Abstract: Reinforced concrete (RC) shed serves as an effective in-situ solution for rockfall protection along mountainous highways and railways. Using the commercial software LS-DYNA, refined numerical simulations were conducted to investigate the damage and failure assessment of a prototype framed T-beam type RC shed under rockfall impact. The simulations considered scenarios both with and without cushions, including 600 mm and 1200 mm sand cushions, as well as 1200 mm sand-expandable polyethylene (EPE) composite cushion. Firstly, a refined finite element model of a prototype framed T-beam type RC shed located on the Shanghai-Kunming railway under rockfall impact was developed, of which the rockfall masses ranging from 1 t to 30 t and impact velocities ranging from 10 m/s to 57 m/s. Secondly, by comparing with the results of existing impact tests on bare RC slab, as well as RC slabs with sand and EPE cushions, i.e., impact force-time history of the drop hammer, acceleration and penetration depth-time histories of the rockfall, as well as reaction force and deflection-time histories of the RC slabs, the accuracy and reliability of the adopted material constitutive model, mesh size, contact algorithm, and corresponding parameters were validated. Furthermore, the damage patterns and dynamic responses of the prototype shed without cushion, with sand cushion, and with sand-EPE composite cushion were compared and analyzed. Finally, taking the maximum penetration depth of the rockfall reaching the total thickness of the roof slab and cushion as the failure threshold of the shed, the corresponding relationship between the rockfall mass and the critical impact velocity was established, which enabled rapid assessment of protective performance of sheds. It indicates that: (1) Under the impact of a 15 t rockfall at velocities of 10 m/s and 25 m/s, the damage to the shed without cushion is primarily concentrated in the impact area of the roof slab. On average, the use of sand cushion and sand-EPE composite cushion reduces the peak impact force by 92.8% and 91.6%, respectively. The maximum deflections, ranging from 2 mm to 20 mm, indicate only slight damage to other flexural components of the shed; (2) At impact velocity of 10 m/s, the sand-EPE composite cushion exhibits superior buffering and energy dissipation performance compared to the sand cushion. However, with impact velocity increasing to 25 m/s, the EPE in the composite cushion is rapidly compacted, leading to a diminished protective effect. In this scenario, the impact force and energy transferred to the roof slab with the composite cushion are 89.3% and 37.8% higher than those with the sand cushion, respectively; (3) The critical impact velocity of rockfall corresponding to the failure damage of the shed follows an exponential decay trend as the rockfall mass increases. The application of cushions can increase the critical impact velocity by 52% to 155%, significantly improving the protective performance of the shed.