Need a book? Engineering books recommendations...

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

RE: Redundancy factor 1.0 for single story wood framed?

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Ben,
I think that there should be some attention paid to the fact that wood
framed residential structures have inherent redundancies that we have
debated on numerous occasions. Gary Searer's paper does an excellent job of
identifying the ambiguity in this portion of the code by providing 10 or 12
examples from wood frame structures to multi-story concrete rigid diaphragm
structures that the calculation of Rho yields inappropriate results.
My point is that I don't believe a residential wood structure should have a
redundancy factor greater than 1.0 - especially since there is no evidence
of failures occurring because of a lack of redundancy.
Few residential wood framed structures are design as lofts - with a lack of
interior partitions. The current code ignores the interior partitions at the
expense of the home owners.

I have read the San Jose simplified design approach, and as with the L.A.
City
ST-12, it does simplify the design methodology, but again, at the expense of
the public by increasing the strength of the uplift resistance on shearwalls
and an assumption of Rho at 1.5.

I am starting to see a trend in my area where homes, historically designed
by engineers for middle income families, are beginning to adopt prescriptive
methodologies by eliminating the few irregularities in order to allow the
developer to profit much more. Unlike three or four years ago, the same home
is selling for what the market will bear - often 2 times the selling price
from last year with no change in construction cost, property cost and less
than a 10% increase in material costs. This is 'glut' as a low income home
in my area is now afforded by only middle income families.

I continue to protest the adoption of a code that feels the public will just
have to dig deeper into their pockets to afford a home which is designed to
standards that have yet to prove their effective gains for the increased
investment in cost. Rather than improving the quality of construction, we
are improving the quality only for those who can afford it and creating
incentive for decreasing quality for those who have no control over what is
being constructed (i.e.., spec homes).

An engineer made a comment to me recently that the dissatisfaction within
the structural community had reached its peak about four months or so ago
and the community is becoming complacent and accepting of the new methods.
This is a real shame as it isn't support for the method but rather
frustration as the code writers are simply unwilling to do anything ease the
burden of their over-design on the public who can least afford the changes.

These are my opinions on the issues and I certainly wish the engineering
community would not forget were the responsibility lies.

Regards,
Dennis S. Wish, PE
The Structuralist Administrator for:
http://www.structuralist.net
AEC-Residential Listservice
admin(--nospam--at)structuralist.net
(208) 361-5447 E-Fax
ICQ # 95561393

From: "Youseffi, Ben" <Ben.Yousefi(--nospam--at)ci.sj.ca.us>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Redundancy factor 1.0 for single story wood framed?

Lynn

There was a code change proposal to the IBC by SEAOC, that was approved last
April. It added the following at the end of the definition of r(max) for
shear walls:

"...In light-frame construction the value of the ratio of 10/L(w) need not
be greater than 1.0."

What this change does, is to eliminate any penalty for having a shear wall
that is shorter than 10 feet when calculating the value of  Rho. This is a
great help in wood light frame construction where many walls are normally
less than 10 feet.

However, since we are not going to adopt the IBC, something needs to get
done in the interim about the Rho factor in light frame buildings. One
possibility is to work with the state agencies (such as HCD) to see if they
would be willing to add this to their list of amendments to the state code,
when the 97 UBC goes through the re-adoption process next year. Individual
persons or groups can also submit code change proposals to the Building
Standards Commission, but I don't think there is any precedent on getting
anything amended to the CBC through that channel. The dead line for
submitting proposals to the BSC is January 3, 2001.

Ben Yousefi, SE
San Jose, CA