Diaphragm Load Transfer

[tclat]

Hi,

I have a single storey steel structure 220'x 80 feet. The roof structure has cold formed z-purlins at 32"c/c, 3/4" plywood and standing seam roof. I plan to use the plywood as the diaphragm (because its there) but will still require cross bracing in the plane of the roof. I am a bit concerned about the load transfer between the plywood and the diaphragm chord. Typically for timber construction, I would add blocking between the roof joists to satisfy the diaphragm fastener spacing requirements. For most metal buildings I've seen, there is no blocking between the purlins at the support. Can anyone provide advise as to how load transfer is typically handled?

Thanks

 

[JAE]

Set a Z purlin "piece" between the other purlin bearing ends to create a similar blocking type condition?

 

[msquared48]

The purlins are transverse to the mainframes. Seems to me that the edge purlin at the mainframe becomes the chord member by default and has to transfer the chord to the "X" bracing in the long walls.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[tclat]

Mike,

I am concerned about the other direction. I don't have any moment frames at the moment. Only a combination of x-bracing and masonry walls along the perimeter. But having said that, I don't recall ever seeing blocking between the purlins at the mainframes for the type of building you are describing. How does the load really move from the steel deck into the mainframe?

Thanks

 

[JAE]

tclat,

For typical metal buildings, there is no diaphragm sheathing but rather a series of roof X-bracing rods that extend across the bottom of the Z purlins. Thus the purlins, and the metal roofing above them, do not participate in the lateral "diaphragm" load path. There is usually an eave strut between the moment frames that serves as a kind of diaphragm chord. The rods are placed just under the top flanges of the main bent beams.

In your case, it appears that you have a Z-purlin system and you are attempting to create a plywood diaphragm on top of them. You mention also having in-the-roof-plane X bracing but not sure how the two can properly be combined (I have to think about that)

As I posted above, you could very simply install edge-of-roof cross purlins between the spanning roof Z purlins to create an entire perimeter of Z shapes. These would serve two purposes - 1) chords for the diaphragm bending and 2) a mechanism to allow the plywood to screw down to the purlins and transfer lateral shear in to shearwalls or whatever lateral brace system you have that starts and the bottom of the purlins and extends below.

The perimeter purlins, whether continuous spanning members or interrupted blocking purlins, would have to be strapped together to ensure a continuous chord across the full diaphragm width.

 

[tclat]

JAE,

Thanks for the comments.

You are correct. The architect wants plywood so I figured it would be a shame not to use it as part of the lateral load resisting system. It isn't strong enough particularly for the 220 foot span so I'm adding the x-bracing. I would need to calculate the relative stiffnesses to determine how the load is shared between the two.

I undertand the concept you describe of adding z-purlin blocking pieces. Because of the cross section of a typical z-purlin, I'm concerned these pieces will not act as proper blocking as if it was all wood construction. I therefore feel a continuous member will be more effective. Working up the detail.

Thanks again.

 

[JAE]

You could use a C8 channel perhaps to have a more competent member to drag down the forces.

 

[RFreund]

If you're using the z-purlin as blocking than would seem to be ok for them to be discontinuous as your wood blocking would be. The purpose of the wood blocking is to transfer the force to the top plate which is acting as the tension/compression chord. If your z-purlin (or channel) is acting as the chord then you may want it to be continuous.

EIT

http://www.HowToEngineer.com