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RE: Rigid diaph. analysis with diagonal walls

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I know that the code rejects a skewed shear wall from flexible diaphragm
analysis. However, what is the rationale behind this. The direction of force
is rarely orthogonal to the structure - especially if the home is located on
the end of a cul-de-sac. In the past I have taken the skewed wall and
calculated the orthogonal shear in both directions and designed the wall for
the worst case condition. It is true that the effect of the wall is more
accurate if the diaphragm is considered rigid, but if this were to occur in
a residential or light-framed structure that utilized interior walls for
shear and bearing, then the decision to use rigid analysis would prevent
future modification unless all records were maintained for the next engineer
to know exactly where the and how the load path is defined. A similar
problem exists if the wall is curved as well as skewed.
In your example, assume for the moment that the skewed wall is the only wall
in the home parallel to the direction of the epicenter of the seismic load
(or the direction of a static wind load). How would you treat the all other
walls in the home that are, essentially, skewed to the direction of lateral
force.
It seems that we are attempting to over-compensate for what I've discussed
on numerous occasions as the problems in construction rather than the
problems in design and detailing.
In this real world, how many of you have noticed a properly designed and
constructed custom home suffer collapse or major structural damage from the
use of a skewed shear wall? I've never seen one after Northridge, Loma
Prieta, Whittier, Sierra Madre, Landers, Imperial and every other earthquake
I studied since becoming licensed 22-years ago.
I do, however, agree that if you attempted to break the skewed wall into
vector components, you will be weak in at least one direction - but
historically, I've over compensated by taking the worst condition (wind vs.
seismic and east-west vs. north-south) and used this as my shear capacity
while still considering the structure to be loaded orthogonally.
The other issue that arises from this is when the home or structure is
designed similar to an "L" shaped geometry, but where one leg is not 90
degrees to the other, but the entire block is skewed. In this case, the idea
of designing to rigid analysis when the actual direction to the epicenter of
the worst case active fault is no more accurate than spitting in the wind
and making the assumption that the direction it hits you in the face is the
direction the wind will blow for all present and future events - not an
accurate choice considering that winds, like seismic events, can occur from
virtually any direction.
If you do choose a standard of professional practice that includes skewed
walls in flexible analysis, then I would agree with Dave on the issue or
overcompensating the wall, but not for rigid analysis where it can have a
negative effect on other walls by drawing more shear because of this walls
rigidity, but in flexible design where you can negotiate orthogonal
components that are parallel to the direction of applied load used for all
other walls. We are not dealing with an exact science here - only
speculation and the proven theories of physics in nature.

Dennis

-----Original Message-----
From: David Merrick [mailto:MRKGP(--nospam--at)winfirst.com] 
Sent: Thursday, March 13, 2008 9:15 AM
To: SEAINT
Subject: Re: Rigid diaph. analysis with diagonal walls

Attention!

IF    a skewed wall is analyzed by resolving each diagonal wall into 
equivalent X and Y length walls,
THEN    the resulting error can be as much as 30%.
This method should only be used by adding 30% of the Y forces to the X 
forces and, visa versa.
in other words
By using 100% shear plus 30% of shear affects from the perpendicular 
direction.

Know the method of your program.

Skewed wall stiffness in the direction of the base shear varies with the 
stiffnesses of walls perpendicular to that base shear direction.

Example
All skewed wall stiffnesses reduce in the X direction when  any one 
non-skewed wall, in the Y direction is reduced.

I wonder how the seminar by-Gosh for skewed walls worked out? Was it not 
given in the East Bay area near San Francisco?

David Merrick, Structural Engineer
WEIGHT, Conserves Energy, reduces noise and PROTECTS.
MRKGP(--nospam--at)winfirst.com



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