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Old 04-21-2015, 11:31 AM   #1
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Measuring stress on angle iron

Hi,

I am building my tank mounts and have been nervous the whole tie about whether or not my tank mount design will be strong enough. I have four 46 gallon tanks, two fresh and two grey (no blackwater for me). The two fresh tanks will be tied together with a line on opposite sides of the bus, but the two grey will be next to each other between the frame rails. When these are full it will be nearly 800 lbs of water. Does anyone have any designs for good tank mounts handy? I have some in my head but don't have a good means of drawing it out. I have a bunch of 1.5"x1.5"x1/8" angle iron which, if done in a certain way may be strong enough, but I haven't worked with it enough to know what its load capacity is like.

Thanks!
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Old 04-21-2015, 05:05 PM   #2
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I would use thicker, or deeper material.

Wider, or deeper material has much less deflection.

Old houses used 2x6 as floor joist. They held the weight, but the floor bounced alot. This is deflection.

New houses use 2x12 or I joist. We get much less deflection.

Nat
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Old 04-21-2015, 10:19 PM   #3
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You can never have too much overkill.
I would use heavier material.
Lots of weight there and bouncing down the road is hard on things.
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Old 04-21-2015, 10:36 PM   #4
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Do any of these tanks have baffles? Shifting that much fluid around during take-off and braking can create some significant forces. Both front to rear and side to side.
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Old 04-22-2015, 02:08 AM   #5
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Here's a photo looking straight up from below my black water tank. It isn't exactly the Ameri-Kart H002 tank, but it's pretty close. You can kind of see in this photo, and better on Ameri-Kart's drawing, how this style tank has a projection on the long sides. Ameri-Kart has a "holding tank installation information" document which recommends using that projection to carry the tank.

I hacked a crude Z profile late before leaving on a trip and bolted it up through the floor. All four corners of the tank have a duplicate of this bracket. Each Z has a flange that wraps around the corner to prevent the tank sliding out forward or back. I put a piece of threaded rod across the narrow direction and just above the top surface of the tank to tie the brackets together so they can't wiggle apart and let the tank fall through. The nut on the end of one of those rods is visible in the photo. I guess that rod also creates a clamping force on the sides which also helps prevent the tank slipping forward or back. I think I used 16 gauge for those.

This tank is shallow (8 to 10 inches), drains forward, and holds less than 30 gallons so I haven't worried much about the forces of sloshing inside.

As an alternate to using that projection, Ameri-Kart suggests 1-2" wide straps at 12-18" spacing for support of tanks without the projection. I infer "strap" to mean a strip of sheet metal in the 20-16 gauge range -- something the size of the earthquake straps sold for residential tank-style water heaters for example. Judging by that recommendation I'm thinking they're not too worried about sloshing in the holding tank profiles.

Some tanks, especially the fresh water, might be more cube-ish. I haven't entirely figured out what I'll be doing with my two 40 gallon fresh tanks, which are cube-like and nearly twice as tall as my black tank. Ameri-Kart has a separate set of recommendations for fresh water tanks. Below 30 gallons they say support one side and bottom with 1/2 to 3/4 plywood; over 30 they say support all sides and bottom. I get the idea that sloshing is more of a concern for these more rectangular and larger tank shapes.

As a really rough idea, I'm thinking about threaded rod hanging down from the bus floor with a length of angle iron spanning under the tank. Use several copies of that to get the spacing right. Insertion of plywood on the sides and bottom is easy. Ends not so easy. But plywood isn't preferable because of the weather-exposed location under the bus, so maybe use sheet metal instead? Whatever the sheet is, it can be secured to the framework to stiffen it and prevent sliding forward/back from sloshing. The right kind and attachment of the sheet can fill the role of the threaded rod, too. Come on, it's late and I'm not thinking very well.. somebody step up and improve the idea!


Regarding the 1.5"x1/8" angle -- if it's arranged such that it spans the narrow direction of the tank, and is repeated at 12-18" spacing, then it's plenty thick. If you box the tank like Ameri-Kart suggests then simple sheet metal or flat bar straps could do.
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Old 04-22-2015, 11:24 AM   #6
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Quote:
Originally Posted by family wagon View Post
Regarding the 1.5"x1/8" angle -- if it's arranged such that it spans the narrow direction of the tank, and is repeated at 12-18" spacing, then it's plenty thick. If you box the tank like Ameri-Kart suggests then simple sheet metal or flat bar straps could do.
Thanks. I had a decent design down (in my head) for the single 46 gallon tanks (taken from VonSlatt):



But my concern was mounting 2 of these tanks next to each other on the same span, in between the frame rails. About 28" wide, almost 800 lbs of water to support. I will try and do a mock up of an idea I had to mount those when I get home on SketchUp.

Thanks!
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Old 04-22-2015, 03:39 PM   #7
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I only have exposure on the bluebird chassis I'm working on, but the frame cross members have holes that line up vertically through them that look like they are intended to hang heavy things from via threaded rod that is up to 1/2" diameter.

1/8" thick hot roll angle iron is pretty strong stuff, but I would certainly build some triangulated trusses to hold the 1000 lbs of water tanks.

Ultimately, the frame rails of the truck body are supported by the spring perches, and that span is going to give you an idea of how far you'll need to distribute the weight across the frame fore to aft.

A point load of 800 lbs mid span is really different than 4 point loads of 200 lbs spread across an 8 foot span, for example.

When you start looking up the young's modulus of hot roll .125 2.5" flange angle iron, then applying that to multi-point spans you have to put on your civil engineer hat a little bit.

It's also very easy to get too worked up about it, and if done wrong can also give you false comfort about how something is designed or fitted, so take care to really think about how what forces you're going to be encountering.

Slamming into a speed bump at a higher speed than you wanted? Think of the acceleration forces applied to both the frame structure you've built and the tank itself.

Will that poly talk pop like a water balloon with 800 lbs of liquid in there, because you didn't add enough side support straps?

There's certainly some things to ponder. I don't have many answers for your question but this is the thought process I'm going through currently for a few things.
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Old 04-22-2015, 04:04 PM   #8
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this brace would stiffen it up alot

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Old 04-22-2015, 07:18 PM   #9
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Interesting points, aaronsb. I wish I had a civil engineering hat to wear.. it's really fascinating stuff.

The suggestion about the tank wall failing (popping) correlates with the Ameri-Kart recommendation to box the tank with plywood. It says to me the tank's purpose is to be the leak-free liner inside a structural container provided by others. What can we use instead of plywood? Plate metal would work, but at great weight or cost. Is there a corrugated material.. smaller than corrugated roofing or metal roof deck? taskswap mentioned H45 and H100 in another thread recently. Not having worked with fiberglass myself I'm not certain what those are... but a web search turned up Divinycell, which at first glance looks like a cellular board.. a hard structural foam of sorts? Maybe something like that would be a good choice for covering the space between the metal members.
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Old 04-22-2015, 08:51 PM   #10
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The tanks I have been looking at have ribs cast into them. This adds the rigidity needed to contain the weight of the water without needing more support to the sides.

If the plastic tanks without ribs need plywood or the equivalent to support them, I would be building tanks out of stainless steel with ribs formed into the metal sheets to add strength.

I like where this topic is going. Great info so far.

Nat
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