# Forum Home Renovation General Odds & Sods  Help with design for fire bunker

## GOM

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).  bunker8.pdf 
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. 
Thanks, GOM

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## Dan574

What are you using for a door? 
How deep will the earth be on the roof? 
Take a look here for more info,  http://candobetter.org/node/1047

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## rrobor

You are overlooking the biggest killer and that is the force exerted by the fire. Your shelter must be airtight for as long as the fire is above you otherwise it will suck out all the air from your shelter. Building something like that creates another problem, airtight doors and children dont mix. Thats another problem to solve. Please consider these issues and dont build a death chamber.

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## GOM

Thanks for the comments: 
Dan574; the door will consist of a metal square-tube frame with sheet metal skin and insulation in the core. Yes, that would get hot, but the door will open inwards and have refractory bricks attached to the outer skin, fitting into the door jamb. That way the door would not be directly exposed to heat. (I want the door opening inwards so it can't be blocked by debris. I think this is a failing in many of the commercial designs). There will be about 300mm of earth on the roof. 
Rrobor; you're right, I need to be very careful with the door. I will be making a steel door handle that can be operated from both sides, and can't be locked from either side. There will also be a 400 x 600 mm window at the far end (similar in design to the door) which will be large enough for emergency exit. The door (and window) will be sealed with the fibreglass rope used for slow-combustion stoves. As far as I can see, if the fire-front produces lower air-pressure outside, that will assist in keeping smoke out of the bunker. I will also have an air-compressor pre-filled with air inside the bunker from which I will slowly release air. This should assist in maintaining positive pressure inside the bunker. Once the fire-front has passed, I would expect any pressure differential to dissipate, then we can come out. 
Cheers, GOM

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## rrobor

Dont worry about keeping smoke out, if you can get smoke in, the oxygen will get out and within a few minutes you will be dead. For me the back window would be at the end of a 2 m passage, that way its not going to explode due to heat and probably the front door as well with a passage to that. All this though depends on the fuel you have for the fire, A grass fire and its overkill, in the middle of a forrest its not

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## woodbe

Good for you for taking action on your own behalf. 
The crazy thing about this is that a bunker is a simple device and could be mass produced by the concrete tank manufacturers and dropped on site for a few grand. Instead we have this crazy industry with architects and fancy designs that cost a fortune - and no building reg support. 
I agree that the doors should open inwards, or open outwards under a heavy overhanging roof. An emergency exit is a good thing too. Remember that your steel door jambs will get very hot, if you need to exit the emergency exit while it is hot you will need protection of some sort or you will fry. 
Also, what about the doorhandles. If the fire is outside, they will probably be red hot.... 
With the doors sealed and opening inwards, if you need to open them during a fire, you might need some way of de pressurising the room as the internal pressure may hold the doors shut. 
How big a compressed air cylinder will you have? It may need to be quite large if you want to maintain positive pressure not just supply breathable air... With a bit of tinkering, you could possibly bury this outside the room in the bank or between the bank and the bunker. 
I don't know anything about the structural requirements. I have thought about building one out of hebel, or home-made tilt-up panels. Mine would require an excavator, but I do have access, and a position close enough to the house that it could be completely underground and connected to the house by a short underground accessway from the basement. 
woodbe.

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## Gaza

the current issue of owner builder magazine has a good write up on the do's and dont's of building a fire bunker

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## seriph1

Hi GOM and welcome to the forum   -   feel free to PM me requesting contact details if you'd like to have a discussion regarding this issue. I don't sell fire bunkers (or anything else really) but may be able to offer some insights, being one of the regional community reps for our area's fire victims. AREA: Mitchell Shire.

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## GOM

Thanks for the further replies. As there seems to be some interest in the door arrangement, I have attached a plan:  Bunker3_70.pdf 
Woodbe; I won't have steel door jambs. The door will seal on the rear face of the besser blocks. The doors are designed to be like an inside-out industrial or laboratory oven, with refractory bricks on the side of the door facing the fire. For the concrete blocks surrounding the door I will probably use Boral's "Scoria Blend Blocks", which have a 240 minute fire rating. The rest of the blocks will be normal blocks, either covered by earth or up against the water tank. 
I don't think the door handles will get red hot. The door will be under a porch with a concrete roof and floor. More importantly, the natural slope of the land means that the door will not be in the direct line of any significant burning vegetation, so the door should not be subject to overwhelmingly high temperatures. I may make some additional block "baffles" in the entrance corridor. I think I need a steel door latch so that it is pretty well indestructible, but I may be able to come up with some form of refractory cover for the handle (I have seen some hollow refractory tubing). 
My air compressor is relatively small (24 litres), which should release about 190 litres of "decompressed" air. This is not a great volume, but I think even a small slow release of air from within the bunker will help to keep it positively pressurised. 
Gaza; thanks for the tip - I bought a copy of the magazine today. (I have had some previous correspondence with the author). 
Rrobor; I think that the design will allow me to keep smoke out and breathable air in (at least long enough for the fire front to pass. 
Seriph1; I'll try to make contact shortly. 
Cheers, GOM

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## Gaza

> My air compressor is relatively small (24 litres), which should release about 190 litres of "decompressed" air. This is not a great volume, but I think even a small slow release of air from within the bunker will help to keep it positively pressurised.

  chances are you will have no power to run the comperssor.

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## GOM

Gaza,   

> chances are you will have no power to run the comperssor.

  My plan is to charge up the compressor when I know a high fire-danger day is approaching, disconnect it from the hoses in the shed, then dump it in the bunker. It should hold its charge for several days, then if nothing happens I'll drag it back to the shed. The compressor's only used infrequently, so that shouldn't be a problem. I'm not planning on any power being available on the day 
I also have an idea about converting some of my old out-of-date 9kg lpg bottles so that they can be filled with compressed air for the same purpose. (Yes, I know, several dangers there). They should hold a charge for years, and could be just left in the bunker permanently. 
By the way, I have read reports about the oils in compressors possibly being a health hazard (although less hazardous than suffocating from smoke inhalation!). If I think that's a problem, I may pipe the air through one of the CIG gas mask canisters rated for vapours. 
Cheers, GOM

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## woodbe

GOM, 
It sounds like you have given this a lot of thought and it shows in the design choices you have made. I understand about the doorframes now, but I would still have some heavy hessian or similar sacks around in case you needed to escape via the window. 
The biggest deficiency I see with the design is the compressor. A challenge for you will be achieving an adequate seal on the door and window to allow you to pressurise the room by even a small amount. 190 Litres of air is not enough in my opinion to keep that room pressurised even with a good seal. I think you would need thousands of litres.  
The air out of the tank will have odours to it. One way of fixing that would be to use multistage filtration to remove water/particles/oils, but that would cost money and probably still smell. Perhaps you could buy some scuba tanks on ebay and get them filled with air each season - at least then you would have breathable air. You could also get a decent sized medical air cylinder from BOC - annual hire would probably cost more than the air inside, but again, it would be guaranteed breathable and lots of it. 
woodbe.

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## rrobor

Im not keen on your idea as to the air compressor, its one of those things that could be missed in a panic. How about a row of old gas bottles in a trench covered by heavy concrete pavers  coupled to a hose in your bunker. My local tip has a good selection of those. a bit of cleac out and a bit of plumbing they should be fine at medium pressures.

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## GOM

> 190 Litres of air is not enough in my opinion to keep that room pressurised

  Woodbe; I may have misled you with the term "positively pressurised". I'm not aiming to maintain a very high pressure in the bunker, just make sure there is an outward flow of air. All else being equal, if I release 100 litres per hour into the bunker, it should slowly leak through the door seals, making it impossible for smoke to come in. You're right that scuba tanks could be used. They are under much higher pressure, and so would release a much greater volume of air.   

> How about a row of old gas bottles in a trench covered by heavy concrete pavers  coupled to a hose in your bunker

  Rrobor; I had mentioned old LPG bottles earlier. I'm not keen on hoses/cables etc passing through the bunker walls. Some people have reported flames burning through 240v cables passing through bunker walls, producing flames "like a blowtorch" inside. I prefer to leave the bottles inside the bunker. 
Cheers, GOM

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## woodbe

I think you would be right to make the room as airtight as you can, and have a clean air source in the room. During a firefront, there will likely be very high winds. If your room is not airtight, and there is minimal pressurisation, it is possible for the wind pressure on one side of the bunker and vacuum on the other to cause smoke ingress. So for my money, I'd be looking for airtight door and window with a controlled leak away from the doors so that the room does not overpressurise or become toxic with Co2. 
To achieve airtight door and window, you could use a rubber or silicone seal on the flange but outside the firebox seal you already have shown. 
If this is not possible, then you could equip the room with breathing masks and just let the smoke leak in. You would still need an adequate quantity of compressed air. It shouldn't be too long before it will be safe to go outside again anyway. 
You could even put tanks under the floor. 
woodbe.

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## Dan574

another option for your air supply would to go for scuba tanks.  Just looked on ebay and there are some there.  Im not sure how long the oxygen will last in the tank, as far as posing a danger, if they are in danger of exploding then I would suggest you wouldnt be around to know anyway. 
You could also make a small fire proof box to put them in with the hoses coming out.  Just another thought.  Alot easier than a compressor.

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## GeoffW1

> Thanks for the further replies. As there seems to be some interest in the door arrangement, I have attached a plan:  Bunker3_70.pdf 
> My air compressor is relatively small (24 litres), which should release about 190 litres of "decompressed" air. This is not a great volume, but I think even a small slow release of air from within the bunker will help to keep it positively pressurised. 
> Cheers, GOM

  Hi, 
I would forget the compressor and use a scuba tank or 2. They will hold the air fill indefinitely (although they must be in current test before filling) and the average tank will release 2500 litres. 
Cheers

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## GOM

Woodbe;   

> you could use a rubber or silicone seal on the flange but outside the firebox seal you already have shown

  Not a bad idea. I had thought of the sealing rubber used on kitchen ovens (probably only rated to 300C, but that should be high enough). You mentioned breathing masks, and we have 2 sets of these which we plan to use before the fire front hits. No doubt we will take those with us into the bunker when we retreat. 
Dan574, GeoffW1;   

> I would forget the compressor and use a scuba tank or 2. They will hold the air fill indefinitely (although they must be in current test before filling) and the average tank will release 2500 litres.

  Thanks for the info on scuba tanks. I had not realised that they would need to be in current test in order to be filled. 
Nevertheless, an alternate air supply inside the bunker is just the cream on top. People have survived the bushfire by sheltering in wombat holes (see http://www.babble.com.au/2009/02/11/...n-wombat-hole/ ) or in home-made bunkers where the door failed completely (see http://www.babble.com.au/2009/02/09/...from-bushfire/ ) 
All the comments have been useful and relevant, but doesn't anyone have any thoughts on the original questions? Basically I want to know whether you think the whole thing is going to fall down on top of me, and is there a simpler way of doing the roof? 
Cheers, GOM

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## Dan574

Forget about using concrete I think I may have found you a solution, hebel blocks for the walls and hebel panels for the roof.  http://www.hebelaustralia.com.au/res...eshelters.aspx  http://www.hebelaustralia.com.au/why...fireareas.aspx

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## seriph1

suitable products include: Hebel; concrete in a range of styles/forms; RAPIDWALL  -  locally made, super fast, inexpensive - I have personally seen an oxyacetylene torch held on it for 10 mins without failure; timbercrete  -  phenomenal fire resistance and locally made; BENEX block  -  made in NSW and rates well 
for the roof  -  RAPIDWALL will do it I believe, but cannot confirm 100% at this stage. I believe it is being used for flooring and if you filled it with concrete, carries a 240 minute rating from memory. It has been used on one build near Kilmore, as a roofing/ceiling material, with colorbond fixed to it. 
hope this is helpful

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## seriph1

the best fire plan we have come up with is to be somewhere else next time..... but a bunker is a great idea for anyone who can't leave, which was definitely the case for a lot of folks around here

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## rrobor

Fire on black Saturday was movning  faster than some people could outrun it in a car. Its very easy to think your safe till its too late, so a backup plan is a great idea and gives you the choice to stay and fight. If or when it goes pear shaped to get cover. My  belief is heat is not your issue so any fireproof material  with some depth will do. I believe to be safe you need 20 minutes of air in a timber fire and 10 in a grass fire.

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## Dan574

I was up in the kinglake area after and saw a number of  plastic water tanks that half survived.  They melted down to the point where the water was.  It was amazing what survived and what didnt there didnt seem to be any pattern so anything you do will surely help. 
Correct me if im wrong but you need to survive until the front has passed, the radiant heat  and lack of oxygen are the killer .  Tackle those 2 and you have a chance of survival.   
So your idea of a bunker is a great idea and it sucks that it has taken the deaths of 200 odd people for the powers that be to do something about it.   
I hate to say it but I bet that 20 years down the track it will happen again as complacency sets in, I hope not though. 
Have alook at that hebel, I reckon thats the way to go, light weight, easy construction.  They just glue together, put down some strip footings, you wouldnt even need a proper floor, crushed rock would suffice.  You could even use the hebel panel for a door on a steel frame, make up a frame using angle or u shaped steel wide enough for the hebel to sit in then weld it in.

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## GOM

Seriph1;   

> the best fire plan we have come up with is to be somewhere else next time

  The problem with planning to leave is that it's impractical to depart on every day declared as one of high fire danger. Alternately, it's too dangerous to leave when you think a fire's about to hit. The fire might sneak up on you (for example at night). Our property was completely burnt out in the 1983 Ash Wednesday fires. The previous owners decided to leave when the power cut out (they only had an electric pump). Two ways out of the valley, and they were going to head down the valley. If they had continued that way they would have died. They were stopped at the front gate by the CFA and told to jump in the dam while the fire front passed through, which they did. The house survived because it had a favourable design. We plan to stay and defend, knowing that the house is inherently defendable. We have put in a pump (yes, petrol) at the dam, with 2" poly pipe underground, leading to 5 standpipes (all metal) surrounding the house. We have 2 x 25m fire hoses which will be used before and after the fire front, but we will retreat to the house during the fire front. The bunker is there just in case the house starts burning uncontrollably. It's close enough so that we can reach it fairly easily from the house (although I'm aware that some people have described such a feat as being quite difficult). I think it will encourage us to do everything we can to defend the place, knowing that we have a fallback position. 
The RAPIDWALL system looks very interesting as it could be cut to fit an odd shape. Do you know how you'd be able to fill a floor piece with concrete, given that it would be horizontal? 
Dan574;   

> hebel blocks for the walls and hebel panels for the roof.

  Thanks for the links. Yes the AAC hebel blocks look good for fire resistance. I'll probably use them (or something like them) for the exposed blocks around the door. The Hebel Powerwall would make a great roof, but the pre-existing steel reinforcing would seem to be a bit of a problem if I have to cut them to fit an odd shape such as I have (please don't tell me to change the shape - I've just spent the last 5 days digging it out with a pick and shovel).   

> You could even use the hebel panel for a door on a steel frame

  Now that's an idea.   

> you wouldnt even need a proper floor

  No, I'm no structural engineer, but the floor provides a "foot" to hold up the wall on the left, which is really a retaining wall. It needs to hold back several tonnes of water when the ground becomes saturated in the middle of winter (that's if we get rain again).   

> it sucks that it has taken the deaths of 200 odd people for the powers that be to do something about it.

  Yes Dan574, it sucks. In the USA there is a government agency FEMA that not only gives accreditation to commercial providers of tornado shelters, but issues its own designs (see http://www.fema.gov/plan/prevent/rms/rmsp453.shtm ) There are similarities to fire bunkers, but some crucial differences.   

> Correct me if im wrong but you need to survive until the front has passed, the radiant heat and lack of oxygen are the killer

  I think that radiant heat and smoke inhalation are the main dangers, not so specifically lack of oxygen. My very limited knowledge says that inhalation of smoke can kill you by causing damage to the airways and lungs, leading to difficulty in getting oxygen. I don't have any specific information, but I wouldn't think that the lowering of oxygen levels due to combustion would kill you (as I said previously, people survived by sheltering in wombat holes and dodgy bunkers). I think that the main aim is to provide a large, sealed space containing about 10 cubic metres of fresh air and protection from radiant heat, that won't collapse if a tree falls on it. Tricky little additions like air compressors and scuba tanks might be fun, but they're not the main aim. (Gee, I haven't even told you about my ideas for a periscope yet). 
Thanks for all these valuable leads. I have some more research to do!
Cheers, GOM

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## pauljygrant

Some great ideas.
As for the air compressor, would all members of your family in a panic situation know and be able to transfer the compressor to the correct place and operate it in a controlled manner?  The worst always happens when you are least prepared so any design must be operational by the least capable member of your family who may be home alone.
In my opinion, if you are going to the effort and cost of building the shelter, then buy and keep filled some scuba tanks.
One final comment - do you have any neighbours?  Assuming they know you have a fire shelter - human nature will encourage them to seek shelter when the fire front is close by.  Will it accommodate them and have you calculated extra oxygen allowances for more people?   If they got to the shelter before you, what happens if they release all the air from the compressor too quick and there is none left by the time you get there?
Paul

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## seriph1

RAPIDWALL could be filled by capping one end and filling with a sloppy slurry while the panel has a slight tilt. Once it cures, it can be simply lowered into its final position. They are also developing a reinforced flooring/roofing system now I am told, that has triangulated sections inside each cell. 
 -  I agree it isn't always possible to leave and I clearly stated that  -  but we will not be staying next time. We live on a main road so our capacity to go elsewhere is better than many and while I am not prepared to talk about it here, through my community work I am very familiar with each condition people faced in our region.  
Importantly, this was not just a day of extreme fire danger (though they are bad enough and I am NOT looking forward to the alerts that will come this year)  -  reports stated that what was coming was a day of convergent conditions rivaling the worst that had ever been experienced. 
  -  Bunkers are the way to go I feel. Thoughtful design and conversations like these are vital. Whether there are "officially approved" bunkers is irrelevant .... sensible, thoughtful people with skills will design/build what works. I for one will be amazed if governments come out saying they have a bunker that is approved .... 
On a far brighter note: I am typing this while warm and dry in my office as it absolutely PELTS down outside   -  has been raining super-hard for several hours now and we are most thankful.  :Smilie:

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## chipps

Have you considered it's use more than once? 
So many horrible scenarios, (Yep Ash Wednesday here also) but hate to see you holding tight in there and get a wind change away and then back again. Materials may not tolerate more than one exposure and then there's the pressure source already used. 
Mate. 
I'd be digging a bloody deep shaft with ladder and filling the bottom with water. 
Or buy a used armoured personel carrier. 
Best of luck

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## GOM

Paul;   

> buy and keep filled some scuba tanks

  Yes, well, a tank "in test" is around $400 second-hand. "Out of test" tanks are cheap, but can't be refilled. My plan is to leave the bunker unlocked (and in fact unlockable), so that I'd be leaving quite a bit of gear that would be vulnerable. At this stage I think my best bet will be to convert several dead LPG bottles to hold compressed air. A standard 9kg bottle has a volume of about 22 litres, not that far shy of my air compressor (24 litres). If I use out-of-date LPG bottles I could charge them with air, which should stay put for years, then leave them in the bunker. As they're practically worthless there'd be no temptation.   

> do you have any neighbours? Assuming they know you have a fire shelter - human nature will encourage them to seek shelter when the fire front is close by

  I see it as a great opportunity to weed out some of the riff-raff in the community. Maybe I'll throw a BYOO party (bring your own oxygen). In reality, the bunker holds enough air for 4 people for 6 hours. As it will probably only be needed for 30 minutes while the fire front passes, we can probably squeeze a few more in. I won't be advertising it though. 
Chipps;   

> Materials may not tolerate more than one exposure

  I think after the fire front has passed by the first time the fuel load will be minimal. Most of the effort in shielding the bunker is taken up by the earth on top and one side. The water tank is on the other side. I've read reports of concrete water tanks spalling off in the heat, but not failing altogether (and even if it did, there'd only be wet broken concrete left next to the bunker the second time around). That leaves the two end walls, but these are protected by being situated down a long trench. If they are made from 240-minute fire-rated blocks, I think they'll survive quite well. 
Seriph1;   

> RAPIDWALL could be filled by capping one end and filling with a sloppy slurry

  I'll look into RAPIDWALL a bit more closely. Your comments on leaving vs. staying are well made. Everyone faces different circumstances. 
Thanks for the input, 
Cheers, GOM

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## Gaza

this "bunker" will only need two hour structual fire rating at the very most, this is very easy to get using fire rated plasterboard, steel door frame and a fire door. 
for 120min you will need to sheet the walls & roof with four layers of 16mm fire rated plasterboard, to weatherproof i would cladd the exterior with 9mm compressed fibro cement or even 9mm villaboard & set all the joints with james hardie external compunds. 
the door frame will need to be custom made to suit the overall thickness of your wall, being 4x16mm plus 90mm timber frame plus 10mm CFC = 164mm.  the door will be 40mm thick it should be "free" on the inside so no one can get locked in.   you can buy 120min smoke seals that stick into the rebate of the door frame. 
i would suggest you order the door frame, door & lock (hardware) drop seal and smoke seals from a fire door company in one package. 
in owner build mag that suggested that you have vents that can be closed when the fire front is approaching. 
i dont think you will need air tanks if the shelter has enough air inside it to wait out the fire.

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## Ausyuppy

I saw this article. 
Good to see that the "powers to be" are aware of the problem of dodgy bunkers. By no means am I suggesting that your idea is GOM.   http://www.news.com.au/story/0,27574...-29277,00.html 
Cheers
Steve

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## rrobor

Sorry but as far as I am concerned the people that say you only need this or that can get stuffed. If I am a mole in a hole with a fire raging overhead  and Im in there with my family I want to know that the stuff above me will survive in hell for an hour. Was at the tip on Sunday  and they had large oxy bottles going there. For me Id put something like that in a trench, hard plumb them with copper and lead a pipe into your bunker with a compress valve and a release valve

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## GOM

Gaza;   

> for 120min you will need to sheet the walls & roof with four layers of 16mm fire rated plasterboard, to weatherproof i would cladd the exterior with 9mm compressed fibro cement or even 9mm villaboard & set all the joints with james hardie external compunds.

  I'm not sure how the plasterboard fits in with the design I posted. The floor, walls and roof are reinforced concrete with 300mm or more of dirt on top of the roof.   

> you can buy 120min smoke seals that stick into the rebate of the door frame.

  Do you know the name of these seals or where to get them? At the moment I am looking at the 20mm diameter fibreglass rope that is used to seal the doors on slow combustion wood heaters. 
Ausyuppy;   

> Good to see that the "powers to be" are aware of the problem of dodgy bunkers

  Yes, this topic is one of the focal points of the next phase of the Bushfire Royal Commission. Unfortunately, as the article you highlighted points out, "standards and protocols for accrediting personal bushfire bunkers ... could be fast-tracked in six months". Six months takes us through to the end of February. I (and many others like me) want my bunker finished before November. Hence the dilemma - there are no standards against which to judge any commercially available bunkers or any building proposal. Councils won't issue building permits because they have no proper way of assessing them. Additionally I know at least one supplier of the concrete-water-tank style of bunker who is worried that people might not install them underground as intended. 
Anyone who is trying to get an operational bunker before the fire season starts is therefore flying blind. Hence I greatly appreciate the comments that people have offered so far. 
Rrobor;   

> Was at the tip on Sunday  and they had large oxy bottles going there

  I'd caution anybody NOT to use oxygen under such circumstances. Oxygen in the presence of anything combustible can produce a very dangerous flame or explosion if there is a spark. Compressed air (either from scuba tanks or converted dead LPG bottles) is the go I believe. 
Thanks again for the responses. I've now just about finished digging the foundations (about 6 tonnes of dirt excavated by pick and shovel (poor access), so the next step will be to pour the concrete floor). 
Cheers, GOM

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## Dan574

How about posting some pics of your progress? 
Saw today at the royal commision that the chairman of standrds australia is reluctant to set standards for bunkers due to them becoming death traps after people neglect them.

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## woodbe

> the chairman of standrds australia is reluctant to set standards for bunkers due to them becoming death traps after people neglect them.

  Imagine that. 
Just like, let me see... 
Cars, electricity, roller skates, etc. etc. What a nanny state we have become! 
We can't do anything because if we do it wrong it will fail... Just like the fire prevention and protection services and devices we had last year. 
Woodbe.

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## rrobor

I and everyone else can understand that. You are making  life saving chamber. There are always those who want minimum code and those who want maximum. When it comes down to it, its the people who are going to seek shelter from hell who matter, so sorry some polly doesnt do it for me and I doubt the guy who posted here could care as to minimum code. He is protecting his genetic code, and man will move mountains to do that.

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## GOM

Dan574 asked for some photos of progress so far (I suspect out of a secret desire to witness someone else's pain and suffering). There's nothing much to show of the bunker, other than what I have dug out, but the photo below may give you a better idea of the overall setting. The bunker will end up slightly higher than the concrete water tank on the right. All the dirt has been stockpiled so it can be spread on top of the bunker once it is finished.   
It is difficult to get machinery into the site, so it has all been dug by pick, shovel and wheelbarrow. As the ground is passing into weathered granite at the bottom of the excavation, I've had to use a jack hammer. (I bought a very solid jack hammer on Ebay - cheaper than hiring one at $100 per day!)   
Against the wall of the excavation, a trench has been dug in two steps - down 300mm and 200mm below the top of the slab. Once filled with reinforced concrete, this will provide the strength to hold back the soil bank which will butt up against the left-hand wall. On the right-hand side there will be a separate Besser block wall abutting the water tank (as shown on the plans I posted on page 1). The finished bunker will be quite narrow, but long enough to give me sufficient air volume inside.   
The next step is to put up the form work for the concrete slab. This will be tricky as the slab will go virtually from the edge of the trench to the edge of the water tank. I haven't worked out how to put up some boarding that will allow me to screed off the concrete to a flat surface. Yes, I know, normally you'd have a foot or two of free space outside the formwork to allow your screeding board to be manouevered around, but I can't do that in this case. I'm tempted to place some boards within the area of concrete, use them to screed off flat, then take them out and patch up the resulting mess. Any ideas gratefully received. 
Cheers, GOM

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## chipps

Is it possible to use the tank's water to have a sprinkler system? 
Something that'll saturate the ground above the bunker and it's doorways just before or during a firestorm. 
Perhaps even design / incorporate a weak-spot into the side of the tank, triggered manually (Chain / wire in bunker) to dump. 
Probably best if tank was on high side of bunker though. 
Hopefully if you can make it airtight and strong enough, then the weight of the water dump and concrete debri will be ok. 
Yes my ideas are a weird, but offered with best intentions.

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## woodbe

I'm thinking about your retaining wall. 
We had to build one when we built our garage close, and lower than the road - so that the road didn't slip onto the garage. 
The engineer designed a concrete slab tied to a concrete filled block wall. The wall and the slab were tied together by L shaped reo bars which ran the width of the slab and to the top of the wall. The principle being that the strong shape would not tilt, and the slab was butting the garage slab which would be a tad difficult to move too. 
I'm not an engineer, but maybe you can use some of these concepts in your slab and wall? 
woodbe.

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## GOM

Chipps;   

> Is it possible to use the tank's water to have a sprinkler system?

  Well, it's possible, but as the water pressure is supplied by an electric pump, I don't want to be reliant on the electric grid during a bushfire. We do plan to plant native groundcovers and succulents on top of the soil. These will be watered during the summer, so the soil cover should be quite damp. 
Woodbe;   

> We had to build one when we built our garage close, and lower than the road - so that the road didn't slip onto the garage. 
> The engineer designed a concrete slab tied to a concrete filled block wall. The wall and the slab were tied together by L shaped reo bars which ran the width of the slab and to the top of the wall. The principle being that the strong shape would not tilt, and the slab was butting the garage slab which would be a tad difficult to move too. 
> I'm not an engineer, but maybe you can use some of these concepts in your slab and wall?

  The website was having problems when I first started this posting. The Plan and Section I originally put up were therefore done as PDF documents which might have been hard to crank up. Here is the current plan in an easier format:   
And here is the cross-section:   
You can see that the left-hand wall, which acts as a retaining wall, incorporates L-shaped reinforcing bars in the base as you describe.  These then overlap in the wall with other L-shaped reinforcing rods which continue on to reinforce the roof. I'd be interested in how this all compares to your retaining wall. How tall was it? What no-one has been able to tell me is whether there is enough reinforcing (or too much). Is this thing going to fall down on me? 
Cheers, GOM

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## rrobor

How about having a water barrel above the bunker, one of those plastic ones. If it gets too hot the barrel melts water spills out and due to evaporation it will help keep the bunker cool. As to doing the concrete put stakes in and screed to the stakes, as you pass the stakes pull them out and screed to the next. The thing about the wall falling down  is, the wall slowly gets pushed down by soil getting wet and dry. So put an agi  drain in along the bottom and fill the between the bank and the wall with gravel.

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## woodbe

> You can see that the left-hand wall, which acts as a retaining wall, incorporates L-shaped reinforcing bars in the base as you describe.  These then overlap in the wall with other L-shaped reinforcing rods which continue on to reinforce the roof. I'd be interested in how this all compares to your retaining wall. How tall was it? What no-one has been able to tell me is whether there is enough reinforcing (or too much). Is this thing going to fall down on me? 
> Cheers, GOM

  Ours doesn't have the roof, but otherwise: 
We have a box section in either side of the floor - probably 3-400mm.
The floor butts up to the garage slab. Yours looks like it is lower. Might be worth putting starter bars in the slab to pick up the load and transfer it to the wall. 
I'll see if I can find a plan for ours in case it is helpful. 
woodbe.

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## woodbe

Ok, found it. 
Time has blurred my memory, but as soon as I saw the plan it all came rushing back. There is a thumping block of cement below the slab. This is probably overkill for your situation because you're not parked next to a road (are you?) 
Anyway, here is the plan for our monster. If we were doing it again, there would be a fire bunker built in there somewhere...   
woodbe.

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## GOM

Woodbe;   

> There is a thumping block of cement below the slab. This is probably overkill for your situation because you're not parked next to a road (are you?)

  Thanks so much for taking the trouble to post a copy of your plans. It really helps, as it gives me another point of comparison. As you say, there is a huge block of concrete under each pier (1000 x 1000 x 1400 - eek! Your engineer didn't have a brother-in-law whom he recommended as the concreting contractor did he?) 
Can you tell me any more about the vertical reinforcing bars shown in the "Wall Section" (i.e. the central figure). At the top of the figure it notes "N10 @ 600 CRS. EVERY THIRD CORE". These are shown extending through the height of the wall, but I can't quite see if they overlap with some starter bars in the "key" below the wall, or whether they extend right through the wall and down on into the "key" themselves. Also, it appears that trench-mesh has been specified for the concrete immediately below the wall, but I can't see any specifications on the Sections. Are there any specifications on the trench-mesh elsewhere on the plans? 
For your interest (and that of others) I have just found a very useful document on building retaining walls at http://www.reao.com.au/forum/attachm...2&d=1233576984 It also has designs for "propped" basement walls, which may be closer to my situation than retaining walls. As they say, "a concrete floor slab (e.g. the roof of my bunker) will provide a 'prop' to the top of the wall, simplifying the wall details". Here's a copy of one of their propped wall designs:     

> Might be worth putting starter bars in the slab to pick up the load and transfer it to the wall.

  Thanks for the suggestion. That wouldn't bee too hard to achieve. 
Rrobor;   

> As to doing the concrete put stakes in and screed to the stakes, as you pass the stakes pull them out and screed to the next

  Well, I'm not quite sure if we're on the same page, as I need a horizontal surface to screed off against. But, you've set me thinking. If I had vertical stakes with something like star pickets attached horizontally at the level of the top of the concrete, I could screed off against that and then pull them out without disturbing the surface so much. Is that what you had in mind? 
Cheers, GOM

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## woodbe

> Can you tell me any more about the vertical reinforcing bars shown in the "Wall Section" (i.e. the central figure). At the top of the figure it notes "N10 @ 600 CRS. EVERY THIRD CORE". These are shown extending through the height of the wall, but I can't quite see if they overlap with some starter bars in the "key" below the wall, or whether they extend right through the wall and down on into the "key" themselves. Also, it appears that trench-mesh has been specified for the concrete immediately below the wall, but I can't see any specifications on the Sections. Are there any specifications on the trench-mesh elsewhere on the plans?

  Nope  :Smilie:  
Actually, I have pages of engineers calcs, but it's all gibberish to me. I can't see the answers to these questions in there or in any of the drawings I have though. The wall was against a slope across it's width, so we had Retaining walls Type 'A', 'B', and 'C'. 'C' being the extreme because of the height at that point. It became one of those things you just have to do even though the builder and the architect and the trades all thought it was way over the top. 
Wall Tye 'A' had a max height of 1650. It was unsupported for shear by the building and it has a 250x600 footing and a 200 slab.  Type 'B'  had a max height of 1850 and it was supported for shear by the building - just a 200 slab. If you need to have a look, I could scan them too, but probably not until Monday... 
woodbe.

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## Dan574

in todays herald sun there is a photo of your bunker.  almost exactly the same.

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## GOM

Woodbe;   

> I have pages of engineers calcs ... I could scan them too, but probably not until Monday..

  . 
Very kind of you, but please don't go to the trouble. They are probably a bit divorced from what I have at hand. For your interest I have "finalised" the design I will be following. It is summarised on the Section below. Basically it follows the "propped wall" design in the Hanson document I gave a link to earlier. It means I have to reinforce and core-fill the right-hand wall, as the roof transmits part of the shear load onto the right-hand wall. The benefit though is that it negates the need for a massive excavation for the "key" under the left-hand wall. According to the Hanson specifications, an 1900mm high wall would have required a "key" 400mm wide and 600mm deep. As I have reached weathered granite in my excavation, that would have been difficult in the extreme.   
From what I can see, my design will be far stronger than an ordinary concrete water tank used as a bunker. A standard water tank design I have seen uses 85mm thick concrete reinforced only with SL72 mesh. 
Cheers, and thanks to all who have helped so far. GOM

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## woodbe

I'm not an engineer, but it looks good to me.  :Smilie:   :2thumbsup:  
woodbe.

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## Ian Pullar

<o:smarttagtype namespaceuri="urn:schemas-microsoft-com<img src=" images="" smilies="" sad="" shock.gif="" border="0" alt="" title="Shock" smilieid="413" class="inlineimg"></o:smarttagtype>
Hi  Gaza and everyone else (hello again GOM). I wrote the Fire Bunkers article that appeared in Owner Builder magazine. 
Actually, it's just a summary of the detailed 59 page article I wrote on the subject, which you can get from  http://home.iprimus.com.au/ianpullar/firebunkers.htm 
If you can't get it from there, send me a message, including your email address, and I'll send you the WORD document back to you. 
It contains links to just about every bunker supplier, and lots and lots of other stuff, including commercial doors and door seals, ventilation hints, signage and more. 
What is DOESN'T contain is the structural info that GOM is seeking - but I think I've learned more about all that in one night from the excellent contributions and links on these last 4 pages! 
One link that is missing from my document is 
Wildfire Safety Bunkers  many aspects similar to what Ive written in my document. Well worth a look. 2 sizes. Also sell accessories. http://www.wildfiresafetybunkers.com.au/index.htm 
Cheers - Ian

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## eddyb

Hi
Like many other people i believe that there needs a back up plan for when the fire plan you initially had fails. My vision is for a 2.4m steel frame cube with compressed fibro inside and out covered with point 7mm stainless steel sheet to reflect the 90% radiated heat. These would be approved, sold through Bunnings and delivered on the back of a truck to be lowered into a shallow level trench which, with a fireproof door, would create an airtight seal.
In times of emergency the mower would be thrown out and 12 people could stay safe until the fire had passed.<O :Tongue: </O :Tongue:

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## Bodge

Just a quick thought, it might sound silly but make sure the bunker is NOT downhill from your house.
Might seem obvious, but I know of a situation where an entire family perished because of this. By the time they realised that they had lost the battle to save the house they could not reach the bunker. 
The bunker was unscathed, but was built downhill from the house in the direction from which the fire front was bound to come from.

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## autogenous

The problem with concrete exposed to fire is it explodes with granite shrapnel unless shrouded with dirt. 
I don't think anyone is going to sanction a fire shelter because there is too many variables. 
People don't even bother keeping fire breaks up and in some cases aren't allowed to clear trees from near there house under the greenie guideline.   
Even burning off by the authorities has been disallowed in the past creating the big human barbecue.  
With the extent that the fires have been without oxygen tanks, your stuffed.. 
Todays Australian "cover your ass" standards means there may not even be someone prepared to offer a classification solution.. 
Lets face it, there is fire, then there is fire bomb.. 
Maybe one of these built to protect against Napolean attack?  Martello tower - Wikipedia, the free encyclopedia

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## autogenous

A poor diagram as some had the ability to fill the wall cavities with water..

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## Black Cat

Personally, oxygen is the last thing i would be taking into a fire bunker with me. Do you mean air, perhaps?

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## elvy

Could I put a concrete water tank under my house and use it as a cellar come fire bunker which is closed off with the use of two sealable doors and a tunnel leading away from the house with a door at each end. So if the cellar is damage you still have the tunnel to go through. which leads down the hill about 50 to 100metres away from the house. Advance would be most people are caught in their house so you have a get away. The underground water tank could be pumped into the bunker so small fire can be put out.
I am going off the designs of 100 years ago when all they had were tunnels, they would enter one end and the air had a clear flow through.
It would be good if fire bunkers had two uses then more people would install them. What do you think??? :Confused:

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## Moondog55

I am going to comment because fire season is almost with us again. 
A nuclear bomb shelter needs to have multiple purposes as its use is in the barely possible category, here in OZ a bushfire shelter is more in the probable category so dual use isn't really a good idea in my opinion, by the time you get a full set of PPE for each person, a decent first-aid kit; a few 20 liter drums of drinking water, 200l of fire fighting water and an empty drum for toilet purposes there wont be much room left in the bunker most of us can afford to build.

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