Improved Cross Frame Details
Researchers: |
Graduate Students: |
Undergraduate Students: |
Todd Helwig Michael Engelhardt |
Anthony Battistini Wei Wang Sean Donahue |
Guy Bergel |
Sponsor: Texas Department of Transportation
Project Number: 0-6564
Project Summary:
The critical stage for the stability of straight and curved steel I-girder systems generally occurs during placement of the concrete bridge deck. The capacity of the girders is improved by incorporating bracing in the form of cross-frames that restrain twist of the girders. The cutting, fitting, and welding of the cross frame members are very labor intensive. Cross frames are the most expensive component per unit weight on the steel bridge and a significant component of the final cost of the superstructure. Conventional cross frames are usually fabricated using two diagonals and two horizontal struts. The cross frames are typically fabricated from steel angles for the diagonals and either angles, WT, or W-sections for the horizontal struts. The angles are connected along one leg producing eccentric connections that cause bending of the angle and relatively poor structural behavior because of their low bending stiffness. In addition, connection requirements for the intersecting diagonals and top and bottom struts typically necessitate turning over the cross frame during fabrication. Torsional deformations in the girders often result in a tension diagonal and a compression diagonal of the cross frame; however because of the relatively low buckling strength of angles, traditional cross frames are normally designed by conservatively neglecting the compression diagonal thereby relying on a "tension-only" diagonal system. Better structural behavior and a reduction in the handling requirements may be possible if members with improved buckling strength are used for the cross frame. Tubular members are a possible choice and since the sections have a substantial buckling strength, cross frames with a single diagonal member are possible, resulting in fewer connections and a reduction in the handling requirements for the braces. Other potential single diagonal members might be built-up double angles that also have good buckling resistance. Tubes or double angles may lead to better structural behavior since the connection can be made concentric with the centroid of the symmetric member. However, the difficulty in utilizing these members is developing a practical connection method that is simple and reliable.
A commonly-used connection for tubular members consists of slitting the tube to accommodate a gusset plate that is welded to form a concentric connection. Although the eccentricity is eliminated, the fabrication of the connection is time consuming since relatively price geometry is required for the slits to produce good fit for the weld. The connection also leaves the end of the tube open to the atmosphere and can attract debris, birds, and insects that over time might compromise the corrosion performance of the tube. The resulting connection will likely have higher fatigue strengths than fillet welded angles; however fatigue performance is still an issue. Fatigue in the slit tube will be controlled by cracking at the end of the connection plate providing at best a category E fatigue detail. A simple connection solution that blocks moisture and debris from entering the tube is to use a steel casting that fits into the end of the tube and is welded to provide a sealed connection. Although steel castings were considered in the testing program, relatively poor fatigue performance was achieved and therefore the option was eliminated from consideration.
The research in this project is focused on designing and developing improved details for cross frame systems. In addition, the performance of existing TxDOT details for cross frame systems are being evaluated. The project duration was originally set at three-years and began in September of 2009. The study has included laboratory testing and computational parametric studies. The laboratory tests consisted of fatigue and tension tests on cross frame components to evaluate various connection details. The tests gave a measure of the fatigue performance of double angle and tubular members and helped the researchers developed recommendations for connection methods. Single angle members were not tested in fatigue due to potential issues with damaging the testing machines due to the impact of the eccentric connections; however stiffness ultimate strength tests were conducted on the single angles.
A testing frame for conducting stiffness and strength tests on full-scale cross frames was also fabricated. The tests showed that analytical solutions that have been relied upon for estimating the stiffness of many cross frames can grossly overestimate the stiffness of cross frames with large connection eccentricities such as single angle members. The large errors were not present in members with concentric connections such as tubular members and double angles.
Based upon the results of the project, TxDOT gave a one-year extension to the project so that fatigue tests on full scale cross frames could be carried out. The researchers are currently testing a wide variety of cross frame systems to gain a measure of the fatigue performance. Both existing cross frame details and proposed details are being tested. The project will terminate on August 31, 2013.