Frame and Cable Structure - Diaphragm Shear 1

[cslater]

I'm working on a redesign/retrofit of an existing metal shop building. The structure is made up of 4 rigid frames, 40 feet wide and spaced 25 feet apart.

The roof covering is supported by longitudinal purlins at 48" o.c., covered by metal roof sheeting. The existing design uses 3/8" diameter cable cross bracing in the roof, but only in the sections that directly line up with shear bracing in the walls.

I have the as-built calculations, but its not clear how the shear is being transferred to the shear resisting wall panels for the portion of the roof that doesn't have cross-bracing.

It doesn't seem like sheet metal roofing would have the shear capacity needed of the diaphragm, but I can't see anything else providing it. Can anyone shed any light on the design process behind this type of structure?

Sorry for the long-winded, confusing description, and thanks in advance for any help on this.

 

[csd72]

cslater,

This sounds like a fairly typical framing scheme for where I come from. are you sure they are cables and not rods?

The roof sheet acts in tension and the purlins can take some compression to transfer your wind loads to the roof 'trusses' (i.e. where the bracing is). The roof trusses are then designed to transfer all of the shear load to the supporting shear walls.

The purlins will also act as your compression struts in the trusses if there are no other members there. They can usually take about 1 or 2 kips in compression.

csd

 

[cslater]

csd,

According to the as-built calcs, the shear frames on the long side of the building are rods, but the bracing at the roof, and for the panels on the short side of the building are cables.

What you said makes sense (and will make more sense when I sit down and sketch it out to get it all straight in my head).

Thanks for the response!

cslater

 

[haynewp]

Check the attachments of the sheeting to the purlins. We never considered standing seam roofs as diaphragms. I like a diaphragm to work in shear instead of tension. If the roofing attachements can't take the load, then the roof shear may be (undesirably) going through the weak axis of the rafters to get to the braced bays.