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Connection Design

Connection Design

Connection Design

(OP)
I have s a situation where I have a pipe column (2-3 kips) bearing on the top chord of an open web truss. There is no guarantee that it will be able to hit a panel point, so I have to provide reinforcing between the panel points.

The detail I came up with needs three roof penetrations per pipe column to do this, and I do not like it. Does anyone have a detail they have used that is simple and uses fewer roof penetrations to install?

I can post the details I have sketched when I get to work if needed.

RE: Connection Design

For window washing tiebacks, our standard detail is to run the post past the joist and connect to the side of the joist at the top and bottom chord, then add kickers to deal with the torsion. It's a little different situation because we're trying to deal with the moment and it sounds like your situation is axial, but maybe that will spark a connection idea for you.

RE: Connection Design

(OP)
steellion:

With metal deck over the joist, how did you connect to the side of the joist without welding or bolting to the top and bottom flanges of the joist, or did you?

The joist is existing and this is a retrofit, so I am worried about compromising the capacity of the joists by welding or bolting to the flanges. Currently, I am trying to literally sandwich the flanges between two angles and three bearing plates, shimming and grouting as needed.

Mike McCann
MMC Engineering

RE: Connection Design

Try using a C6 channel instead of the grout. A C6 matches most ribbed profiles (or other shape is needed) and use the flanges as short columns.

For the bottom connection, try using a beefy 8" long angle spanning to either side of the joist (perpendicular to chords) these can take the bolts easily and then can be welded/bolted back to back for a (4) bolt configuration to keep them from walking, and tack them to the chord if you want.




RE: Connection Design

First of all, is the existing joist good for this load? Why not add additional web members to the joist as you typically would for a concentrated load not at a panel point, instead of adding the 4x4 angles? Personally, I would not support the column base plate on the deck. I would use a relatively small base plate, cut out the deck at the base plate, and weld the base plate directly to the top of joist. If losing a little deck support at the base plate is a concern, you can add minor supplementary supports to the joist top chord either side of the cut.

RE: Connection Design

(OP)
Eric:

Thanks for the channel suggestion. I'll look into that.

Placement of the transverse 8" deep angles could be interfered with by the chord member of the truss though, particularly at the 8" depth plus the depth of the angles. I am trying to make this connection as adaptable as possible to any field condition, but I know that is a longshot. Have to try though...

I am supporting a steel equipment platform (600#)and about 900# of equipment in a high seismic zone, so I am concerned with developing some moment capacity at the diaphragm level too. Not much, just some.

I guess the vertical reaction is only 650# or so per post considering some additional platform live load.

Mike McCann
MMC Engineering

RE: Connection Design

Our typical detail is to use the C6 as suggested above. This detail can also be found in Jim Fisher's book "Designing With Steel Joists and Joist Girders". This detail should help with the detail where the post does not hit the joist. If things don't line up quite right, use a C 12.

Where you end up hitting between the panel points we will add the web members discussed above. I would not try to develop any type of moment at the base of the column. If necessary develop the moment in the steel above the roof.

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