Need a book? Engineering books recommendations...

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

Re: Effects of the New Code on Wood structures - good or bad?????

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dennis:
I am in complete agreement with Mark on this subject.  You seem to have lost
your perspective on this problem.  Wood frame structures are extremely
complicated under dynamic loading.  Material properties such as strength and
stiffness are dependent on the rate of loading. The structures have a great deal
of internal damping and the loading history is nearly completely unknown.  Any
attempt to accurately model the behavior of such a system will either be
extremely complex, highly inaccurate, or both.  It will always be easy to
criticize any proposed analytical model. We shouldn't take the numbers we
calculate quite so seriously, they probably are not accurate to one significant
figure.
The fact of the matter is that many the wood framed structures did not perform
acceptably during the Northridge earthquake.  As a consequence, some very
knowledgeable and experienced engineers have proposed changes in the Code in
order to improve the performance of wood frame construction.  Design loads have
been increased and we are now being asked to evaluate our structures a little
more carefully.  That is going to include an evaluation of rigid and flexible
diaphragm performance, like it or not.
Clearly we need more information on the performance of shearwalls.  We need more
test data from walls of differing aspect ratios.  We need more test data
regarding tie down deflections.  We need more information regarding typical
construction defects.  IMHO APA should have provided us with this test data a
long time ago .  Simpson has also been negligent in not providing us with usable
information regarding tie down performance.  Be that as it may, until such time
as the CUREE project has generated more test data, we are being asked to do the
best we can.
The hard part of this analysis is making an inaccurate determination of
shearwall deflection.  The fourth edition of Breyer?s book "Design of Wood
Structures" contains a very clear example of current methodology.  The
unfortunate part of the calculation is in the lack of good information regarding
tie down deflection.  The example given sites a "typical" tie down deflection of
.125".  In narrow shearwalls the tie down deflection and nail slip are what
govern, and Simpson has been unwilling to provide information regarding tie down
deflections with wood posts.  This is a good reason to avoid narrow shearwalls
and bolted tie downs.
The problem of perforated shearwalls has been addressed in tests by Dan Dolan at
VT.  APA Report 1#57 provides a usable methodology for the calculation of
deflection.  I personally do not believe that the  detailing recommended are
adequate, and would suggest using Ed Diekmann's analysis as shown in Keith
Faherty?s "Wood Engineering and Construction Handbook" chapter 8.
We have been given an opportunity to upgrade our craft.  Then new Building Code
provides a good rationale for doing a better job and charging a little more
money.  It does not seem responsible, or prudent, to attempt to continue to
ignore the requirements of the Code.
Chuck Utzman, P.E.