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===General features===
[[File:PanoBunker.jpg|thumb|250px|right|A bunker on the island of [[Texel]], in the Netherlands.]]
The doors must be at least as strong as the walls. The usual design is now starting to incorporate [[Vault door|vault doors]]. To reduce the weight, the door is normally constructed of steel, with a fitted steel lintel and frame. Very thick wood also serves and is more resistant to heat because it chars rather than melts.{{Citation needed|date=December 2008}} If the door is on the surface and will be exposed to the blast wave, the edge of the door is normally counter-sunk in the frame so that the blast wave or a reflection cannot lift the edge. A bunker should have two doors. Door shafts may double as ventilation shafts to reduce digging.
In bunkers inhabited for prolonged periods, large amounts of [[ventilation (architecture)|ventilation]] or [[air conditioning]] must be provided in order to prevent ill effects of heat. In bunkers designed for war-time use, manually operated ventilators must be provided because supplies of electricity or gas are unreliable. One of the most efficient manual ventilator designs is the [[Kearny Air Pump]]. Ventilation openings in a bunker must be protected by [[blast valve]]s. A blast valve is closed by a shock wave, but otherwise remains open. One form of expedient blast valve is worn flat rubber [[tire]] treads nailed or bolted to frames strong enough to resist the maximum overpressure.<ref>{{cite web |url=http://www.oism.org/nwss/s73p939.htm |title=App. D: Expedient Blast Shelters |publisher=Arnold Jagt |access-date=2010-06-10 |url-status=live |archive-url=https://web.archive.org/web/20110525040356/http://www.oism.org/nwss/s73p939.htm |archive-date=25 May 2011}}</ref>
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