Which is stronger? 2x4 vs 2 strips of 3/4" plywood

[ronl]

I have read the debates about 2x4 framed stands covered in plywood vs plywood only construction. Uncleof6 has expressed his dislike for the use of 2x4, including several disadvantages of using dimensional lumber, such as warping, twisting, expanding, shrinking, and not being able to get a perfectly straight 2x4.

Assume for now that the design of a particular stand will require the use of a 2x4 frame. Would it be better to use 2x4's or 3 1/2" wide strips of 3/4" plywood glued and screwed together. The expense of plywood would not be a factor because the side panels will be 31 1/2" wide, so I would have 16" x 8' of leftover plywood that I could rip into 3 1/2" strips. Glueing 2 of these strips together would give me the same dimensions as a 2x4.

So which would be better for the stand frame?

Thanks,
Ron

 

[peppie]

Rocketengineer will be able to tell you the load comparison of laminated plywood vs 2 x 4
But I am pretty sure you would be better off with the 2 x 4 as a vertical load bearing member. I think the laminated plywood would crush easier.

 

[Rancherlee]

Personally I'd take double 3/4" ply over a 2x4 any day for strength. Basically it's what they call "engineered lumber". Just make sure it's quality exterior grade plywood, not some cheap 4 or 5 ply interior stuff. I've been using Arauco 7ply 3/4" for a lot of different projects and its very good plywood that's ACX grade.

 

[sirreal63]

The difference between the two is grain. With solid lumber the grain all runs in one direction, in plywood it is opposite for every layer, reducing strength, engineered lumber like Parallam has all the grain running in one direction, just like solid lumber, but the wood is comprised of strips oriented and compressed, but the grain still runs in one direction, hence the strength. MicroLam, or LVL is made up of strips like plywood, but all of the grain runs in one direction, giving it compressive strength, but is as floppy as a wet noodle when laying flat. Plywood is not engineered to hold a load in a vertical orientation, it is engineered for horizontal loading. It still works fine for our purposes though.

I am on the road today so I won't have access to load tables, but the solid wood is stronger. As far as the difference in interior or exterior plywood, the difference used to be mainly the glue, but all plywood is now bonded with the same glue, they stopped switching out the glue long ago, it is cheaper to use exterior glue than it is to switch out, at least in this country, I cannot speak for import plywood.

As far as solid lumber twisting and shrinking, it can happen but is dependent on the wood species. Southern Pine is very much less dimensionally stable than SPF, which is very stable. Wood can move from moisture leaving the cells, most lumber is dried to a moisture content of 19%, (construction grades, not furniture) which is an average of all of the wood in the kiln, some may be more, some may be less. As soon as it leaves the kiln it will continue to dry until it reaches equilibrium with the environment it is in. Less dimensionally stable wood can and frequently bows, twists and warps as it continues to dry but the change in moisture content is slow which lessens the issues with warping etc. Unless you are using Southern Pine, I would not worry too much about warping, twisting or shrinking.

For the record, I have been in the wood industry for almost 30 years, managed a sawmill and treating plant and have distributed lumber all over the the Americas.

 

[KCombs]

I've done a couple of "edge on" plywood stands...
the plywood definetly takes up less space, but a MAJOR caveat nobody mentions is the cut edge can absorb salt-water and expand/warp....
on my 2nd build I sealed the heck out of the edges (and/or used glue)

 

[StaghornE]

2 pieces of plywood laminated together will not be stronger. If you insist in having these columns in your stand made out of plywood then you could make some 3x3 or 3x4 columns using 4 pieces of plywood glued and screwed together to make a "box" which would support a great deal of load. I would build the stand without any dimensional lumber your 3/4 plywood sides will hold up whatever aquarium you put on them as long as they are held in place vertically. Good luck

 

[PowerNap]

Laminates have a superior strength to weight ratio, but dimensional lumber is easier to work with.

Follow the KISS principle.

On my build I glued and screwed, 2x4 framing. The verticals were doubled up. Once the frame was complete I sheathed the entire thing in particle board, then in 1/8" oak ply, and then trimmed out with decorative touches. In the event of an earthquake, seek shelter under the tank.

 

[BigAl2007]

Great write-up Jack! Well said.

I happen to "Be in that industry" and I can assure you most of the tanks we build stands for are not anywhere near "failure weights" on materials.

With that being said.. if you want a VERY good and VERY straight piece of dimensional lumber ask your local lumber dealer for "Framers Series" from iLevel (Weyerhauser). Every inch of every board has been tested and is so straight (not just now but for the life of the product) that they have to mark the boards for crowning (construction term) because to the human eye there is no crowning. They cost a few cents more and not every lumber yard carries them (although they should) but they are more than worth the difference.

http://www.woodbywy.com/lumber/framer_series.aspx

I have several customers now who ONLY use Framers Series for the home builds. That says a lot for the lumber.

 

[BigAl2007]

Personally I'd take double 3/4" ply over a 2x4 any day for strength. Basically it's what they call "engineered lumber". Just make sure it's quality exterior grade plywood, not some cheap 4 or 5 ply interior stuff. I've been using Arauco 7ply 3/4" for a lot of different projects and its very good plywood that's ACX grade.

Plywood in and of itself is very much NOT engineered lumber. It's merely "oriented" and in no way engineered.

 

[ronl]

Thanks for all of the feedback everyone. I have used fir 2x4s in the past from HD because they look nicer than the standard 2x4s. I think this would be the SPF that sirreal63 mentioned. Not sure if I would have access to a lumber yard to get the iLevel framer wood that BigAl2007 mentioned. I was always under the impression that lumber yards only sold to contractors, not 2 or three pieces to a retail client.

Let me explain what I am trying to build. As background, I had a 58g tank leak last year and ended up with about 15-20g of water soaking the carpet, floor, and leaking into the finished basement below. Obviously, the wife was very unhappy. Much to her dismay, I still have not painted the repaired ceiling and wall in the basement. I promised her that any future tank would be smaller and I would somehow build a stand that would hold the water if the tank leaked so no damage to house.

I am planning a Deep Blue 30g Reef Ready shallow cube (24x24x12). I planned on building the stand with a 30x30 frame then covered in 3/4" plywood. So stand will be 31.5 x 31.5 and the top of the frame would be covered with a sheet of 3/4" plywood. The sides of the stand will come up 3" higher than the top of the frame. The tank will sit on a 3" high platform on top of the frame and 3/4" plywood sheet. So I will have a 3" wide and 3" high channel around the platform on which the tank sits. The top will be sealed so it can hold water and there will be a bulkhead under the platform with a hose to drain into a 5g bucket in case the tank leaks. The bottom of the frame would use 2x6s and also sealed so that the bottom of the stand will hold water. So inside bottom of stand would be 27 x 27 x 5.5 and sump and bucket will sit on platforms to raise them up. Bottom of stand should be able to hold about 17g of water in case of leak.

I have attached sketches of what I have described. First pic is a cutout of the side view of the stand. The second picture is the top view of the framing on which the top sheet of 3/4" plywood would sit. Since the tank is 24x24 and the inside of the frame is 27x27, the edges of the tank will not sit directly on the vertical supports. So I need to attach additional material to the top of the frame to provide added support to the top sheet of plywood. Looking for the best way to do this so the plywood sheet does not sag. Since the tank is fairly small and shallow, I do not think I will have too much trouble with sagging.

Thoughts?

 

[ronl]

The two holes shown in the top view picture are for the return plumbing, which will come up over the back of the tank (doing a Herbie using the two hoe in the tank) and for the light support, which will be bent conduit. These two holes will be sealed and the bulkhead to drain any tank leaks will be in the middle under the raised platform.

 

[PowerNap]

Build the sand to contain 10-15 gallon of water, then throw in a $15 leak water leak detector from home depot. For a minor leak the alarm will give you plenty of time to react. In the absolute worst case, where you have another major leak, you can always deflect blame to the “POS LEAK DETECTOR!!!”. Nice and easy!