Diaphragm and Continuous Strut/Tie Design 1

[TTUengr51]

I am currently working on a large warehouse structure utilizing tilt-up construction for the perimeter walls. I have an open web steel joist roof structure covered with metal decking. Due to the size of the structure and a step in the roof plane, I have 3 individual diaphragms to design.

I am in Seismic Design Category C (2006 IBC) and seismic is definitely controlling due to the weight of the tilt-up panels (45' to top of parapet).

The question I have concerns the out of plane forces generated by the tilt-up walls. In transferring the forces into the diaphragm, I am creating smaller sub-diaphragms to collect the load to my continuous struts/ties. My continuous strut/tie will be either an open web joist or joist girder at my column lines.

My questions are as follows:

1) Do the strut/tie forces need to be carried across the entire length of the building, making sure each connection to the column and next strut/tie in line can carry this force? Or do I account for them until I have an adequate diaphragm length to dissipate the force, similar to how a collector element between shear walls or braced frames?

2) How should I contend with the strut/tie force in the design of the open web joist/girder? Is this as simple as noting an axial load on the plan for the open web joist/girder designer to content with? This seems right, but I have never seen such forces noted on other structural drawings before. The sub-diaphragms will require a joist/girder to be a chord element. I would imagine this force should be shown on the plan as well.

Any help and/or thoughts would be greatly appreciated.

 

[DaveAtkins]

1) The fasteners between deck and joist take the axial force out of the joist, and put it into the deck diaphragm. The number of fasteners required to do this will determine how far the force in the joist must travel.
2) Joist suppliers are accustomed to seeing special load requests. What you are proposing is not that unusual.

DaveAtkins