Return to index: [Subject] [Thread] [Date] [Author]

Re: Unblocked Plywood Diaphragm Deflection Calculation

[Subject Prev][Subject Next][Thread Prev][Thread Next]
At 06:55 PM 5/19/99 EDT, Frank E. McClure wrote, in connection with BLOCKED
versions, for which code deflection formulas have long existed:

>Is it not correct to assume that the "nail slip" will be greater at large
forces per nail?

>I would welcome any responses to this email message.

Frank (and others), I looked into these deflection matters in 1984-85 while
a member of SEAOC-Central Section's Seismology Committee, which wrote the
wood section of the 1988 Blue Book/ 88UBC. I used the 1976 UBC Stds for nail
slip values, and they appear to agree with what you and Mr Seegert quoted
from the 94 UBC version of same.  Nail slip values were given for a broad
range of load-per-nail, and are markedly non-linear across that range. I was
amused to note that the nail slip value is essentially constant for every
nail size and type, as used with sheathing grades and thicknesses listed as
suitable for that nail, at the UBC-listed allowable shear value in lbs per ft.

Said differently, the nail slip value is the same at about 0.030 for any
nailing, spacing, and sheathing combination that is fully stressed to the
allowable shear listed. 

The nail slip value is about one third as much (0.010) when the stress is
half the UBC allowable, but grows fast as shear stress rises above the
allowable. The implication in this is that real benefits in deflection can
be had with intentionally understressed (ie, overdesigned) diaphragm
construction.

Another implication is that allowable shear values may have been based on a
standard nail slip figure rather than on margin against failure.

I can't relate this to FEMA 273, which I don't have and am not interested in
yet, in observance of the Satchel Paige principle.

The Gd figures reported given for various types of straight board and
unplocked panel diaphragms in FEMA 273 have only one significant figure, and
only even ones of 2,4,and 8 hundred thousand. These look like rank guesses,
not like authoritative results of research. I hope they are not supposed to
be used for other than very rough estimates.  

It appears that the old UBC basic horizontal diaphragm deflection formula is
for a truly simple span diaphragm where the ends are not restrained in
rotation. Where several spans are continuous, or a single span diaphragm is
fixed ended (such as for a tilt-up building where ends can't rotate and
chords can't slip along side walls near the ends) then the chord stretch
components of deflection will be down to around one-fifth as much as for a
true simple span.

And the free end of a cantilevered, fully fixed-ended diaphragm (in classic,
open-end "rotation") would have less deflection than midspan of an equally
loaded and stressed, pure simple span diaphragm of twice the length. All
that came up in Seismology Committee in 1985, but went over like when
schoolteacher Scopes taught evolution in Tennessee in the 1920's.
Blasphemous to the faithful; subversive to the rest. 

Engineering science still has a hard time competing against engineering
dogma, especially in fundamentalist code work. Where's FEMA in this? Church,
state, or is it all the same?

Charles O. Greenlaw SE   Sacramento CA