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Old 02-25-2012, 11:35 AM   #1
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Today's Math Question: Sizing a Beam


I need to size a beam for a patio shed roof off the back of my house. The slope will be a 3:12, the length will be10’ from the house wall to the support posts plus a 16" overhang, and the roof width will be 18' and carried on two 4" x 6" posts. The rafters will be spaced 16” oc.

The snow load requirement in our area is 30#. What is the minimum verticle dimension using a 4" x ?" beam?

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Old 02-25-2012, 12:26 PM   #2
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Today's Math Question: Sizing a Beam


Have to check your local building code

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Old 02-25-2012, 12:33 PM   #3
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Today's Math Question: Sizing a Beam


This is not a proper question for a forum like this. And none of us (even if we think we know the answer) should even attempt to give you an answer.
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Old 02-25-2012, 01:04 PM   #4
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Today's Math Question: Sizing a Beam


Quote:
Originally Posted by chrisBC View Post
Have to check your local building code
Yes Chris, as I said in the post, minimum live load is 30#.

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This is not a proper question for a forum like this. And none of us (even if we think we know the answer) should even attempt to give you an answer.
There are standard formulas and tables for figuring dead and live load for buildings. Carpenters with the knowledge to use them and make the calcs don't need to guess. I am assuming someone on this forum has the education and skillset that could save me a lot of research time and quickly compute the beam size for me. Why don't you give those with the appropriate experience an opportunity to offer their assistance without going negative on me right off the bat?
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Old 02-25-2012, 01:09 PM   #5
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Today's Math Question: Sizing a Beam


You will be better off using a glue-lam beam or some other engineered wood product. You will need at least a 3 1/8" x 12" 24F-V8 or V4 DF GLB. Or you could use a 5 1/8" x 9" 24F-V8 or V4 DF GLB. This assumes a 15# dead load and a deflection of LL= L/240 TL=L/180. These assumptions must be checked with your local building codes. Most local lumber yards should be able to tell you what is required for your area and what is the most economical choice.
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Old 02-25-2012, 01:12 PM   #6
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Today's Math Question: Sizing a Beam


Yes, you can size it, or you can have it done locally. Not being negative, if you are so familiar with the building code then it shouldn't be an issue.

I am not about to tell someone over the internet what I would do thousands of miles away.

Honestly, (and no offense meant) if you are doing structural loadbearing work on your home, the internet is not a place to look for sizing a beam, There are local codes and processes in your area you need to follow. You may have to submit plans to get approval as well, you would here.

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Old 02-25-2012, 01:50 PM   #7
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Today's Math Question: Sizing a Beam


This topic comes up repeatedly on this forum. How to size a beam, how to size a joist, how to size a truss, you name it. There are almost certainly half a dozen, maybe a dozen people on this forum who are capable of correctly sizing the beam for you. But in the immortal words of Clint Eastwood, "you feeling lucky today?".

How are you possibly going to determine if the individual responding to your request knows anything, or is merely blowing smoke? Are you actually going to put in a beam based on the size you got over an internet chat forum, from someone you will never meet, who has never seen your site, and who may or may not know anything about structural engineering? And exactly what are you going to do if the beam size selected turns out to be incorrect? Are you going to sue them over the internet?

Conclusion: Performing mission critical design work over the internet on a site you will never visit is way outside a DIY chat forum. Hire a local engineer, size it yourself and take your chances, or go to a local lumber yard with your plans and let them size the beam for you as part of the cost of an engineered lumber header.
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Old 02-25-2012, 02:14 PM   #8
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Today's Math Question: Sizing a Beam


Okay, I get it. I was expecting an answer like that from claymcc (thanks). Didn't see some of the issues you guys are bringing up. Thought it would just be a simple inquiry, but your points are all well taken.
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Old 02-25-2012, 02:26 PM   #9
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Today's Math Question: Sizing a Beam


GLB seem to be a regional product. I've checked my LLY and found that they do not stock GLBs but instead only offer LVL beams.
Another thing, is there such a product as exterior use GLB or LVLs or do you just wrap them in some sort of membrane to protect from moisture damage?
Sorry, don't mean to derail but I'll be going through this for the summer 2012.
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Old 02-25-2012, 02:55 PM   #10
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Today's Math Question: Sizing a Beam


Knowing the modulus of elasticity of the wood (usually between 800,000 and 2.2 million) and using the engineering toolbox site you might be able to step through this. Going from lbs/sq. ft to beam loading per linear inch is a little tricky and for your pitched beam you need to know vectors.

If you find a formula where a quantity is raised to the fourth power you are on the right track.
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Old 02-25-2012, 03:05 PM   #11
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Today's Math Question: Sizing a Beam


A GLB is a glue-lam beam. They are available with a pressure treated preservative. I have also use treated parallams. I don't know about the LVL's however.
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Old 02-25-2012, 03:12 PM   #12
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Today's Math Question: Sizing a Beam


lvls are just very densely pressed plywood layers that are glued with very strong adhesives, they then coat them in nothing more than a thin layer of wax. they do need to be protected, easiest way to do it is with aluminum coil stock
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Old 02-25-2012, 03:22 PM   #13
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Today's Math Question: Sizing a Beam


Also, check the 30 PSF live load. If you are constructing a one-story, low-sloped shed roof addition up against a two-story structure, in a climate with snow, you may need to add live load for drifting snow load.

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