit provides sufficient flexibility at the connection without producing high stress ranges at critical locations.