Moment connection in wood portal 7

[struct_eeyore]

Hi all,

Need a sanity check. I'm generally against moment connections in wood, but here I have to evaluate someone else's work, so obviously don't want to make too much of a fool out of myself. Please see the attached image below (or wherever it ends up). We've got an open building; the trusses rest on 2x beams which are then secured to columns with 2 thru bolts. The trusses themselves are secured to the 2x beams with 2 USP clips. Now, I can get the portals to work if I resolve the moment across all the straps, of all the trusses. The question is, will that happen in reality, or will the moment likely to want to distribute itself thru the closest straps to the posts? Also, despite the fact that the clips can carry the couple moment, do you think they are rigid enough

 

[oldestguy]

Looks damn skimpy especially with screws just in from the edge. How about some angles instead and bolted similar to the bolts shown.

 

[r13]

The truss chord is subjecting to a moment, which is resolved into a couple as you did. The metal strap delivers tension to the 2x below, and the truss chord press against another 2x. The rigidity depends on the strength and deformation characteristics of the strap, its interface with the truss chord, and the tightness of the thru bolt connections.

 

[struct_eeyore]

do you have any experience with the deformation characteristics?

 

[Ron]

This is not a moment connection. I would probably give it about a 20 percent fixity.

 

[r13]

The rigidity evaluation is quite complicate, I think you can only relying on the data provided by the manufacture with sufficient safety factor. Will the truss fail if assume pinned connection (zero fixity)?

 

[struct_eeyore]

@Ron - I agree with you, but what is the rational explanation. The numbers fortunately/unfortunately, imply that the couple can be resisted.

 

[r13]

I don't quite understand the failure mechanism. If the post is supporting a beam, I can see the beam might fail in the positive moment region, but you said it is a truss...

IMO, the connection was designed to prevent the slippage of the post due to rotation of the truss chord and translation.

 

[struct_eeyore]

@reitired13, the posts acts with the truss to form a portal frame. except that i need the entirety of the trusses to act in unison to create a single "beam" that will be shared between a set (2) of portals. the failure mechanism is likely tensile failure/ductility of the strap

 

[r13]

Did you check Simpson for strap capacity? Could the tension capacity controlled by the nails?

 

[civeng80]

Why can't a knee brace be put in to stiffen the joint?

 

[phamENG]

You're going to have to contact your Simpson rep. They're generally really helpful and, if they have the answer, they'll provide it. That said, don't tell them what you're doing - just what you need. I can all but guarantee that they'll tell you not to do this. Ask if they have stiffness data for their hurricane straps in tension. Then you can use that to determine the rotational stiffness of the joint and see if it's anywhere close to what you're looking for.

Another thing to look at - what is this moment doing to the bottom chord of the truss? Does this occur at a panel point, or at the middle of a chord member?

This looks like one of those times where, under reasonable service loads, it'll probably have enough fixity. But sometime before you reach full design loads it's going to deflect too much and you'll loose stability and a strap or two.

 

[SteelPE]

Cross grain tension loads into wood, yikes!

Good luck with your review. Seems like you are required to justify someone else’s mistake.

 

[KootK]

The question is, will that happen in reality, or will the moment likely to want to distribute itself thru the closest straps to the posts?

You're instincts with that are sound; that's your silver bullet right there. Regardless of the deformation characteristics of the connectors, the flexibility introduced by bending in the two, out of the page 2X beam things should render the interior connections useless. It likley also means that you'd compromise the connections closest to the post long before you got even close to fully mobilizing those in the interior.

Some questions for you:

1) How long are the out of plane 2X and how many connectors need to be engaged?

2) Does the through bolt connection work? I'd have guessed that to be a weak spot.

 

[r13]

I think you can assume full fixity if,

1) the applied tension is well within the tensile capacity of the strap, its end connectors, and the wood it connected to.
2) the shear friction between the 2x and the post exceeds the intensity of the couple with some margin - no slip will occur.

 

[KootK]

Cross grain tension loads into wood, yikes!

I don't know that cross grain direct tension is quite the deal breaker concern that cross grain, flexurally induced tension is. Many common wood fasteners and connections simply could not function without reliance upon direct cross grain tension capacity and do, in fact, account for it in the requisite design procedures. Which is not to say that tension parallel to grain isn't to be preferred by a wide margin when it is available as an alternative.

 

[KootK]

2) the shear friction between the 2x and the post exceeds the intensity of the couple with some margin - no slip will occur.

I'm skeptical of that mechanism's viability retired13. All three members in the connection will experience moisture related shrinkage and any reliable shear friction will drop to zero.

The very best that could be achieved in this situation regarding connection restraint would be full restraint with lateral "bleed out" similar to the equivalent column design approach in two way, concrete slabs. The classic problem of trying to "fix:" a wide thing to a narrow thing.

 

[oldestguy]

Why not change the thing so no cross grain tension is present especially at the tension edge? Locate it farther back in. The catalog doesn't show this use with tension cross grain a short distance from that edge. Otherwise expect an edge split failure. Catalog shown capacities surely don't apply here.

 

[KootK]

Is anybody under the impression that a classic hurricane tie doesn't involve cross grain tension close to an edge?

 

[oldestguy]

Maybe start with the required working value of moment needed.and safety factor.