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Old 12-22-2011, 04:05 PM   #1
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another couple of water tank questions

1. I'm putting in a freshwater 180L tank under the deck of my housetruck. I've had a look at a few different mounting systems and am thinking of using two pieces of angle-iron across the bottom of the tank, and 8mm bolts up through each end of the angle iron to the deck rails on the truck. Just wondering if the 8mm bolts will be strong enough or should I get something that is rated for the load? Is two bars enough? The tank is 595mm across and 910mm long (angle iron would be going across the 595).

2. for the greywater tank (no sewerage) is it ok to run the pipe from the sinks to the tank so they go up and down. I can attach very easily to the bottom of the wooden deck, but will need to dip down under the deck rail once or twice. The inlet on the tank will still be well below the outlets of the sinks. I guess this means there will be water sitting in the pipes, which is not good, but I do have a hepvo valve below the sinks to stop smell coming back up.
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Old 12-22-2011, 07:56 PM   #2
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Re: another couple of water tank questions

#1 -
I guess you're thinking something like this -

The lowest grade of threaded rod has a yield strength of about 36,000 psi (permanently deforms) and a tensile strength of about 60,000 psi (breaks).
Your 8mm is about 5/16", so I'll leave you to work out the aritmetic.
The rule of thumb for nuts is that thread engagement of 80% of bolt diameter is sufficient to cause the bolt to fail in tension before the threads strip.
Whether the angle bar you use is strong enough, it depends of course on the size of the bar. You didn't mention this in your post. 2 bars would probably be strong enough, but the tank is much happier if it's supported continuously around it's perimeter and it's absolutely ecstatic when supported over it's entire bottom.
If you suspend it with threaded rod don't forget something to prevent lateral motion in both directions.

#2 -
True about the standing water, but you wind up with the 'stranded solids' problem, when the water stops in the pipe everything that was suspended in it falls to the bottom of the pipe. Same way when a pipe is pitched too steep, the water 'outruns' the solids and leaves then behind in the pipe, crud dries in the pipe and sooner or later a clog. That'a why there's minimum as well as maximum fall on a pipe.
Probably wouldn't mean much in a gray water pipe, just thought I'd mention it.

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Old 12-22-2011, 10:15 PM   #3
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Re: another couple of water tank questions

Yeah similar to that, but with the angle iron up the other way (an upside down L), and across the bottom of the tank further in rather than on the edge. The angle iron is 40mm x 40mm. I only have two bits, but I have some other, lighter angle iron I could use in the middle as well, or possibly on the ends to prevent movement (but that would have to be screwed into the deck rather than bolts to the deck rails)... I had thought that the flatness of the tank top against the deck rails, tension on the rods and the weight of the tank would prevent lateral movement.

I got some 11mm threaded rod today. I also have two long bolts which are 12mm. I didn't follow your maths at all sorry, can you please spell that out?

My biggest limiting factor is that all the engineers are closed for a couple of weeks, so it would be hard to buy something else and get it cut now. I need to be on the road in a couple of weeks, but won't drive unless I have something safe.


Sounds like I will have to rethink the greywater pipe install, thanks for the heads up.
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Old 12-22-2011, 10:40 PM   #4
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Re: another couple of water tank questions

Sounds like you've got enough good stuff to work.
The angle bar's plenty. I'd get a piece of plywood under the tank tho' as soon as practicable. Over the long haul the bottom will sag and be liable to fail. Also the plywood will prevent the angle bars from rubbing holes in the bottom of the tank.
The 8mm bolts will hold about 9,000 pounds each before they 'stretch', so they're plenty. Use the 12mm bolts if you want but they're overkill - might want to save 'em for when you need 'em.
The plan to bolt it up tight to the truck frame is just the ticket to prevent lateral motion - put something between the truck frame and the tank to prevent abrasion of the tank - some weatherstripping, sponge rubber or something like that - old garden hose would work fine.

I'm guessing that you're using a plastic tank of some sort, you didn't mention. If you're using a metal tank forget the abrasion stuff.

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Old 12-23-2011, 12:55 PM   #5
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Re: another couple of water tank questions

Quote:
Originally Posted by Iceni John
Your 180 liter tank will hold about 170 kg of water when full, or about 200 kg complete with supports and framework. Factor in at least twice this weight to allow for dynamic forces, such as driving over a bumpy road, and you're talking about almost half a tonne. This is why you must make any tank supports as strong as possible. Now think what would happen if one of your under-tank cross-supports broke, or one of the hangers gave way - would the tank still be secure?

I put my two fresh water tanks in last year, and I'm in the process right now of installing my black and gray tanks. Each fresh tank holds 110 US gallons (about 450 liters), so that's up to a ton of dynamic load each. For this reason I hung each tank from ten lengths of 1/2" (12.7 mm) threaded rod, with double nuts top and bottom (and Nylok on the bottom). Each tank sits on a full-length frame of 1.5 x 3" (38 x 75 mm) Mil-spec angle steel that sits on five cross supports of heavy channel steel, and to counteract any bowing out when full all four sides and the bottom are sheathed in 1/2" plywood. To absolutely prevent any lateral movement the tanks are located by the frame rail on one side, a structural cross-member in the front, and on the other side and the rear I made a lip of the same angle steel that is massively screwed and bolted to the bus floor and frame. In other words, the tanks aren't going anywhere! This means that the hangers need do only that - they do not need to take care of any lateral forces whatsoever. The tanks are completely restrained in all six directions of possible movement. After I filled both fresh tanks I took my bus for a drive, and there was absolutely no movement or settling at all.

Tank supports are one area where more is better. If in doubt, make it stronger. Imagine how Isambard Kingdom Brunel would have made them if he were converting a bus!

Good luck, John
Gee, why not a set of wheels on the bottom of the tank too, just in case all that wasted material doesn't give way?
A rose is beautiful, 1000 roses isn't a thousand times more beautiful.
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Old 12-23-2011, 01:15 PM   #6
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Re: another couple of water tank questions

Don't forget to protect the top of the tank from abrasion as well. We held our slightly off the rails in the floor with foam insulation pieces and shot Great Foam around them to "glue" them in place.
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Old 12-23-2011, 04:17 PM   #7
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Re: another couple of water tank questions

The tank is gavlanised steel. It's second-hand and has come off a house bus. I have a piece of 9mm ply I can put between the tank and the angle iron at the bottom. Would I still need some protection from abrasion at the top? The place where the tank meets the deck rail is a welded join, slightly raised, which means it will be taking the tension on those 4 or 6 points. Would it be useful to put a piece of ply on top too?

John, my top priority is to not have the tank come off while driving down the highway Second priority is to be legal (although I doubt that the authorities here in NZ will have a problem with what I am doing), and third is to have a functional water system.

My similar sized steel diesel tank sits in two brackets mounted horizontally, off the chassis. It does have some friction protection on the top straps. They wrap around the body and attach to the brackets at the bottom with two 10mm bolts. I guess the main difference is that the brackets are super solid, and the whole thing will have been certified by an engineer.
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Old 12-23-2011, 04:56 PM   #8
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Re: another couple of water tank questions

Quote:
Would I still need some protection from abrasion at the top? The place where the tank meets the deck rail is a welded join, slightly raised, which means it will be taking the tension on those 4 or 6 points. Would it be useful to put a piece of ply on top too?
Abrasion protection would be good on top - the ply on top would be perfect.

John -
Quote:
Roses may be pretty, but a strong piece of engineering is prettier!
That's just my point - there's no 'engineering' in there - just some arbitrary decisions about what you thought was 'strong enough' and then some assembly.
Why 1/2" bolts - why not 3/4"?
Why 1 1/2" x 3" angle bar - why not 6"x6".
Why 5 supports - why not 6?
etc. etc.
None of these is based on any 'engineering' - just your decision that these were 'strong enough'.
Always when I see this it reminds me of when we used to build tobacco barns when I was a kid. Made 'em outta HUGE timbers, took forever to put one up, just like they'd been building them for as far back as anyone could remember. They'd last a coupl;a hundred years - we had ones around that had.
After some time in engineering school I thought back on those and realized that with that material and manpower you could have built 6 barns in half as much time that would last just as long.
Engineering creates a solution that's appropriate to the problem - no more, no less. That's the beauty of engineering.

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Old 12-23-2011, 06:11 PM   #9
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Re: another couple of water tank questions

welding might be a good idea
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Old 12-23-2011, 06:42 PM   #10
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Re: another couple of water tank questions

Quote:
Originally Posted by Iceni John
The reason I overbuild (sic) is because I like the end result, pure and simple
The only reason you need, just don't confuse your prediliction for overbuilding with good engineering practice, or any engineering for that matter. It's actually throwing materials at a problem until you're satisfied with what it looks like.

Merry Christmas to you as well !


Quote:
Originally Posted by chev49
welding might be a good idea
.
Suggestions were made to the OP based on the materials that OP had on hand or were available to him.

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