Another Beam size

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  #1  
Old 11-05-08, 06:11 PM
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Another Beam size

I have another one of those girder/beam span questions. But what I dont understand is with the ease of creating your own beams and the availability of engineering software why I cant find any tables on line for girder size versus span length for something like shown at the link below. Please look at the drawing at this link for the following girder questions. If you have one of those software programs could you please run a few numbers?

drawing

1. Is the shown beam overkill to support the 2nd floor loft bedroom joists as shown? Figure 40psf not 30psf, please.
2. How far could the 24 span be increased with shown girder?
3. Would it be possible to use 2, 2x8s instead of 2x12s for the girder, resulting in a beam 3x16x24 instead of 3x24x24?
4. How does it affect all the above questions if the bedroom joists are 2x8x12 instead of 10 ft long, leaving a 1 foot overhang like shown?
5. Would it substantially affect the above if the lumber in the center of the girder were replace with two more plywood layers instead of the 2x12s or 2x8s?
6. Would the use of OSD instead of cdx substantially affect the results, and how?


P.S. It also seems that someone should post some gluelam span tables for some common arch configurations.
 
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Old 11-05-08, 09:21 PM
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This would likely fail inspection. Beams and columns must be continueous.What you have is two beams. You cannot substitute a stick for ply wood, you may however make it stronger by adding. The only thing that you can do from your list is cantelever the floor joists, one foot overhang is nothing. Usually 4/1 for cantelevers, but Ive had engineers allow 3/1 for some applications.

My span table is different than yours. I dont want to give you false information. Buy a code book from your city. A span this large would have to be steel in my area, or engineered(anything over 16').
 
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Old 11-06-08, 09:36 AM
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first, (2) 2X12's stacked, sandwiched between plywood to form a 3x24 is rather ridiculous. in fact, standard lumber won't make this span at all, and will not pass inspection. so i ran the numbers for you, LVL is what you will need:

assumed values: (your drawing is unclear, so they're assumed)
tributary length: 5'3"
@ 40 psf DL+LL

SO, this span will require a 2-ply 1.75x16 LVL, so yes you can get down to 16" beam depth, you could even do 14" with 3-ply.

also, there'll be over 2700 lbs. coming down @ each beam bearing. your best bet would probably be some sort of steel post at each end.

hope this gets you on the right track.
 
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Old 11-07-08, 04:46 PM
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ok

Thanks for the info.

So if I can rephrase. If I took 3 or 4 layers of plywood 3/4inch by 16" and laminated them to form a 24 foot beam it should work in the discussed scenario?

If that is so, then how far would a 24" span rather than 16".

Obviously you guys really dislike the 2x12 sandwich. But any glue lam is stacked wood. 2x12 is just unconventional because of the height to width ratio, but the plywood sandwich helps with that. It is certainly not necesssary to have one continuous piece of wood for a glue lam beam. I have seen many in commercial buildings. You just have to stagger the joints.

I don't know what you mean by tributary length.
 
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Old 11-08-08, 05:03 PM
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the only convention i see people using as far as plywood and beams go is sandwiching a 1/2" layer between two 2x's to form a beam that is 3.5" wide like a standard stud wall. it stiffens the beam a bit, thats all.

But any glue lam is stacked wood.
sure, but i think you're over generalizing. and it is obvious that your are trying to save as much money as possible. you cant create your own glu lam sitting in your garage with a bucket of adhesive and pieces of plywood. this stuff is manufactured with epoxies, that first i doubt you could get ahold of, and second probably wouldnt have the means to handle properly. but if you are really adament about this talk to a structural engineer, i wont go down that path (likely they wont either).

like i said, you are going to have to purchase and engineered lumber beam as a solution for your problem. that is the minimum.

I don't know what you mean by tributary length
google tributary length, beam sizing or something and i'm sure you could readily find a diagram that would make it clear to you.
 

Last edited by OhioDraft; 11-08-08 at 05:05 PM. Reason: added qoute
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Old 11-15-08, 04:41 AM
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Thank you again. Let me rephrase the question in a way the enables a more palatable answer. Given a standard engineered LVL 3"x24" what span would it support for a floor of 40psf DL+LL, and 50psf DL+LL.
you cant create your own glu lam sitting in your garage with a bucket of adhesive and pieces of plywood.
We will just have to disagree on this one. I have glued wood "in my garage" where the wood ripped apart before the joint. There are plenty of good strong wood glues available and easy to use. You don't need any stronger than that.

The mentality on the internet and the industry on this subject amazes me. There are plenty of tables on the internet that show span, girder size, and load relationships for std 2x? at std lengths. But surely it would be a heinous dangerous sin to do the same for a beam glued together from std. plywood with std. glue. Since there may be some variability in strength of the plywood, glue and bond, factor in 25% increase in strength. It would be a quarter the cost of of paying engineers and beam manufacturers to over charge for their products. I suspect If I were financed to do so I could have a load of plywood delivered with some glue and a few tools and build a hundred beams, and that testing would show relatively little variation in strength.

Engineers do it all the time with concrete. I have read several industry white papers on concrete arches. The fact of the matter is that the mixes, dry time and conditions can vary so widely that they factor in a huge margin of error, because they simply don't know how strong any given cured pouring will actually be. Duh, do the same for wood girders!

google tributary length, beam sizing or something
BTW I tried several googles for it before I asked.
 
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Old 11-16-08, 05:54 PM
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most, if not all engineered lumber manufacturers stop at 18" depth so my software won't do a 24" deep calculation. but a 3.5"x18" LVL will make up to 28' within your situation if that helps at all. but you cant really assume from that information that a 24" beam made out of plywood and wood glue would make that span. basically, you could probably rationalize something like that in your own head and it may work but no one does that so you wont get anyone to say it will work, meaning you wont pass inspection. unless you are going around the whole permit process, then in that case i guess you are foolish enough to use a plywood beam. i just dont think it's worth saving a few hundred bucks.
 
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Old 11-22-08, 09:04 AM
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Thanks for the info OhioDraft. Was that for 40psf or 50 psf?
 
 

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