Partial Diaphpam

[ztengguy]

Got a building where it has a bowstring truss roof. Off to one end, they added on a small addition to make a L shaped building. The addition had a flat roof, wood joists and decking.

Now its being rehabed into a strip mall, and the flat roof will be torn off, and trusses added. Leaving the flat roof is not an option. The new roof will be a regualar truss roof with standing seam roofing.

Is it possible to deck the underside of the truss for a diaphram? I dont see why not, its no different than what is there now.

The big question is, does the whole thing need to decked, or can I do a perimeter 'beam' type pattern, thus the plywood still transfering the load to the sides?

Thanks

 

[msquared48]

Sorry, but I'm a little confused with the description. Can you post a diagram or two?

Mike McCann
MMC Engineering

 

[ztengguy]

something like this.

 

[hokie66]

A flat diaphragm at the ceiling level is always structurally preferable to a sloped diaphragm.

 

[msquared48]

I guess you could as long as the affected truss bottom chords could take the chord force generated by the plywood diaphragm and were designed as such.

However, being in a high seismic zone myself, I would definitely say no if the ceiling diaphragm was sheetrock.

Mike McCann
MMC Engineering

 

[TehMightyEngineer]

Short answer, yes you can. I designed my parents garage and to brace the non-balloon framed gable ends I used a small ceiling diaphragm (which then doubled as attic storage space). It has worked quite well.

That said, you could do a perimeter diaphragm as you've drawn but what are you really saving? As a diaphragm is essentially a deep beam taking strong and weak axis bending loads you're essentially breaking that up into 4 smaller, less efficient beams. You'll need tension and compression chords for each partial diaphragm, edge nailing (or screws) around all the edges, possibly blocking due to the small size, and you'll only save just the diaphragm material and field nailing in the middle. Much better just to make it a normal ceiling diaphragm and call it good.

As Mike pointed out if this has high seismic you're not going to be able to use sheetrock for a ceiling diaphragm which means you'll need wood decking. As you need wood decking on the exterior of the roof trusses you're probably better off just designing a typical sloped diaphragm.

Maine EIT, Civil/Structural.

 

[hokie66]

TME,
He said the new roof will be standing seam, so there is no need for wood sheathing on the slope.

 

[TehMightyEngineer]

Oh, you're right. I'm so used to residential where standard sheathed diaphragms are used I almost never see people span between purlins with standing seam metal roof.

Maine EIT, Civil/Structural.

 

[hokie66]

Depends on where you are located in the world. In Australia, we almost never use "standard sheathed diaphragms". Our roofing material is required by code to be "permanent", so concrete tiles, clays tiles, and steel roofing are the norm. Actually though, "standing seam" means different things in different places, so now that I think about it, standing seam could require solid sheathing.