Steel Roof Deck as a sub-diaphragm 1

[seattlemike]

Hello Engineers,

So, while steel roof deck is not allowed to count as continuous ties for sub-diapragms across the length of the building, it is my understanding that metal deck is typically considered adequate for tension and compression bracing/anchorage of the sub-diaphragm whereas on a wood roof, plywood is not. Is that correct? Are additional struts required for compression anchorage?

Mike

 

[MillR]

The same problem exists of having to distribute the point load into the diaphragm. In the design of steel decks laterally supporting concrete or masonry walls, I always put steel tubes into the flutes of the metal deck, at about 8' on center, for one bay (back to the steel beams). The issue of ties across the length of the building ends once you get that anchorage force into the diaphragm.

 

[seattlemike]

Hi MillR,

Thanks for your input. That is a good point on the point load. Could the ledger angle distribute the point load into the metal diaphragm? I suppose it would need to be very stiff to distribute the load evenly to the puddle welds.

If I only go one bay with the nested tubes, then I won't meet the 2.5:1 depth ratio for sub-diaphragms? That is okay?

Mike

 

[MillR]

Look to IBC Section 1620.3.1. (Unless you're on CBC 1633A.2.9). Subdiaphragms do have to meet the 2.5:1 and can't use the metal deck perpendicular to the span. However, once you distribute the point load and meet the subdiaphragm ratio, I think you can stop.

 

[seattlemike]

MillR,

I agree. I think the tubes nested parallel to the roof deck only have to go back far enough to develop the point load.

The overlap welds and deck can take the tension anchorage from there, and the deck can handle the compression anchorage.

Thank you!

Mike

 

[MotorCity]

What is the difference in function between a diaphragm and sub-diaphragm?

 

[seattlemike]

MotorCity,

For seismic loads, a subdiaphragm anchors a heavy wall (CMU or concrete) to a flexible roof (metal deck or plywood), because the metal deck or plywood alone could not adequately prevent the wall from separating from the roof.

A standard roof diaphragm transfers the seismic (or wind) load from the face of the building (wind) to the exterior walls (or interior walls) perpendicular to the direction of force.

Mike