Performance-Based Retrofits of Long-Span Truss Bridges Based on the Alternate Load Path Redundancy Analysis

Existing long-span truss bridges typically undergo significant retrofits to improve their resistance against extreme load events, such as the seismic loads, for which the bridge may not have been designed or considered originally. However, the conventional seismic retrofit measures recommended in most existing codes and specifications may not be effective in providing sufficient alternative load path (ALP) redundancy to a bridge that is vulnerable to sudden member-loss scenarios because of the significantly different magnitude of demands. This paper proposed a performance-based retrofit (PBR) approach for long-span truss bridges that are vulnerable to sudden member-loss scenarios. The performance of the bridge has been evaluated by using the demand-to-capacity ratio (DCR) or strain ratio (SR) on the member/component level or the displacement factor on the bridge system level as indicators. Based on the analysis results, the argument is made that the proposed PBR approach allows bridge designers to meet the predetermined performance objectives for sudden member-loss loading conditions. Increase in weight of steel because of such the ALP retrofits is less than 10%. The retrofitted bridge would meet the desired performance levels in the event of sudden loss of any members on the primary trusses of the bridge.

Automated summary

Existing long-span truss bridges often require significant retrofits to enhance their resistance against extreme load events, such as seismic loads. However, conventional seismic retrofit measures may not provide sufficient alternative load path redundancy for bridges vulnerable to sudden member-loss scenarios. This paper proposes a performance-based retrofit approach that uses demand-to-capacity ratio or strain ratio as indicators to meet predetermined performance objectives. The retrofit increases the weight of steel by less than 10% and ensures desired performance levels in case of sudden member loss.