Skewed Steel Bridges, Part II: Cross-Frame and Connection Design to Ensure Brace Effectiveness

Abstract

Skewed bridges in Kansas are often designed such that the cross-frames are carried parallel to the skew angle up to 40°, while many other states place cross-frames perpendicular to the girder for skew angles greater than 20°. Skewed-parallel cross-frames are longer and require different connections than cross-frames oriented perpendicular to the girder. As cross-frames lengthen, they become less stiff and less effective at distributing forces between girders if the same connecting elements are used. For the cross-frame / diaphragm elements to be able to brace the bridge girders, the brace elements must possess both sufficient strength and stiffness to restrain the girder from instability. While strength can be addressed by increasing the cross-sectional properties of the brace elements, providing sufficient stiffness is a more significant challenge. Stiffness of the brace system is dependent on both the brace elements and the type of connection made (Yura et al. 1992; Yura 2001). Therefore it is important to determine whether the cross-frames and their corresponding connecting elements placed in a parallel-to-skew configuration are sufficiently designed to resist lateral torsional buckling demands using current KDOT practices. The authors have performed a study to investigate the effect of cross-frame orientation, skew angle, and cross-frame connection upon bridge system behavior and cross-frame stresses. In a suite of detailed 3D, solid finite element analyses models of skewed bridge systems, cross-frame layout, connection thickness and type, and skew angle were varied. Skewed bridge systems with cross-frames placed parallel to the skew angle as well as systems with cross-frames arranged in a staggered configuration were considered. Varying bent plate connection thicknesses and a half- pipe connection were also analyzed. Cross-frame spacing of 4.6 m [15 ft] and 9.14 [30 ft] were examined; severe cross-frame spacing of 13.7 m [45 ft] was also considered to examine behavior at very long unbraced lengths. The models include geometric nonlinearities to assess the lateral deflection and lateral flange bending stresses in different bridge systems. Material nonlinearities were found to produce insignificant differences in the results and were not included in the full parametric analysis. The findings of this study showed that skew angle, skew configuration, and connection type all influenced the strength and stiffness of system. The skewed-staggered configuration produced higher lateral deflections in the girders compared to the skewed-parallel configuration. With a couple of exceptions, the skewed-staggered configuration also produced higher cross-frame stresses compared to the skewed-parallel configuration. Larger skew angles resulted in lower lateral deflections. As the skew angles increased, cross-frame compression stresses generally remained the same or increased while maximum cross-frame tension stresses generally decreased. Thicker bent plates produced higher lateral displacements, with the 12.7 mm [1/2 in.] and 25.4 mm [1.0 in.] thick bent plates producing similar lateral displacement values. For skewed configurations, cross-frame stress generally increased with thicker bent plates, with 12.7 mm [1/2 in.] and 25.4 mm [1.0 in.] thick bent plates producing similar cross-frame tension stresses. For the non-skewed configuration, cross-frame stresses decreased with thicker bent plates. The half-pipe connection was shown to correspond with smaller magnitudes of lateral deflections than bent plate connections. Finally, the data showed that cross-frame placed parallel to skew up to an angle of 40° performed similar or better than cross-frames oriented perpendicular to skew for every given skew angle and connection type.