Steel roof deck with bar joists chord force transfer

[ronster]

I have a one story 50' X 86' building with 22 ga type roof deck on steel bar joists at 5'-6" o/c and spanning 50 feet. I designed the metal deck roof deck diaphragm and it can tranfer the lateral loads to the shear walls on each end. My chord force for the diaphragm is 4.2 kips. I am using the steel roof beam at the perimeter of the building to act as the chord member. How do I check the tranfer of the chord forces from the roof deck to the steel bar joists into the steel beam? Can the force be transferred directly from the deck to the joists to the beam (every 5'-6"o/c) or do I need a continuous angle secured to the deck and beam? The roof joists sit on top of the roof beam leaving a 2 1/2" gap between the top of the beam and the bottom of the roof deck.

 

[JAE]

On the side of the building where the joists are perpendicular to the exterior wall, you would want to place a continuous steel angle (3X3 perhaps) across the top of the joists to serve as a continuous edge support and collector for the deck.

What we do is then add a 2 1/2" steel tube between the joists, perhaps every 3rd or 4th joist, to connect the angle to the supporting beam below, thus by-passing the joist seats.

I've seen where some engineers require the joist manufacturer to design the seats to take a certain orthogonal lateral load through their seats but I've not done this too much.

 

[jike]

You need a continuous edge member so that your fastener spacing matches what your diaphragm requires (not just one every 5'-6"). Then some how you need that force to jump down to the beam level. Since you have a small building, here are two suggestions:

Provide a 2.5" x 2.5" tube or inverted angle welded to the beam between joists that the deck can get welded to.

Provide angle on top of the joists to weld deck to and then specify required joist rollover resistance to transfer force from top of joist to beam.

 

[frv]

FYI, joists are designed with a "rollover" capacity. You may not need to add the collector as JAE suggested, although I'd say it's a good idea.

The numbers I have are 1K allowable rollover capacity for a 2 1/2" seat and a 0.5K rollover capacity for a 5" seat.

So you would not technically need the collectors between joists if your service diaphragm shear is <182 PLF (at 5.5' spacing). I guarantee you this is not sufficient capacity in most cases. Still need to add collectors.

 

[nitnelion]

One thing to be careful of with a 2.5" x 2.5" tube and the deck running parallel to that tube is that there is no guarantee that the bottom of a deck flute will line up over the tube and allow an attachment. Wouldn't a wider tube than 2.5" be better? For a type B deck, the flutes are 6" o.c.

 

[DaveAtkins]

Based on my own quick calculation of your situation, I don't think you need the collectors. Using your chord force of 4.2 kips, the maximum shear to be transferred should be 195 plf. With a 5'-6" joist spacing, your rollover force will be 1.07 kips. As I recall, Canam says their joists are good for 1.8 kips rollover.

DaveAtkins

 

[ronster]

Thanks for your replies. How do you calculate the shear transfer force to the chord remains my question. Is it (2 x chord force/length of the chord) or would VQ/It apply? The building was recently erected and when I went out to look at it, the transfer of the shear force to the chord accross the bar joist it got me thinking. There is no angle installed at the time.

 

[jike]

Reason it out! The chord force is zero at the end and a maximum at the center, so you have a half a chord length to transfer that force. Unit shear = F/(L/2).

Make sure your beam to column connections can transfer the chord force across the connection.