Residents in fire-prone areas face a classic Catch-22 situation. The local Council will not issue a building permit for a fire-proof bunker because no standards or guidelines are available. Professional builders are reluctant to build such structures for this reason, and because these projects are relatively small. It is possible that such guidelines will be available some time in the future, but I want a bunker before the next fire season starts. Therefore I will have to build it myself. As I do not need a permit to build a wine-cellar, I am in the process of building a fire-proof wine cellar.
There are commercially available shelters, but each design has its problems. More importantly, access to my site is difficult for anything larger than a 4WD plus small trailer. Therefore I am building this all by hand (with the assistance of a jackhammer and concrete mixer).
The most promising location for the bunker (sorry, wine cellar) is in the gap between the concrete water tank and the embankment into which it is excavated. Either I'm getting into weathered granite at the outer limits of the excavation, or I'm getting more tired as I go. I am aiming for a concrete structure 5m long, 1.9m high and 1.2m wide internally. This would provide enough air for four people for six hours. It needs to be relatively airtight, and will be positively pressurised from an air compressor during a fire to exclude smoke.
I have cobbled together a design (below) from a couple of sources. By my reckoning what I need is a reinforced retaining wall on one side to hold back the earth, a 100mm reinforced roof to hold up a foot of soil, and an unreinforced wall on the other side to hold up the roof. I don't want to use the water tank for the other wall as there are already several hairline cracks in the tank wall that weep water. (These cracks eventually repair themselves but I don't want excess stresses on the tank wall). The structure needs to be strong enough to withstand a 30m tree falling on it (I have cleared around the site, but the trees just won't stop growing).
I would appreciate any comments on the proposed design, but in particular:
1. Is the reinforcing sufficient?
2. Do I need to reinforce the right-hand wall?
3. The current design uses hollow half-width and half-height blocks at the top to support Bondek to form the concrete roof. Can I get concrete down the hollows of these blocks when I am pouring the roof? (The full-sized blocks in the wall would be poured before the roof).
4. I had intended to embed N12 rods into the top of the wall, then bend them over to reinforce the roof. This means that I have to stop the Bondek short of the half-width blocks as shown in order to leave room for the bars. Will that leave enough room for the Bondek sheets to be supported adequately?
5. Can I lay N12 reo along the very bottom of the Bondek, to get it into the high-tension part of the roof?
6. Is there a simpler way to do the roof?
Hopefully this is a project that I will never need to use, but if it is used, it needs to work.
I would greatly appreciate any help.