beam span?


Old 05-19-03, 07:48 PM
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Question beam span?

I am getting ready to build a low level deck (approximately 14" at its highest point) The basic design is 20' x 16' using 2'x8' lumber.The joists will be ledgered to the block foundation. From there the joists will span 92.25 inches to the first beam and will be attached using joist hangers on the side of the beam. Continuing from the first beam to the second beam will be the second row of joists at the same lenghth and attached in the same manner. Both beams will be built up using 2x8x10 lumber to achieve a 4x8 thickness and spliced lengthwise with a metal strapping plate to achieve a 20' length. The joists will be 16" on center and the decking will be 5/4 x6 pt lumber. My question is about the footings. My plan is to use 8" tube forms with direct bearing post anchors mounted on top for the beams to rest in with 1/2" plywood spacers shimmed in to to make up the full 4" for the beam. The spacing of the footings for the beams would be one set 2' in from the edge of the beam (cantilevered) and one at the center point of 10' (under the splice) and a third 8' from the center point, leaving a matching 2' cantilever at both ends. Does this sound like enough footings for these spans? Some books I've read say 8' spans are fine while others say 7' would be the limit . I would use 2x8x20 boards for the beams but my Home Depot doesn't carry them, so I need a footing under the splice. Also does it sound like the splices will work using the metal strapping plates or would I be better off using 2x8x12 boards cut to overlap each other several inches at the splice. -- Sorry for being so long winded, but I just wanted to be as detailed as possible--Thanks in advance for any replies to my post -Terrence
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Old 05-20-03, 05:43 AM
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Beam span is typically a creature of the area of the deck that the beam must support. The center beam in your design is the critical structural element, because it must be designed to handle the load halfway to the ledger (4') and halfway to the outer beam (4') across the entire width of the deck (20'). This amounts to 160 square feet. Assuming local code requires 40 lbs. live load and 15 lbs. dead load (55 lbs. total), that beam must be designed to support 8800 lbs. Across 20 feet, that's 440 lbs. per linear foot. At an 8' beam span, doubled up 2x8's are not going to cut it, especially considering that you are planning on cantilevering the beams. To keep the 8 foot spacing, you would require tripled 2x8's. And forget the plywood. You don't want to put anything in that beam structure that isn't rot and decay resistant. You could stay with doubled up 2x8's if you added a fourth footing, giving you a beam span of like 5.5 feet. I personally would go the latter route since it puts less stress on each footing.

For the beam splice points, make sure they fall over a footing. Strapping is okay, but I would also suggest through bolting (on both sides of the splice) in the vicinity of the splice. That'll make that beam rock solid.

As for those footings/piers, the 8 inch tubes for piers are fine, as long as you make the footing (the bottom 8 inches) bigger. Good soil can handle about 2000 lbs/sq. ft. at 2' depth and probably over 3500 lbs./sq. ft. at 4' depth. In NY, code require 3' depth, which gives you in the neighborhood of 2800 lbs./sq. ft. If you use 4 footings/piers, you need each to handle up to 2200 lbs. (8800/4), so you want close to a square foot of footing (pardon the pun). The .35 sg. ft. of an 8" tube alone will not suffice. You could go to a bigger tube (12") or use the 8" and flare out the bottom 8 inches of the hole by 2 to 3 inches. The latter approach also helps with frost heave because you have a lot of weight on top of that flared-out portion of the footing/pier (if frost heave is even an issue).

Good luck,

John P.
Old 05-20-03, 01:43 PM
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Thanks John for the reply! I think I'll take your advice and add the extra footings -Terrence
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