Garage joist support

I have a tall garage, and want to put a storage space above the garage doors, for about half the garage's depth. The garage is 20' feet wide; so I need a 20' beam across, then joists to support the deck. There's a serious steel beam above where my new beam should go.

Question: does it make sense to drill and tap the steel beam and run a threaded rod down to support the middle of the 20' span? Will one 3/8" rod in the middle suffice? More? Thicker? Or is there a better way I've completely missed? Putting a post down to the floor dead center in the garage would interfere with car doors...

Also, for fire protection should I wrap the rod(s) somehow? The beam is wallboarded because of living space above.

I can figure the joist loadings, but I'm not sure how to calculate for the beam spanning across the joist ends....

Thanks for any advice!

 

Just thought I'd throw my 2 cents in here. I'm not a contractor, just a practical guy. I don't think you're going to get an answer here, at least a specific answer. Your question involves structural calculations and considerations beyond this forum. I can tell you what I would do, and that's all it's worth. The beam you want to add is not structural in the sense of the I-beam above it. All you really need to support is the joists, the structure itself, storage load and one or two people on the platform. I would run a double 2 x 12 across the span to support the joists, supported by posts at either end. I would then place a ledger board on the door wall, and connect the joists to the beam and ledger board using joist hangers and hanger nails. I wish I could tell you the deflection for this, I can't. I can't imagine it would be much though... You are adding a load to the garage door header. I guess I would at least call around and get an architect's advice or maybe a structural engineer. If you plan on building it yourself, save on labor and invest a little money in their advice.

 

Sderose & Handyone. Handyone, you are wrong.
I am a civil engineer, and I figure deflections every day.
Sderose: You are on the right track, just the wrong material. What you want to and down from the beam to support the middle of your long beam is a piece of 2" angle iron, instead of rods. Rods do really not have much structural strength, but angle iron does. I would use the 2x12's as handyone suggested, however good 20' ones may be hard to find. Bolt them together and when hung, with crown up, then bolt your angle iron with three 3/8" bolts about 3" apart and the other end of the angle iron to your beam. If you have trouble bolting to the beam, you may have to have the angle iron bent to bolt to the web of the beam. You deflection on a 20' 2x12 over a 20 foot span, will be approximately 1" depending on the type of wood. with 2 2x12's bolted together, about 1/4". Good Luck

 

Jack,
Sorry, I'm not wrong. I suggested he get advice from an engineer or architect and he did. (namely you). I guess I was wrong when I said he probably wouldn't get it here.
Anyway, like I said, I just wanted to throw my 2 cents worth in.
Help me out, is this right:
If we say he has a dead plus live load of 50PSF (which I think is a high estimate), and uses #2 DF, a single 2 x 12 can span 19'-6" with 1/360 deflection. If I'm figuring this right, that's a deflection of about 5/8" over the span. And that's a single 2 x 12. So a double 2 x 12 should be a little less, let's say your 1/4" calculation. This is with no stress value adjustments and no middle support. Adding your angle iron to support the middle will drop the deflection to almost nill. I just want to know for my own reference. Is this how you figure the span? What I'm doing is using joist span tables. I didn't know (and maybe you can't) use them for the beam in this application, but our numbers seem to be pretty close for it not to be.

 

Thanks for the replies, HandyOne and Jack the Contractor!

Am I right that the problem with HandyOne's calculation is that for joists, the 50PSF is divided across multiple joists, whereas here the one beam at the end of the joists ends up taking about half of the *entire* load (the other half being on the ledger board at the other end of the joists?

The change to 2" angle makes sense; wouldn't have thought of it myself. If as you hinted the 2*12s are hard to find, could I use 2*10s and put supports at 1/3 and 2/3 instead of just the middle?

I understand why the angle iron is much stronger laterally, though I didn't realize that tensile strength is related to more than cross-sectional area; glad to learn better.

Just so I don't have this fall on my head, when you mentioned using 3 3/8" bolts at the bottom, I'm guessing the same would be adequate at the top, even though the would be stressed lengthwise instead of shear. Am I fooling myself?

Thanks again, I'm really glad I posted and asked!