How deep should my support posts be?

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Old 08-30-14, 01:02 AM
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How deep should my support posts be?

Ij'm building a 12' x 4'(2" x 6")pressure-treated wood rectangular frame, which will be bolted to the side of a concrete structure with three lag bolts. The frame will be anchored appproximately 26" above a grass-topped ground base(at least 1-2 feet deep). I'm planning to nail 3 2" x 4" pressure-treated and pointed wood posts, driven into the ground(how far?), as well as 3 angled 2" x 4" braces, from the ground up to the bottom of the frame. So I need to know how far into the ground should the posts be and whether the angled braces will insure the stability of the frame.
What's resting on the frame? Across the 12' I'll be nailing about 26 2" x 6" by 4' long pressure-treated wood. One adult will be walking across the boards at any one time and a total of 40 lounge chairs, totaling less than 150 lbs. could be also resting(spread out) on the boards.
I'll be happy to add any additional information on request.
 
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Old 08-30-14, 04:36 AM
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You don't want to 'drive the posts into the ground' It's better to dig/pour a footer and then secure the post to the top of it. How deep/wide the footer needs to be depends on local conditions, I doubt yours would need to be deep but your permit office can tell you the dimensions they require.

Lag bolts won't securely attach to concrete or anything masonry.
 
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Old 08-30-14, 05:03 AM
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When building a deck where the load on the posts will be mostly straight down a footer and bracket is the way to go. But if doing something with an overhung or side load I think it's better to go like a fence post and bury the post in the ground. The wood will not last as long in the ground though.

Look at the treatment level of your wood. Most 2x lumber is only treated for non-ground contact. You usually have to go to 4x or larger to get ground contact treated lumber. Coastal areas though use higher treatment levels so in your area you might find higher treated 2x wood.

I'm not sure how you're planning to do the support posts into the ground. Just three 2x4's driven into the ground in a straight line? I don't think that will be enough. With 4x4 posts set in holes with concrete or compacted gravel it might or if you double your legs to create more of a "table" design with legs on each corner and a pair in the middle.
 
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Old 08-30-14, 05:07 AM
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I question the dimensions. 4' is not wide enough to provide much walking space, nor the ability to hold all those lawn chairs. Is it a deck or storage shelf? No wood below grade. Make it free standing with no connection to the concrete.
 
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