Garage- doubling up on joists

[BigguyZ]

OK, after a long delay in working on my new garage shop, I'm finally free to resume work. My garage is pretty large, and is about 24ft wide. Currently, there's no real access to the half story above the joists that go from wall to wall. However, I will be making an access bay to the upper level so I can use it for storage. I'm sure the long joists are code, and I'm not too concerned about any extra weight on the upper floor (I'm not storing concrete up there or anything), I'd still like to double up every 2 or 3 joists for additional support.

I have a couple of questions:
-Is this necessary, or is it just a waste of money and time?
-If it is a good idea- do I go every-other joist, or every third joist?
-With the long run, I'd have to special order and have the boards delivered if I were to get single boards that would cover the entire span. Would two boards butted together still provide support and help prevent the upper floor from sagging?

Thanks,
Travis

[chopnhack]

Is the run of the joists 24' unsupported? Also, how long is the building? What size are the joists, 2x or truss style? If you will not store too much, you may be able to get away with doubling up the plywood floor, should spread the weight more uniformly, but above 30 psf you would need to consult engineering tables.

[BigguyZ]

actually, I think it's more like 20", but yes, it is unsupported from wall to wall.

They're 2x8s, beams. So if I used two 10' 2x8s glued and screwed/ bolted to the original structure, would that be effective?

I'm not planning on having a whole lotta crap up there, but I'd like to not have to worry just the same.

Thanks,
Travis

[master53yoda]

http://www.msrlumber.org/spantables.pdf

The above site has a PDF with load tables that will give you some info.

2x8s 16" on center at 20 ft are at max for load without anything on top.

storage is a a long term load and would need to be calculated the same as a dead load. What I have seen done quit often is a center beam in the back 10 ft for support of storage above. and then use a tighter span in the storage area.

good luck

[BigguyZ]

OK, so what are the F and E numbers in each column? So if I have ceiling joists that are 2x8" with 16"oc spacing- I can't put any load on it? That doesn't seem to make sense. That would mean I couldn't even sheetrock it.

Am I mis-understanding something here?

[BobSch]

The pre-made trusses in our garage aren't rated for any load. I'm not even sure if I can put up a drywall ceiling safely.

[pelligrini]

Most codes list a minimum loading of a 5 psf dead load and a 10 psf live load for uninhabitable attics without storage, and more loading is designed for limited storage.

According to the the tables in the 2003 IRC (International Residential Code) most species and grades of 2x8 ceiling joists will handle the above loading at 16" OC and 20' spans (except for #3s)
http://www.alr6x6.com/pelligrini/BT3...nalResCode.pdf

f is the bending strength and E is the Stiffness in the other linked tables. This seems to be a pretty decent article on Machine Graded Lumber http://www.cwc.ca/NR/rdonlyres/CD5CF...adedlumber.pdf

[woodturner]

http://www.msrlumber.org/spantables.pdf

Those tables are for msr lumber, not the normal #2 found in the lumber yard or a box store.

TABLE R802.4(2) in the residential code pelligrini linked would be the minimum for this application. DougFir 2x10's 12" OC would be the minimum - but they have to be #1 grade, and good luck finding that retail. It will likely be special order and expensive.

I used 2x12's 12" OC for my 20' wide garage, and even that is marginal - but it's a flat roof with no storage possibility, so is acceptable.

The more common and better solution is to put a steel beam down the center, supported at the front and back of the garage with lolly columns. In retrospect, I wish I had done that. The cost would have been less than the 2x12's and there would be less "bounce" in the roof.

Another option is laminated beams - LDL or something like that. The issue I found with those, though, is cost - they are expensive.

[chopnhack]

Well, even at 20' you are pushing the limits of #2 2x8's. As woodturner mentioned, a great solution would be to install a steel beam down the center of the shop to pick up the weight at the center of the floor. This would effectively double the stiffness and allow you to increase the load. I would assume that the center may have a doorway, at that area you could build a steel header around the door that would avoid a column in your doorway

[parnelli]

The pre-made trusses in our garage aren't rated for any load. I'm not even sure if I can put up a drywall ceiling safely.

While we're kinda on the topic- How were you able to determine the rating?

I've always wondered about how the second, third, etc owner is supposed to know/find out what manufactured trusses would be rated for.

[iceman61]

Engineer specs for the ceiling of my 20x24 shop were for 2x8's on 16" centers with a double 24' - 14" LVL down the center all free span (no columns in occupied space). This is for "light" storage in the attic with a snow load for west TN figured in. This was for a 7/12 pitch roof. Lower pitch roofs require wider joists due to the ability to hold more snow weight. Higher pitch roofs are more friendlt when it comes to snow loads. I don't know what the cost offsets are for steel beams now vs LVL's but 4 years ago it was more cost effective to utilize LVL's. The 14" double LVL's center out over a 10' garage door header. I know for a fact this setup has no deflection at all, all free span with a 9' ceiling.

[jking]

While we're kinda on the topic- How were you able to determine the rating?

I've always wondered about how the second, third, etc owner is supposed to know/find out what manufactured trusses would be rated for.

There's probably a rating stamped or tagged onto the truss.

[toolguy1000]

BigguyZ...just out of curiousity, what did the architect or engineer who designed this structure when you submitted plans for a building permit say about this? i ask because, back here in the NYC tri-state area (1 hr commuting time to NYC from NJ and CT), the building depts, AIAs and PEs take a dim view of any "addendums" to their designs, for obvious liability issues.

[BobSch]

While we're kinda on the topic- How were you able to determine the rating?

I've always wondered about how the second, third, etc owner is supposed to know/find out what manufactured trusses would be rated for.

I haven't found any tags or other markings showing any rating but I've been told by a couple of contractors and our local building inspector that the trusses in our garage weren't designed for any load. I''m not all that interested in finding out.

[jking]

BigguyZ...just out of curiousity, what did the architect or engineer who designed this structure when you submitted plans for a building permit say about this? i ask because, back here in the NYC tri-state area (1 hr commuting time to NYC from NJ and CT), the building depts, AIAs and PEs take a dim view of any "addendums" to their designs, for obvious liability issues.

That view is not limited to your area. Alterations that happen post construction that are not approved by the engineer of record become the responsibility of the one making the alteration. If someone is altering a drawing that has been sealed by a AIA or PE without their consent, that is a different situation & is a much bigger deal.