From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Fri, 21 May 1999 08:25:26 -0700
At 08:04 AM 5/21/99 -0400, you wrote:
>Just a question. If the diaphragm is correctly engineered to begin with
>(assuming loads not to exceed the allowables), why would one need to
>worry about nail slippage past the elastic capacity. I can understand
>if analysis is being performed on an existing structure.
>Also, I agree with the negligible nail slip and chord slippage in the
>majority of all relatively small diaphragms. But to every
>generalization there are exceptions. Some of the roofs that I have
>designed in the industrial and warehousing areas were so large that nail
>slippage did become a sizeable concern when looking at story drift.
C.G.: Several sides of the question come to mind:
One reason is to discover, for developing one's personal "feel" for when to
worry and when not to, what building sizes and configurations matter. You
are already doing that.
Another reason is for an old or unusual building, as you noted, where doubt
exists or where not all the customary conditions are present.
For seismic loads, rough background knowledge of what happens "past the
elastic capacity" is relevant as a general rule, because seismic loads are
generally understated so much that the code-expected earthquake will load
everything way beyond that "elastic capacity". The building is expected to
be beaten up like at a summer Saturday night's bar fight during a full moon.
You're the "trainer" preparing your man for the brawl, but the traditional
code approach is that you do it by somewhat overpreparing him for tennis at
the country club.
Yet another reason is self defense. Many say the code now requires
full-blown diaphragm deflection calculations in all cases. Whether those
persons are correct or not, their beliefs are a major menace to you if they
are willing to allege or testify that you violated code. The cost of
clearing yourself of such charges in a legal venue is staggering, and you
are stuck with it.
Part of the question I disagree with a little. "Nail slip" is a colloquial
term for all the sheathing fastener deformation effects. It is not literally
a slippage that happens or doesn't, nor does it have a clear-cut elastic
limit or capacity up in the useful code allowable shear range, or way out
There is always a human desire to have every design consideration reduced to
code-approved precision and certainty, and a reluctance to deal at all with
unruly matters that remain untamed. Watch out for that reluctance and its
Charles O. Greenlaw SE Sacramento CA