Reagan Herman

Buckling Behavior of Steel Bridge I-Girders Braced by Permanent Metal Deck Forms

Permanent metal deck forms (PMDFs) are often used in the bridge industry to support wet concrete and other loads during construction. Although metal formwork in the building industry is routinely relied on for stability bracing, the forms are not permitted for bracing in the bridge industry, despite the large in-plane stiffness. The forms in bridge applications are typically supported on cold-formed angles, which allow the contractor to adjust the form elevation to account for changes in flange thickness and differential camber between adjacent girders. Although the support angles are beneficial toward the constructability of the bridge, they lead to eccentric connections that substantially reduce the in-plane stiffness of the PMDF systems, which is one of the reasons the forms are not relied on for bracing in bridge applications. This paper documents the results of an investigation focused on improving the bracing potential of bridge deck forms. Modifications to the connection details were developed to improve the stiffness and strength of the forming system. Research included buckling tests on a 15-m (50-ft) long, twin-girder system with PMDFs for bracing. In addition, twin-girder tests were also used to validate computer models of the bracing systems that were used for parametric finite-element analytical studies. The buckling test results demonstrated that modified connection details make PMDF systems a viable bracing alternative in steel bridges, which can significantly reduce the number of cross-frames or diaphragms required for stability bracing of steel bridge I-girders during construction.

Use of lean-on cross-frame bracing in steel girder bridges

Four steel bridges in Texas have been designed and will be constructed using lean-on bracing systems where individual cross-frames provide bracing to multiple girders. The detailing used in these bridges includes top and bottom lateral struts in girder bays adjacent to a cross-frame to allow several girders to lean on the cross-frame. Utilization of lean-on concepts should reduce fabrication costs since there will be fewer bracing members that need to be constructed, and also provide maintenance benefits over the life of the bridges since there will be fewer cross-frames to inspect. However, the most significant benefit from use of lean-on cross-frame concepts in bridges with skewed supports is that the bracing system can be laid out such that the cross-frames will pick up smaller forces under truck traffic in the completed bridge, than those that would occur with a conventional cross-frame layout. The bridge systems in which the lean-on bracing system is being implemented in Texas include three severely skewed bridges and one bridge with normal supports. This paper provides details of the specific system implemented in one of the skewed bridges.

Using Metal Deck Forms for Construction Bracing in Steel Bridges

AbstractMetal deck forms are frequently used in the steel building and bridge industries as formwork for the wet concrete slab. Although the forms are often relied on for stability bracing during construction in buildings, conventional connection details between the forms and girder top flanges in the bridge industry possess an eccentricity that limits the bracing performance of the forms. A research study to assess and improve the bracing potential of permanent metal deck forms (PMDFs) used in bridge applications was conducted. Recommendations from the research study were implemented on two steel I-girder bridges located on the IH-610 north loop in Houston. The use of the PMDF bracing resulted in the elimination of 680 intermediate diaphragms from the bridges. Before the implementation project, laboratory buckling tests were conducted on a twin-girder system with members and bracing that were identical to those used in the implementation project. To the authors’ knowledge, this project represents the fir...

Trapezoidal Box Girder Bridge Bracing: Results from Field Monitoring and FEA

Trapezoidal box girders are being used more frequently, particularly in urban interchanges, based on the aesthetics and substantial torsional stiffness of the completed bridge. The closed section formed by the combination of the steel box and the concrete deck is attractive in applications where the horizontally curved layout results in significant torsional demands on the completed bridge system. However, the open bare steel section requires a number of bracing systems to maintain stability during transport and erection, and casting of the concrete deck. Some of these bracing systems are permanent, while others may be removed before the bridge is put into service. This paper discusses results from a field instrumentation and FEA study of the bracing elements of a steel box girder bridge with a skewed support. The field investigation included monitoring of girder stresses, as well as the stresses in each type of bracing element typically used in box girder bridges, including external and internal cross-frames, the solid diaphragm at a skewed support, and the top lateral bracing system. The paper also discusses recommendations resulting from the field study and companion parametric studies using details found in the instrumented bridge.