Load Rating Assessment of a 56-Year-Old Short-Span Composite Steel Girder Bridge in a Coastal Environment: Brai 2 Bridge

Abstract

Brai 2 Bridge is a 5.00 m span composite steel girder bridge built in 1970 and located approximately 30 m from the shoreline, where its steel structure is exposed to a highly corrosive marine atmosphere. By the 2026 evaluation, the bridge had been in service for 56 years, exceeding the 50-year design service life commonly adopted for similar bridges in Indonesia, yet no formal load rating had been performed. This study evaluates the structural capacity of the bridge superstructure while accounting for long-term corrosion-induced section loss. Atmospheric corrosion was predicted using the Albrecht and Naeemi power-law model, and Rating Factors (RFs) were calculated in accordance with SNI 1725:2016 and Indonesian Ministry of Public Works and Housing Circular Letter No. 03/SE/M/2016. The assessment was independently verified using manual Lever Rule calculations, a 2D grillage model, and SAP2000 finite-element modeling. The predicted steel thickness loss was approximately 3.40 mm (17%), reducing nominal composite flexural capacity from about 1130 kN.m to 973.5 kN.m. For this short span, the two-lane "T" Truck load governed over the "D" Load. The overall governing RF was 1.64 for support shear using the official procedure, while flexural RFs ranged from 1.77 using the Lever Rule to 2.90 using SAP2000. Because all RFs were at least 1.0, the superstructure remains structurally adequate without load restriction, although its safety margin has decreased. The study demonstrates the value of multi-method verification for aging short-span bridges in corrosive coastal environments.