Steel plate girder bridges make use of traditional cross-frame diaphragms to stabilize the compression flange of girders. These braces are required during construction, especially during deck placement, to prevent lateral torsional buckling of bridge girders. Girder buckling capacity is a function of cross-frame diaphragm spacing as well as strength and stiffness. Recent developments in bridge design may cause the governing girder limit state to shift from one of strength to one of stability. These developments include the elimination of in-plan bracing, composite girders, High Performance Steels, and phased deck replacements. In addition, the American Association of State Highway and Transportation Officials (AASHTO) has changed its code requirement for cross-frame diaphragm spacing in the 1998 AASHTO LRFD Bridge Design Specifications. The requirement for 25-foot maximum brace spacing has been removed. The current requirement is for a "rational analysis" to determine cross-frame diaphragm spacing. Explanations of the problems these changes cause in design are discussed. A case study is presented of a bridge that suffered construction difficulties during deck placement. This investigation found that the cross-frame diaphragms were not stiff enough to brace the plate girders during the deck placement. Suggestions are given as to an efficient, economical design and spacing for cross-frame diaphragms on plate girder bridges.

When analyzing steel I-girder bridges, the approach used to model cross-frames can significantly impact performance predictions for girder stability during construction and for cross-frame fatigue under in-service traffic loading. A common practice is to model cross-frame members as truss members subject to axial forces only. Recent research has shown that this approach can lead to erroneous predictions of cross-frame stiffness and cross-frame member forces. Actual cross-frames are typically constructed using single-angle members with gusset plate connections that introduce significant out-of-plane eccentricity and in-plane rotational restraint. These connection effects combined with the complex bending behavior of single-angles results in significant bending of the cross-frame members. This bending behavior can significantly change the axial stiffness of the cross-frame member and potentially introduce large errors in truss element models. The objective of this research is to study the behavior of cross-frames in steel I-girder bridges to better understand their stiffness and internal force distributions during in-service traffic loading on the completed bridge as well as during construction of the bridge. The research involves development of high-fidelity three-dimensional finite element models of steel I-girder bridge systems, with predicted cross-frame response validated using laboratory experimental data as well as data from field instrumentation of in-service bridges. The validated models are then used to conduct parametric finite element studies to examine a wide range of bridge and cross-frame geometries. Based on the results from the parametric studies, stiffness modification factor for truss element models is developed to improve the analysis of cross-frames in steel I-girder bridges

In recent years, bridge engineers and researchers are increasingly turning to the finite element method for the design of Steel and Steel-Concrete Composite Bridges. However, the complexity of the method has made the transition slow. Based on twenty years of experience, Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges provides structural engineers and researchers with detailed modeling techniques for creating robust design models. The book’s seven chapters begin with an overview of the various forms of modern steel and steel–concrete composite bridges as well as current design codes. This is followed by self-contained chapters concerning: nonlinear material behavior of the bridge components, applied loads and stability of steel and steel–concrete composite bridges, and design of steel and steel–concrete composite bridge components. Constitutive models for construction materials including material non-linearity and geometric non-linearity The mechanical approach including problem setup, strain energy, external energy and potential energy), mathematics behind the method Commonly available finite elements codes for the design of steel bridges Explains how the design information from Finite Element Analysis is incorporated into Building information models to obtain quantity information, cost analysis