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3_D analysis non-parallel walls

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I missed the begining of conversations for the subject on 3-D anaysis, But
thought 
some conversation on non-parallel walls could help.

The principle axis is the key to combining perpendicular base shears to
create a needed 
resultant. Does anyone know of a 3-d program that will automatically adjust
for a rotated 
principle axis?

Diagonal walls will rotate the principle axis of a building. Recall that a
force is in line 
with the principle axis when the resulting deflection vector is equal to
that direction of the 
applied force. 

Don't make "local" wall vectors for the non-parallel wall and ignore the 
                     "global" principle axis affects on the structure's
analysis. 

A code "minimum base shear" is assumed to be applied in-line with the
global principle
axis. 

The rigid diaphragm analysis that is modified for the attachment to Dennis
Wish's wood 
shear wall program has a principle axis check. This can find a principle
axis by trial and 
error, or the first set of deflection results can be used to calculate the
principle axis angle.

For a symmetrical and triangular plan shape, all angles that the horizontal
force can be 
applied results in at least one of the walls receiving maximum possible
resulting forces. 

Here's how to apply a force to a principle axis that is not parallel to a
computer model
reference axis, if one knows the principle axis.
   Apply two loads, sized to create a resulting vector that is
       parallel to the principle axis.  The resulting vector needs
       not to be greater than code required. 
   A second set is required for a perpendicular resulting vector.

In the early 70's, I increased a 2-D base shear analysis by a factor of
1.41 to 
compensate for not knowing the exact principle axis alignment. It was a
small job and the 
extra reinforcement was insignificant. This factor could be lowered, if one
knew of 
an approximate range of the principle axis angle. It may not have covered
all
possilbe combinations.

As mentioned by others, using the both, directional code base shears,
together in a
3-D analysis and not reduced, is too much but works. Using 2 perpendicular
base 
shears, equal and 70.7% of code is the 45-degree case and is too low. 

UBC: 1631.3 Mathematical Model.  .  "A three-dimensional model shall be
used for the 
dynamic analysis of structures with highly irregular plan configurations
such as those 
having a plan irregularity defined in Table 16-M and having a rigid or
semirigid 
diaphragm."  .

For the rigid wood diaphragm users: Are we going to do a 3-d dynamic 
analysis for a system with a diagonal wall?

UBC 1633.1 General. This section requires uplift considerations for
irregularities defined 
by table 16-m type 5. That is the diagonal wall plan irregularity.

These are ramblings from memory and hunches. If you need to know, check the

statements for yourself.

David B. Merrick, SE
MERRICKGROUP(--nospam--at)compuserve.com