# I Can't Do The Math

Discussion in 'Boat Design' started by ElGringo, Dec 23, 2016.

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### ElGringoSenior Member

I want to make a light weight removable cover approximately 4 feet by 8 feet. It would need to support people walking on it. Then I started thinking about engineered wood "I" beams. The kind made with 2 X 4's for the top and bottom and plywood for the web. If I made them out of 1 X 2's and 1/4" plywood with the overall height of 6 inches, a free span of five feet, how much approximate weight would each beam support?

I looked everywhere I could think of but could not find any information on compression of the plywood and probably could not do the math if I found it. Can someone point me in the right direction or better yet just tell me how much a beam of that size would support?

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### upchurchmrSenior Member

Build one and do a test.
Use some extra weight to give yourself a margin of safety.

Relatively cheap and more sure than a calculation.

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### BarrySenior Member

You are making an 8 foot by 4 foot panel but you said the span is 5 feet. So the hole then is 5 feet?
Does this mean that there is a 1 1/2 foot overhang on each end of the cover

How many people does it have to support at one time. Need to get a load value.

Is the side of the panel supported along its longer length?

For a 5 foot open span and if the sides of the cover are supported I would not be leaning toward a plywood I beam.

If the sides are supported, I would run the stiffeners crossways, ie across the 4 foot opening if you want to minimize the thickness of the beams

Also, you need to have thick enough plywood between the beams so you do not have an unacceptable deflection between the beams. Ie if you used 1/4 inch ply for the cover, and say 12 inches between the beams, the 1/4 ply would flex to much to make a comfortable floor to walk on, ie too flexible.

So perhaps you might be looking at 1/2 inch ply to minimize this flex/deflection, now if you add a number of 6 inch beams to carry the load, you are adding unnecessary weight building the beams, ie 1 x 2 flanges, 1/4 inch webs.

Ie the actual hole size that you are trying to cover, is the side of the cover supported, how many people might be standing on the cover at any time. And do you have a weight of cover target. IE can you handle a 50 pound panel, or 75 pound panel or

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### ElGringoSenior Member

I think you may be right. How would you do the test? Throw a rope over it, tie a hoist to it and start lifting things until it broke?

You would think that the strength values of plywood would be available. I might have to wait until after the Holidays to do it and of course I want to know right now.

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### ElGringoSenior Member

Barry, it has two beams running across the opening five feet apart, but the hole I want to cover is a total of a little less than eight feet. Probably wouldn't be more than 3 people in a four by five space? They use 1/2 plywood in I joists for houses, surely I wouldn't need it that thick? I will be the one lifting it and I have Degenerative Spine problems. It will cover an old, unused stairwell that I might want to have access to at a later time.

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### TungstenSenior Member

Pending on how big you are,i would just frame in the opening and lay plywood ontop.This way your only lifting the plywood when you need to get in.
If you need a bigger opening then frame in 2 parts, hinged on each end,center support for the doors to rest on.

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### RurudyneSenior Member

You should be able to find something of use on this page: http://www.apawood.org/structural-composite-lumber

Another approach is to go to your local Half Price Books, or equivalent, and look for old handbooks on wood structure design.

Finally, there will be building codes in your area that may give you a ready made solution, as these tend to be over engineered.

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### ElGringoSenior Member

Rurudyne, it didn't have anything about plywood but I didn't know they made Oriented Strand lumber 2 x 4's. I've never seen any and it was interesting.

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### BarrySenior Member

Are the beams going through the opening part of the floor structure on either side of the opening? IE do the go through the opening then on through to another beam that holds the rest of the floor up
Are you putting a covering on the plywood
If you can put hinges on the 8 foot side of the sheet, the weight that you have to lift, ie the force will be half the weight of the panel

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### ElGringoSenior Member

The stairwell was torn out and structural beams added by someone. If you were standing like you were going to walk down the stairs, the beams would be running from left to right, and several inches below the floor level. The rent contract will not allow me to make any changes to the building.

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### upchurchmrSenior Member

ElGringo,

If your issue is walking on the cover and you expect the beams to support most of the weight, just make a beam, block it up off the ground about 6" then walk on it.
If it seems good, then get another person to step on it in the middle with you. Doubling the weight.

Don't try it over the hole for the test.

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### tom28571Senior Member

There are several people here who could develop what you need but complete requirements would be helpful. Feeding the information bit by bit like this is exasperating to anyone trying to help you. It appears that you would be best served by a torsion box design. I can say that 1/2" ply top and 1/4" for the bottom with cross beams of 1 1/2" square spaced on 12" centers would be plenty strong enough with no additional support. Even 3/8" on top would be good. One 1 1/2" interior beam on each side would be nice but not required.

A torsion box is really a connected parallel set of I beams and way stronger than just beams under the bottom.

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### ElGringoSenior Member

Tom, building it as a box is not a problem, I just really didn't know what I needed. I do know that a lot of people here deal with some very strong beams so I decided to check here first I will also need to stop the ends from moving down when stepping on the very ends of the cover and I can't remember what it looks like or how to attach something. I wont be able to get back into the building until next week.

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### tom28571Senior Member

Just make the perimeter like the beams with 1 1/2" square stock. If you need to anchor it, you might make the perimeter of 2 X 4 laid flat but its not necessary for strength.

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