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35' span suggestions

Jeff Matthews's Avatar

Join Date: Sep 2004
Posts: 155

07-25-08, 06:52 PM   #1  
35' span suggestions

Going to demo old carport attached to house and put garage in. Would like to build up 2-stories for living space above new garage. Rough dimensions 35x30 or so.

Will have follow-up questions, but first, what is the standard depth of a garage - i.e. the depth needed to accommodate the length of full-sized vehicles (like Suburban), plus a few feet or whatever so you don't bump the back wall when you pull the vehicle into the garage and you can still close the overhead door?

Follow-up questions will deal with best materials for accommodate span requirements. Thanks.

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formula1's Avatar

Join Date: May 2007
Posts: 115

07-26-08, 06:12 AM   #2  
It's easy enough to measure the Suburban length, add the front space you want, and an extra foot in the back and you have the garage depth you want. Our attached garage is 45' x 24' (4 car) and I can pull my '94 Chevy pickup in with some room to spare (no work area in front of it, though). The ceiling is a clear span with ceiling joists of 2x12 douglas fir running in the 24' direction. BUT, THIS WON'T SUPPORT A LIVING AREA ABOVE! Code requires living space to be designed for 40 PSF live load plus 10PSF dead load. You can't get there with 2x12 lumber and a clear span. For a clear span garage with a living area above, the only reasonable option is to go with deep (tall) manufactured I-joists. There are floor vibration issues with long I-joist spans and the causes can be explained mathematically involving fundamental vibrational frequencies, etc., etc. The rule-of-thumb that one article proposed to avoid detailed calculations is to design for acceptable L/360 deflection using 40/10 PSF design loads at 24" O.C. spacing and then drop back to 16" spacing. The net result is this raises the floor stiffness and gets the natural floor vibrational frequency out of the irritation zone.* Talk with several local lumberyards and their design people should be able to get you the I-joist design that will work for you.

Wirepuller38's Avatar

Join Date: Feb 2006
Posts: 6,188

07-26-08, 08:55 AM   #3  

Another option may be floor trusses. Trusses would add height, which may be undesirable.

OhioDraft's Avatar

Join Date: Oct 2007
Posts: 157

07-26-08, 10:36 AM   #4  
30' deep garage seems a little unnecessary. The suburban is close to 19' no more than 20' with a trailer hitch or something. so you could easily get away with a 23' deep garage. The method formula1 details is pretty interesting since i've never heard of it. IMO you should just go to a website like TJI (link below) and look and their span tables. The main consideration here is using L/480 deflection limit (recommended) as opposed to L/360 (code minimum which could result in bounce). Choose the depth that will make the 23' span comfortably, meaning with a few feet to spare. I.E. 14" deep 360 series @ 12" o.c. would work. simple as that.


cwbuff's Avatar

Join Date: Mar 2007
Posts: 1,316

07-26-08, 04:22 PM   #5  
I have a 24' deep garage that gives me space to work in fronty of my pickup with enough room behind to walk around it.

I have 14" deep I joists spaced 12" OC that span 24' in my garage. Above the garage I have a workshop with typical woodworking tools. I have not had any noticeable deflection or vibration.

My original plans called for 2X6 joists supported midspan by a built up beam. I did not want to have posts mid floor in the garage so I chose to go with I joists. I simply went to the lumber yard, showed my plans and asked them to size I joists for the span I had. The lumberyard called an engineer at GP to get the correct size.

septicguy's Avatar

Join Date: Nov 2007
Posts: 123

07-26-08, 06:27 PM   #6  
I've done it two diff ways in the past.

1. Large steel I beam running the width of the garage. Reg fir joists spanning on top of it. Very solid with almost 0 deflection, vibration, etc.

2. Large flush LVL or steel beam w/ side lamination. Then reg fir joists to hang off of beam. Also very stiff.

If you use TJI's or engineered joists, they will "bounce" to some degree. It all depends on length between supports, center to center spacing and joist depth.

formula1 is on the money with the vibration thing. All engineered joists "vibrate" even if slightly. I have a 23' deep addtion with TJI 560 at 16" o.c. It "bounces" for an instant. On top is advantech 3/4" osb screwed and glued. Carpet on top. I have no issues with the bounce. I mean SLIGHTLY. Other side of my house is 2x10 doug fir 16" o.c. 16' span. NO DEFLECTION, BOUNCE, VIBRATION, ETC. I realize the span is shorter with the fir, but IMHO a solid fir joist with bridging is the way to go if you can. Either way, stay inside the span charts and you should have no problems.

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