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Re: Questions about Residential design and 1997 UBC

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In a message dated 10/1/98 6:07:35 PM EST, wish(--nospam--at)cwia.com writes:

<< 
 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?>>

I would use the Rw of 8, since near field factors are going to drive up the
base shear for the building.  Using an Rw of 6 would be conservative, and I
would expect the building to perform better, because the walls have been
designed for a higher force level (I wouldn't be surprised that when you
consider near field, that the base shear would be the same as if you had used
an Rw = 6).
 
 <<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?>>

The diaphragm will still probably be rigid even when the aspect ratio is
greater than 2-1/2 to 1 when using the definition in the UBC, particularly in
the longitudinal direction.  I currently do not believe that the diaphragm
will be able to transfer loads by rotation in the true sense as is assumed in
a rigid concrete diaphragm analysis,  particularly if the diaphragm is
relatively long.

Example:   150 foot x 50 foot building with shear wall lines in the transverse
direction spaced 30 feet on center (total of 6 shear walls lines).   Assuming
that there are limited number of  shear wall elements along each shear wall
line due to door openings and other architectural requirements, the diaphragm
being 50 feet deep between shear walls at 30 foot on center will most likely
calculate out to be rigid between shear wall lines.  I do not currently
believe that the diaphragm will actually act as a rigid element and transfer
forces by torsion to the extreme end walls 150 feet apart.   Instead I believe
the diaphragm behavior would be more along the lines of a continuous
foundation where the shear wall lines are the springs restraining the
diaphragm from moving.  Since the shear wall lines most likely have different
stiffnesses, I would then expect the 6 shear wall lines to have different
translational movement.  Since the deflections are different, as one wall
deflects more than the other wall, the diaphragm will drag load back to the
stiffer adjacent wall, causing an increase in deflection of this adjacent
wall.  The question is then, that when designing by tributary area, some walls
will be underdesigned for shear (the stiffer shear wall lines) and other will
have excess capacity (the less stiff shear wall lines which deflect more).

The example above relates more to commercial buildings, but custom homes, etc
can have closely spaced shear walls, the diaphragm will most likely behave
more rigidly than has been previously assumed.

The impact on residential framing could be minor, depending upon what
standards are developed to account for a redistribution of load (true rigid
diaphragm rotational analysis or other simplified method).  Additional time
will be required for calculating the overall deflection of individual shear
walls along each shear wall line, but computer programs can be written for
this.  I believe a simplified method can be developed that would only require
one redistribution of seismic loads to adjacent shear wall lines based upon
deflection without having to perform a rotational analysis that more help to
minimize additional design time.  Yes we are going to have to ask for higher
fees since we are spending more time, which will be difficult to do unless
building officals enforce the requirement for checking shear wall deflections
on all designers.  Hopefully the owner can be made to understand that this
additional effort this will help improve the performance of ths structure
during an earhquake, and is worth the additional fees.

In order to protect the architects requirements, we will probably require the
use of more braced frames (not moment frames) in custom residential homes in
order to get deflection compatibility between frames and shear walls.  Aspect
ratios for shear walls will have to be decreased furthur ( 1.5:1, 1:1, etc)
depending upon wheather the building is two stories or a single story, or
proprietary shear walls such as Simpson Strong-Wall, STS or Hardy Frame are
going to need to be used which have been tested for deflection. 

Before doing shear wall deflection analysis, we are going to need
manufacturers of holdowns to provide deflection of their holdowns under loads
(I believe only Simpson publishes this information),  wood crushing loads and
corresponding deformation on sill plates, and design shrinkage factors for
wood.  Without this information being standardized (or methods) it will be
hard to justify this design criteria.

<<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 provi
 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?>>>

For custom homes I don't think developers and architects will move towards
conventional framing.  For Tract Homes, this is a good question, I would
assume this is going to be a problem in areas were the building department is
not very strong, and the developer trying to save money, will lean towards the
conventional framing procedures.
 
 <<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?>>

I have not reviewed the document, but I would be interested to see how it
performs using the SEAOSC testing proceedures.  Currently, I would tie the
wall down each side of the door opening.

My thoughts,

Michael Cochran