Plywood Triangle Gussets for Wood Structure: How does DPW do it?

[XaOs]

I have done a lot of searching but can't get quite close enough to the details I need, so I'd better ask: how best to brace a wood shade structure to prevent racking (leaning / collapsing) under wind load?

The structure: 16' x 20', 12' high in front & 10' in back. 4x4 posts with 2x6 beams. Tarp roof, about 60% open on the sides. Bolt-together construction, metal footings for the posts lag-screwed into the playa (I don't want to have to use rigging to lash it down).

I saw a mention in one of the ePlaya posts that DPW builds wood structures that use plywood triangles (gussets) for bracing. Does anyone have any details about that? I want to know the size & thickness of the triangles, and the size and types and number of fasteners per triangle (bolts? screws? nails?).

Here are a couple of pics to help illustrate what I am talking about:





I'm thinking of possibly metal triangles instead of plywood but the idea is the same.

[some seeing eye]

What you are trying to prevent is your rectangular structure from becoming a trapezoid. The DPW sinks their posts into the ground. Because you are bolting to the ground, your resistance to the post tilting provided by your ground bracket would be much less than a DPW design. You can add diagonal steel cables for strength too. Bottom line, perhaps you can find someone on ePlaya who has built the same structure if you provided more details on the roof and sides, and ePlayans answering had the same structure - maybe they have photos of their structures?

[XaOs]

What you are trying to prevent is your rectangular structure from becoming a trapezoid. The DPW sinks their posts into the ground. Because you are bolting to the ground, your resistance to the post tilting provided by your ground bracket would be much less than a DPW design. You can add diagonal steel cables for strength too.

The posts won't be sunk but will have 18" lag screws into the playa: 2 or maybe 4 per post - not as good but maybe not so bad. Don't know why DPW would need to use gussets at all if they're sinking the posts.

I was thinking of using compression bracing (crossed cables as you described) but wanted to avoid the expense of the additional hardware and the additional build time of running the cables and fiddling around with the turn buckles. Also, since there would be doorways front and back, I don't know how well the compression bracing would work if it wasn't between ALL of the posts.

I'm out of my depth here, structural-engineering-wise. Those treated wood posts are heavy, and the 2x6 beams won't be light either. I really don't want it to fall on anything / anyone.

[Traveller in Time]

if using (ply)wood make sure the grain is along the stress direction.

For these triangles the grain should be with the long, diagonal side.
(even plywood has its grain only in 90degree angles )

Take a fresh square and start by cutting it over the diagonals.

[XaOs]

Take a fresh square and start by cutting it over the diagonals.

Sorry, not quite clear on what you mean there. Care to elaborate?

[Ratty]

XaOs don't feel all alone. He lost me there too. Tell us what you're building. So ambitious.

[maladroit]

You'd want to align the hypotenuse of your gussets with the edge of a sheet of plywood, so that (some of) the grain runs along the longest side of the triangle. I'm not sure if this is ultra necessary though.

No one can tell you if your structure will be strong enough. At least, not until you collect the actual dimensions, materials, and conditions you expect to withstand, and give all that info to an engineer.

If you don't want to do that, get out in the garage and make a test setup. Try making part of a frame and a way to generate a couple thousand pounds of force, and see if it breaks. If your structure would be dangerous if it fell on someone, you owe it to yourself and the public to invest the effort needed to make sure it doesn't fall. Otherwise, stick to monkeyhuts.

[ygmir]

there are also pre made metal brackets of different configurations, sold at hardware stores, that have shear ratings and all sorts of info. Not super expensive and easy to install with screws or TICO nails.

[Traveller in Time]

Sorry for the confusion, was already third attempt to formulate that sentence.

What I wanted to say is what 'maladroit' explained more clear. I should have made a drawing

[Paperman]

You need to secure all of your right angles with the gussets. if the posts are sunk or not. And in the horizontal roof plane so it doesn't twist. Maybe 3 bolts , off set in the 2"x 6"s. If you use gussets on both sides of the door than thats perfect, with a single top rail. Make the gussets big enough. And you can put them on both sides if you wish. I built something simular , 16'x16'x12'high But with no gussets but with wood screws and 16' x 2"x6"s . Was a platform that held 30 plus. Maybe it wood help to build a scale model. Good Luck

[Traveller in Time]

The artery (TPR and Le Wrench) revealed some structures for their trusses.

Go check it out in the dust

[Token]

if using (ply)wood make sure the grain is along the stress direction.

For these triangles the grain should be with the long, diagonal side.
(even plywood has its grain only in 90degree angles )

Take a fresh square and start by cutting it over the diagonals.

This is nonsense. Plywood by design is built by gluing layers of orthogonal grain veneer, so there isn't a primary grain direction. Hence, plywood doesn't present different directional strength.

[Traveller in Time]

if using (ply)wood make sure the grain is along the stress direction.

For these triangles the grain should be with the long, diagonal side.
(even plywood has its grain only in 90degree angles )

Take a fresh square and start by cutting it over the diagonals.

This is nonsense. Plywood by design is built by gluing layers of orthogonal grain veneer, so there isn't a primary grain direction. Hence, plywood doesn't present different directional strength.

Othogonal as in 90degree angles . . .

Try a strip of an inch wide cut out at 45degree angle to the grain.
Compare to a strip with the same dimensions at 90degree angle.

The first can barely support its own weight, the latter can be used for something.

NB I really admire the trusses in the Journal, light but all angles covered.

[Token]

Show me the direction that is "weak".

[trilobyte]

I'm with Token. Even a plywood with a fancy finish top layer is almost certainly going to be made up of a number of layers angled in different directions. For stronger parts, I'd recommend looking for the most layers in the given thickness of wood. On a side note, on a 45-45-90 triangle the orientation wouldn't really matter even if you had some special non-orthogonal plywood.

[Traveller in Time]

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It is a polar diagram, the distance to center is strength. At 45degree it is only the glue keeping it from falling apart.

[the parser has eaten all my spaces, was nicely indented]

[GreyCoyote]

Token: I think I see some of the confusion on both sides. You are correct that plywood is a laminate. The 5 layer type you show is only very marginally stronger in the one direction, but for all practical purposes its almost a push. And 7-layer plywood is going to behave even more symmetrically. Higher lams help this omnidirectional performance even more.

The problem comes with 3-layer plywood. This stuff is fairly common in some places (esp in 1/4 inch thicknesses), and it makes a whopping 2:1 difference in strength depending on the orientation of the surface grain. Add to this that middle layer is often full of voids (precisely because its hidden from view), and you can have an even more lopsided performance vs orientation curve.

So depending on the layer count, both sides of the discussion may be correct.

The devil is in the details.

[Token]

What cheap FKO uses 1/4 ply for anything bigger than a finch birdhouse?

If you are concerned with loads on 8-10' span, spend the $$ and get a good grade of 3/4" ply.

If you are doing this at BM, ditch the ply and go OSB cuz it's stronger against shear, has less air spaces and more glue, doesn't warp and suffer as much in 5% humidity and playa desiccant.

The only place where ply outperforms OSB is nail/screw holding force and marine applications.

[Traveller in Time]

Any results from the playa test? Field research at DPW constructions?

I got some strips along the grain and some diagonal, force measuring device, tackle system. . .
Only figuring way to attach the strips of plywood (3,5,7 and12 layered samples) without compromising them.

[Traveller in Time]

Managed the grip using steel wool whipped in two lengths of rope.

8 fold tackle levering, 16 to one lever. gauge at tip says 4kg as some ropes snatch. back to drawing board.

Have reduced width of 12 layered strips to 10mm.