Initial shear demand in beams and one-way slabs subjected to blast load

Abstract

Experimental evidence shows that a beam that can develop a flexural mechanism of failure under static load may fail in shear under blast load during its initial stages of response. Shear failure limits the ability of the beam to “absorb” the energy imparted by blast load. Because energy-absorption capacity is the key to the survival of a structural element to blast, initial shear failure must be avoided. To do so, the likelihood of a change in the failure mechanism (from flexure to shear) of a beam under blast load must be quantified. This can be done by evaluating the ratio of initial shear demand to shear capacity for the beam. The matter is not as simple as it may appear because, for dynamic and impulsive load, shear demand depends on inertial forces and shear capacity depends on loading rate. This paper presents the results of analytical and numerical investigations conducted to characterize the initial response and provide simple means to estimate initial shear demand in beams subjected to blast. Two simple numerical methods are used to estimate shear demand: (1) a finite difference-based solution of the equations of motion of a linear Timoshenko beam; and (2) a method proposed by the authors based on an equivalent linear single-degree-of-freedom (SDOF) system and the work done by Biggs (1964) to compute dynamic reactions. The results for maximum shear obtained with both methods are compared with results from finite element analysis (FEA) of beams subjected to blast. Initial shear response of one-way reinforced concrete slabs tested under blast load is also analyzed using the two simple methods. It is shown that these methods provide results that are in reasonable agreement with FEA results and experimental observations, which makes them suitable for preliminary blast-resistant design, especially considering their simplicity.