Open Web Joists and Shear Transfer

[Galambos]

i attended a seminar on open web joists recently that covered shear transfer from the diaphragm to the lateral frames. SJI had suggested placing HSS2.5x2.5's between joists and welding them to the supporting member and welding the deck to the hss. while this makes complete sense to me, a steel fabricator spoke up and said that he had seen the detail once in the last 7 years.

this got me thinking and I wanted to conduct an informal survey to see how others are transferring diaphragm shear when using open web joists with steel lateral frames. have you ever used this detail? if i were to use it, my clients would scream that i was overdesigning. your thoughts would be appreciated, but please...i understand the mechanics of the detail.

 

[msquared48]

I have used single 2.5" angles welded to the deck and member below to transfer the shear (One leg flat at underside of deck to weld to). Can also use angles welded back to back for odd depths, or small channels. Seems like HSS tubes would be more expensive.

Mike McCann
MMC Engineering

 

[frv]

We use the HSS detail.

If the shear is low enough, you can use the rollover capacity of the joist seat (I believe allowable is conservatively taken as 1000# for 2.5" joists).

 

[youngstructural]

I think I am picturing your question correctly, however I'd really like to be certain (and perhaps learn a thing or two as I have more than once before from Galambo...). Any chance someone could sketch this up, or post a copy of their detail?

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[DaveAtkins]

I also use the rollover capacity of the joists whenever possible (I think it is higher than 1000# per joist, but I don't have the Fisher textbook in front of me). Bear in mind that when this is done, you must use the diaphragm shear value for no sidelaps, because there is nothing to attach the deck to (unless you lay a continuous angle across the top of the joists--then you have something to attach the deck edge to, and it is OK to use shear values with sidelaps).

DaveAtkins

 

[JAE]

We lay a continuous angle along the end of the joists, perpendicular to the joists, at the deck edges.

Then we use the 2 1/2" tubes between the angle and the supporting wall or beam below the joist seats to transfer lateral forces from the continuuous collector angle to the wall or beam below.

So the continuous angle is the collector and the tubes are used intermittently only to the extent needed....not at every joist space.

 

[jike]

As others have said, use joist rollover capacity for small buildings. We also use a collector angle so that we don't have to use the zero sidelap fasteners as DaveAtkins described. As JAE, we use tubes only where forces are large enough to justify their use.

 

[strguy11]

Since the rollover capacity depends alot on the actual joist chord design, Is it possible to specify the required rollover force and have the joist designed for that?

Here is a link for rollover design

http://www.pdhonline.org/courses/s116/s116.htm

 

[Gumpmaster]

I typically use either an HSS tube or a small channel between joists to transfer load to my lateral system. 2.5" deep for K joists and 5" deep for LH joists. It's probably more common here on the west coast where I commonly have more that 1kip/foot diaphragm shear.

 

[Lutfi]

This is one of my typical details. I get all kinds of arguments from fabricators because (may be) they miss it during takeoff and bidding. My response is either to provide it or ask the joist designer to design the joist seat to transfer the load parallel to the wall. I also add it over steel beams to eliminate the torsion to joist seats.

Regards,
Lutfi

 

[JAE]

We've not depended on the roll over capacity of the joist seats in the past, simply because I had concerns over the durability of that deflected condition and whether or not the joist manufacturers really designed for those loads.

 

[GalileoG]

Tagging this thread for later reading.

 

[frv]

There is actually some info on this (rollover capacity) topic in the Vulcraft Design publication.

It's worth obtaining.