vertical slip connection

[Gopher13]

I have a question about a pretty common detail where I wish to transfer lateral diaphragm load into a wall but not gravity load. Please see the attached drawing. How do I assure the joist and angle will be able to move up/down without loading the wall? Do they not tighten the anchor very much so there is minimal clamping force to overcome? Should there be some sort of bond breaker between the vertical leg of the angle and the masonry or should I put a steel plate between the two at the anchor locations to decrease the friction? I have also seen details where there is teflon or graphite between two steel members at the slipping surface. Is this something that should be done?......so many question.

 

[JAE]

You can try to use a vertical slotted hole and that detail I suppose, but a number of comments:
1. Don't have them bridge the joists into the wall - no need - and the bridging isn't shown allowing the vertical joist deflection anyway.
2. Why is there a joist so close to the wall? Can't you load the wall with a 1/2 joist spacing roof load? That way the deck could flex a bit up and down with the first interior joist deflection and still "grab" the wall laterally.





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[XR250]

I was taught to put the first joist 6" from the wall which makes NO sense as JAE responded. Keep it a joist spacing away, let the deck flex and forget about the slip joint.

 

[Gopher13]

JAE: Great feedback on the bridging. Seems so obvious now that you say it. The wall to the left is taller than the roof on the right resulting in a large snow drift load. The wall is existing and is composed of 8" cmu + 4" brick with a header course every 24" vertically. I am anchoring into the brick which I am not a big fan of doing, and there is far less lateral load that gravity load. So I guess I am okay sending a minimal amount of lateral load into the brick but not the larger (it's really not that large though) gravity load.

What I am more interesting in know is how the slip plane actually works. Per my original post, do they not tighten the fasteners down very tight? Does there need to be some special slippery material between the slip planes?

 

[JAE]

I think it is generally difficult to get a good slip joint (using teflon or not) in a situation like this. I always suspect that the interface would bind up eventually just due to irregular movements, dust/dirt, etc.

I think teflon slip joints were used more extensively years ago - 1970's-1980's perhaps - but any more you don't see them as much except for bridge bearings maybe - and then you see more flexible elastomeric pads that allow the movement.

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[Deadblow]

I have often run into this same situation. I want to transfer the loads from the roof diaphragm into masonry shear walls. For what it's worth, I have been taught to add the joist close to the wall similar to the OP's detail. That joist is therefore only receiving half the load of the other joists and the deflection will only be half as well. I call for 1.5" slotted holes in the angle and fasteners "torqued to allow vertical movement."

EIT

 

[JAE]

I just think that any minor rotation (about the joists longitudinal axis) will bind the angle and prevent much slip at all.

Then the angle and wall will try to take all the joists load (force follows stiffness) and there you go. Why not space the joist out further and be done with it?

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[Gopher13]

Thanks for the replies! I like the "torqued to allow vertical movement" language. I also like the comparison to elastomeric bearing pads for bridges. I might try that.

 

[Ron247]

On the actual tightening, I never tighten them much at all. We used to call out "Finger-tight and upset threads" or "Finger-tight and use Jamb Nut". The nut on the bolt just controls "steering" the angle up and down. You need anything reasonable to insure they slide on each other. Install in the summer and in the middle of winter it can become tighter.

I assume you do cannot load the wall vertically for some reason.

Out of curiosity, what "holds" the top of the wall in place? Are you allowing it to move laterally but no more than the "looseness" of your slip-joint? From what I see in your detail, the angle is welded to the bar joist.

 

[Gopher13]

See the attached drawing. It is a mechanical penthouse addition that sits on the low roof of an existing building. As far as I can tell the existing roof is not really connected to the wall at all. The joists bear directly on the wall. No bearing plate. No anchors. There is a course or two of brick above the joist seats that I guess holds it in place.

The penthouse has a moment frame on the right side and a light gauge shear wall on the left side. I was trying to avoid putting out of plane loads on the wall. It is unreinforced and ungrouted. Now that I think of it though, when the wind blows on the long face of the penthouse, the roof diaphragm will deflect pushing on the edge angle which will then load the wall.

 

[dhengr]

Gopher13:
Are the slots perpendicular to the longitudinal axis of the angle, or are they vert. in their final position on the wall? How do you rationalize vertical movement/deflection with the roof sloping, which may induce some transverse movement of the angle w.r.t. the roof diaphragm. Bolt the darn angle to the mas. wall and eliminate the stl. jst.at the wall, for the savings of the cost of two stl. jsts., and to eliminate a bunch of complexity which you can’t justify or explain. Then do a good job of flashing and counter flashing at the wall/roof intersection.