Dimensional Lumber Variations (2x4s of different weight/density) - Woodworking Talk - Woodworkers Forum
 
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post #1 of 8 Old 08-02-2011, 10:11 AM Thread Starter
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Dimensional Lumber Variations / Aquarium Stand Design

Hello...first post here but I plan on asking you folks a lot of questions as I embark on my first woodworking project (besides a decade ago helping my father with decks, sheds, and home improvements, i have not done any such work myself as an adult)

I am building an aquarium stand for a fish tank that will weigh around or north of 400 pounds after including water, rock, equipment etc. 36" wide x 18" footprint.

I have already cut out the 2x4s for the frame, which will consist of an upper and lower rectangle matching the tank footprint and legs in between. Not using any joint technique other than vertical load is transferred down onto the leg below via a butt joint and not shear-force on the screws. In fact the boards are held together via Simpson Strong Ties (just holding things in position; again, only wood in a vertical configuration is actually bearing any load) and additional strength will be provided by 1/2" or 3/4" plywood skin to prevent racking. This particular design has been used in dozens of tank stands I can find on the internet and has held up well. Unsure if I can link to other forums or show pictures I didn't take/create but I searched the web for "40 breeder stand" in order to find the design plans.


Now to my question:

I was assembling the frame last night so I can seal and waterproof and move on the upcoming weekend, and during assembly I noticed that the 2 x 4 26" leg in one hand was heavier than the leg of the same length in my other hand. Closer inspection revealed that one leg had a denser wood ring pattern.

Should I be concerned about the other leg being weak? If I had only gotten the "lighter" 2 x 4 lumber, I never would have known there would be a difference, so I tend to think I'd be OK using it. Or should I be on the lookout for "dud" wood?? I know to avoid large through-knots, cracks, etc, but wasn't really thinking about checking the densities of the individual boards.


Thanks for input

Doc7

Last edited by Doc7; 08-02-2011 at 11:24 AM. Reason: title change
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post #2 of 8 Old 08-02-2011, 10:39 AM
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I wouldn't worry about it personally, stud lumber is pretty variable and there's no way for you to know what will and won't make it stronger or weaker. It would be important for it to be identical wood type - occasionally they change sources, offering pine instead of hemlock or fir. That's not to say pine is weaker than fir or vice-versa, it's just different.

Personally for a 400 lb tank, I'd double up the legs to 4x4, a 2x4 gives a pretty narrow mounting surface. I don't see it as being too weak though. I would NOT rely on Simpson StrongTies.

Rob

"Sometimes I lie awake at night and wonder - what have I done wrong. And then a voice says to me, 'This is going to take more than one night.'"
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post #3 of 8 Old 08-02-2011, 11:22 AM Thread Starter
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I don't want to link to another forum or someone else's photos, so here is the sketchup I made for my original design. Please note it is different in actual construction - the bottom frame is now identical to the top frame instead of the legs going to ground (ie, legs are now 3.5" shorter but total height is unchanged because they end up on top of the lower rectangle). It's the latest SketchUp I have though. The "actual" build is identical to the one I found elsewhere online.


I waffled a lot about changing it to my personal design shown in this sketchup and I still think it might have been better however, almost all tank stand designs on the internet even for MUCH larger tanks incorporate this frame. It will make shimming for level more difficult (will have to shim evenly across frame I think instead of just dealing with 4 legs).

The 90 degrees between horizontal pieces and legs have strong ties holding them together that wrap around 2 sides of each pieces. Here is an image from Home Depot of them:




Additionally it will be skinned with plywood to prevent racking. As you can see no vertical forces are transferred to the strong ties or screws.

Last edited by Doc7; 08-02-2011 at 11:24 AM.
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post #4 of 8 Old 08-02-2011, 03:04 PM
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if it is going to way 400 pounds you should realy think about doubling up the legs
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post #5 of 8 Old 08-02-2011, 03:17 PM
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I definitely wouldn't worry about the density differences of the legs if it will be skinned - the compression rating of stud lumber is considerable, much greater than the 100 lbs you'll put on each one. the strong ties will take some shear unless you can get those leg dimensions as tight as you can. Mind you I don't have anything personally against shear ties like a Simpson StrongTie, in fact I've used them extensively in non-critical structure or structure where looks are unimportant, I just tend to think a well built joint only subject to static forces doesn't require them. I use them out of laziness more than anything.

If you're comfortable about the design then don't worry about my comments on doubling up the legs, with the added skin it shouldn't be nearly a problem I thought it would - just remember that the skin accepts the shear forces so tying it to the underlying frame is important. The overall structure should be very sturdy if everything is cross connected.

The moderators may have their own opinions on pics (and listen to their word), but if it clarifies your post then by all means.

Rob

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post #6 of 8 Old 08-02-2011, 04:04 PM Thread Starter
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Thank you for your comments.

I do have one more question and I am glad I thought to ask now prior to staining and waterproofing the 2x4 frame (which I wanted to do before skinning in order to ensure protection of the most important part of the structure)

I plan on routing out 1/4" deep of the back side of the plywood skin where it will lay over the Strong Ties (should be about a 4 inch by 4 inch "L" in each corner of the skin pieces for the east and west sides) - this way, the skin will lay flush against the 2 x 4 framing.

Should the skin be nailed, screwed, or glued into place? I was originally thinking of using small nails but given the structural integrity it provides, should it be glued?

same goes for the Face Frame which I will overlay with solid (1x4s or similar) boards at the top, bottom, and down the middle where cabinet doors will meet.


Or is there no avoiding Kreg Jigs?

Last edited by Doc7; 08-02-2011 at 04:06 PM.
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post #7 of 8 Old 08-02-2011, 04:54 PM
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I would not use pocket screws for assembly. Structurally your best bet is to construct so the load is supported by the floor. Preventing racking and twisting will keep you from a lot of mopping.

You can use 2x4's for framing and clad with 3/4" plywood. I would screw perpendicularly through the 2x4's into the plywood. Here is a quick layout to make your stand completely from plywood, with no 2x4's.
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Examples of aquarium cabinets done with just plywoods, clad with either veneer, or tambour.
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post #8 of 8 Old 08-02-2011, 05:38 PM Thread Starter
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Thanks again for the comments!


I definitely plan on having the 2 x 4 frame transmit the load to the floor. The plywood skin is there for rack/twist prevention (and of course, aesthetics) The 2x4 frame will be shimmed at intervals on all 4 sides both to achieve level which is important in this application, and to prevent plywood from taking vertical load-bearing (which, in this case, would actually be upward, caused by contacting floor and "pushing")


If I am using 1/2" or 3/4" plywood to skin the sides, and screwing through a 2x4 into the plywood, how much of the skin do i need to "catch" to provide the rack-prevention I am aiming for?

With 1/2" Ply, I am looking at a total of 1.5" 2x4 + 1/2" ply or 2 inches of wood ... is a 1 3/4" wood screw, maybe every 6-12 inches around the entire frame, enough for this application? Is that enough grab?

If screws are the proper tool and not wood glue for the plywood I will be staining/waterproofing the entire 2x4 frame prior to skinning/trimming.

Thank you again!
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