Blast load

In summary, an object s blast loading has two components. The first is a transient pressure distribution induced by the overpressure of the blast wave. This component of blast loading is determined primarily by reflection and lateral rarefaction of the reflected overpressure. The height and duration of reflected overpressure are determined by the peak side-on overpressure of the blast wave and the lateral dimensions of the object, respectively. The Blast loading of objects with substantial... 

A control building is located 125 ft (38 m) from a facility that handles highly reactive materials having the potential for explosion. The building can withstand a 12 psi (0.83 bar) side-on overpressure, 20 ms blast load. 

The company standards classify this facility as "high explosion potential," requiring control buildings that are located at this distance [125 ft (38 m)] from a potential explosion be designed to a minimum load of 10 psi (0.69 bar) side-on overpressure and 20 ms blast load. It was determined that this... 

Impulse or duration of the blast load (Impulse is the area under the pressure-time curve. Duration is the length of time of the overpressure phase of the blast wave.)... 

Mass and stiffness. These relate to the natural period of vibration or natural frequency of the components. The relationship between the dynamic characteristics of the structure (i.e., natural frequency) and the dynamic characteristics of the blast load (i.e., duration or impulse) affects the magnitude of the structural response to blast pressures. 

This section has been primarily concerned with the response of structural elements to which the blast pressure loading has been directly applied. This is termed primary (local) response to blast loading, and it is commonly... 

Evaluation of Building Response and Tolerance Criteria Principles of evaluating building response to explosion loads are presented in Section 5.5.1. The resulting responses are to be compared to structural capacities in accordance with ultimate strength provisions of the governing material standards (e.g., Refs. 94,99, and 100). Blast loads are large compared to conventional loads and less likely to occur than are such conventional loads,... 

The ideal side-on parameters almost never represent the actual pressure loading applied to structures or targets following an explosion. So a number of other properties are defined to either more closely approximate real blast loads or to provide upper limits for such loads. (The processes of reflection and diffraction will be discussed later.) Properties of free-field blast waves other than side-on pressure which can be important in structural loading ares... 

Normal Reflection. An upper limit to blast loads is obtained if one interposes an infinite, rigid wall in front of the wave, and reflects the wave normally. 

Because these loads are usually suddenly applied, and because they last from fractions of a millisecond to at most seconds, the response of or damage to loaded structures or objects is almost always dynamic. So, usually structural response or damage is dependent not only on the amplitude (peak overpressure) of the applied blast loading, the loaded area and the structural strength but also on the mass or inertia of the structure, and either the duration of the transient pressure loading or the applied specific impulse. 

Although the curves in Figures 13-15 were developed for transient loads defined by total applied forces and inpulses, we could as easily have developed them by initially specifying an applied pressure transient loading, with its accompanying specific impulse, plus a loaded area. So, the concept certainly applies to simple structures under blast loading. The inportant inferences to be drawn from the simple- analyses are that structures respond primarily to peak overpressure if their vibration periods are much shorter than the blast duration, while they respond primarily to specific impulse if their vibration periods are much longer than the blast duration. 

If these two times are about equal, then both blast loading quantities are important. 

Figure 14. Shock Response for Blast-Loaded Elastic Oscillator.

Figure 16. P -I Diagram for Blast Loaded Rigid-Plastic System.

The P-i concept can also be used to collapse the results of a number of dynamic response calculations for structural elements into compact dimensionless design curves. A number of illustrations are given in Ref. 15, with one for blast-loaded beams with various boundary conditions appearing in Fig. 20. These curves give predictions of maximum dynamic bending strains and displacements for beams with a variety of boundary conditions. Details appear in Ref. 15, so we do not try to define all parameters here. 

Figure 20. Elastic-Plastic Solution for Bending of Blast Loaded Beams. (Reprinted with permission from ref. 15. Copyright 1983 Elsevier Science.)...

For this simplified model of spalling, graphical boundaries have been determined for incipient spall for normally reflected air blast loading in Ref. 28, as shown in Figure 22. In this figure, terms not already introduced are defined as follows ... 

In preparing this figure, the authors of Ref. 28 assumed no wave attenuation through the wall thickness H, so Pr and ir are the normally reflected blast loading parameters on the loaded side of the wall or slab. 

Hokanson, J. D., Esparza, E. D., Sandoval, N. R., and Baker, W. E., "Effects of Combustibles on Internal Blast Loads on the Damaged Weapons Facility, Final Report for Phase IV," Final Report on P.0. F0913400 for Mason Hanger -Silas Mason Co., Inc., Pantex Plant, Amarillo, TX, Oct. 1982. 

Reinforced concrete is the most commonly used construction material for structures designed to resist explosive blast loads. It is used extensively in blast hardened structures because of its strength, ductility (when properly designed), mass, penetration resistance, relative economy, and universal availability. Its strength, mass, and ductility provide high resistance to the extreme blast pressure (psi) and impulse (psi-ms) loads. It is important to remember that (unlike in static load design) in the... 

---