Loft framing question

MushCreek · January 24, 2010, 04:44:17 PM

I got some plans for a 28X48 gambrel roofed barn. I'd like to change the loft framing a bit, but I'm not sure how. The joists run cross ways (28') with a 4X12 in the middle to support them. The loft deck is part of the roof structure, in that it prevents the bottom of the scissors trusses from spreading. I think that's why they run the joists that way. Anyhow, I'd like to make the middle bay 14 or 15' wide, rather than the 10' the plans have, so I have room to work all the way around a truck. I could easily run the joists the 14' way, but I'm worried about the roof spreading, although the plywood diaphragm should prevent that. If I keep them the way they are, is a 4X12 that's 14 (or 15') long strong enough to support the joists in the middle? What about a triple or quadruple 2X12? Or should I go with a steel beam? What if I divided the 28' into thirds, and added another crossmember? The other thing I noticed is that the joists are 2X12's on 12" centers. In #2 yellow pine, that calcs out to 100 lbs/sq/ft live load! Seems excessive for a loft. Anyhow, to simplify- I want a 14 X 28 open space with a loft overhead, and the joists need to run in the 28' direction. Any advice? The good news is, there's no permit or inspection for an agricultural building in SC. The bad news is- I don't want to join the short list of builders crushed to death by their creations!

Don_P · January 24, 2010, 10:32:58 PM

Any way you can post a pic of the frame?

January 25, 2010, 05:23:10 AM

I'll draw it up at work today, and post drawings tonight.

MushCreek · January 25, 2010, 03:43:28 PM

Okay- here ya go. The first drawing is the truss, which spans 28', and they are on 24" centers. The second drawing is what the area in question would look like. Note that in the first view, the 2X12's are not part of the truss. You build your walls and loft, then build the trusses on top. They are attached with framing clips.

Squirl · January 25, 2010, 04:21:31 PM

I am still trying to figure out what you are changing. I will keep reading it to better organize my thoughts. One thing that did pop out is the loft framing. No, 100#/sq. ft is not overkill if you are putting hay up there. That is a lot of space and a tall loft. You can pack a lot of hay in there and hay is heavy. A 5x5 bale can weigh 1000 lbs. You could easily stack three on top of each other in a loft that size. That would be 3000 lbs over 19.6 sq. ft or 153#/sq. ft.. Most farmers I know fill their hay lofts.

I am more concerned about the center beam of a 4 x 12 and how far the spacing is for the posts for the 4 x 12. Off the top of my head from the framing book I don't have in front of me 4x lumber isn't recommended for heavy loads. It was always 6x for girders.

January 25, 2010, 05:34:15 PM

As designed, the spacing on the support posts for the 4X12 is 10'. That's what I want to change- I want 15' free span, if I can get it. I don't need anywhere near 100# load rating. I won't be storing hay, or much of anything up there. Probably just household stuff, and I maybe use it as a rec room, with a ping pong table or the like. I suppose I should build it pretty stout, in case a future owner wants to store hay up there. My concern is the 4X12. Would it help if I had 2 of them, thus dividing the span into thirds? Or should I go with a more massive beam? I can stack together a bunch of 2X12's, but going deeper than that will require a laminated beam. I just don't know how to calculate the load on that beam. Thanks for your help!

Don_P · January 25, 2010, 06:09:47 PM

No roof load on the 4x12 so the load is easy;
Half the floor load is bearing on the walls. The 4x12 is supporting a swath 14' wide (I'm rounding)x15' long or 210 square feet. If we call the load 100psf then the beam is supporting 21,000 lbs and is spanning 15'. I don't get there with a 12x12 in #2 SYP. A 4 ply 14" deep LVL would work, the truss supplier can spec and supply it. Ours also deals in metweb beams that would be shallower, then there's I beam.

Don_P · January 25, 2010, 06:43:42 PM

I've got a 4x12x10' working with a 40 lb total load, this is a light sleeping room, sumthins wrong

MushCreek · January 25, 2010, 09:26:06 PM

These plans are also used as houses; a 30# live load plus 10# dead works out then for the 10' 4X12. Is it really necessary to have all those 2X12 joists on 12" centers then? Any idea what that 14" LVL would cost (roughly)? And does having 2 carrier beams help at all? Then the joists would be about 9' long, and the load would be distributed over 2 beams rather than one, or am I wrong in this thinking?

January 25, 2010, 09:38:03 PM

No, right on the money. A 30' wide barn here is typically 3 aisles.
9x15x40=5400lbs...4 ply #2 syp, 2 ply handles the 10' spans handily, all at a 40lb total load.

MushCreek · January 26, 2010, 08:30:49 AM

What do you mean by '4 ply #2 SYP'? Would that be 4- 2X12's? In 2 places?

MountainDon · January 26, 2010, 12:31:41 PM

#2 SYP = grade 2 Southern Yellow Pine

MushCreek · January 26, 2010, 03:11:26 PM

I got that- what do you mean by 4 ply?

MountainDon · January 26, 2010, 03:15:18 PM

4 ply = four of the 2x together would be good for a 15 ft span; two 2x would be good for a 10 foot span at the same 40# load

January 26, 2010, 04:07:01 PM

All right- just so I got this straight- I'm going to have 2 beams, each consisting of 4- 2X12's, right? Do I still need 2X12 joists, being that they'll only be 9' long? Do I still need to put them on 12" centers? All of this in keeping with the 40# calc, that is. I really appreciate the help with this!