In this work, the influence of a fire event on the dynamic structural characteristics of concrete members is investigated. A set of reinforced concrete beams have been subjected to a fire exposure of moderate structural severity. The beams were simultaneously loaded in a four-point bending test setup to mimic structural loading. Before and after the fire test, the beams were subjected to a modal test, using both accelerometers and fiber-optic Bragg grating strain sensors. While the fire 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. This effect can therefore be exploited when assessing the capacity of a structural member in post-fire condition.