Multiple types of diaphragms on the same floor

Question

Multiple types of diaphragms on the same floor

JIMEY (Structural)

(OP)

27 Mar 12 03:00

Historically, I have always treated wood diaphragms as flexible diaphragms because that's what I was taught to do. However, I was reading in ASCE-7 that they should only be treated as flexible if they deflect more than twice as much as the supporting shearwalls.

That is all well and good. My question is what do you do if you have a diaphragm that spans over say 5 shearwall lines. Following the code, some of the longer spans may be flexible and some of the shorter spans may be rigid. If this is the case, how do I handle the load distribution?

Thanks for any help.

msquared48 (Structural) · Answer 1 · 2012-03-28 14:16:15

That's a very good question.

Personally, in that case, I would look at both the rigid and flexible numbers and take the worst cases for the diaphragm and shearwall shears. More work, but your bases are covered.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

frv (Structural) · Answer 2 · 2012-03-28 16:30:26

I guess this is not what you are asking, but it makes the question a moot point.

The IBC allows the idealization of wood structural panels as flexible diaphragms per 1613.6.1.

RFreund (Structural) · Answer 3 · 2012-03-28 18:01:15

I would have suggested similar to above however I guess for a more accurate and involved answer you could treat them as semi-rigid where you account of the diaphragm stiffness (deflection) and shear wall stiffness (deflection). Like a multispan beam on springs kinda deal, maybe?

EIT
www.HowToEngineer.com

JIMEY (Structural) · Answer 4 · 2012-03-28 19:54:49

All good points.

Personally I don't agree with the IBC idealization of all wood diaphragms being flexible because if you do the math (meaning if you check diaphragm and shearwall deflection relative to each other) most of the time I get quite low diaphragm drifts compared to the shearwalls.

And more importantly, I live in Canada, so the IBC has no bearing up here. In fact, we have no code that prescribes when to use flexible/rigid diaphragms.

I think I will try looking into each of the methods that you guys suggested. Thanks.

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Multiple types of diaphragms on the same floor