Numerical simulations of shear reinforced concrete beams subjected to blast loads

Abstract: Historical accidents and experimental investigations have made apparent that blast loaded concrete members are prone to fail in brittle shear rather than a ductile flexure mode. Air blasts from accidental detonations or explosives may cause severe damage to buildings and infrastructure and it is of great importance that load carrying members can withstand the impulse that arise to avoid progressive collapses. The aims of this thesis are, through explanation of blast loads as dynamic loads acting on structures and measuring of the effects of blast loads on reinforced concrete beams with shear reinforcement, to understand the mechanisms governing shear failure. Two hypotheses are therefore tested: That for a reinforced concrete beam with shear reinforcement, the mechanism governing dynamic shear failure is similar to that of static shear failure and that blast induced shear failure in reinforced concrete beams can be prevented through a sufficient amount of shear reinforcement. To meet the stated aims and test the hypotheses, a literature study was conducted together with numerical simulations using explicit non-linear finite element analysis software LS-Dyna.Previous experimental investigations on blast loaded reinforced concrete beams have displayed a possible shift in failure mode from a ductile flexural failure at static loading to a brittle shear failure at dynamic loading. The shifting may be a property of higher exciting frequencies of blast loads, inducing modes of vibration with larger portions of shear energy. The results obtained from the numerical analyses indicated that an increased ratio of shear reinforcement reduces the risks of a brittle shear failure as well as decrease beam deflections and concrete strains, while increasing strains in the tensile reinforcement.Analysis of the shear capacity and shear reinforcement design through methods given in Eurocode 2 and FKR 2011 were considered as supplementary to the FE analysis. FKR 2011 provided accurate estimations of the maximum dynamic support reactions. Eurocode 2 uses a more conservative approach resulting in lower values of the design shear strength.The conclusions are that for the given beam and blast load, brittle shear failures may be prevented through reduction of the spacing and increase of the bar diameter of the shear reinforcement. The increased plastic strain of the tensile reinforcement as well as measurements of shear crack widths, support reactions and strains in the concrete suggests that the beams with large ratio of shear reinforcement exhibit more ductile behaviour without reaching failure.