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RE: Questions about Residential design and 1997 UBC

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-----Original Message-----
From: Dennis S. Wish PE [mailto:wish(--nospam--at)cwia.com]
Sent: Thursday, October 01, 1998 4:01 PM
To: SEA International List
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? 
 
Based on the way I interpret the Code, you can use Rw=8 if the entire lateral force resisting system consists of plywood shear walls, Rw=6 if not (i.e., gyp. board and/or stucco). However, if you have a frame (OMF) at the garage opening, you have to use Rw=6. If you use flagpole columns (i.e., "inverted pendulums"), you have to use Rw=3 for all the lateral resisting elements in that direction of analysis. IMO, this section needs to be re-worked since the plywood shear wall at the opposite end of the garage will be overly designed. I believe COLA has a more practical interpretation of this approach, but it does not appear in 1997 UBC (nor 2000 IBC).
 
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? 
 
First, I believe you make quite an assumption that our fees can automatically go up because suddenly the Code is more complex. There is evidence that, on several occasions, code writers are totally insensitive to the business impact of their efforts. From my point of view, it has been a monumental task convincing my clients that my outrageous fees are justified for the prior code changes. At this point in time, I feel that I have discuss this issue with my clients and give him/her a choice: Simplified Static w/rediculously high design forces and "normal" feel or Static with higher fee.
 
Second, I believe that probably most SFRs perform as if they had rigid diaphragms anyway based on the aspect ratios of the diaphragms and the relative stiffnesses of the shear resisting elements. Analysis by flexible diaphram methods have just been more convenient. Looking at the code specification for flexible vs. rigid, I doubt if many SFRs would have diaphragm deflections twice that of the shear wall deflections particularly when considering the contribution due to the hold downs.
 
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? 
 
Personally, I believe the Conventional Framing Provisions are inconsistent with paradigms of other prescriptive codes. For example, when I used to do energy calcs (a long time ago), there were three methods: Prescriptive, Point System and Energy Budget. The Prescriptive method specified exactly how much glazing you could have, how much insulation to use, etc. It was/is the most restrictive of the three methods. The Point System allowed the designer to do some moderate trade offs (i.e., a little more glazing compensated with more insulation, etc.). the Energy Budget allowed the most flexibility but required a computer analysis to model annual energy budgets and to compare them with a standard budget.
 
In contrast, the Conventional Framing Provisions is less restrictive and less costly (from an engineering fee point of view) than a full blown engineering analysis. Of course, the cause of this paradox is due to the fact that houses were built long before engineers got their hands on them and have had (relatively) few failures (Frank Lew, are you out there?).
 
To me, it seems that there is just too much inertia (provided by the builders associations) behind the Conventional Framing Provisions and it seems unlikely that an engineering group by itself (w/o help from someone like the insurance industry for example) can affect this inertia.
 
 
My $0.02
Bill Allen