Experimental analysis of the influence of moderate fire damage on the dynamic and incremental static behavior of reinforced concrete beams

Since structural design requirements related to fire safety focus on safe evacuation and the avoidance of structural collapse, post-fire condition assessment has received relatively little attention so far. However, an adequate post-fire assessment methodology is required if one wants to avoid unnecessary structural refurbishment and demolition, while still ensuring structural safety. In this context, reinforced concrete beams have been subjected to a fire exposure of moderate severity, with the aim of investigating its influence on their dynamic and incremental static behavior. The fire was experimentally simulated with a radiant panel, such that a temperature–time profile similar to the Eurocode parametric fire could be induced at the beams’ surface. During fire testing, the beams were also loaded in a four-point bending test setup to mimic the structural loads that occur in practice. Before and after the fire test, the beams were vibration isolated from the environment and subjected to a modal test, using both conventional accelerometers and fiber-optic Bragg grating (FBG) strain sensors. While the fire event did not result in significant changes in the eigenfrequencies and displacement mode shapes, the strain mode shapes were clearly affected. The neutral axis positions under bending deformation, which could be derived from the strain mode shapes, exhibited significant shifts in the zone that had been exposed to fire. The nature of the shift can be explained by thermal and subsequent nonlinear structural finite element modeling of the fire event. Both the finite element model and additional quasi-static experiments confirmed that in post-fire conditions, the neutral axis under bending in the damaged zone may shift significantly with increased structural loading. This effect can therefore be exploited when assessing the capacity of a structural member in post-fire condition.