Beam size

   / Beam size #1  

ch47dpilot

Gold Member
Joined
Mar 26, 2010
Messages
258
Location
Ozark Alabama
Tractor
07 Kubota L3400
Designing a 40x60x14' high garage and am thinking about building a small 20x20 office/workshop inside it in one corner. The problem I'm not sure how to solve is how to make the top on the shop. I don't want any supports in the middle so I need to somehow span the 20' gap. I don't plan to store anything heavy on top except maybe some Christmas decorations, a car fender or 2 ect. I was thinking about putting a beam across and then span the 10' section with 2x8x10'. Not sure the best way to make the center beam. As I said it won't be holding a lot of wait so I'm thinking I could maybe spice 3-4 2x10 together. What does everybody think, any different ideas, anything you've done. I'm not all against a steel beam but seems like over kill.
 
   / Beam size #2  
Locate one of these books, answers to all your questions are in this little book.;)
 

Attachments

  • Framing Guide.jpeg
    Framing Guide.jpeg
    5.8 KB · Views: 289
   / Beam size #3  
Take a look at the lumber span tables:

Span Tables :: Southern Pine

From the ceiling joist table, you can span 20 feet with a 2x8 on 24 inch centers, but, for your application (assuming you don't have to meet specific building codes) I would probably be comfortable with 2x6 on 16 inch centers.

I think a center beam is an unnecessary complication.
 
   / Beam size #4  
I would over build it so you can use it as a mezzanine for serious storage, will only cost you a little more if done now.

I don't know the span tables, but 20' x 2x6 is not what I would do.

I would do the beam and the 2x8, Make the beam HD maybe three x 2x12, so you can lift from it in the future.

JB
 
   / Beam size
  • Thread Starter
#5  
I only thought of using the center beam so I could get shorter boards, haven't seen many 20' 2x8 at the local Home depot, so I thought about double or tripling up some 2x8x10 for the center beam then I'd only need 2x8x10 for the floor joist.
 
   / Beam size #6  
You might also think about using the I joists that they use in houses (2 x with plywood/osb in between) or the 2x4 web joists that they also use.

wood-i-joists.jpg


PB1800632.jpg
 
   / Beam size
  • Thread Starter
#7  
I thought of using the I joist but what are the cost. I'd think they are pretty expensive. Anyone bought some lately to get me in the ball park
 
   / Beam size #8  
2 suggestions for ya...

1. Plan for the future. You may, or a future owner may want to put more weight up there.

2. Evaluate OVERALL cost for the flooring.

An ALL wood floor is going to feel spongy and sag much more overtime. 20' is pushing the limits of dimensional lumber.

And by running a steel beam in the middle (allthough may gost more than wood initally) will allow you to use smaller lumber for the 10' spans. This is why I say to evaluate the OVERALL cost of said floor.

Heres some things to think on...

The following will all be using 30PSF live loads and l/360 deflection

To clearspan w/2x10 joists would have to be on 12" centers. So you would need (21) 2x10x20's

To clearspan w2x12 joists, they would have to be 16" centers. So you would need (16) 2x12x20's

Personally, I would use a steel beam. I am not a fan of homemade laminated beams. Steel isnt going to break the bank either.

For steel, you would need a beam to handle a 300lb/linear foot and not deflect more than 0.667" And this would allow you to drop to a 2x6 joist on 19.2" centers. Which you would only need 28 of them.

The beam would need a moment of inertia(i4) of 56 or greater.

Now all you need to do is look at I-beam specs and find a beam that is 56 or greater. Since the root price of steel is usually in pounds (heavier is more $$$), look for the lightest beam that meets the i4 spec of 56.

A W10x15 beam would do it. It has an i4 of 68.9:thumbsup: And the total beam would only weigh 300lbs. And it is 10" deep. Pricing varys so it would be best to call around. Last time I priced beams, they were 60-70cents per LB, so you are only looking at ~$200 for a 20' stick of W10x15 beam.

Cheap peice of mind IMO.

Like I said though, weigh all of your options. But I would use steel if it were mine.
 
   / Beam size
  • Thread Starter
#9  
Never worked with steel beams before so I'll assume your math is right. I'm sure someone out there will be sure to correct you if your wrong. So how do you attach the wood to the beam? Does it just sit on top or does it need to be anchored someway, I'll assume you can't drill through the beam as that would weaken it.
 
   / Beam size #10  
Never worked with steel beams before so I'll assume your math is right. I'm sure someone out there will be sure to correct you if your wrong. So how do you attach the wood to the beam? Does it just sit on top or does it need to be anchored someway, I'll assume you can't drill through the beam as that would weaken it.

Typically there is a sill-plate bolted to the top of the i-beam. For example, if the beam top flange is 6" wide, you would use a 2x6 laid down on it and bolted down.

The minimal effect of "weakening" the beam with the holes is more than bade up for by the strength of that 2x6 being attached:thumbsup:

Then simply toe-nail the joists on the sill, or use some of those special simpson connectors, like hurrican ties, or whatever they are called:confused2:

A couple of suggestions if this is the route you take. These are only suggestions and my personal preference, but check your local codes if they apply.

First, I like to over-lap the joists a bit, OR use a full length 20' that rests on top of the beam. If you dont use the 20' length one, use 2 12'ers (I know I am contradicting my previous post) and overlap them. OR you can butt them end to end and use a 2 or 3 foot peice to tie them together.

Second, you need to make sure the floor joists stay vertical. There natural tendancy is to want to lay (flop) over on their side when a load is applied. There are two ways you can do this.
1. use those metal X connectors that go from the top of one joist to the bottom of the other. (If your house has a basement, you will probabally see these). or 2. use a block of wood nailed in that is the spacing of the joist and the same size. For example, If you use 2x6 joists on 16" centers, you will cut a block of 2x6 that is just under the 16" (to fit snug up in between the joists) and nail it in place. This prevents the joists from "twisting" or flopping over.:thumbsup:

And wether you use steel or something else, I would reccomend this.
 
 
Top