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# Plywood over 3x T&G decking--Rigid or Flexible diaphragm

## Plywood over 3x T&G decking--Rigid or Flexible diaphragm

(OP)
Would you consider a diaphragm constructed with plywood over 3x T&G decking to be rigid or flexible? Any references would be appreciated! Thanks in advance.

Nancy H. Devine, P.E., S.E.

### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

A lot of that would depend on the aspect ratio in my eyes.

### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

And on the deflection of the diaphragm relative to its supports

### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

(OP)
The building is basically square.

How would you estimate the deflection of the diaphragm in this case?

### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

Assuming a USA project (you might specify which country you are in)....

The IBC has equations for estimating diaphragm stiffnesses and both the IBC and ASCE 7 have definitions of rigid vs. flexible diaphragms that depend on the building size, aspect ratio, etc. - diaphragm deflection vs. shear wall deflections (per MotorCity's comment).

The trick in your case is that you also have a T&G decking under it which certainly adds to the diaphragm stiffness above and beyond the plywood by itself.

There have been posts here on E-T that mentioned new information on T&G decking shear capacities and stiffness - do a search.

You might then have to estimate each and combine into one overall shear stiffness (G') and then see how your diaphragm rates vs. the shear wall deflections.

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### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

Boat hulls might have 3 plies of T&G.
Why would a building have that many plies?

Mike Halloran
Pembroke Pines, FL, USA

### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

Mike - it may be 3" thick T&G, not 3 plies.

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### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

Please forgive me if I am more confused than usual.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

Or a 3 lam T&G product where instead of routing out the tongue and groove it is manufactured with three lams offset to form a tongue and groove.

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### RE: Plywood over 3x T&G decking--Rigid or Flexible diaphragm

I was thinking that if I found 3 plies of T&G in a building, I would suspect that two are rotted.

Mike Halloran
Pembroke Pines, FL, USA

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