Proposed Modification to AASHTO Cross-Frame Analysis and Design (2021)

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1 Proposed Modification to AASHTO Cross-Frame Analysis and Design Cross-frames are important structural components that serve many functions throughout the service life of steel I-girder bridges. They primarily act as stability braces to enhance the lateral-torsional buckling resistance of the bridge girders during erection and deck construction. Among other functions, they also distribute live loads to girders across the bridge width in the final composite condition. Under repetitive load cycles caused by heavy truck passages, cross-frames and their connections can be susceptible to load- induced fatigue cracking if not properly designed. Despite recent changes to design speci- fications that signal cross-frames as potential fatigue-sensitive details given substantial live load force effects, little load-induced fatigue cracking has been observed in existing steel I-girder bridges across the country. Cross-frames have historically been detailed and fabricated based on general rules-of- thumb and experience. In recent years, however, developments in bridge design specifica- tions have necessitated updates to 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, bridge designers have generally lacked adequate guidance on the appropriate (i) fatigue loading criteria, (ii) analysis procedures, and (iii) stability bracing requirements for cross-frame systems. With that in mind, NCHRP Project 12-113, “Proposed Modification to AASHTO Cross-Frame Analysis and Design,” was under- taken to address many of these knowledge gaps in an attempt to improve the reliability and economy of cross-frames in steel I-girder bridges. The fundamental objectives of NCHRP Project 12-113 were to produce quantitatively based methodologies and design guidelines for the following items: a. Improved definition of fatigue loading for cross-frames in skewed steel I-girder bridges (straight or curved) analyzed using refined analysis methods; b. Investigation of the influence of girder spacing, cross-frame stiffness and spacing (including staggered layouts), and deck thickness on the force effects in cross-frame systems; c. Additional guidance on how to evaluate the influence of end connections on cross-frame member stiffness in refined analysis models; d. Evaluation of commercial software programs and their ability to accurately predict cross-frame forces for various bridge geometries and cross-frame configurations; and e. Evaluation of stability bracing strength and stiffness requirements for implementation in the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications. S U M M A R Y

2 Proposed Modification to AASHTO Cross-Frame Analysis and Design The objectives were systematically addressed through a series of field experiments, para- metric finite element analyses, and weigh-in-motion studies (considering high-resolution data from 16 sites across the country). This report summarizes the results of the research effort undertaken as part of this project. An important step in the research was to first validate the computational models utilizing field data from representative bridges. As such, three steel I-girder bridges of varying geometry (i.e., a straight bridge with normal supports, a straight bridge with skewed supports, and a horizontally curved bridge with radial supports) were instru- mented and experimentally tested in two different ways. First, a controlled live load test was conducted. Dump trucks of known axle configuration and weight were statically positioned at different locations on the bridge deck, and the stress response in critical cross-frame members and girder flanges was subsequently measured. Second, stress cycle counts induced in the same critical cross-frame members from truck traffic were moni- tored for a one-month span. The data collected from the controlled live load test were primarily used to validate a finite element modeling approach. The in-service stress range data served as a physical benchmark for computational analysis and weigh-in-motion studies conducted in later phases of the project. Upon validating a finite element approach, a series of extensive analytical parametric studies were conducted to expand the breadth and depth of knowledge gained from the field studies. In general, the load-induced fatigue behavior and stability bracing charac- teristics 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 (3D) approaches to simplified, two-dimensional (2D) approaches. Based on the results of the experimental and analytical studies, there were several key findings related to the five major objectives identified above, as follows: • The current fatigue loading model implemented in AASHTO LRFD, which was calibrated based on girder response, does not accurately represent the cross-frame fatigue response to the U.S. truck spectrum. The current design loads and factors generally overestimate the force effects in critical cross-frame members when compared to measured field data or weigh-in-motion records, largely due to the sensitivity of cross-frames to load position on the bridge deck. • Load-induced cross-frame forces are highly variable and depend on a number of parameters. However, the results of this study indicate that significant live load-induced cross-frame forces are generally correlated with large support skews and tight horizontal curves. Implementing discontinuous, staggered cross-frame layouts represents a practical means to mitigate excessive load-induced forces for bridges with large support skews. In contrast, cross-frame fatigue force effects in straight bridges with little to no support skews are generally small. • The load-induced deformational response of cross-frames in composite systems (appli- cable to in-service bridges) is much different than the deformational response in non- composite systems (applicable to bridges during construction). Additionally, traditional 3D modeling approaches of cross-frames commonly used by commercial software programs (i.e., cross-frames treated as pin-ended planar trusses) can misrepresent the true stiffness of a cross-frame panel and its connections. As such, in-service stiffness modification factors are proposed to improve analysis procedures for cross-frames. • Two-dimensional models, which represent cross-frame panels as equivalent beam elements, are prone to predict erroneous top strut forces and substantial errors in diagonal member forces when evaluating the composite condition. This is largely

Summary 3 attributed to the assumptions inherent with the postprocessing procedures (i.e., con- verting equivalent beam forces into cross-frame member forces). Although these dis- crepancies were observed for bridges of all geometries, 2D analysis methods tend to perform worse for heavily skewed and/or curved bridge systems. • Previously established bracing stiffness and strength requirements published in the American Institute of Steel Construction (AISC) Specifications were reviewed and modified for implementation into the AASHTO LRFD Specifications. Additionally, stability-related force effects in cross-frames can be combined with other construction- related load cases via linear superposition. Design examples were developed to demonstrate these procedures. Based on the major conclusions of NCHRP Project 12-113, changes to the specification and commentary language in select AASHTO LRFD Articles are proposed. Design examples were also developed to support and demonstrate the major findings of this research. Ballot items were subsequently prepared for consideration by the AASHTO Highway Subcommittee on Bridges and Structures. In general, the guidance developed from this study is expected to result in improved reliability and economy of cross-frame design in steel I-girder bridges.