Comprehending Performance of Cross-Frames in Skewed Straight Steel I-Girder Bridges

Abstract

The effects of support in steel bridges can present significant challenges during the construction. The tendency of girders to twist or layovers during the construction can present a particularly challenging problem regarding detailing cross-frames that provide bracing to steel girders. Methods of detailing cross-frames have been investigated in the past to identify some of the issues related to the behavior of straight and skewed steel bridges. However, the absence of a complete and simplified design approach has led to disputes between stakeholders, costly repairs and delays in the construction.The main objective of this research is to develop a complete and simplified design approach considering construction, fabrication and detailing of skewed bridges. This objective is achieved by comparing different detailing methods, understanding the mechanism by which skew effects develop in steel bridges, recommending simplified methods of analysis to evaluate them, and developing a complete and simplified design procedure for skew bridges. The main conclusion of this research is that lack-of-fit effects for the Final Fit detailing method at the steel dead load stage are equal and opposite to the lack-of-fit effects for the Erected Fit detailing method at the total dead load stage. This conclusion has helped using 2D grid analyses for estimating these lack-of-fit effects for different detailing methods.

Steel Structures Research Update: Advances in Design with Hollow Structural Steel Members

“…Recent study has shown that the lack-of-fit effects for the TDL fit detailing method at the SDL stage are equal and opposite to the lack-of-fit effects for the SDL fit detailing method at the TDL stage (Gull et al, 2017;Gull, 2014). Lack-of-fit effects for the SDL fit detailing method at the TDL stage can be obtained from grid analysis and reversing their sign shall give the lack-of-fit effects for the TDL fit detailing method at the SDL stage.…”

“…Table 1 shows maximum values of lack-of-fit effects; each lack-of-fit effect is obtained from different methods of analysis and compared to recommend a method of analysis for calculating the lackof-fit effects. Detailed comparison of the results is provided in research reports by Gull andAzizinamini (2014a, 2014b).…”

“…It is worth noting that Δ is obtained from the concrete dead load camber calculated from deflection of the system of girders and cross-frames attached together in NCHRP 725 (White et al, 2012). This is an incorrect way of calculating Δ for the TDL fit detailing method at the SDL stage as explained elsewhere (Gull et al, 2017;Gull, 2014).…”

“…Reasonable estimates of lack-of-fit effects are obtained by reversing the improved 2D GA for Bridge B and Bridge C. Bridge A uses staggered framing with a small stagger distance that results in a small unbraced length and overestimation of torsional stiffness of the girder, which consequently results in overestimation of lack-of-fit effects. Detailed comparison of the results is provided in research reports by Gull andAzizinamini (2014a, 2014b).…”

“…This is because the cross-frames are holding the girders into the twisted positions or are responsible for the lack-of-fit. Recent research (Gull et al, 2017;Gull 2014) has shown that cross-frame forces for the TDL fit detailing method are equal and opposite to the cross-frame forces at the TDL stage for the SDL fit detailing method. Therefore, cross-frame forces for the TDL fit detailing method at the SDL stage can be obtained by reversing the sign of the cross-frame forces for the SDL fit detailing method at the TDL stage.…”