Raft Structural Integrity

[Jonathan Hall]

Hey Everyone,

I am looking for some advice. At the point of planning out my DWC/Raft for my greenhouse build. There is a particular size I am trying to achieve while maintaining a low wall/structural width. The less I consume the bigger my walk ways can be...

The preferred internal surface area of the raft is is 8' x 38' x 2'(tall) 

I will partially bury the raft down in the ground about say 10 inches.

One of the keystones in my attempt to keep the reinforcements to a minimum size was to use 18 guage coiled strap around the framing I build. like this - http://www.homedepot.com/h_d1/N-5yc1v/R-100375224/h_d2/ProductDispl...

Has anyone used this or a product similar? I am thinking with a strap around the top, and the middle should provide some decent support.

Additionally, after the placement of the straps I will add 3/4 inch plywood stakes at the beginning of each new board for added support.

Connecting each of these supports I foresee putting a small 1 x 1 piece of lumber across and nailed down to keep it together as well.

I am no carpenter and have limited building experience so I sort of just made this all up. Really trying to achieve the 2 foot tall tank at that larger size due to placing prawns under the rafts. Want to give them at least 2 foot depth to swim in.

Would love to hear about your designs and recommendations. I am really up in the air with this and will take any help I can get!

[Dan Ponton]

I am also building a DWC system and am curious why you are burying them? I am going to raise mine up 2 feet so I can work an harvest standing up, not bent over. For me this is an important issue since I am almost 60 years old.

I do not think you need to give them 2 feet to swim in since them will be feeding at the roots of the plants almost all the time. I am going to build mine out of 3/4 inch plywood 12' deep.

[Jonathan Hall]

On mine, being 8' x 38', it would seem to me that it will need a decent support system. Not sure on the exact height yet, it is two feet of water I am aiming for. To fit that in my greenhouse, I have three walkways 1-1/2' each perfectly. I can deal with that foot space, but it is definitely the minimum.  So I have to find ways to support my structure while keeping it as thin as possible. It will be lined with pond liner.

So what I will do is bury the 3,400 gallon+ prawn tank to reinforce the walls near the base. So that eliminates a big portion of what can break apart under the pressure.However, there is a giant boulder about 10"-12" so I wont be going that deep.

Today I purchased a Uline steel strapping kit. It will allow me to run a steel strap around the tank and tighten up with the tensioner. It has a test of about 1200 lbs (I think, bought it used, so guessing off their site. The product gauge is not labeled on it.. Now that I have this I need to go and find out what sort of calculated pressure I will be dealing with. Math was never my strong suit. I expect that I will be strapping it about 6 times just to be safe.

I believe that the straps will gain even more strength once I put on the wooden stakes on every, #(undecided), feet. I will screw them in tightly around the straps so that should give each section in between the strength of 1200 lbs. Or at least whatever the friction holding power is of the wood in steel. Then the final step will be a crossbar linking the two stakes together from either side. Just a small 1x1 piece of lumber.

Another reason would be, that the rocket heater I am making will be in close proximity. The thermal mass will be about 1' away. This is going to heat the ground up substantially (I hope). With it in the ground, which is now heated, will have more surface area to transfer the heat in. I intend to heat my fish tanks through the winter so everything helps.

The reason for going 2 feet deep is that is what I read in a training booklet on raising Malaysian giant prawns. And again, all that water makes a great thermal battery. It will be hard to change that temperature.

The prawns should feed on the dead roots but not the living. Going to build them some PVC jungle gyms as well so they can hide in and hopefully increase survivability. That design will take some thinking still...

Dan Ponton said:

I am also building a DWC system and...

[Jon Nose]

if you are down in the ground what makes you thing that the dirt around the tank will not be sufficient to support the walls ..?

[Jonathan Hall]

Do you think it will be?

I really do not know. However, my thought process was that since more than half of the tank will be above ground, It will have the outward pressure that may compromise the structural integrity. Perhaps just the stakes would be sufficient. Maybe even with the cross bars.

I am definitely all ears from those who may know something I don't. (I am sure there are lots of those here!)

Either way I have already purchased the steel strapping, needed or not. Luckily I bought it used and got it for a steal. Just the strapping kit without the straps retails for over $500. I got it all for $150. Which is still less than I was going to spend to buy the off brand stuff from Northern Tool (but not by much). Thanks Craigslist!

I purchased a few IBC's without cages on them so this strapping will also come in handy when reinforcing them when used as sump tanks.

And again thank you for pointing me towards your RMH pics. They were very helpful.

Jon Nose said:

if you are down in the ground what makes you thing that the dirt around the tank will not be sufficient to support the walls ..?

[Jon Nose]

Strapping may get in the way of your floating rafts ... would need to run at a minimum some 2/4 or 2/6 on edge for support on the top rail ... remember to line the outside of tank the you are digging in to preserve it longer ... condensation ?
Jonathan Hall said:

Do you think it will be?

I really do not know. However, my thought process was that since more than half of the tank will be above ground, It will have the outward pressure that may compromise the structural integrity. Perhaps just the stakes would be sufficient. Maybe even with the cross bars.

I am definitely all ears from those who may know something I don't. (I am sure there are lots of those here!)

Either way I have already purchased the steel strapping, needed or not. Luckily I bought it used and got it for a steal. Just the strapping kit without the straps retails for over $500. I got it all for $150. Which is still less than I was going to spend to buy the off brand stuff from Northern Tool (but not by much). Thanks Craigslist!

I purchased a few IBC's without cages on them so this strapping will also come in handy when reinforcing them when used as sump tanks.

And again thank you for pointing me towards your RMH pics. They were very helpful.

Jon Nose said:

if you are down in the ground what makes you thing that the dirt around the tank will not be sufficient to support the walls ..?

[Jonathan Hall]

I was actually thinking of putting the straps around horizontally. So the single strap would be about 93' long. The supports spanning the top by the stakes are there for more for rigidity. These won't be too large maybe1x2's. This would be the only thing that could get in the way, but I don't expect it to be an issue.

Really I am hoping to avoid 2x4&6's. 3/4 would be great! Keeping my fingers crossed for that steel strap(s)... perhaps a really wet greenhouse.

[Kate Mink]

Strapping wrapped horizontally around the trough will not keep the trough from bellying out in the middle. I have had good results from 2 approaches -

- nailing a length of 2 x 4 across the top of the trough, and

- putting stakes outside the trough walls, extending above the trough, one on each side, and using ratchet ties to hold the tops of the stakes together above the floats. You could use your steel strapping for this.

I strongly recommend against making your troughs too deep (anything over a foot is unnecessary) or too wide. More than 4 feet wide and it's all but impossible to reach into the middle to set pots, harvest, anything.

good luck.

[Jonathan Hall]

Thanks for the advice Kate.

I plan to do more or less exactly what you are saying.

Jonathan Hall Said: "Then the final step will be a crossbar linking the two stakes together from either side. Just a small 1x1 piece of lumber."

As far as not going any higher than a foot I have to disagree. I feel it is very important for the following reasons;

1) 2' Recommended Minimum Depth for raising the prawns.

2) The water is also acting as a thermal battery. The rocket stove will be heating my water (and possibly some electric heat). During the day the sun will help to heat the water and additionally I will be applying artificial heat. The large quantity of water will be very difficult to lower the temperatures of.

In regards to being four feet wide. I actually changed the design to be 6' as I realized I just did not give myself enough walking room, nor enough room for the rocket stove and required venting. It will require some basic tools in the fashion of a broom stick or a hoe to pull and push the floating rafts to me.

I agree that this may not seem as comfortable but that is not my goal here. Sacrificing comfort for production.

Kate Mink said:

Strapping wrapped horizontally around the trough will not keep the trough from bellying out in the middle. I have had good results from 2 approaches -

- nailing a length of 2 x 4 across the top of the trough, and

- putting stakes outside the trough walls, extending above the trough, one on each side, and using ratchet ties to hold the tops of the stakes together above the floats. You could use your steel strapping for this.

I strongly recommend against making your troughs too deep (anything over a foot is unnecessary) or too wide. More than 4 feet wide and it's all but impossible to reach into the middle to set pots, harvest, anything.

good luck.

[Jonathan Hall]

3D Aquaponics Greenhouse Model

This may help get the general idea across.

[Kate Mink]

Jonathan, I hope it works well for you. Please let us know!

[Jonathan Hall]

I will certainly do that! Hope the Prawn are worth all the extra effort! (and not die over winter)

[Kate Mink]

For an experienced perspective on prawns go here

http://www.friendlyaquaponics.com/support/newsletter-back-issues/

and look at issues 45 and 46.

Also http://www.fao.org/docrep/005/y4100e/y4100e00.htm#TOC