# Re: Diaphragm Rigidity

• To: njineer(--nospam--at)att.net, SEAint(--nospam--at)seaint.org
• Subject: Re: Diaphragm Rigidity
• From: Mlcse(--nospam--at)aol.com
• Date: Thu, 28 Jun 2001 23:14:42 EDT
Hi Nels,

I think Dennis is on the correct path.  Aspect ratio of the diaphragm is
going to influence the rigidity, the closer to a square shape, the more rigid
the diaphragm in both directions. I think you can show that the unblocked
diaphragm is rigid by calculating the diaphrgam deflection as rigid and
multiplying it by 2.5 as Dennis suggests.  The ceiling and soffit framing
will probably help stiffen the roof system.  The partition walls that are
attached to the roof framing members will also help stiffen the roof
diaphragm (sort of like taking the lid off of a shoe box and putting dividers
inside the shoe box and trying to twist it, the dividers make the shoe box
more stiff).

So how much money is the framing going to give back since he bid the plans
that said the diaphragm is blocked?

Mike Cochran

In a message dated 6/28/2001 8:22:08 AM Pacific Daylight Time,
njineer(--nospam--at)att.net writes:

I a 3 story residential addition project that is under construction to have
rigid diaphragms because the available shear walls would not work if they
were bracing the diaphragms based on tributary load -- I needed a rigid
diaphragm to distribute loads, past the relatively weak walls to the
available strong shear walls.

I plywood panels of the diaphragms are specified to be blocked all edges.
My intention in requiring blocking was to assure that the diaphragms really
are rigid, not flexible.  At the architect's request, the plywood was
specified on the drawings to be tongue-in-groove.

Yesterday, I was called to observe the diaphragm nailing.  The contractor
has installed no blocking.  He said that he has never constructed a
residence with a blocked diaphragm, and thought that the tongue-in-groove
panel edges took the place of blocking.  He didn't ask me about that.
Roofing installation, scheduled for today, has been called off because I
want the blocking installed -- everyone involved is upset.

There is no place in the diaphragm at which the shear exceeds the allowable
for an unblocked diaphragm.  The cost to install the blocking as the
framing was being erected would have been nominal; the cost to install it
now, with plywood, ceiling and soffit framing, and ductwork in place is
going to be a big deal.  Is there any justification for considering an
unblocked diaphragm rigid?  Is it possible to calculate the rigidity or
displacement of an unblocked diaphragm?

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net