Balloon framing question

[GBsCards]

Pretty straightforeward. I am building a 24x32 garage. I want full use of the

1st floor with no posts so I am going to use wood I's or I joists. whatever they are called in your neck. I also wanted full use of the 2nd floor. I wanted a knee wall of about 3ft. The roof pitch was 10/12, and I didn't want to put collar ties low. How can I frame the walls so I can have a knee wall without structural collar ties and the wood I's for joists? I had it all planned out by balloon framing with a mortised ledger and big beam, but the building dept won't let me span the 32 ft. with a big beam. Such is life, I guess. Any ideas>

[Yaofeng]

What do you mean the building department won't let you span 32 ft? You hire a structural engineer to provide the required calculations and design. If I understand you correctly, with a 32 long ridge beam, you don't need collar ties. Your rafters will be supported by the knee wall and the ridge beam. I think you realize the roof loads are on both ends of the ridge beam and the knee wall.

[default]

A 32' span with a single beam is a hell of a long ways. On the other hand, the ridgepole isn't supporting any significant weight, anyway. It's a compression member. The problem is that there's no way the three foot kneewalls are going to resist the outward thrust of the rafters without help. The usual solution to this problem is a sissor-truss; I'm not entirely sure you can do that with I-Joists, but if you can do collar-ties, you can do sissors, they're about the same thing.

I think OP should (A) Use engineered trusses, instead of I-joists, (B) Use regular wood, and lift the roof-pitch enough to use collars, (C) Raise the building enough to put in a complete second floor, or (D) redesign the roof as a cross-gable with no kneewalls.

I must confess, after saying that, that I still have no freaking clue how a cross-gable roof works. What holds the valley-rafters up?

--Goedjn

[Chris Lewis]

According to [e-mail] :

I think the I-joists are intended for the floor joists of the second floor. Can't use trusses for that. The collar ties were mentioned in context with the roof construction, not floor joists.

I think what he wants to consider is using I-joists for the first floor ceiling and engineered scissor trusses for the roof.

Tho, of course, with only a 2-3' kneewall on the perimeter, and a 10-12 pitch, he might not get much headroom even in the center if he uses trusses.

Yeah, needs to have engineering sign-off on the floor joist design/layout. That's a _big_ floor span. The scissor trusses are no big deal (except for headroom). I'd have the floor trusses going the 24' direction, (the trusses would naturally go that way too). Makes things a lot simpler than building for 32' joist span....

An engineer may be able to finesse the roof trusses. Ie: steel stretchers in the floor beams allowing thinner roof trusses.

I was involved in a retrofit that converted a second floor into a cathedral ceiling. Engineering required stretcher beams (enclosing threaded rods with steel plates to spread the load) every 12' to keep the masonry walls from spreading.

[default]

Umm.. lessee.. 5' rise to the center, so the peak is 8'.. so there's about a 2' channel down the middle that's 7' or more, you're right, that's not going to translate into a BOCA complient habitable space...

If I read correctly, he does have the joists running that way, that's why the RIDGEPOLE would need to be 32'. If this is an attached garage, the side against the house is probably driving the orientation of the roof. And the garage doors will limit where you can put posts down to support a timber frame or any other massive beam.

What *IS* driving this particular roof design?

--Goedjn

[default]

Arg. Can't do geometry without pencil.. Clear-space w/7+ feet of headroom is ~14' wide, and more than half the room... clear space over 5' is 18' wide, it will be fine.

[RB]

In most areas I'm aware of even architects can't design spans of greater than 25 feet. To span 25 feet or more all you need to do is hire a licensed engineer to review (and concur with) the design. Once his seal is on the drawing the building folks should be happy. This shouldn't be a big expense.

RB

GBsCards wrote:

[Yaofeng]

If you make the ridge beam load supporting, the rafters will not open up.

[Chris Lewis]

According to [e-mail] :

That's not taking into account roof truss (or rafter for that matter) depth.

Either the rafters are going to be fairly large dimension (for a 12' span, you need what, 2x8? 2x10 _here_ I think), or it's going to have to be carefully finessed thru the use of a specialized truss.

[default]

The depth of the rafters doesn't matter, because it's the bottom edge of the rafter that sits on the wall-plate, and it's the bottom edge of the rafter that determines headroom. But ultimately, I think the answer is that if OP wants both knee-walls, AND no collars, then he needs professional design help.

--Goedjn

[The Firm2]

A 30'+ long true ridge beam will be huge! Not practical. I don't think he can have everything he wants.

[Yaofeng]

Well... If money is no object and he is crazy enough, a garage suspended in air can be built, you know.

[RB]

I've got a two story, 4 car garage (2 cars up and 2 cars down), 26 x 28 ft with no columns, just lots of WF steel beams. It's not difficult but it needs to be done correctly.

RB

Yaofeng wrote:

[Yaofeng]

Lemme guess. You live in New York City, pull in 7 digit income, and has more money than you can spend?

[The Firm2]

Sounds like a good situation to be in, I'm jealous.

[default]

If you figure the roof needs to support somewhere around 40psf, (this will vary widely with expected snow-load, roof pitch, etc.. then the centerbeam of a 32 x 24 garage is supporting 384 sq.ft, or 15360 pounds, for which a 15"x5.5" American standard I beam will be more than adequate. Of course, the beam itself will weigh almost 1400 pounds, and you need a post and footing at either end that will hold around 8400 pounds...

[Mike Lamond]

I don't know if this will give you what you want, but my parents did something like this when they built a 26'x28' detached garage using attic(?) trusses. The trusses are on 24" centers, with a 12/12 roof pitch and a 10' wide x 6'-6" high open area down the middle. There is no carrier beam or columns on the 1st floor. Most of the bays between the trusses outside of the open area have low rolling carts for storage. Four bays have been closed in to make a cedar closet.

Mike