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I'm designing an outdoor bench. It will be made out of 1in plywood and pressure treated 2x4s. I'm wanting an 8ft length (give or take). The seat will be 7 2x4s. Can I get away with an open span? or do I need to add another bracing section?
iso view

Here's a fly around of the bench if that helps https://youtu.be/OLPQDtLKUG8

Sorry if this is a stupid question. I tried the sagulator, but it didn't have pressure treated as an option.

UPDATE

So I realized this design had two fatal flaws regardless of the weight on the seat.

  1. There is no real horizontal shear support for the base
  2. Having a long flat base as this does, will not sit well on an unlevel surface like my yard.

Here is what I ended up with. This thing could support an elephant!! Final Product

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    There should not be any appreciable difference in strength between pressure treated and plain. – Ashlar Dec 28 '17 at 4:07
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    "Can I get away with an open span?" That depends on the expected loads. The maximum central load is the key one, people seated off to one side produce far less of a bending force on the wood, and two people sitting side by side can also be less (even if their weights total to slightly more than that of the person who might sit in the centre). Can I just check, is this for a domestic setting? If so it'll see lighter and more occasional use so I can't imagine this won't be fine. For a public setting it would be different, and how the ends were fixed might also have to be chosen with more care. – Graphus supports Monica Dec 28 '17 at 7:38
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    Plugging in some approximate numbers into the Sagulator I get what it classes as excessive load, and it would be for a shelf which just sits there holding books. For periodic loads things are slightly different, and how you feel about a slight bow (enough to be visible) would likely be different as well. It's a big deal visually if a shelf bows, not so much with a bench. Also here it might be felt that it provides a modicum of springiness which could be seen as a positive, not a negative (your call though). – Graphus supports Monica Dec 28 '17 at 7:42
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    Not sure what numbers you used in Sagulator, Graphus. I used 96" span, 24" wide, 1.5" thick of douglas fir. With 500 pounds of center load, or 1000 pounds of load spread evenly, I get a sag of 0.18, which is acceptable. – LeeG Dec 28 '17 at 15:45
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    @AloysiusDefenestrate the bench will have cushions on it. – Doug Chamberlain Dec 31 '17 at 1:41

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