I don't have an argument with anything you stated. The only point I wanted
to make in my original post was that there are many issues surrounding wood
construction that we simply do not consider and that, we should at least
consider to some degree when designing new codes. We are consistently
making wood design methods either excessively restrictive OR we are trying
to circumvent the greater restriction with short-cuts. A prime example of
this is that we determined that shearwall panels with high aspect ratio's
deflect much more than we knew - the reason was we never calculated
deflection or drift but designed walls to be overloaded at the greatest
aspect ratio. When damage occurred, the professional community acted quickly
to change the ratio from 3.5:1 TO 2:1. At the same time, manufacturers
started developing shearwall systems which would reduce deflection and allow
for the high aspect ratio's. Many of us jumped on the proprietary bandwagon,
but failed to consider the foundation design to compensate for the uplift
and the greater tension in the panels due to the NEED TO COMPENSATE FOR
HIGHER COSTS BY CONSOLIDATING shearwalls into fewer walls with higher loads
per wall that have been empirically tested to deflect within allowable
Codes keep compensating for creative proprietary designs and this becomes a
vicious circle with the public at the losing end.
Returning to my original examples, I was trying to show that there is a
"redundant" (if you will excuse the analogy) system within wood construction
that provides additional reserves. Most of these unexplored "systems"
account for the reasons why wood design can have such a high factor of
safety. The codes attempt only to look at the basic materials and their
capacities and ignore the "system" which I tried to discuss in the e-mail
post. The rim joist that some see no value to can still act as a "web" of a
built-up member to create greater strength in bending. The web need not be
continuous as long as the flanges are [continuous], they need only maintain
the depth separation between flanges. The rim joist is of value, but
neglected for the most part.
Finally, we still have Conventional Construction at the other end of the
issue. The difference between engineered and prescriptive methods is growing
much farther apart - the next code suggests we design wood structures using
Flexible, Rigid AND NOW LFRD methods so as to find all of the boundaries of
potential failure. If we consistently make the code more restrictive and at
the same time rely upon proprietary products to compensate for the loss of
creativity (less openings) we succeed only in raising the cost of building
and owning a home. What benefit do we gain by demanding greater restriction
in wall stiffness and shearwall design when we will soon adopt a new method
called "Perforated Shearwalls" that also work to reduce the hardware demand?
We need to start paying more attention to construction quality issues as
well as design drawings and the need for less generic and more specific
detailing for lateral load transfers within a structure. This requires
common sense rather than more restrictive building codes. IMO, we do not
need three methods of design in order to better the quality of homes, we
simply need more enforcement of compliance to the design drawings and
minimum code requirements.
Sorry, I did not intend to preach in the process.
Dennis S. Wish, PE
The Structuralist Administrator for:
(208) 361-5447 E-Fax
ICQ # 95561393
Jason Kilgore responded:
This is probably the most common complaint I hear from contractors, usually
phrased as something like: "I live in a house in the country that's 75 years
old, and an inspector never looked at it. Why do you need to look at this
one?" My standard answer is "For every house built by a competent
contractors that stands for 75 years with no problems, there are fifty
houses that are built by incompetent contractors that are torn down or have
cracked sheathing and sticky windows and doors." And yes, I consider those
to be structural failures.
The main problem is that you can't find a contractor that "appropriately
follows" historic conventional construction. Any ya-hoo who can hold a
hammer thinks he can build a house.
Jason W. Kilgore
Leigh & O'Kane, L.L.C.