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Eccentricity in single shear connection

Eccentricity in single shear connection

Eccentricity in single shear connection

I'm curious as to how moment is resisted at a joint in a single shear bolted connection. I'm specifically concerned with wood, but I imagine the same question applies to steel members that are bolted together. See the attached pdf. If you look at the Mode II yield mode, is that what is checking and making sure that the moment of the connection can be resisted (Since that one doesn't apply to the symmetric double shear connection)? I assume there is a simple answer to this question, but I can't seem to figure it out.

The specific example I have and reason I ask is for a wood ledger screwed to the thin face of wood studs. Do I need to consider tension withdrawal values in any of those screws or just shear?

RE: Eccentricity in single shear connection

Moment in a ledger may be resisted by the supported joists acting in compression, tension in the fasteners or by a combination of those actions. If you are relying on screws alone, you must consider withdrawal in combination with shear.


RE: Eccentricity in single shear connection

I believe your assessment is correct. I have wondered the same thing--and with steel connections as well (as you mentioned).

I think a few things are at play here:
* The eccentricity is small enough to be ignored for practical situations
* The rotation of the fastener (bolt, screw, nail, etc.) takes care of some of the moment (Mode II yield mode, which probably applies to steel in some ways as well)
* The lap of the two members creates the ability for the moment to be resisted by a couple, which puts compression between the two members, and tension in the fastener


RE: Eccentricity in single shear connection

Since wood is not very ductile, I imagine that the fasteners would fail in shear or cause tearout (fracture) of the wood. Since steel is ductile, I think the first thing you would see is a rotation of the lapped portion. Almost like a prying action of sorts.

RE: Eccentricity in single shear connection

For both steel and concrete connections, looking only at the connections, we're making the assumption that the eccentricity related moments are a small fraction of what the connected members can take. There's no doubt in my mind that the moments exist. I remember dealing with a field problem on a very serious steel connection that ended up requiring lots of shimming to make a go of it. It was astonishing to me how much shimming you can do without actually running afoul of standard AISC bolt design methodology.

For your specific ledger, there's more at play. As I see it, your shear enters the connection over the seat of the joist hanger and leaves it at the face of the supporting stud work. As such, your real eccentricity is really the distance between those. And it can add up, especially with multi-ply ledgers. In that case, I feel that the moment does need to be explicitly resolved somewhere, most plausibly by using your bolts in combined shear and tension as mentioned by others. For what it's worth, it is my impression that very few designers are taking this into consideration.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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