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Old 04-28-2008, 10:19 AM   #1
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Beam Size


Right now I have a detached garage, it currently has an attic ladder. I seriously hate the ladder and want to put in some stairs, deck the floor above, and use it more seriously for storage, with the possibility of a future dormer for a bonus room.


This is my floor plan, and my side profile, nothing fancy, pictures speak a thousand words.

So, the rafters, they are 2x6, with 2x6 ridge beam (2 15' sistered together with 3/8 ply). Currently there are 2x4's nailed to the ridge down to the joists and toenailed in. There are Angular supports running along these from the outside up to the center. This supports the roof. I don't mind building a central wall with a couple of doorways in it to continue to carry the load of the roof.

Could someone calculate the beam size for me? I would prefer it to be 4" wide to keep clearance to a maximum. I don't care if it is steel or laminate, I don't want any supports in the center, although there could be one under the stairs at the side.

Thanks
Steve

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Old 04-28-2008, 02:31 PM   #2
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Beam Size


Does anyone know of an online calculator to use then by chance?

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Old 04-28-2008, 03:18 PM   #3
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Steve, For something like this you really need to get an engineer involved. 2x6s aren't going to span 14 feet no matter what kind of beam you install. You are probably needing a beam and new joists and something to tie your rafters together. When you start taking out the lumber that is in that attic to open it up you are weakening the roof system and that needs to be addressed. And you'll need the right support for the beam all the way down to proper footings. Your door headers might also not be adequate to support the joists that are now floor joists instead of ceiling joists (different load requirements).

A few hundred $s for an engineer is well worth it.

Sometimes a good lumber yard will have engineers that can size beams etc for you for free if you are buying material from them. I don't know if they would be up to designing your whole project but you might check. Keep in mind that you need more than a beam sizing. If that's all you ask for that's all you'll get and you'll have a mess on your hands. Make sure they understand that you need someone to specify all required changes to do this project.
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Old 04-28-2008, 04:01 PM   #4
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Beam Size


There is already a ceiling, with Drywall on it, it is already being used for storage. It does not have a beam right now. And it is already spanning 28 feet, with no beam. So, putting a beam in, along with tongue and groove decking, screwed and glued to the 2x6's, to me at least would be mountains better than what I have right now. If I can't make it a usable living space with 2x6 joists then I can live with that. Mainly if I were to use it at all it would be for a bonus room. If it would not be useful for that, then an attic with a proper floor is right on too then.

Getting a structural engineer involved to calculate a beam size for an attic seems like a teeny-tiny bit of overkill to me. I hoped that this would not be one of the response I would get though. I just hope that someone else is up for more than a cop out.

Thanks though.
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Old 04-28-2008, 04:06 PM   #5
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It wasn't a cop out by jogr, it was very good advice. Attacking the first person that takes the time give you a response (when he doesn't tell you what you want to hear) won't get you many more responses.
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Old 04-28-2008, 04:27 PM   #6
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Well, I have to say, that he did not answer my question. I wanted to know a size of a beam that would be required, I understand that this is not ideal.

I don't really see how my putting a central beam in is going to affect the headers on the doors in the existing part of the garage... If anything, it would alleviate some of the load exerted on them. Maybe the complexities here are more than I am imagining.

I called a lumberyard, and they told me that I would be fine using a 1 3/4" x 11" lam beam. Does that sound like it makes sense? Really, this is something that I am totally willing to calculate on my own. I figured that this would be a good place to try to get some help. I have poked around a bit for an online calculator, or some formulas and haven't come across any.

I don't know, to me, this is all known variables, dimensions and existing lumber sizes etc. You should be able to plug these things into something, be it a formula, a table of some sort, or some sort of a calculator that would give you some options.

I am sorry if I came across as rude. I am really good with computers, I own my own business, and I am also active on some tech type forums. And I would NEVER tell someone coming for help to take the computer into a shop so that someone qualified could work on it without trying to help him first.
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Old 04-28-2008, 09:53 PM   #7
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I hear what you're saying, but someone giving bad advice on a computer forum has different ramifications than giving engineering-related advice on a construction forum. The tech guys might make your computer start smoking and crash, but if someone doesn't give you good structural advice, lives are at risk.

Based on my structural experience, your joists are drastically undersized as they are currently framed (span as well as added roof load). Sure, they haven't crashed in because they're not stressed to the point of failure. I would imagine that they're deflecting beyond normal limits.

I also believe that you got some very, very bad advice at the lumber yard. 30' is a very long span, and is way beyond the capabilites of an 11-7/8" LVL, PSL, or similar size gluelam unless you involve at least a couple posts to make it a multiple span as opposed to a simple span. If you're limited to a 4" width, I'd probably suggest looking into a steel beam. At a 30' clear span with the amount of tributary load, you're talking about a pretty healthy piece of steel.

Whatever beam you put in will add to the rigidity of the floor, but will change the way the building performs structurally, hence the recommendation for an engineer. There may or may not be online calculators or programs to assist you, but (especially) if your project involves the local building inspector, don't waste your time with anything but an engineer.
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Old 04-29-2008, 06:13 AM   #8
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Actually the advice I got from the lumber yard is what made me want to come and see for myself what I had going on here. Thanks, I will put some more thought into it.
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Old 04-29-2008, 08:06 AM   #9
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Just for kicks I went up and RE-re measured the joists, and sure enough they are 2x6, and they are also full span 28'. This has always made me nervous, especially keeping stuff up there. Right now there is a flooring made out of random pieces of wood, and whatever would work put in by the previous owner. I was hoping that I could get away with bracing it with a beam. Now I am wondering if maybe I need to use either 2 beams, or should I just get an engineer in there before I dump any time and money into it. The house is a bit bigger and it uses 2x10 joists, plus has a bearing wall.
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Old 04-29-2008, 08:44 AM   #10
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The 2003 IRC code gives the following spans for 2x6-#2 used as floor joists 24"oc....

Doug Fir: 9'1"
Southern Pine: 9'4"
S-P-F: 8'11"

That is based on a 10# dead load and a 30 pound live load. It is basically the chart that is used for sleeping areas, and wouldn't be anywhere near the live load needed for storage.

I just re-read your original post that says there are two 15' 2x6's end-spliced with plywood. That pretty much made my heart stop! When I first read it, I thought you had two 2x6's slapped together side by side. Obviously, end-joining two pieces of lumber in a structural situation is a big no-no. You need a beam under that splice, and probably need to add two others to break up the span.
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Old 04-29-2008, 08:49 AM   #11
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Quote:
Originally Posted by thekctermite View Post
The 2003 IRC code gives the following spans for 2x6-#2 used as floor joists 24"oc....

Doug Fir: 9'1"
Southern Pine: 9'4"
S-P-F: 8'11"

That is based on a 10# dead load and a 30 pound live load. It is basically the chart that is used for sleeping areas, and wouldn't be anywhere near the live load needed for storage.

I just re-read your original post that says there are two 15' 2x6's end-spliced with plywood. That pretty much made my heart stop! When I first read it, I thought you had two 2x6's slapped together side by side. Obviously, end-joining two pieces of lumber in a structural situation is a big no-no. You need a beam under that splice, and probably need to add two others to break up the span.
Yes, when I recently went up there to look at that, it about made my heart stop too. I am no engineer, but I can bang things together, and I have a pretty good feel for what is a solid job and what isn't, and that is just junk. I will have to get an engineer to look into this I suppose, if I need to replace the joists to make it right then I will do it. It isn't like it will cost the world, maybe I could go ahead and dormer it now too.
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Old 04-29-2008, 09:14 AM   #12
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Hopefully an engineer can help you find a way to make it both safe and usable without having to buy anything but beams!
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Old 04-29-2008, 02:15 PM   #13
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Quote:
Originally Posted by thekctermite View Post
Hopefully an engineer can help you find a way to make it both safe and usable without having to buy anything but beams!
Amen to that, I would imagine that I would have to remove only a few joists at once, and replace e few at a time, replacing the verticals to the ridge beam each time until it was done, then putting int the beam, then bracing the roof, and changing out the ridge beam. Seems like it would be easier almost to build a new garage.
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Old 04-29-2008, 02:32 PM   #14
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Keep in mind that those joists are not only picking up the vertical roof load through the king posts (amazingly given the span), but also any horizontal load (thrust) the roof might be exerting on the wall top plates through the rafters. Since you indicated there was little or no deflection to the joists even with the indicated span, I wonder if any of the vertical load is actually transmitted through the posts or if it's all loading on the top plates. In that scenario, the joists are working in tension to take up that load and prevent the walls from rotating out.

Last edited by MacRoadie; 04-29-2008 at 03:13 PM.
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Old 04-29-2008, 03:09 PM   #15
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Who knows, I may just leave well enough alone at this point. If it ain't broke don't fix it. I have options, I have wanted a patio behind the garage, I could go the rout of pouring a pad, extending the garage into a screened porch, pulling the roof off, and buying truss' to go the other way over the whole lot. I need to mull things over here. I would love a second story over it, but the question becomes is it worth it. And, if it is working now, and I don't abuse the storage as far as weight goes, then I probably should not put a beam in.

Maybe my neighbors tree will fall on it and I can start over again.

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