AbstractThe dynamic response of shear deficient (SD) beams constructed with ultrahigh performance fiber reinforced concrete (UHPFRC) is evaluated in this study. Additionally, a comparison is made with adequately reinforced (AR) beams when subjected to sole impact and blast loads and their combinations using finite-element (FE) simulations in LS-DYNA software. To explore more efficient and optimal strengthening designs using UHPFRC under extreme loads, the influences of the thickness of a UHPFRC cover (tU) and different strengthening schemes of a UHPFRC cover are investigated through a parametric study. Also, a new approach is proposed for calculating the damage index, based on the residual shear capacities of the beams, by performing a multiphase loading procedure to describe the damage states of the UHPFRC-constructed beams quantitatively. From the FE simulations, it is found that the use of UHPFRC in the whole cross-section of the beam has more positive effects on the strength enhancements of the SD beams compared to the AR beam, especially when the beams are exposed to combined actions of impact and blast loads. Furthermore, the tU of 10 and 30 mm are recognized as the optimal UHPFRC thicknesses in strengthening the SD beam under the sole impact and combined impact-blast loads, respectively. However, tU=20 mm is accepted as an optimal thickness for the AR beams under sole and combined loads. Also, the applications of UHPFRC on the two sides of the SD beam and as a U-shaped cover represent more efficient designs in the strengthening trend of the SD beams when subjected to the sole impact and combined impact-blast loads, respectively.