Min beam dimensions for ~20' span for a loft support - Woodworking Talk - Woodworkers Forum

 
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post #1 of 17 Old 02-14-2011, 05:42 PM Thread Starter
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Question Min beam dimensions for ~20' span for a loft support

Hi all,

I'm looking to put a sub floor/loft into a garage that I have. The span is 6 meters (approx ~20 foot). I want to put a single cross beam in as the main support, without having to place any vertical support columns in the center. While this can be done using a steel beam, I'd prefer to use just a single wooden beam.

The floor would be ~20 foot across and ~12 foot deep. The max load would be approx. 2200 pounds (1000 KG). Three of the sides would be fixed to the walls (concrete block) using steel fixings. This new sub-floor/loft area is only going to be used as storage.

Presently the floor space is totally used up and I'm toying with the idea of utilizing the space for a new workshop. The overall dimensions of the garage is 6m wide by 5 meters deep by 4.5 meters high. The subfloor would be 2.25 m off the ground (high enough for me to walk under without hitting my head).

Has anyone done anything like this or have any advice on material to use, dimensions, what material to not use?

Is this even possible, without having to go to some extreme size for the beam?


Thanks.

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post #2 of 17 Old 02-14-2011, 05:51 PM
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I would suggest 3-2x12's with 1/2" plywood flitches staggering the butt joints, you can make the top of your beam flush with your floor joists to gain head clearance below. I AIN'T NO ENGINEER BUT I DID STAY AT A HOLIDAY INN EXPRESS ONCE. And I have framed many houses too.

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post #3 of 17 Old 02-14-2011, 05:56 PM
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Hey, this is gonna be great. I'm makin' some popcorn and sittin' back waiting for all the structural engineers to chime in.








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post #4 of 17 Old 02-14-2011, 06:01 PM
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This same topic is discussed here

Structural Engineer in the House???
And the 20 ft dimension is the same!
"I" joists in my opinion are the best solution rather than a single beam with a 20 ft span which is a long span.
http://www.internationalbeams.com/i-joists/
They do make 2 x 14's on special order, but you need to consult a floor load table, plug in the floor load per square foot, span, type of wood etc, and see what size the chart recommends ...for a start. All free advice, not responsible for any catastrophic failures... bill
floor load calculator:

http://www.awc.org/calculators/span/...rcalcstyle.asp

The answer to your question will only be as detailed and specific as the question is detailed and specfic. Good questions also include a sketch or a photo

Last edited by woodnthings; 02-16-2011 at 11:14 AM.
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post #5 of 17 Old 02-14-2011, 07:49 PM
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I'm in the process of structural repairs in my house (with basement shop). The main beam on the first floor is failing, and our design for replacement took into account my intended workflow. I asked my structural engineer about the max span allowed for wooden beams. Not engineered beams, just dimension lumber. He said our code (and I think we adopted the UBC without tweaking this part) requires a vertical support a minimum of every 10 feet for dimension-lumber girders.

We looked at other options and after factoring in cost decided we could arrange the shop around the new posts. But we still went with an LVL to save ourselves a couple of inches of headroom (and at that size, the LVL also required a support every 10 feet.

Whether you need supports closer than that will depend on what you plan to put in the loft, plus the rest of the load. Will the increased load increase your need for rafter ties?

Structural engineers aren't cheap but IMO they're worth it. Specing the 1st floor framing plus a hidden beam in the attic and post footings ran us $750. Given the time and stress for me trying to do it myself, and do it safely, I consider that price a bargain. Plus we're in a jurisdiction that requires permits, and my insurance frowns on DIY repairs without the city's paperwork.

Suggest some professional help, and staying legal if your're in such a place. Also check the insurance fine print for its rules about these changes.

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post #6 of 17 Old 02-14-2011, 08:54 PM
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first thing you need to do is find out what your local code is and that will tell you. contact your city/county, or state building department.
you looking around 4x14, 4x 16 for a 20' span. i would use a glue lam and not a solid beam ( 1 pc ). steel would be best for strength and space.
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post #7 of 17 Old 02-15-2011, 02:52 AM Thread Starter
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Thanks everyone for the suggestions.
@woodnthings: Thanks for that link, the I-Joists look interesting.

At least I now know where to start looking.

Thanks again.

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post #8 of 17 Old 02-15-2011, 06:00 AM
Huh?
 
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I-Joists are great. I used them on my home addition.
21' without supports (except on the two ends ) using 9-7/8" I-Joist 12" OC. They did the calculations, not me. I wouldn't trust myself on that.
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post #9 of 17 Old 02-15-2011, 06:21 AM
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Are laminated wood beams (Glulam) available in France? In the nordic countries this is a typicall application for a glulam beam. Here is a link to the Nordic Glulam Handbook http://www.svensktlimtra.se/en/limHTML/
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post #10 of 17 Old 02-15-2011, 06:27 AM
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post #11 of 17 Old 02-15-2011, 09:43 AM
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Min Beam

I will pass on providing engineering on line but I will say that 20' for regular wood members is aggresive. Depends on the loads that it has to carry and other factors. Also the connections at the ends would be critical--not to mention the load on walls and foundation. woodnthings had a good suggestion about using engineered trusses/joists to span the 20' direction.
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post #12 of 17 Old 02-15-2011, 12:25 PM
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I think I'm confused. Can you run 2x16 floor joists @ 16'' centers from one wall to another. Distributing the load evenly across all joists and along the walls. Why hang this whole floor on one beam?

Last edited by Master Tinker; 02-15-2011 at 12:41 PM.
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post #13 of 17 Old 02-16-2011, 09:37 AM
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I probably was confused too. My post above was about a beam under a loadbearing wall, which isn't your situation.
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post #14 of 17 Old 02-16-2011, 11:21 AM
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Floor span Table showing 2" x 14"'s

http://www.engineeringtoolbox.com/fl...an-d_1479.html
It's possible to use those for your span, but not 2" x 12"'s which limit out at 17" 6" in the previous link I posted above. bill

I still think "I" joists are the best answer....JMO.
http://www.internationalbeams.com/i-joists/

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post #15 of 17 Old 02-16-2011, 05:28 PM
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why are you not spanning across the 12' dimension with regular joists? That being said GP and others make wood i joists that span 20'
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post #16 of 17 Old 02-17-2011, 09:33 AM
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I think there's only one wall in the 12' span, the other side would have to sit on a massive beam. I THINK.

Was at the local big box store last night. 2X14 I joists can span up to 19'8''.
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post #17 of 17 Old 02-17-2011, 09:59 AM
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It may just get down to $$$

Sure the 12' span could be made with 2 x 8's, cheaper, but then there's the cost of a fairly stout beam to hang them on or off.
The "I" joists may be comparable in cost, I donno? The beam could always get a post in the center "if" future use or heavier loads demand it. The I joist solution will not permit that easily. A slightly deeper I joist will rule out any issue with future additional loads. It's a cleaner approach in my opinion, and it's done all the time these days for similar conditions. Hi Tech vs Old School kinda thing. bill

The answer to your question will only be as detailed and specific as the question is detailed and specfic. Good questions also include a sketch or a photo
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