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RE: Use of WOOD: Single Family Housing Design

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Neil Moore brings up some valid issues. The city of Los Angeles makes it
very difficult for a developer to conform to conventional framing standards.
Even still, their inspectors are some of the most well trained in the field.
Plan Checkers, as well, are engineers.
As you get farther away from the major metropolitan area's you find that
available well trained professionals are harder to come by. Many of the site
inspectors are equally lacking in load path training and rely upon the
contractors ability and track record from the past. I was supprised to find
that the inspectors in my own town were rather ignorant of structual issues
when I first moved in. As the 94 UBC approached, the inspectors were
required to complete ICBO inspection training. Today, I find them to be very
capable - some of the best around. I still would not suggest that they be
considered the expert in conventional framing.
Load path training, although simple for most of us who have developed a
"feeling" for how materials behave - are more advanced than most inspectors
should be able to work with. Here is an example of what I mean. I do many
cold-formed steel homes in my area - all for low income housing. The
contractor shortened a shear panel from 4' to 3'-6". The inspector, wanting
to help, suggested adding screws and decreasing the panel screw spacing.
What he failed to understand is that the mode of failure in steel stud is
buckeling, not panel connection failures. His suggestion may be valid for a
wood stud wall, but is incorrect for metal stud construction. In this case,
the wall provided greater capacity than the demand. Therefore, it was a
simple matter of allowing for a 6" shorter panel as long as the boundary
screws were in tact on continuous members.

Contractors are, in my professional opinion, the weak link. I returned from
a site visit to a large custom home remodel that created a great roof 35
feet wide with radius truss roof - reaching close to 20 feet high at the
peak (a 41' radius).
The contractor made a number of field changes without my knowledge because
he "understands some load path idea's" unfortunately not enough to validate
his changes.
He could not receive a 5.5" x 11.875" Parallam in time and substituted a
6x12 DF #1. the beam was actually a header that took a 6 kip reaction from
the 5.5 x 18" parallam that supported ten radiused beams. To make matters
more complicated, he cantilevered the 6x12 out about 4' to pick up a
concentrated load from one of the radius beams.
without getting involved, the concentrated load may have saved his butt,
however, once I calculate his beam, I expect it to be in jepordy in shear -
not bending. However the concentrated load reversed or diminished the
deflection mid span.
He also left out the shear transfer detail from the high roof to the
continuous parallam below.
He also placed a smaller parallam below the existing main beam and above the
shear wall near one end. The problem is that there is a discontinuity
between the shear wall and the upper 18" Parallam created by the
installation of the header which is about 4" below the Parallam. Rather than
framing the header to a double king-stud, he extended the header past the
shear panel.
Next he decided to use the column supporting the 18" parallam in lieu of an
HD5 at the side of the shear panel. This will probably work, but what gave
him the authority to make these changes without my knowledge or approval.
Other changes included: changing diaphragm heights and shear transfers.
Removal of one of three flagpole lateral restraints (the owner wanted an
unobstructed view of the golf course)and the installation of a large
skylight in of of the diaphragms.

This is typical of any contractor in the field who redesigns the project on
his own authority rather than wait for an engineered solution. If it happens
to engineered plans, think of how often it occurs when there is not adequate
detailing on a conventional plan.

Construction quality and the knowledge by even the best framers, are not
knowledgable enough to replace the engineer or architect qualified to do
this work. when I called the contractor on the 6x12 replacment for a
Parallam, he flatly stated that I probably beefed up the beam rather than
take the time to determine just how much load really occurs.
I got pissed and informed him that the difference in a conventionally framed
set and one engineered is the time it takes the engineer to calculate all of
the real loads in order to provide the best design at the best price. I told
him that I balance the most ecconomical section with the amount of
deflection I find and choose based upon where the beam is and how visible
the deflection will be.
I took great strides not to loose my temper over a contractor who, armed
with a little structural knowledge, considers himself an authority on how a
load path works

Dennis Wish PE