attended the panel discussion at the wood fair held yesterday in Long Beach.
I think I walked away with more questions than I received answers. I thought
I would pose these questions to the list and ask that you respond to them by
the subject title of each question listed.
PLEASE DO NOT JUST RESPOND TO THIS MESSAGE UNLESS YOUR
RESPONSE IS VERY GENERIC - MY PURPOSE IS TO GENERATE COMMENTS FOR EACH
QUESTION WHICH WILL BE DIFFICULT TO SEARCH FOR IF THE SUBJECT IS NOT
Rw and residential construction - I have always taken the
safe road determining the Rw value I use. Regardless of whether or not I use
gypsum or stucco (which I don't) in a custom home with plywood shearwalls I
assume the Rw to be 6. The code allows an Rw of 8 for a box system with
plywood shear panels.
Am I being overly conservative and how many would stick with
code rated Rw of 8?
Rigid Diaphram and shearwall stiffness analysis in residential
Construction - The 1997 UBC (and proposed Los Angeles Regional
Codes) requires a diaphragm and shearwall deflection analysis. The results
of the horizontal diaphragm analysis will determine if the diaphragm should
be considered rigid. The panel discussion expected that in most cases where
the aspect ratio is less than 2-1/2 to 1 the diaphragm will work out to be
rigid. The distribution of shear into vertical walls is to be determined by
rotational analysis and relative stiffness rather than the current uniform
The proposed changes will create numerous problems and opportunities
in residential construction. The benifit will be more complexity of the
design methods which leads to higher design fee's. The downside is that
architectural designs will either become more restrictive, or we will need
to be more creative in the process of protecting the architects plan.
What are the perceived pro's and con's by engineers who
specialize in residential construction?
Prescriptive Residential Construction Vs New Design Standards
- It was pointed out that the Northridge earthquake caused over $60
Billion in damages (I am recalling from memory). The provisions of the 1997
code are intended to reduce the damage, provide better involvement with the
EOR by the required Structural Observation provisions.
While builders of engineered products are expected to comply to
stiffer code requirements, the Conventional Framing section of the code has
increased in scope rather than becomming more restrictive. Outside of
metropolitan area's such as the City of Los Angeles, developers of homes
which do not fall under the definition of tract developments are allowed to
design and construct using prescriptive measures only.
Finally, ICBO will not revise the known descrepencies or mistakes in
the Conventional framing (prescriptive) section since the IBC is
forthcomming and will replace the UBC.
feel that there are a few potential problems:
Conventionally framed homes will not calculate (by the number - especially
if a raised floor is considered) to be of equal or greater strength than
engineered products. No provisions are made in the '97 code to strengthen
Conventional Framing OR to revise the code to correct mistakes and missing
information in the publication. A loophole in the code exists that will
allow Conventional Framed homes to be at greater risk AND unethical
developers will take advantage of the ommissions of the code to increase
profits on these homes while marketing them as equivalent complying
As the definition of Conventional Framing increases to allow larger and more
irregularly shaped structures (not load path irregularities) more architects
and builders will attempt to make larger scale custom homes conform to
conventional framing standards. This would be counter-productive to the
intent of the new code provisions if the presecriptive measure does not
Conventional Framing does not require Structural Observation provisons which
has historically identified problems in the construction phase and helped to
improve quality of construction subsequent to Northridge and Hurricane
There are no present provisions which requires continuous education and
training for for framers (although the Wood Truss Council of America has
been backing a framer certification program which is also, as I understand
it, supported by NAHB).
Are we putting the cart before the horse by creating
more regulations to prevent construction quality control problems rather
than attempting to educate the construction industry on interpretation and
understanding of the conventional framing section of the
Will the greater restrictions posed by the '97 UBC cause
developers and Architects (designers) to seek closer compliance to
conventional framing which will produce a product designed to a lessor
standard than we presently have in the '94
Perforated Shearwall Design - APA #157: American
Forest and Paper Association (AF&PA) presented a methodology for the
design of perforated shearwalls. The method includes acceptance of a
length of wall sheathed above and below openings that would be modeled as a
continuous shearwall. Force would be distributed to the "piers" by
applying a reduction factor based upon the size of the opening compared to
the total wall (ie, an opening representing 10% of the wall area would have
a reduction factor which will be applied on the panels on either side of the
The methodolgy stems from work by a Professor (excuse my spelling
please) Sugiyama in Japan who has studied this problem for many years. Tests
have proved favorable.
The methodology is based upon Imperical testing and has not been
converted to a mechanical process supported by numbers. The example
presented a wall with a door in the middle. Each end of the wall (excluding
the sides of the door opening) was secured by mechanical holddowns. The
methodolgy explains that the sill plate discontinuity at the door openings
does not require the use of tension ties to the foundation inasmuch as the
reduction factor applied would yield a tensile load of the plywood panel at
the edge of the door, low enough to allow the nailed panel connection to be
sufficient to resist uplift.
Furthermore, the method ignores the potential for cross grain failure
as the plywood panel boundry nailing is allowed to act on the bolted mud
sill connection - creating the same concerns as those of horizontal
diaphragms at concrete or masonry walls (ledgers).
The consensus of the engineers and building officials at the panel
discussion is not to accept the methodology. Cyclic and Monotonic testing
have been favorable to support the theory.
Would you accept the methodology based upon imperical values
from cyclic and monotonic testing done in accord with the SEAOSC testing
What specific concerns do you have for this method if you
have reviewed the APA reports?
anyone would like to ask specific questions regarding the information and
the seminars at the Wood Fair, please feel free to place them on the list
for others to respond to.
Dennis S. Wish PE