Distributed Fiber Optic Strain Sensing is an family of measurement technologies developed to quantify the local axial strain of an optical fiber. In particular, Optical Frequency-Domain Reflectometry is often used in static structural health monitoring applications thanks to its millimetric spatial resolution and medium range in the tens of meters. This provides a very high number of measurement points while maintaining reasonable cost and complexity. The sensor consists only of a thin fiber optic cable installed inside the studied structure or bonded at its surface. This technique is particularly suitable for measuring the strain distribution of reinforced concrete structures, as the sensing cable can usually be attached at a few points on the bar before pouring the concrete, providing accurate strain transfer and simple sensor installation. In this work we study a planar reinforced concrete structure with a length of 18 meters and a width of 2.6 meters. We placed two straight sensing cables inside the concrete along the length, each one near one side of the structure. The structure is supported on pivots at both ends, and dynamically loaded in flexion. Using the dynamic distributed strain measurements, we characterize the different modes of the structure using a space-frequency representation. This allows us to determine in particular their symmetry. We also demonstrate direct reconstruction of the modes shape and movement of the structure using a simplified shape sensing reconstruction method. We believe this method can be an interesting alternative to discrete conventional sensors when line-of-sight methods cannot be used.