Principles of shelter design
How to design an effective and safe underground blast shelter

There are standard practices used in the design and construction of shelters that are fundamental to effective underground shelters. Please allow us the opportunity to introduce these practices:

Location - choosing the perfect spot for your shelter
We see a lot of shelters going in under and adjacent to houses. We believe the advantages of having shelter access from within your home outweigh the disadvantages. If you can enter your shelter without leaving your home, you greatly expand the utility of your protected space. Your shelter can also function as a panic room in the event of a home invasion robbery and can be more easily utilized for food storage and a vault for valuables. You also have electricity and water right at your shelter site. The only major disadvantage is the possibility of a structural fire in your home while you are in the shelter.

It is standard practice to draw air from the outside - away from the home, and discharge the air into the home - preferably near the door between the shelter and the rest of the basement. If you are discharging filtered air from your shelter into an adjacent basement room or up a stairwell to the first floor of the house, you will have that room protected from airborne threats as well because all of the air in that room will be flowing outward from the shelter. If you have an airtight door at the other end of that adjacent room, you will have created an airlock. As long as you only have only one of the doors in that airlock open at a time - and the NBC filter is on, you will ensure that airborne toxins do not enter your shelter.

We believe that every shelter should have at least two egress (exit) locations. One of these can be an emergency escape hatch. It is much easier to equip and supply your shelter through a blast door, than through a blast hatch and down a ladder - this is another advantage of having basement access.

Construction - what you should know before you build
There are many types of underground blast resistant shelters. All have their advantages and disadvantages. Here is our take on the three main types available: steel reinforced concrete, prefabricated steel, and prefabricated fiberglass.

Steel reinforced concrete shelters are utilized in every well known bomb shelter constructed by a government with a nearly unlimited budget to build a protected space. The materials are relatively inexpensive, but the installation time and labor costs are more than the other two major types. An arched roof is desirable as it conducts the load to the walls more efficiently and it is harder to create an arched roof with this concrete, but the less expensive materials mean that you can use more of them – and they are very good materials at stopping radiation. The porosity of the concrete demands that waterproofing is an issue that must be dealt with – but this is a known issue that can be prevented.

Prefabricated steel shelters can be designed and built to create a very effective shelter. They feature low installation time which lessens your exposure when installing a shelter. Corrosion becomes an issue that must be dealt with - but this is a known issue that can be prevented. Shelters constructed out of steel culverts have internal space limitations, but they have a couple of very real advantages – culverts usually have an engineered load rating from the manufacture and they have an inherent arch to their roof.

Prefabricated fiberglass shelters have a low installation time and can easily integrate an arched roof in their design. They do not suffer from corrosion or water issues, but their main disadvantage is price and size limitations. They are very labor intensive to build and must be shipped from the factory.

A majority of the shelter projects we get involved with are integrated with new home construction. If you are going to build a home, building a shelter underneath or adjacent to your home and incorporating the cost and onsite skilled labor into the home construction will save you time and money.

The layout
We like to see shelters have the following features:

• At least two egress (exit) points. One can be an emergency exit that you have to dig your way out of, but only having one egress point is dangerous. Most people do not notify anyone when entering their shelter and debris or a malicious person blocking your only egress point leaves you without options
• An airlock. The easiest way to incorporate an airlock is to put your NBC filter as far away from the door as possible, flow the air through your shelter toward your door, then out a blast valve adjacent to - or above - the door and out into a stairway or a hallway that has a gas tight door on the other end. You will need another overpressure valve adjacent to that gas tight door, but if you set it up this way, all the air in that hallway or stairway will be flowing one way - out of your shelter.
• No dead spaces in the airflow. The air comes in through the intake pipe, goes through the Safe Cell, and is introduced into the shelter out the top port of the Safe Cell. From here, it finds its way to the overpressure valve on the outflow blast valve. Where you place the filter and the outflow blast valve determines exactly where it "finds it's way." They should be at opposite corners or opposite ends of your shelter. If you have multiple rooms that the air flows through, it is standard practice to have an overpressure valve between the rooms. There is a limit to how many overpressure valves you can push air through - you may be doubling the pressure with each valve and more pressure means less airflow. The valve should be mounted alternately high and low. Storage rooms need less airflow than living spaces.
• Command post. There should be one place in the shelter where your detection, observation, and communication equipment is located. One spot that one person can be stationed at to deal with anything going on outside the shelter.
• A mechanical room. This is optional. Most large shelters that we consult on and provide equipment for have a mechanical room where the filter is located. One advantage of this design is that your critical life support equipment can be locked up. If you have someone in your shelter that you do not complete trust or do not completely trust their judgment and mechanical aptitude, you can ensure that they do not decide to "change the filters" in your Safe Cell while you are asleep.
• Kitchens and bathrooms. Every shelter should have at least one of each. Things to look out for: vents and drains that may leak filtered air out, vents and drains that are not protected from nearby detonations with a blast valve, and carbon monoxide being generated by cook stoves. If you place your toilet by the outflow blast valve, we can plumb the toilet vent between the blast valve and the overpressure valve. This allows the toilet to draw air into it so it flushes properly and saves having to install a dedicated blast valve.

Blast protection
Most underground shelters have the structure to resist blast pressures, but their portals to the outside need protection as well. It makes no sense to have a shelter that will withstand high pressure blast waves and not install blast doors and blast valves. A detonation imparts several pressure waves – incident (direct), reflective, and a combined incident and reflective wave called the Mach stem. These high pressure waves blow the atmosphere out from the point of detonation creating a momentary vacuum - a dramatic drop in air pressure. This vacuum can be dangerous to equipment and occupants of a shelter.

Blast valves are devices that install over the ventilation pipes inside your shelter. They are normally open to low pressure air moving in both directions. When a high pressure wave comes over your shelter, the valve automatically shuts. When the vacuum that follows the pressure wave comes over your shelter, the blast valve will also close. Then they will return to their neutral position - letting the NBC filter move low pressure air into and out out of your shelter.

Blast doors should have a pressure rating and a rebound load rating. To handle as much pressure as possible, all swinging blast doors should swing outward so the pressure load is transferred directly from the door leaf to the frame. If the door were to swing inward, the load would have to travel from the door leaf, through the latches and hinges, and then to the frame. It is more efficient to have the door leaf act as a bridge between the sides of the frame and take the load in the seated condition.

The rebound load rating need not be greater than negative 14.5 PSI (1 bar). This is the standard air pressure at sea level - when you remove the air, you can only go down to a vacuum (zero pressure). The latches and hinges need to be engineered to take this negative 1 bar load without failing.

Air Filtration
Once you go to the time and expense of constructing an effective blast shelter, air filtration must be considered. The only real choice for collective protection is a Nuclear, Biological, and Chemical (NBC) overpressure air filtration system. The NBC filter elements consist of a pre-filter to collect the large particles, an HEPA filter for the fine particles, and a carbon adsorber for the gases and vapors. It is connected to the blast valve that is mounted over the air intake ventilation pipe.

The HEPA filter is tasked with removing the particulates and aerosols from the airstream. There are a lot of types of “HEPA” filters available. Not all of them are suitable for protection against radioactive fallout. The best filters are individually scan tested. They are individually challenged with an inert aerosol that duplicates the properties of what you are trying to protect from – such as dioctyl pthalate (DOP). If you have a HEPA filter without a serial number, chances are good that you have one that has not been individually tested.

The carbon adsorber is placed after the HEPA filter. It’s purpose is to adsorb the toxic gases present in the airstream that flows through it. American Safe Room uses the same carbon as the US military uses in their collective protection filters. The residency time of the air in the granular carbon bed must be engineered so that the carbon has enough time to adsorb the toxic vapors present in the air. Room air filters with shallow carbon beds for "odor control" are not suitable for shelters.

An effective NBC filter will have a way to operate the filter when the power is interrupted. Most have a manual (hand operated) backup system. The Safe Cell also features a battery backup system that will keep the filter in operation for up to 24 hours with a user supplied battery that is maintained by the power supply inside the filter housing. It has been evaluated by the US State Department and found to be a Class IV Munition because of it's level of air filtration. Another credible set of standards are the ones that are met by Israeli air filtration equipment.

Once you have proper filtration, what you do with that stream of filtered air is critical. To ensure that all of the air in your shelter is flowing outward - and not letting toxins migrate inside, you need to have positive pressure in relation to the outside. This is known as overpressure. The US Army Corp of Engineers considers .3 inches of water gauge a Class 1 Toxic Free Area. This is relatively low pressure – 0.0108 PSI, but it is enough to ensure that toxins do not migrate into your shelter through any cracks.

Ventilation
Ventilation is how you move air from the outside, flow through, and then back out of your shelter. You can do this with filtered or fresh air. Proper ventilation replenishes the oxygen supply and removes the carbon dioxide and moisture that the occupants exhale. Ideally, you should bring the air into your shelter in one corner and expel it in the opposite corner. This is not always possible, but you should position the intake and outflow ventilation pipes as far apart as possible to avoid short circuiting the airflow and creating dead spots that do not get sufficient air exchanges.

If you have multiple rooms in your shelter, the air should be routed through as many of them as possible. The room that the NBC air filter is located should have an overpressure valve between it and the rest of the shelter in case you lose your grid power, then exhaust the battery, and are down to hand powered filtration and ventilation. When powering your filter by hand, you want to have as small of a space as possible to protect. The overpressure valve helps the filter to create and maintain overpressure by restricting and regulating the outflow of air - it opens to release air only when the pressure in the room gets up to a certain pressure. It also acts as a check valve, not allowing air to flow back into the protected space.

Once the air leaves that room, it should flow throughout the shelter until it is expelled through the outflow system – which should consist of an overpressure valve, a blast valve, and an outflow ventilation pipe if the air is expelled outside of the shelter. If there are multiple rooms, the vents between the rooms that the air moves through should be on opposite corners and at different heights in order to get the best ventilation.

Underground shelters are notorious for being damp and have odor issues. We manufacture an automatic shelter ventilation system that utilizes the same intake pipes and valves that the Safe Cell uses to provide daily ventilation of your shelter. It features a flexible 24 hour timer that will automatically give your shelter several air exchanges a day – relieving the moisture and odors while replenishing the oxygen.

Putting it all together
If you are planning a shelter, we can make recommendations on equipment and airflow. A plan view drawing of your shelter is the best place to start. You can contact us by e-mail at sales@mailasr.com or telephone 541-459-1806.

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