Criteria for blast-resistant design of structures for explosions above ground
1968 Edition

This code divides structures into two main categories for blast resistance:

1. Diffraction Type Structures:

   • Enclosed forms without apertures.
   • Entire surface area faces the blast pressure.
   • Experiences both shock wave overpressure (p) and dynamic blast wind pressure (q).
   • Examples include solid walls and fully enclosed buildings.

2. Drag Type Structures:

   • Open frameworks like beams, columns, and trusses.
   • Small projected area facing the blast.
   • Primarily subjected to dynamic pressure (q) from blast-induced wind.
   • Examples include open lattice structures and frameworks.

This classification aids in selecting the proper load considerations and design methodology for each structural type.

The standard employs the cube root scaling law to adjust blast pressures and time durations for explosions differing from a reference charge. The approach involves:

• Calculating the scaled distance as the actual distance divided by the cube root of the explosion yield ((x = \frac{R}{W^{1/3}})).
• Calculating scaled time similarly ((t = \frac{t_{actual}}{W^{1/3}})).
• Using tabulated data to find peak pressures and scaled time values for this scaled distance.
• Converting scaled time back to actual time by multiplying by the cube root of the explosive yield.

This method ensures consistent blast parameter estimation across varying charge sizes.

Under blast conditions, design stresses are adjusted downward from static values to account for dynamic effects. Typical recommendations include:

• Structural Steel: Allowable stresses range from 60% to 75% of the yield stress (fy), considering strain rate sensitivity and ductility demands.
• Reinforced Concrete: Compressive stresses limited to about one-third of characteristic compressive strength ((0.33 f_{ck})).
• Steel Reinforcement: Designed generally per static codes (around 87% of fy) but with attention to ductility and dynamic performance.

These adjusted stresses accommodate the increased strain rates and energy absorption requirements during blast events.

The standard allows plastic deformation except where permanent displacement impairs functionality, enhancing energy dissipation capacity. Key points include:

• Plastic deformation extends the effective time period of loading, reducing peak design loads.
• Permissible deflections are linked to the energy absorbed by the structure, represented by the area under the resistance-deflection curve.
• The resistance-deflection relationship is idealized as elasto-plastic to simplify design calculations.

This approach balances safety with economical design by permitting controlled inelastic behavior.

To safeguard against flying fragments from bomb blasts with bare charges detonating at 15 meters, the standard recommends minimum wall thicknesses as follows:

• Reinforced Concrete: 30 cm thickness for 50 kg charges and 38 cm for 100 kg charges.
• Plain Concrete or Brickwork: 34 cm for 50 kg charges and 45 cm for 100 kg charges.

Additionally, the use of non-splintering glass panes is advised to minimize injury risks. These thicknesses ensure adequate protection in blast scenarios.