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Re: Questions about Residential design and 1997 UBC

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Dennis:
 
Doug T. and I presented a seminar on this topic back in february.  personally i'm not sure if the rigid analysis is justifyable for single family resdences but i can tell you that as of now it is the position of the wood siesmology sub-committee that this level of analysis will be required under the 1997 UBC.
 
Bill Nelson
-----Original Message-----
From: Dennis S. Wish PE <wish(--nospam--at)cwia.com>
To: SEA International List <seaint(--nospam--at)seaint.org>
Date: Thursday, October 01, 1998 4:07 PM
Subject: Questions about Residential design and 1997 UBC

I 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 CLEAR.
 
1. 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?
 
2. 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 distribution methods.
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?
 
4. 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.
 
I feel that there are a few potential problems:
A) 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 products.
B) 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 correct ommissions.
C) 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 Andrew.
D) 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 code?
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 code?
 
5. 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 opening).
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 proceedures?
What specific concerns do you have for this method if you have reviewed the APA reports?
 
If 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.
 
Sincerely,
Dennis S. Wish PE