Understanding the dynamic loading characteristics of granular flows impacting structural elements is essential for designing resilient protective systems against geophysical mass movements. This study presents new insights into the fluctuating impact stress and the induced structural vibrations resulting from dry granular flow interacting with cylindrical obstacles, based on controlled laboratory experiments. Rather than relying on conventional time-averaged forces, we decomposed the impact stress into mean and fluctuating components and quantitatively assessed their respective roles. The results show that the fluctuating component can elevate the peak impact stress to nearly 1.5 times the mean under the tested conditions, with its intensity primarily controlled by particle size, flow inertia represented by the Froude number, and obstacle geometry. In contrast, traditional hydrodynamic formulations systematically overpredict peak stresses in steady granular flows, by up to about 2.5 times, highlighting their limitations when applied to coarse, collision-dominated flows. Spectral analyses of acceleration signals further demonstrate that the obstacle's vibration response is governed mainly by fluctuating stresses rather than by mean loading. These findings emphasize the dominant role of fluctuations in vibration excitation and indicate that structural vibration records can serve as practical proxies for granular flow dynamics. The results offer a clearer understanding of impact-induced structural vibration and support the design of protective structures that prioritize mitigation of fluctuating dynamic forces.