Reef Central Online Community - View Single Post - Wood base

kalare · 09/18/2017, 10:21 AM

I'm a structural engineer. I don't think you need 3 layers of plywood at bottom, but if you have it cheap, I suppose why not? How high is the water? I wouldn't span the plywood more than 2 feet without a 2x4 support joist, and I wouldn't span the 2x4 support joists more than 4-5 feet without a load bearing wall/post at each end. This will result in a super overbuild stand, but it would be my preference to limit deflection of the plywood bottom.

I'm curious, you have what is effectively a solid 2x4 wood bottom at one end where the spans are shorter, and at the other end, you have no joists? I really recommend adding at least 2x4 at 16" on center at the other end, especially since it has the longest span. Having a super strong base on one side of the tank and very light framing on the other side makes no sense, what works on one side should work everywhere, and I would note that I do not trust it to work. I would also recommend having the center load bearing support at the other end as well...if that's not possible due to sump or other issue, provide a network of 4x beams (deeper than 3.5" section).

The user that mentioned the pipe penetrations through the 2x4 members is absolutely correct. The amount of load those members can support will be tremendously reduced. shear capacity of those members at the penetration will be reduced by what looks like 80% or so, bending capacity, most like 50%.

For a tank this size, I would personally use 2x6 (or possible 2x4 @ 12" on center, with more support lines), and drop the plumbing below the framing, with absolutely no penetrations through the joists. Can you not add a support line on either side of the central plumbing line? The way it's currently framed you have a central double 2x4 spanning from center support out front, to the single 2x4 in the rear that is penetrated. You really need to add more support in this area somewhere.

Another note, your support lines are 2x4 and they will be clad with plywood? You should construct your supports similar to a typical wall, they will be much stronger that way and you'll use less wood. The way you have your 2x4 oriented makes them weak. The plywood should be attached to the small dimension of the 2x4, not the large dimension. As shown, the support walls can buckle out of plane much more easily. (Note, I've just seen some renderings further down the first page that appear to correct this issue)

Sorry if I've been blunt or offensive, just want you to be safe! I'm sure some of the above was not clear, let me know if you need more clarification. Good luck!

09/18/2017, 10:21 AM	#36
kalare Registered Member Join Date: Mar 2003 Posts: 399	I'm a structural engineer. I don't think you need 3 layers of plywood at bottom, but if you have it cheap, I suppose why not? How high is the water? I wouldn't span the plywood more than 2 feet without a 2x4 support joist, and I wouldn't span the 2x4 support joists more than 4-5 feet without a load bearing wall/post at each end. This will result in a super overbuild stand, but it would be my preference to limit deflection of the plywood bottom. I'm curious, you have what is effectively a solid 2x4 wood bottom at one end where the spans are shorter, and at the other end, you have no joists? I really recommend adding at least 2x4 at 16" on center at the other end, especially since it has the longest span. Having a super strong base on one side of the tank and very light framing on the other side makes no sense, what works on one side should work everywhere, and I would note that I do not trust it to work. I would also recommend having the center load bearing support at the other end as well...if that's not possible due to sump or other issue, provide a network of 4x beams (deeper than 3.5" section). The user that mentioned the pipe penetrations through the 2x4 members is absolutely correct. The amount of load those members can support will be tremendously reduced. shear capacity of those members at the penetration will be reduced by what looks like 80% or so, bending capacity, most like 50%. For a tank this size, I would personally use 2x6 (or possible 2x4 @ 12" on center, with more support lines), and drop the plumbing below the framing, with absolutely no penetrations through the joists. Can you not add a support line on either side of the central plumbing line? The way it's currently framed you have a central double 2x4 spanning from center support out front, to the single 2x4 in the rear that is penetrated. You really need to add more support in this area somewhere. Another note, your support lines are 2x4 and they will be clad with plywood? You should construct your supports similar to a typical wall, they will be much stronger that way and you'll use less wood. The way you have your 2x4 oriented makes them weak. The plywood should be attached to the small dimension of the 2x4, not the large dimension. As shown, the support walls can buckle out of plane much more easily. (Note, I've just seen some renderings further down the first page that appear to correct this issue) Sorry if I've been blunt or offensive, just want you to be safe! I'm sure some of the above was not clear, let me know if you need more clarification. Good luck!