E GIRDER SECTIONS UNDERFIRE EXPOSURE USING FINITE ELEMENT METHOD

Pedestrian bridges are essential components of urban transportation systems, yet their structural safety is significantly threatened when exposed to fire hazards. Recent incidents and limited available research on the fire performance of pedestrian bridge girders have highlighted the need for systematic research into their thermal response and post-fire behavior. This study was therefore conducted to examine the temperature distribution, degradation patterns, and fire endurance of three common girder types; steel I-beams, concrete I-beams, and concrete rectangular sections; together with a real-world evaluation of the composite girder used in the University of Benin pedestrian bridge. The overall aim was to understand how these girder systems behave under fire exposure. A finite element modelling (FEM) framework was developed using Abaqus CAE, employing transient thermal analysis under the ISO 834 standard fire curve. Temperature-dependent material properties were defined according to Eurocode 2 and 3 provisions, and 2Dthermal models of each girder section were created for computational efficiency. Boundary conditions included convection and radiation on fire-exposed surfaces, with analysis conducted at 1200s, 2400 s, and 3600 s to capture progressive heat penetration. For the University of Benin case study, a composite girder was modelled to evaluate real structural behavior under elevated temperatures, focusing on heat migration, cracking zones, and the thermal protection offered by the concrete slab. Results showed that steel I-beams heated rapidly and reached critical temperatures earliest
due to high thermal conductivity, resulting in rapid loss of stiffness and structural stability. Concrete I-beams demonstrated moderate resistance, while rectangular concrete beams performed best, maintaining a cool core even at long exposure times. The composite girder exhibited heat concentration along its underside, with spalling and reinforcement weakening in exposed regions but retained strength in upper concrete zones.