CRITICAL DISTANCE FOR BLAST-RESISTANT DESIGN

Blast loads have, in the recent past, become important service loads for certain categories of structures. AN important task in blast-resistant design is to make a realistic prediction of the blast pressures. The distance of explosion from the structure is an important datum, governing the magnitude and duration of the blast loads. The current practice is to choose some arbitrary distance for design purposes. This paper presents some results of analytical studies to show that such a notion is likely to be erroneous, particularly for tall and slender structures. The elements of the blast phenomenon are review, before going into the formulations leading to the 'critical blast distance' at which the transient dynamic response rises to a maximum. Based on the principle of Mach stem growth and consequent transformation of the spherical shock front into cylindrical or plane shock front, an expression for the distance at which the structure is fully engulfed by the Mach region is derived. This is the distance at which the cumulative blast effect reaches a maximum, and hence can be identified as critical distance. To verify this theory, certain numerical experiments are conducted on structures of different heights and diameters, such as cylindrical towers, a chimney and a cooling tower. The results of these studies have convincingly proved the existence of the critical ground-zero distance at which the cumulative blast effect reaches a maximum. It is concluded that this critical distance should be used as the design distance, particularly for tall structures. It is also advisable to use a realistic type of shock front and shock reflection coefficient, consistent with the height of Mach stem, incidence angle and pressure magnitude.