Clear Span, Girder or I-Joist

[MapGuy]

Hi All,

I'm designing/building a two story garage and trying to determine the best method to support the second floor with a clear span. I searched the threads and didn't find quite the help I was looking for.

The idea is a 30 wide by 26 deep two car garge. The garage area will be 26x26 with the remaining 4' used by the stairs going up to a multi purpose room.

I want to avoid columns in the 26x26 area so I reviewed the span data supplied by a couple manufacturers for their I-Joist products. They both appear to be able to make the 26' span with an 18" or 20" deep joist. I'm concerned about flex though and wondered if supporting smaller I-Joists with two LVL girders would provide a less bouncy floor. I'm designing with 40 psf LL and 10 psf DL.

Any thoughts or suggestions?

 

[JAE]

Are you sure 10 psf is enough? Total up everything such as the plywood (or whatever type of) floor, the self wt. of the joists, lighting hung below, ceiling, flooring, partitions in the upstairs, etc. 10 just sounds a bit slight.

Bounce in joists - depends on their stiffness and the type of wood. Your live load deflection should be kept to the traditional L/360 limit, though to be sure, perhaps consider L/420. These are still in the 3/4" deflection range and with a clear span and no partitions above, you may get vibration regarless.

A supporting LVL may not do much, or perhaps make worse, the potential for vibration.

If this is for storage - don't use 40 psf for LL - you would need 125 psf.

 

[FalsePrecision]

I agree with JAE - 10 psf dead is too little. I use mostly 15 psf for residential. The I-joist products generally are tabulated with conservative live load deflection allowables, but if the floor area is without partitions (non-load bearing walls), you have little damping, and the floor is bouncy. Solution: use 1-1/8" thick tongue & groove APA-rated sheathing, or use 3/4" to 7/8" sheathing with 3/4" to 1-1/2" gypcrete over it.
Breaking the span of the joists with a girder is not a good solution, and very ineffficient structurally.

 

[jjeng2]

Id use 15psf dead load min. Headroom dosent seem to be an issue so use joists as deep as you can and make the subflooring/flooring as heavy as you can. Of course the weight needs to be weighed against the strentgh of the joists but the most efficient way to reduce vibration would be to make a heavy floor and deep joists.