Some of Louisiana’s bridges built in the 1950s and 1960s used two-girder or truss systems, in which transverse floor beams are carried by main longitudinal members and continuous (spliced) beams supporting the deck are supported by the floor beams. The main longitudinal members are either two edge (fascia) girders or trusses. Continuous-beam bottom flanges are in compression in the negative moment region, which could result in lateral torsional buckling. When the continuous beams are load rated, $C_b$ is calculated in accordance with a standard specification that does not account for potentially beneficial bracing effects from a noncomposite concrete deck and could therefore underestimate actual flexural strength. As a result, the rating may require a restrictive bridge posting or closure. This issue affects bridges that are key parts of Louisiana’s highway system, i.e., longer, multispan crossings with high average daily traffic (ADT) counts. The calculated load rating could require expensive, and possibly unnecessary, bridge rehabilitation or replacement with significant traffic disruption. This research reassessed the methodology behind load rating these continuous beams, with efforts focusing on deriving more realistic values for $C_b$ . Its main objective was to evaluate the capacity of bridges built with continuous beams and noncomposite decks and to develop a new approach for rating those beams by more accurately representing the moment gradients using $C_b$ . An experimental study was conducted to evaluate lateral torsional–buckling resistance of a reduced-scale, two-span grillage system that included three lines of continuous beams supporting a noncomposite concrete deck. A significantly higher moment gradient factor as compared to the existing codes and standards was recommended by accounting for the bracing effect provided by a noncomposite deck.

A number of large-scale tests of a grillage system are summarized and reduced data are provided. The tests were completed in association with a Louisiana Transportation Research Center (LTRC) project for the Louisiana Department of Transportation and Development (LA DOTD) whose goal was to better quantify behavior and limit states of steel bridge floor systems to refine longitudinal member (stringer) load-rating calculations. A number of tests focused on the behavior of the stringers resting on transverse members (a floorbeam), a system akin to a grillage. The system was proportioned with the expectation that stringer lateral-torsional buckling, a key steel bridge beam design and load-rating limit state, would occur. The provided dataset includes specimen descriptions and output from 58 tests. Investigated parameters included: stringer unbraced lengths; bracing types (i.e., bolted steel diaphragms versus clamped timber struts); load locations; and support conditions. Sample tests are described and reduced results summarized and presented.