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RE: Questions about Residential design and 1997 UBC

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Bruce,
As one other member stated, this is a great reality check. I can't argue
with any of your comments as they reflect my own personal observations as
well.
The damages paid out by insurance carriers after Northridge, are not
reflective of shear transfer problems - except, possibly, shearwall
deflection issues. In the past few years we came to the following review of
noted damage:
1. Narrow shearwalls, designed to a prescriptive aspect ratio approaching
3.5:1, deflect more than code allows.
My opinion: require deflection calculations for every shearwall and provide
alternative shear devices for walls that exceed minimum drift criteria.
Currently, there are three companies that I know of who have addressed this
issue - Simpson, Hardy, and STS.

2. Shrinkage of wood creates greater initial deflection in mechanical
tension connections on Shearwalls.
My opinion: Either redesign the connection or use a newer type of product
such as the Pre-Deflected Holddowns that don't require a bolted connection.
Using manufactured wood at the sill and King-studs should diminish nail slip
and initital deflection due to shrinkage since manufactured wood products
are dimensionally stable.

3. Oversizing holes in sill plates (and not tightening nuts prior to closing
walls) tend to lead to plate splitting or cross-grain tension problems.
In my opinion: follow L.A. City guidelines and install thicker plates for
shear in excess of 300 plf, and provide larger plate washers to distribute
bolt tension to a greater area of the plate.

4. Assure that shear-resisting panels are not damaged or penetrated in the
field by other utilities.

5. Creating mandatory training requirements for anyone wishing to carry the
title of "Framer" and require at least a minimum understanding of
Load-Path's. Inasmuch as our profession requires knowledgable persons to
design the resisting system, we should expect the same of those building the
system. As Bruce pointed out - rarely does the final design reflect the
initial design. Field decisions are made every day that could have serious
consequences on the lateral stability of the structure - we need the Framers
who build the structural systems to take some responsiblity for their work
and to assure that other trades do not degrade the work that the framer
does.

There is much more, but my opinion of the failures I saw could be associated
with lack of proper field observation or inspection which leads to
misunderstandings and errors by unknowledgable Framers and sub-contractors.
The resulting problems were numerious, but most stem from the same problem -
a lack of understanding of the structural system. When a shearwall 8 feet
long and 8 feet high (sheathed both sides with plywood and close nailing) is
connected to a massive foundation with HD20A's at each end - but stops at
the ceiling joist, something is wrong. Either the contractor did not
understand how to complete the load path, the engineer who designed this
massive interior connection did not observe or detail the connection, or the
city inspector failed to catch the problem. In no way was this a design
problem - the failure came from lack of follow-through.

I looked at a design I'm currently doing for an 11,000 square foot home. The
shape is irregular, diaphragm depths vary in every line of shear etc. Parts
are "L" shaped, others "U" shaped, radiused roof diaphragms, cantilever
roofs which frame above glass to glass corners, etc. If I break the
structure apart in blocks - I generally need to drag one block with
insufficient shear resisting elements into another block using drag struts,
straps and ties. I have no idea how to model a rigid diaphragm that
accumulates shear from an adjacent diaphragm (something I will need to
learn). Portions of the diaphragms contain large cut-outs for skylights,
atriums and more, other parts contain soffits that step up to a raised roof
in the center of another diaphragm.  I have diaphragms that stack at the
edges - California framing. I might be creating a nightmare of a design
problem if I have to design the diaphragms as rigid.

I appreciate the work that others have done to try and overcome these
failure and damage issues by addressing the structural model. However, I
would wonder if the style of home considered (by those who believed that a
change in methodology was needed) in the model was as complicated as the
homes I have had to design in the last twelve years?  I'm not talking about
traditional home styles. High end homeowners want "High-Tech" styles that
are anything but conventional. Even with the traditional programs that I
have created to design flexible diaphragms - I find that I need to be
creative in their use in order to establish proper distribution. I often
accompany my analysis with a letter of design assumption and intention so
that the plan reviewer will understand why and how I distributed diaphragm
shear where it might not otherwise be clear.

Bruce was very clear in his breakdown. I added my comments in hopes that
others might evalute the types of residential models they design and add to
this discussion about what they perceive the problems to be.

Finally, I believe that better education within our industry is key to
damage reduction as I've stated in every post from Conventional Framing to
Contractors Training programs.

Dennis Wish PE