Experimental and analytical evaluation of cross-frame fatigue behavior in steel I-girder bridges




Reichenbach, Matthew Craig

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Cross-frames are important structural components that serve many functions throughout the service life of steel I-girder bridge systems. They primarily act as stability braces to enhance the lateral-torsional buckling resistance of the girders during erection and deck construction, but also distribute live loads in the final composite condition. Under repetitive load cycles caused by heavy truck passages, cross-frames and their connections are susceptible to load-induced fatigue cracking if not properly designed. Cross-frames have historically been detailed and fabricated based on general rules-of-thumb and experience. In recent years, however, developments in bridge design specifications have necessitated the modernization of cross-frame design and analysis practices. Cross-frames are now designed and detailed based on rational analysis for all stages of construction and service life, which has further emphasized the importance of accurate and reliable analysis techniques and design criteria. Although considerable research over the past several decades has improved cross-frame design and analysis, the design industry has generally lacked quantitively based guidance on load-induced behavior of cross-frames in composite, in-service bridges. As such, this dissertation explores two major concepts: (i) the influence of skewed and curved superstructure geometry on the fatigue response of cross-frames and (ii) the limitations of simplified analysis techniques commonly utilized in commercial software programs with respect to estimating cross-frame force effects. Field experiments were performed on three steel I-girder bridges in the greater Houston area, and the stress ranges induced in key cross-frame members from truck traffic were monitored for one month each. Upon validating a finite-element approach with the measured data, an extensive analytical parametric study was conducted to expand the breadth and depth of knowledge gained from the limited field studies. In general, the load-induced fatigue behavior of conventional X- and K-type cross-frames were examined for a variety of bridge geometries commonly found in the United States. These analyses were performed with different levels of computational refinement, ranging from sophisticated three-dimensional approaches to simplified two-dimensional approaches. Based on the data collected and processed from the experimental and analytical studies, recommendations are proposed to improve the design and analysis of cross-frames in composite bridge structures. Because cross-frames represent a costly component of fabrication and erection, these recommendations ultimately lead to improved efficiency and economy of new steel bridge construction


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