When can a deck diaphragm be omitted?

[spieng89]

I have a project where every structural bay will be a lateral frame going in both orthogonal directions due to member size constraints. It's an array of columns and beams in a typical frame pattern, 1 story. I am trying to justify not having a diaphragm element e.g. metal deck as well as no rod bracing as seen typically in PEMB structures? My justification is there are no collection elements, therefore a diaphragm is not needed to transfer load to the lateral frame. Am I envisioning the load path correctly?

 

[KootK]

I would say that you are envisioning the load path correctly and that the situation that you've described is precisely the scenario where a diaphragm is not technically required. So long as your design doesn't rely on being able to distribute lateral load laterally to multiple framing lines, you should be good to go.

 

[driftLimiter]

Im not sure how you can ever justify this. There are distributed wind pressures, and inertial seismic loads out in the plane the of the roof that need to migrate back to the frames. How does your proposed load path address this?

 

[KootK]

Im not sure how you can ever justify this. There are distributed wind pressures, and inertial seismic loads out in the plane the of the roof that need to migrate back to the frames. How does your proposed load path address this?

1) for wind, that would be weak axis "girt" action of the perimeter beams receiving the wind.

2) for seismic, that would be a little weak axis bending in all of the beams perpendicular to load. In most of the industrial situations where I see this coming up, you'd likely not bother with this because:

a) the load would be so small relative to weak axis beam capacity as to be negligible OR;

b) in reality, you have some kind of diaphragm, such as bar grate, that obviously can serve as a diaphragm for it's own lateral support.

3) For general gravity stability, same as #2.

 

[driftLimiter]

I would be willing to entertain weak axis purlin action, no one ever said that . The purlins have a strong tendency to rollover so would be looking for some blocking or something like that on the frame lines. Normally I see only some dog-ear tabs sticking up. I suppose you have some capacity for weak axis bending of the tab as well.

Not my favorite load path but its something.

 

[KootK]

The purlins have a strong tendency to rollover so would be looking for some blocking or something like that on the frame lines.

Yeah. I'm reaching here a bit in terms of my assumption that this is a bar grate kind of thing but, in that situation, often one does not consider the grate to provide LTB support to the purlins (removable clamp attachments etc). So you often start off with fairly stocky sections to begin with.

For certain, though, when there is no diaphragm one needs to carefully consider the bracing of all of the things that would normally be braced by the diaphragm.

 

[driftLimiter]

Glad you're back

 

[KootK]

Thanks dL. It's good to be back. I've got a hunch that I'll be a "come-and-go" presence around here until I'm retired now however. I picked up a couple of new hobbies over Covid that have proved infinitely distracting: bicycle mechanics & canine scent training.

 

[spieng89]

Hmm thanks for the insight so far. Yes, this system would have a purlin and standing seam roof system. I could see how the addition of blocking between purlin connections at the beams would help justify load transfer.

 

[driftLimiter]

If its pemb they also make a little triangular brace component to stabilize the prulin at the connection to the main frame, this could be your main reaction on the purlin for weak axis loading.

 

[KootK]

For PEMB with standing seam, you usually want:

1) No diaphragm perpendicular to the frames. Your wall and roof purlins distribute the wind directly to the frames.

2) Discrete, horizontal bracing as your "diaphragm" in one or more bays between frames, usually both the the end bays. End bay roof trussing is the most robust way to deal with incoming end wall wind load.

 

[human909]

I think the deck roofing you use for roofs you use in the US is e fair bit stronger than what we use down here in AUS. (0.42mm thick sheet)

I never rely on the deck for a diaphragm (well except for LTB reasons for purlins, and I suppose for drag friction). Generally I would have one or two braced bays in each wall and struts or ties to transfer the forces.

 

[hokie66]

Another reason for not using roofing as a diaphragm is watertightness. In Australia, for most profiles, we screw the roofing through the crowns. Or if standing seam, it slips.