Design envelopes for cantilever slabs to resist blast loads

Blast resistant structural designs are becoming essential because of the upsurge in terrorist attacks throughout the world in recent years. A lot of research has been done since 1940s to develop design philosophies against blast forces. As a result, a number of methods have been introduced to estimate design parameters and procedures have been developed to carry out blast resistant designs. Conventional designs codes consider risks such as excessive wind, floods, water waves, earthquakes, crashing of vehicles or aircraft on to buildings, collapse of masses, explosions (gas pipes, gas containers, pressured water lines etc) and consequences of human error. However, designs for explosions using explosives (bombs) are not included in commonly used conventional design codes. Therefore, developing design guidelines which can be easily used for designing blast resistant structures is important. Using results of previous research works and numerical methods, design envelopes can be developed by means of which, the position of conventional structural elements in a blast loading environment can be identified. Design envelopes for cantilever slabs were developed using Kingery and Bulmash’s (1984) empirical method (for estimations of blast parameters) and the procedures described by Cormie D et al. (2009), for blast resistant reinforced concrete design, which have been prepared following the codes UFC-3-340-02, EN 1990, BSEN 1992 and BS 8110, Part 1, 1997 and Part 2, 1985 etc. Research was done for impulsive regime (protection category 2) and quasi static & dynamic regime (protection category 1) for a range of cantilever slabs. The relationship between the scaled distance (Z) and steel to concrete ratio (As/[bd]) for different effective depths (d) were plotted graphically. The position of the conventional design was also plotted on these graphs. Analyzing the envelopes developed, it can be observed that pushing a conventional design towards quasi-static & dynamic regime needs a greater amount of tensile and shear reinforcement. However conventional designs can be pushed towards impulsive regime with minor improvements. It can be observed that increasing effective depth (slab thickness) is more effective than increasing reinforcement for blast resistance. It can also be seen that there is a maximum limit to the amount of steel needed for an element to be in the impulsive regime and therefore the design must be done with great care