Bracing systems serve a number of important roles in both straight and horizontally curved bridges.
The braces provide stability to the primary girders as well as improving the lateral or torsional
stiffness and strength of the bridge system both during construction and in service. Depending on
the geometry of the bridge, braces may be designated as either primary or secondary members. In
the AASHTO LRFD Bridge Design Specifications (hereafter referred to as the AASHTO LRFD
BDS) [1], the member designation as primary or secondary is typically assigned depending on
whether or not the member or component transmits gravity loads through a necessary as-designed
load path. For example, a first-order analysis on a straight bridge with little or no skew during
construction will often result in little or no force in the cross-frame members, which are designated
as secondary members in such bridges. However, in some situations during construction, the
removal of one or more of these bracing members can potentially result in a partial or complete
collapse of the structure due to instabilities that can develop as a result of the larger unbraced
lengths that result from the removal of these members. In cases such as this, the engineer needs
to recognize the importance of the brace and design the members accordingly despite their
designation as secondary members. This volume provides an overview of the design requirements
of the braces so that engineers can properly size the members to provide adequate strength and
stiffness.
