Maximum Beam Span Question

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Old 08-01-07, 01:22 AM
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Question Maximum Beam Span Question

I have been reviewing the maximum beam spans between posts and supports tables in several books and none of them address beam cantilever. All of them state beam span is measured from center of support to center of support. While reviewing deck specifications I downloaded from different counties across the US I noticed that beam cantilever is limited to 1 or 2 depending on that particular counties building codes. Is beam cantilever treated just like joist cantilever?

Also, in one section of the deck I am designing, a 9 long 4x8 PT Dug Fir beam rest directly in two Simpson PBS44 post bases. The span of the joist is 6-6 and the beam span is 6-8 with a 1 cantilever on each side of the beam supports. I see a 7 joist span allows a maximum 7 beam span. Am I cutting it to close to the maximum 7 allowed? If interpolation is allowed I guess the maximum beam span could be increased to 7-6.
 
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Old 08-01-07, 12:30 PM
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I don't think that you mention the joist size.

Generally speaking, my rule of thumb is that you can span about the height of your beam or joist. Thus a 2x6 joist I will span 6' on 24" centers. Same goes with the beams. These are usually well within the maximum spans for dimensional lumber. This is using douglas fir, pressure treated, of course.

Rule of thumb on cantileavers is 1/3 of your span. Thus a 6' span can then cantilever another 2'.

It is actually a lot more complicated then that, but it usually works for me.

This doesn't work if you have very large spans on your joists, thus placing a much higher load on your beams.
 
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Old 08-01-07, 06:22 PM
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Whether it's beam cantilever or joist cantilever, I don't go more than 24", PERIOD!!

I have turned down a couple of $1/2 million jobs because the owner wanted to cantilever his floor joists 4' or 6' "because that was the "architectual design" of the house when it was built 40 years ago. Didn't work then, and it ain't gonna work now!!
 
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Old 08-01-07, 11:45 PM
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Thanks everyone. I went with a maximum 12 inch cantilever for the beams and maximum 24 inch cantilever for on the joists. Here are links to pictures of the deck drawings I am working on.

Paste picture link: http://www.fototime.com/211B599811F7E82/orig.jpg

Paste picture link: http://www.fototime.com/BFACEC312F17912/orig.jpg
 
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