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 lawdealer 10-07-2009 08:48 AM

How much load weight can a 4x4x14 redwood beam bear

I have an old house with a second floor with 3x3x14 rough cut redwood as the floor joists. I attached to them 4x4x14 redwood and put on top tongue and grove sturdy floor 3/4 inch. So combined, I have basically a 7x7x14 floor joist. My son wants to put a 90 gallon fish tank in his upstairs bedroom. It would lie on a single beam, maybe 2 (i cannot tell as floor is covered) running length wise on the beam. As water weighs 8.3 lbs per gallon, I estimate the load to be around 800 lbs US. Will that support the load?

 cellophane 10-07-2009 09:23 AM

somewhere in my notes from structures i have the formulas for figuring that out. if i can find them tonight i can drop them in here for you, unless someone else knows it off the top of their head.

in theory however a Wx14 beam should be fine as the fishtank is a distributed load over your floor supports which are (in theory) spaced at 16" O.C. max. if your tank is 48" wide it spans 4 beams and your 800 lb load is divided by 4 and then is factored by the distribution. i am however not an engineer and its been 5 or more years since i took structures.

those links have some beam load calculators. the formulas are correct but i dont remember which ones to use :blush:

 wrangler 10-08-2009 07:07 AM

Either I am misunderstanding your post, or your math is in error.
A 3x3 atop a 4x4 would at best give you a 3x7 since an inch of the 4x4 is not supporting anything, and depending on how they are attached to each other, I'm not sure that it even has that much lateral strength. An architect/engineer could better answer your question, but I'm sure they would need more info as well.
JMTCW

 Scuba_Dave 10-08-2009 08:04 AM

Usually recommendation is that larger fish tanks sit perpindicular to the joists & on an outside load bearing wall
I have a 125 tank & I installed it directly over a beam in the basement

 MI-Roger 10-09-2009 07:31 AM

Any one do any shear plane calculations?

The joint between the 3 x 3 and the 4 x 4 does not lie in the center of the composite beam. All loads placed on this combined member will result in a shear force parallel along this joint. The only thing resisting the shear force is the quantity and thickness of the nail/screws or whatever is holding the composite beam together.

The limiting factor may not be strength of the wood.

 Scuba_Dave 10-09-2009 07:39 AM

Quote:
 Originally Posted by cellophane (Post 337490) as the fishtank is a distributed load over your floor supports which are (in theory) spaced at 16" O.C. max. if your tank is 48" wide it spans 4 beams and your 800 lb load is divided by 4 and then is factored by the distribution. i am however not an engineer and its been 5 or more years since i took structures.
Except he stated the tank will run parallel to the joist & thus only sit on one joist - maybe 2

 Daniel Holzman 10-09-2009 10:00 AM

I could not tell from the OP whether the 4x4x14 was attached to the bottom or the side of the 3x3x14 beam. If attached to the bottom, the post regarding the need for proper shear connection between the two beams is spot on. If attached to the side, the connection is not as important. The moment of intertia of the composite section is completely different if the beam was attached to the side versus the bottom.

I also was not clear whether all of the joists were added on to in the same manner, or possibly only the one that is going to support the fish tank. Since the tank is apparently aligned along the axis of one of the joists, it may lie entirely on one joist, in which case it would have to be treated as effectively a point load. If it is a point load, the critical factor is the distance from the support ends, along with the weight. Once we know the method of construction of the beam, and the location of the tank, we can compute the bending stress and estimate the factor of safety against failure.

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