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Re: Housing Performance Objectives

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I seem to be missing the point here. There is no difference in minimum
standards of automobiles or virtually any other product on the market. A
minimum standard should be a point of reference that all products are
compared against. Those that don't meet minimum standards are off the
market.
Structural engineering needs to be the same. The performance issue came
about because the community as a whole (architects, engineers, insurance
industry and others) agreed to lower a minimum standard for one slim section
of the market. Rather than doing the job it was intended to do, it became a
tool to allow lowered standards in other areas for only one reason - to
maximize profit. Does anyone really believe that the person benifiting from
the lesser "minimum" will pass the savings along to the people buying the
product - not on your life.
My point is to agree on a starting position - datum. From recent discussions
it appears that this point should be considered using a new philosophy which
includes performance standards exceeding life safety which considers
reduction of damage. This can be determined from studying the types of
damage occuring in the Northridge earthquake AND Hurricane Andrew. However,
I don't arbitrarily support standards that are based upon "compensating" for
poor construction methods. I'm not convinced that reducing shear panel width
to height ratio's (2:1 verses 3.5:1)- for example - are benifiting the
public when story drift design data as we know it from the 1991 UBC Standard
appears to have been supported in the Northridge Quake. Had the panels been
properly connected, nailed and sheathed they may have performed better than
those which were not. I believe the emergency measures were done as
compensation rather than rationalized methods.
For the same token, the current Conventional Framing section of the 1994
code specifies a minimum 2'-8" shear panel 10'-0" tall on the front of a
garage. Where does the standard originate? In my area and many others with a
70mph wind, the resulting shear distributed from the back wall and the two
piers at the front of the garage would yield approximately 1700 pounds of
shear - each line. Distributed over roughly 5 feet total wall we get
approximately 320 plf of shear (give or take). However, if the roof framing
bears parallel to the garage opening, the header won't transfer much
resistance to uplift on the panels. With a 10 foot plate height, the uplift
could be around 3,400 lbs. Section 2326.11.4.1 requires resistance to uplift
of 1,800 pounds - only half of the actual calculated uplift on the panel. So
what is the minimum that needs to be created here.
If the strap fails, who looses? And why shouldn't the strap fail when the
code does not properly PRESCRIBE the correct holddown tension force.
The key word is prescribe or prescriptive. Not a bad design idea but one
that needs to be matched with other design minimums.
To make this work, the code writing authority needs to set minimum standards
and design each component to work within those standards. Included in the
minimum standard must be a maximum allowable design value which, once
exceeded, moves the structure into safer hands for a more complete design.
Finally, we need only one minimum - a standard that everything we do is
compared against.

Dennis Wish PE