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Blast Resistance of Door & Frame Assemblies by Roy Canon, Ph.D.

Vice President Technology Deansteel Manufacturing Co.

A blast event is characterized by a very sudden, almost instantaneous, pressure rise to a peak value followed by a decay in the pressure magnitude to ambient level (see Figure 1). Figure 2 illustrates an idealized blast wave. The blast pressure rises instantaneously to a peak value, Pmax, expressed in pounds per square inch (psi), and decays linearly to zero. The time required for the decay is the duration of the blast wave, td, expressed in milliseconds (ms). The impulse, I, expressed in psi-ms, is the integral of the time-pressure history of the blast wave equation (this is represented by the area inside the pressure-time plot). The magnitude of the impulse can be computed from the following formula for an idealized blast wave:

Pmax td I =

2

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The response of building components such as door and frame assemblies subjected to blast loading is affected by both the magnitude and duration of the blast load as well as the stiffness, mass, and ultimate load bearing capacity of the component. For a given structural member, many peak pressure and impulse combinations will cause a given maximum deflection in the component. This can be understood by considering that a low peak pressure/large impulse (or long duration) blast load and a high pressure/small impulse (short duration) blast load can both cause the same maximum deflection in a given component. Likewise, a mid-range pressure and impulse load can also cause the same maximum deflection in a component. If a building component, such as a hollow metal door, is subjected to a variety of peak blast pressure and blast duration combinations and the on-set of permanent deformation of the doors is measured, the results can be plotted as a Pressure-Impulse, P-i, diagram. The resulting graph would be similar in shape to the one shown in Figure 3 although the actual vertical and horizontal position of the graph would be a function of the strength of the individual door as determined by width, height, thickness and reinforcements.

Figure 3 General P-i Diagram The graph illustrates that the tested doors do not experience any permanent (i.e., plastic) deformation for low pressures and impulses (the region to the left and below the P-i curve). Higher blast pressures and/or impulses would result in permanent deformation of the doors (the region above and to the right of the curve). The degree of permanent deformation would depend on the applied blast loads (both pressure and duration). A series of curves can be developed for a structural component by determining the blast pressure and impulse levels that result in a similar degree of deformation (e.g., maximum deflection or rotation) of the component. Figure 4 shows a series of three curves for an actual 3-0 X 7-0 Blast Resistant Door design for various levels of response to blast pressure and impulse combinations. These curves

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present the pressure/impulse combinations that result in the same level of rotation at the edges of the door panels. The resulting zones representing the same level of deformation (or no deformation) have been categorized based on the qualitative performance of the doors.

Figure 4 P-i Diagrams for a 3070 Door Panel There are four blast resistance performance categories for door and frame assemblies defined in ASTM F2247-03. They are summarized in the following table.

Table 1. Blast Resistance Performance of a Door Assembly per ASTM F2247-03

Category I The specimen is unchanged (no permanent deformation) after the loading incident and the door is fully operable. The specimen remains intact and responds elastically.

Category II The door is operable but measurable permanent deformation to the door panel exceeding 2 mm (5/64) has been experienced. The specimen remains as an integral system.

Category III Non-catastrophic failure. No structural failure occurs to the specimen that prevents the specimen from providing a barrier to blast wave propagation. However, the specimen is permanently deformed and the door panel is inoperable.

Category IV

The door panel is severely deformed. For a seating load test, the deformation of the door panel must be limited to a level that does not cause the door panel to forced through the door frame opening. For an unseating load test, the latching mechanism is permitted to fail, allowing the door to swing open; however, the door panel shall remain supported by the hinges and it is evident that the door panel will not become a flying debris hazard.

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From the graphs in Figure 4, it can be seen that an 8 psi blast pressure having a 100 psi-ms impulse on one of these doors would produce a Category II response. The door would be permanently damaged, but it would still be functional. From the foregoing information, one can see that to fully specify the blast resistance level for a door assembly more information is required than the anticipated peak blast pressure. The blast impulse (or blast duration) is equally important. Furthermore, the desired performance level of the door unit must be specified (i.e., how the door unit responds to the projected blast event). A Cateogory I response would allow reuse of the door unit, but it may prove to be an excessively expensive option for many projects where higher blast loads are potentially possible. For most applications, a Category II response would be desirable to provide protection from the blast event and permit operation of the door after a blast event. Additional information on blast resistant door units can be found on our company web site: www.Deansteel.com, or you may contact Sales@Deansteel.com.