Earth Covered Magazines (ECM) are a common feature at munitions storage locations. These facilities are often categorized as “7-Bar” or “undefined.” But what does this mean? In a nutshell, it is the level of blast pressure that was considered in the design of the facility. This article will define these terms and explain why they are important.
ECMs are specialized structures that offer additional protection to stored munitions in terms of physical security (e.g., theft or terrorism) as well as blast and fragment hazards from nearby explosives events. For additional information on the purpose and history of ECMs, see our blog post Explosives Safety 101 – We’ve Got You (Earth) Covered.
The US Whole Building Design Guide webpage for Ammunition & Explosive Magazines provides a qualitative description of a 7-bar ECM as providing, “the highest level of blast resistance” and allowing the use of “the least restrictive siting separation distances,” but it does not define what 7-bar means or why it is used. To understand more, we need to first describe a few different parts of an ECM.
In terms of design, ECMs have two major components: the headwall, and everything else. (The roof also requires specialized design in some cases, but more on that in a minute). The headwall is the door face; the side not covered in earth (See Figure 1). But importantly, it is not the entire front face. Figure 2 defines specific subcomponents of that front face. The headwall is the portion directly in front of the storage chamber. From a design perspective it does not include the wingwalls to the side. The wingwalls are retention walls to support the earth berms. They are not designed to withstand blast loads.
Figure 1 - Typical ECM Headwall
Figure 2 - Typical Components of an ECM (courtesy of UFC 4-420-01)
So What Does 7-Bar Mean?
In the event of an explosion near an ECM, the sides and rear of the ECM are significantly shielded by the earth berm. The roof also experiences some shielding from the mass of the soil above the magazine. In contrast to this, the headwall and doors receive the incoming blast wave without the benefit of the mass of any soil. For this reason, the headwall and doors have to be designed to a higher standard and minimum blast loads.
The level of blast loading considered in the design of the headwall determines the headwall rating. The term “bar” as in “7-Bar” is a metric measure of pressure. One bar of pressure is equal to 100 kilopascals (14.5 pounds per square inch (psi)), which is slightly less than 1 atmosphere of pressure (0.987 atmospheres). The design value of 7-Bar is equal to 101.5 psi. A magazine designed to receive this rating must be capable of withstanding this pressure in a dynamic blast analysis, as well as an impulse load that is based on the donor explosive weight being considered. More information on these requirements, as well as the required conventional design loads, can be found in Section V2.E5.5.2 of DESR 6055.09.
What About ECMs That Don’t Meet This Standard?
“7-Bar” is a designation given to ECMs that have been designed to withstand this extreme loading. What if the facility cannot meet this standard? In this common scenario, the headwall is classified as “Undefined”. The facility is still classified as an ECM and qualifies for reduced required separation distances in most cases, other than when the headwall facing another Undefined headwall ECM or other Above Ground Magazine (AGM) structure. The vast majority of existing ECMs are legacy structures built more than 75 years ago and are classified as Undefined ECMs.
A third category of “3-Bar” exists in the criteria. This classification is given to ECMs whose headwalls have been designed to withstand 3-Bar (300 kPa or 43.5 psi) of pressure.
Facilities with this classification are extremely rare. A new type of prefabricated steel box ECM has been designed by the Armag Corporation and TSG’s Dr. Tyler Ross, and is under final DoD review. This would be the only available 3-Bar ECM allowed for new construction.
What Do I Do if I Don’t Know the Headwall Designation of an ECM I’m Siting?
It is possible that you are analyzing an explosives site plan that does not have complete information. What if an earth-covered facility exists but you do not know if it qualifies to be an ECM, or what its headwall designation would be?
The first thing to consider is that a headwall designation of 7-Bar has to be proven through analysis. It is usually only very robust doors that meet this standard. If the structure is a legacy facility and has a basic security door (e.g., a single ¼” steel plate), it is almost certain that this is an “Undefined” headwall. If you have a facility with a robust door, measurements can be taken for comparison to approved ECM designs. Government organizations can contact the Corps of Engineers for verification. The Schreifer Group is also available to support these requests.
The interior shape of the magazine roof determines the type of analysis required for that component. Currently, if the roof of the magazine is an arch shape, as is common for many legacy structures, there is no blast load analysis required for the design of that component. If the roof of the magazine is a flat-slab, consideration of a minimum blast load is required in the design.
Only 7-Bar ECMs are approved for new construction (unless a 3-Bar ECM type is approved in the future).
The strength rating of an ECM headwall only applies to exterior loading (blasts from nearby that strike the exterior face of the headwall. ECMs are NOT designed to contain blast loads.
Contact The Schreifer Group
If you would like to learn more about these topics or how we can help with your site planning needs, please contact The Schreifer Group via our online contact form or visit our website to learn more about how we can help. Our growing team of explosives safety SMEs are ready to support you. For more insights, check out our other articles on explosives safety topics.