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Tunedport350
Joined: 28 May 2008
Posts: 1
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 Posted: Wed May 28, 2008 5:21 pm Post subject: Joist and Beam Spans and Cantilevers |
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I have a pretty simple question that I seem to be getting conflicting answers for; I was hoping that someone could shed some light on.
I am designing a deck for my above ground swimming pool. Because my property has tons of ledge I am trying to design the deck maximizing the use of allowable cantilever for the beams and the joists to avoid some known ledge for the footings.
My question is this;
If I am using 2x8 (Group A) joists with 16" spacing the specified maximum allowable joist span is 12'10". The maximum allowable cantilever is stated as ¼ of the maximum span or 3'2 ½".
This all makes sense, but if I end up using a 10' joist span instead of 12'10" is my maximum allowable cantilever now only 2'6" (is the maximum allowable cantilever actually dependant upon the joist span used and not the maximum allowable joist span)?
I would like to use a 10' joist span with a cantilever of 3' on each end (to avoid the ledge) of the joist to achieve my desired 16' deck. I know structurally the joist can handle the load (cantilever) and it’s not like anybody is going to step on the edge and have enough mechanical advantage to teader the whole deck.
Thank You |
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EMDesigns
Joined: 28 May 2008
Posts: 4
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| Posted: Wed May 28, 2008 7:52 pm Post subject: |
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| Thats correct. If your spaning 5', you would still use a 1/4. We actually cantilever everything at 1/3 of the max spanning distance. But we always check with an engineer before it gets the seal. Our engineer doesnt have insurance so we tend to have "over' engineered plans...but more is better. |
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nanrehvasconez
Joined: 25 Feb 2008
Posts: 164
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| Posted: Thu May 29, 2008 8:38 pm Post subject: |
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| if you are talking of using a true cantiliver, the safe bit is to use 25% cantilever, but you can increase safely to 50% if you knee brace the joists with a 45 degree 4x4 every 48" cc appart, the brace shall be properly anchored with simpson steel brackets, meaning that the 50% cantilever should have a height not exeeding the 50% lenghth |
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Coffeefiend
Joined: 29 May 2008
Posts: 4
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| Posted: Fri May 30, 2008 12:19 pm Post subject: |
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| If you're cantilivering off of both sides, you can reduce the potential deflection by weighing both ends evenly (for instance, if you place your large gas grill on the cantiliver on one side, be sure to balance out on the other side with some deck furniture). |
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