Lateral bracing helps transfer horizontal forces (like wind or seismic loads) to the vertical plane, which helps ensure stability and structural integrity. It would seem this concept has been adopted globally for bridge construction, but that is not the case. Some countries prohibit lateral bracing in certain circumstances such as the type of structure and the terrain. There are instances where lateral bracing might have limited structural benefit, for example, in Nordic countries with steep embankments and almost unlimited vertical clearance. What are the differences in how countries in Europe and North America apply lateral bracing? What type of bridge would benefit from increased requirements for lateral bracing? Researchers Victor Vestman, Harry White, Peter Collin, Heikki Lilja, Timo Tirkkonen, Mikko Peltomaa, Javier Jordan, Jacques Berthellemy, and Robert Hällmark explore variations in standards in their paper “How Lateral Bracing is Used for New and Rehabilitated Composite I-Girder Bridges across Europe and North America,” for the Journal of Structural Design and Construction Practice.
Their work drew from examples in Finland, France, the U.S., and Latin America. An overall preference for a simpler load rating process was a factor in whether countries permit, require, or prohibit their use. Their research indicates that incorporating lateral bracing, even on existing structures, can significantly reduce anticipated live load fatigue. Delve into the study and its practical application in strengthening existing I-girder bridges with load concerns at https://ascelibrary.org/doi/10.1061/JSDCCC.SCENG-1644. The abstract is below.
Abstract
Something as simple as the use of lateral bracing for a steel I-girder bridge should be consistent across the globe. However, an investigation into the bridge standards of different countries in Europe and North America revealed little to no consistency in how and why lateral bracing is specified or detailed. Most design codes recognize that adding lateral bracing—the diagonal bracing between the bottom flanges of a typical I-girder bridge superstructure—changes the load path of an I-girder bridge sufficiently to mimic the increased stiffness and fatigue life performance of a much more expensive steel box beam superstructure. Still, lateral bracing is rarely used or, if used, not accounted for in the structural capacity of the structure. This is often because the maximum benefit is found with superstructures with few deep girders in the cross section, and far less benefit is seen for shallower multigirder cross sections. Additionally, accounting for the structural benefit of the lateral bracing increases the complexity of the bridge analysis model and precludes the use of simplified line-girder methods. For these reasons, the investigation showed that even when lateral bracing is used for reasons such as construction stability, it is rarely accounted for as a primary load carrying member in new structures. Since the inclusion of lateral bracing provides no structural benefit but also adds dead load and structural analysis complexity, most agencies attempt to eliminate lateral bracing from their structures and simply increase the capacity of the I-girders. However, for existing two-girder composite bridges, an approach is presented in which the careful addition of a new or structural consideration of existing bottom lateral bracing on an existing two I-girder superstructure could improve the live load distribution and reduce fatigue live load stress ranges sufficiently enough for the structure to remain in service without further structural strengthening.
Explore more on the value of lateral bracing for upgrading I-girder bridges in the ASCE Library: https://ascelibrary.org/doi/10.1061/JSDCCC.SCENG-1644.
