Critique my self engineered beam

[Sam Shank]

I work in a lab, and we have very high (about 11' to the drop ceiling, and another 2-3 feet above that for mechanicals) ceiling.

I want to hang some rock rings http://www.metoliusclimbing.com/howto-trainrockring.htm I have 2 sturdy bench shelves that I can put a beam across. The beam has to be 13 feet across. The beam will need to hold my 200 pound weight without flexing.

Here's my thought: rip 3/4" ply into 6" strips. Laminate them together as follows:

8'-5'
5'-8'
2'-8'-3'

So, I'll have 3 strips of 3/4" ply, 6" wide laminated together. Do you think this will be adequate? I will hang the rings in the middle, and then hang off the rings.

Thanks for your input!

Sam

[dan moran]

could you use a pair of 14' SYP 2x10s??

seems like it could save you a lot of thought and glue

[Kyle Kraft]

My engineering professor told me and I quote, "There is always deflection under load." The point being you can't "zero deflection" but you can have tolerable or perhaps even really super hard to measure deflection.

[Sam Shank]

I could use a pair of SYP beams, but I want to be able to transport it up to the 8th floor easily in the elevator, and not attract too much attention carrying a 14 foot beam from my truck, though the lobby, into the elevator, down the hall, and into my area.

If I break it up like that, I can bring it in here without raising too many eyebrows.

Yes, yes, I know that there will be SOME deflection, but I just don't want it to ever break, and to sag as little as possible.

[Jamie Buxton]

You'd get a stiffer beam if you laminated solid lumber instead of plywood. Half the wood in plywood has its grain going up-down in your beam, and it adds almost nothing to the stiffness of the beam. The other half has its grain running horizontally, and that's what resists stretching, to provide the stiffness. The solid-lumber version would be twice as stiff, because all its grain runs horizontally.

You could sneak clear 1x6 boards into the building just as easily as you can plywood cut into 6" wide strips.

[Art Mann]

Why not use pine lumber of the same dimensions instead of plywood? It will be stronger and more rigid. If you are trying to sneak the materials up to where they will be used, isn't 8' a little conspicuous? I think under this circumstance, I would be inclined to use more shorter pieces. By the way, in order to get a good joint, you will need quite a few clamps, say 20 - 30 if you build it all at once, to get adequate clamping. Lugging that many clamps through a building and up an elevator is going to be pretty noticeable. By the way, you will need to attach a fairly wide piece to the top or bottom of the beam as a stiffener to prevent horizontal swing or whipping action.

[David Epperson]

As was touched on earlier, what you have planned MIGHT hold the static load you want, but when you introduce any dynamic load to it, it could buckle on you. At the very least there will need to be a top flange of some sort. A box beam might be simpler to accomplish successfully. Also you mentioned the height of the ceiling, but not the height of the support shelves.
I'm thinking of (2) 2 ply supports built as you described 8'-5' and 5'-8' about 4" to 6" apart with a top strap of 3/4" ply to form an upside down "U" shape. Attatch the ropes to 8" 2x4 stock at each anchor point on top of the top strap to spread the loads to all members. You might even cut the beams to 4" tall at the ends and 8" in the center for a prestressed truss shape. All this could be bolted together as the laminating clamps, and the bolts left in. Carriage bolts with round heads to the outside and nuts/washers to the inside. The lower edge being the tensile edge here could be strengthened with a steel rod/turnbuckle arangement or sreel cable. A wooden lower member would do best with no joints, but since the rest of the structure would then be stout enough without it, the joints wouldn't be a strength issue, simply cosmetic, as there will be "some" movement at these joints.

Just rambling, but hope it opens up some options and thoughts.

[Montgomery Scott]

Having a constant width along the height of your beam only increases weight, not strength. Sections are much more efficient: The upper and lower flanges transfer bending moment and the web sections transfer shear. Your best bet is to make a rectangular section or I beam with plywood for the web(s) and solid lumber for the upper and lower flanges. The plywood will transfer vertical and horiontal shear and will resist diagonal compression due to the shear because of its alternating ply orientation.

Since your beam is 13' long and will have a pinned-pinned end conections (I assume you are securing the ends somehow) the bending moment at the middle due to a 200lbf load will be 1300 ft-lbf. A mechanics of materials book or web pages on this subjet should assist you in determining the minimum section properties required.

[Sam Shank]

Thanks for all the suggestions.

I'm not trying to 'sneak' it up, just not carry a 14 foot single beam which will be really heavy and bulky and clumsy without another person.

My plan was to glue then screw the ply together.

I had thought about mixing solid lumber and ply, but wasn't sure of the best way to go. I plan on doing at least some assembley in the lab. Which is why I thought glue and screws would be most portable and work best. I could even bring my epoxy in to glue it.

It will be just resting on top of the shelves, not really secured in any way. I will probably lay down a 2x4 where the shelf support is to hold the load in the strongest part of the shelf (steel and 2x frame). They're extremely strong - I've climbed on them several times while almost fully loaded with catalogs and boxes of papers with no flexing at all.

I used to do pull ups all the time from my rafters in my old garage. Those were 2x10s or 2x12s and held me just fine.

I like the idea of a box beam. That would be simple and could be done in sections, and assembled up here.

5'-8' 2x10 side
8'-5' 2x10 side
2'-8'-3' plywood laminate (1.5" thick) top
3'-8'-2' ply botom.

Glued and screwed into a box. Ply could be about 12" wide to make a square box.

My main concerns now are not that this will be stong enough overall, but that the seams will be strong enough.

[Brian Walter]

I agree with Montgomery Scott. From an engineering standpoint he is right on. Have you considered simply using a section of aluminum extension ladder or a painters scaffold? Obviously you'd need a good extension ladder to support the weight, but you might not be so conspicuous carrying a ladder.

[Brian Walter]

Sam, you might just consider laminating 4 - 2 x 6 on edge, staggering the joints. That should be plenty strong, might flex a little more than you wanted, but it would certainly be easy enough.

[Sam Shank]

Yeah, I'm probably going to end up doing something like that. I want it easy to assemble, move, and most importantly - CHEAP.

""Sam, you might just consider laminating 4 - 2 x 6 on edge, staggering the joints. That should be plenty strong, might flex a little more than you wanted, but it would certainly be easy enough."

[Jason Roehl]

You might want to just do 2 2x10s, as they will be yellow pine. 2x6s will likely be white pine (much softer/weaker), potentially with knots, unless you get treated lumber. I'd bet you get less flex with the 2 2x10s than with 4 2x6s. I'd also make sure you build some sort of "stand" for each end--like a triangular plywood piece to make sure you don't roll your beam at an inopportune moment.

Don't overthink or overengineer it. You could do a search on span ratings for framing lumber. I think you'd be surprised by how little you actually need. You probably wouldn't even really notice 1/2" or less of flex.

[Paul Simmel]

How about a truss.

http://www.alpeng.com/upload/19269/sy42.pdf

[Per Swenson]

Ahh, Engineers over thinking again.

Here's my advice, build it. Stop thinking.

By the time you stopped worrying about it, you could

have finished the beta model and your Butt would have healed.

Per

PS

And you would have learned more.

[James Suzda]

I’ve always wondered if my idea would work and maybe with this thread I could get an answer.
Would there be an advantage to laminate strips of sheet metal between the 2 by’s when assembling a “engineered” floor joist or in this case a wooden beam? I am of the belief that the tin would never bend if sandwiched and nailed between the wood and thus would add a lot of strength without much additional weight.
Jim

[Brian Walter]

You might want to just do 2 2x10s, as they will be yellow pine. 2x6s will likely be white pine (much softer/weaker), potentially with knots, unless you get treated lumber. I'd bet you get less flex with the 2 2x10s than with 4 2x6s. I'd also make sure you build some sort of "stand" for each end--like a triangular plywood piece to make sure you don't roll your beam at an inopportune moment.

Jason, you are correct that the pair of 2 x10's would be stronger, but you can't piece them together the way he wants to if you only use 2 of them. That's why I suggested using 4 pieces, you can distribute the forces between the laminations better with more pieces. Also, with something that would be 6" wide by 5.5" high, you wouldn't really need to build a stand for the end either.

[Montgomery Scott]

James,

Your idea would work to some degree but probably not as much as you would hope. What you really want is the strongest material on the outer faces to carry compression and tension stresses due to bending. The area in the middle should be designed to carry vertical, horizontal and transverse shear (VQ/I) which is why beams have a thin web and large upper and lower flanges. I don't think that having interlaminar metal members will help much in absorbing transverse shear.

[Jason Roehl]

Ahh..I see what you thought I said/meant/typed, Brian. I really meant two LAYERS of 2x10s vs. 4 LAYERS of 2x6. He does need to fit the pieces in an elevator. 4 chunks of 2x10 glued and screwed together with significant overlap to make a 4x10 beam would be what I envision.

[Brian Walter]

What I was getting at Jason is that your weakest point in the beam would occur at the joint, and if you are only using 2 members, you only have 1/2 of your maximum strength at that point and if you use 4 members and stagger your joints correctly, your weakest point will be 75% of maximum. Obviously you would want to locate your joints to result in the weakest point not occurring in a critical location. In a narrow beam such as he is looking at, it would also be best to use an odd number of laminations, it allows you to stagger your joints more uniformly. All in all, we are way over engineering this rather simple problem, but it does provide a good mental exercise.

[Jason Roehl]

What I'm getting at is that ONE 2x10 is probably sufficient, but he wouldn't be able to get it up to the room or office. Two 2x10s merely allows him the luxury of smaller parts. I've walked on a single 2x10 spanning that distance laid flat. There was a lot of deflection, though! Stand it up, and you won't notice a 200-lb load over a 13' span. Glue and screw 2 of them together, and it may not be quite double strength, but it will be a heck of a lot stronger than a single. The glue and screws go a long ways in overcoming the weakness at any joints.

[Paul Simmel]

James, it’s called a “flitch plate beam”, but tin wouldn’t add any appreciable strength IMO. See these links for further info.

http://en.wikipedia.org/wiki/Flitch_beam

http://www.betterheader.com/

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IMO, Jason is correct. But, going along with the mental exercise, a truss could be constructed with many small internal parts, sandwiched between sections of bb ply skins, all glued up with construction adhesive/screwed. That would be one stiff “beam”.