Flexible Roof Diaphragm Over Multiple Supports

[msp1]

All,

How are most of you designing a roof diaphragm that spans across multiple supports (interior shear walls)? Should I look at each span as simply supported or as a continuous beam? If I analyze as a continuous beam is my chord force (M/B) calculated using the positive and/or negative moment?

Thanks

 

[JAE]

Simple spans.

 

[whyun]

For considerably large flexible diaphragms (metal deck without concrete or wood panel), I treat them as simple spans.

For wood framed structures, typically single family homes, shear walls are relatively close together (i.e. short diaphragm spans). In this case, an envelope solution from both the flexible and rigid diaphragm distribution is used. A good panel discussion on this topic can be found from this link:

http://www.seaint.org/wood.htm

 

[msp1]

Thank you all for a consensus answer, but if a diaphragm is a deep beam with tension & compression chords why would you not assume that there would be some continuity across the support? I guess for simplicity and conservatism we use simply supported. A two span condition (equal span) would yield wl^2/8 for negative moment but the positive moment is reduced and would yield a lesser chord force at the max positive moment location vs. the max negative location. If you have reentrant corners or drag struts this complicates your design because the max moment doesn't always fall at the end of your collector.

I’m currently looking at a one story building, irregular shaped with offset shear walls, reentrant corners and multiple drag strips...a little more complicated than your average box. I certainly want to simplify my analysis but don’t know if oversimplification is going to work here.

 

[JStephen]

To me, the term "diaphram" implies something thin, not a "deep beam".

 

[Lutfi]

For flexible diaphragms use simple beams. For Rigid, distribute based on the rigidities of he supports.



Regards,
Lutfi

 

[DaveAtkins]

The IBC Code does recognize that a wood diaphragm can act as a rigid diaphragm--it allows three sided diaphragms. I am more liberal on this than others, but if you need to assume the diaphragm is rigid to make it work, I say go for it. The only caution I would issue is that a very long, shallow diaphragm would probably not be rigid.

DaveAtkins