Skeptical
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Skeptical
Well color me skeptical. Yes the signpost beam is very strong, but there are several points of possible failure: The lag bolts holding the signpost to the studs are not the one I expect to fail first. The place I expect failure is where the plywood attaches to the signpost.
The signpost can support plenty of weight, downward force on it is not a problem. Pulling it out from the wall is probably not a problem with the lag bolts if you bolt it to every stud or at least every-other stud.
To keep the plywood from breaking around the fasteners and coming off the signpost, you'll have to fasten it at short intervals along the signpost, maybe every 4-6". If you do that, and use large washers, then I think the shelf won't break off. If you don't fasten it using enough fasteners the plywood will break under pressure and leave you with plywood on the floor and a signpost securely bolted to the wall.
So I think you can probably build it strong enough so it won't break. But I think 3/4" plywood will warp down over time if you have a 16" or 22" extension of unsupported plywood hanging out beyond the edge of its support. And I suspect a 16" or 22" piece of plywood supported on only one edge will be entirely too springy.
I think you'd better test a section out full depth before going any further. You can at least test whether you've built a shelf or a diving board.
Where is this "second beam" to be? If you just run it along the front hanging from the bottom of the plywood and otherwise unsupported that's essentially just adding weight to to the edge, which will make the shelf warp down in front sooner rather than later.
If you run another signpost along the front edge of the layout and support that with legs, then you've built a two-legged table bolted to the wall rather than a shelf, but you'll have something strong enough to support a very overweight person climbing on the layout. But I think that's overkill, and you'd be better off using 2x2 dimensional lumber in place of the signposts if you're going to add legs.
Those questions above answered,...
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Eastwind
Well color me skeptical. Yes the signpost beam is very strong, but there are several points of possible failure: The lag bolts holding the signpost to the studs are not the one I expect to fail first. The place I expect failure is where the plywood attaches to the signpost.
The signpost can support plenty of weight, downward force on it is not a problem. Pulling it out from the wall is probably not a problem with the lag bolts if you bolt it to every stud or at least every-other stud.
It will be lag screwed into every stud location, which in this shed are on 24" centers. The studs in housing construction are normally on 16" centers.
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Eastwind
To keep the plywood from breaking around the fasteners and coming off the signpost, you'll have to fasten it at short intervals along the signpost, maybe every 4-6". If you do that, and use large washers, then I think the shelf won't break off. If you don't fasten it using enough fasteners the plywood will break under pressure and leave you with plywood on the floor and a signpost securely bolted to the wall.
As you can see I have MULTIPLE holes to utilize there, so I can easily add more as necessary.
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So I think you can probably build it strong enough so it won't break. But I think 3/4" plywood will warp down over time if you have a 16" or 22" extension of unsupported plywood hanging out beyond the edge of its support. And I suspect a 16" or 22" piece of plywood supported on only one edge will be entirely too springy.
Where is this "second beam" to be? If you just run it along the front hanging from the bottom of the plywood and otherwise unsupported that's essentially just adding weight to to the edge, which will make the shelf warp down in front sooner rather than later.
If you run another signpost along the front edge of the layout and support that with legs, then you've built a two-legged table bolted to the wall rather than a shelf, but you'll have something strong enough to support a very overweight person climbing on the layout. But I think that's overkill, and you'd be better off using 2x2 dimensional lumber in place of the signposts if you're going to add legs.
Here are the possible locations of those 'front edge beams', and the upright posts as I see it now. BTW I don't have a problem with adding additioal suppot along the 'inner edges' as I can mount my LED lighting out there and run my DCC wiring along there, all behind a very 'thin' fascia piece.