[Structural] Water tank frame

[Meester Over]

I have decided to do a homemade camping trailer from scratch and working out the last of the structural frame.

I am wanting to determine how "light" I can go on the inner frame that will hold the 50 gallon water tank.

Tank: https://www.plastic-mart.com/product/5986/50-gallon-portable-utility-tanks-crmi-50rt

Please take a look at the PDF attached and keep in mind that I am not a mechanical engineer by any stretch of the imagination.

I was originally thinking of using 1.5"x1.0" 16ga rect tubing, but if I can get away with some angle iron or 1.0"x0.5" rect tubing (with the longer edge on the vertical), then that will help (slightly) reduce costs, but more importantly, reduce the weight. I do also plan to put a floor on the inner frame with either drawers or storage tubs. Off the very back, I plan to build a kitchen. Pictured in the back middle is the RTIC 110 qt cooler. A smaller 26 gallon gray water tank is underneath the 50 gallon tank, next to the RTIC cooler.

If anyone would be so kind to review the inner frame and let me know what material you would use it to support the load, that would be much appreciated. I am guess there may be as much at 700# total on the frame, but I will try to keep the heavier stuff on the bottom.

 

[Herbie]

There are calculators online that will help you out. What you're looking for is a "beam/load" calculation on your material.

Here's one to start: http://www.atc-mechanical.com/calculators/tube-size-using-structural-properties/

Enter the weight of your load (water+tank+ any extras) and the length of the beam (the span of your tubes across the frame). It will show you which common tubing sizes have what beam loads.

 

[Meester Over]

I found one of those, but that is just based on one beam, right? I am curious how to calculate the load capacity based on the multiple crossbeams and the vertical supports I planned on putting under it. At this point I am just guessing.

Yeah, that is actually the same site I was looking at this morning.

I can also get the 3D file if there is some way to run a structural analysis (notice my user name, my wife calls me "Mr. Over" for a reason), but I am likely over thinking this.

 

[ottsville]

I'm curious as to why you are putting in a gray water tank?

 

[Meester Over]

Should we decide to wash dishes, I want to feed it back. It may be unnecessary, but I do not want to have food remnants rotting around. There are also a few places around here that fine you for dumping gray water. Mainly going with the advice of a buddy that lives in a Class A.

Do you know anything to the contrary? Would you worry about attracting wildlife, or I guess it would be better to use disposable plates and properly dispose of the trash, but I do not want to contribute to the landfills either.

This is my first foray into building something like this. Wife wanted a 27' travel trailer and I talked her down to this. I typically just pitch a tent, but at campsites. I want to take this more backcountry too.

 

[lowrider_rt2000]

You're going to want to keep your fresh water tank close to your axle. The farther back you go, the more chance of your trailer "fish tailing" you will get. Remember, 50 gal of water weighs 417 pounds. Take that into consideration when choosing your steel. Let's say you and the Mrs weigh 200 pounds together, you're looking at, at least 617 pounds with just water and people. Just my 2¢

Sent from my WJ

 

[DaveInDenver]

I can also get the 3D file if there is some way to run a structural analysis (notice my user name, my wife calls me "Mr. Over" for a reason), but I am likely over thinking this.

Of course there's a way to analyze this, just use Solidworks, AutoCAD & ANSYS if you're independently wealthy.

Assuming not, doing it cheap probably means Fusion360, which can do simple static simulations with the basic subscription, although buckling analysis will require the full deal.

There are free/cheaper options to get there, like FreeCAD to generate the model and LISA or Mecway or CalculiX for the FEA. Just be ready to dig into a bit deeper to get things working. But your $100 (you'd need to buy Mecway to analyze something large like trailer parts, FreeCad is free) you get a lot of capability.

 

[Hillbilly Heaven]

Just briefly looked. Is that fresh water tank mounted high in the box? It should be mounted low. That much weight up high is a disaster waiting to happen.

 

[DaveInDenver]

I know there's ways to steal the programs. I'm also aware of the student versions, I have the Autodesk and Solidworks suites from taking a few welding classes at the community college (I used them as continuing education credits for my engineering license).

You need access to a qualified .edu email to activate your Autodesk student account. It's a 3 year license once you activate a tool. And Dassault will give you Solidworks for cheap, but not free, after you fill out an application and they decide if you qualify for a student version.

Just to compare, getting a legal seat of ANSYS to do what you're after is worth about $30,000. Solidworks will be less, probably around $5,000 to $10,000. And won't be loaded with Russian or Chinese malware and viruses.

 

[Meester Over]

I'm curious as to why you are putting in a gray water tank?

Just briefly looked. Is that fresh water tank mounted high in the box? It should be mounted low. That much weight up high is a disaster waiting to happen.

I actually dropped the gray water tank and mounting the fresh water tank on the floor. I was not liking how that design was working, or not working really.

 

[Meester Over]

You're going to want to keep your fresh water tank close to your axle. The farther back you go, the more chance of your trailer "fish tailing" you will get. Remember, 50 gal of water weighs 417 pounds. Take that into consideration when choosing your steel. Let's say you and the Mrs weigh 200 pounds together, you're looking at, at least 617 pounds with just water and people. Just my 2¢

Sent from my WJ

Thanks. I actually changed the design and moved the tank to center directly over the axle.

And ********, my wife and I eat food, so no way we are a combined 200 pounds ;-)

 

[Meester Over]

Any pics of the updated design? Or finished product?

I will upload some pics in the morning.

I am still working on the frame, for the next few weeks most likely. The weather is still nice here in CO and the MTB trails are rideable, so I am still on my bike a lot of the week, but slowly making progress on this. I will make more progress when the white dirt starts to fall from the sky. I do plan to have it done by next Spring.

 

[Meester Over]

Ok, finally. I had the guy that did the model for me, do some renders. I still cannot figure out how to use Blend (and have only put in about 10 minutes max total minutes of trying to figure it out).

These renders are 95% accurate of what the final structural design will be.

It will have:
110 qt cooler
50 gallon water tank
kitchen in the back
generator
2 batteries
2 30# propane tanks
instant water heater shower
rooftop tent

I still need to add up what I think the wet weight will be as I need to order new leaf springs. The ones I currently have are good for 1500#. Two options raise the spring limit to either 2500 or 3000 #. Axle is 3500#