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Re: Include parallel wall weight for shear?

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In a message dated 9/9/99 3:12:37 PM Pacific Daylight Time, 
dth(--nospam--at)red.primextech.com writes:

<< When summing the moments of loads which a wall has to be designed for, I 
 notice it's common to ignore the effect of the wall itself when the 
 direction of the EQ force is parallel to that wall. (method used in M. 
 Baradar's exam example book). Half the wt x ht of the perpendicular walls 
 are included which makes sense but I feel half the wt x ht of the wall 
 itself also contributes a moment which that wall has to react against. Any 
 comments? Thank you. DH
  >>
In plywood sheathed walls, the weight of the wall may be relevant when 
designing to the maximum tolerance of the sheathing. Enercalc allows the user 
to consider a percentage of the weight of the wall into the analysis. It used 
to default to 30% of the wall weight (assuming the code requirement for 
analysis of portions of a structure) but allowed the user to change it (which 
I often did to the same base shear that I designed the rest of the structure 
to). 
Some engineers consider the weight of the interior partitions (and thus the 
shear resisting walls) into the lateral load of the story under consideration 
(actually 50% of the current story height plus 50% of the upper story and any 
accumulated loads from mulitple levels. This would compensate for the wall 
weight alone and may reduce the need to add the single resisting wall's 
lateral contribution into the analysis.
At 15 psf typical for an interior gypsum or plywood wall, the lateral 
contribution considering the distribution of tributary shear would seem 
negligable unless you are designing (as I mentioned) to the maximum capacity 
of the sheathing. Even still, there is a factor of safety involved that would 
more than compensate for the single wall weight.

Now, if masonry or concrete walls are considered, I would NOT disregard the 
contribution of the lateral weight of the wall section. The sum of walls in 
the line of shear will most likely contribute significantly to the lateral 
load applied UNLESS the lateral weight of the wall was already calculated 
into the diaphragm weight.
It is not uncommon to see calculations where the weight of the walls is 
arbitrarily calculated into the diaphragm load - this represents a 
significant figure that does not differentiate the direction of the wall and 
may assume walls normal to loading. This becomes a judgment call as 
considering the weight of the wall alone may be duplicating the process.
If only the walls normal to the load were considered in the lateral 
distribution, then it is reasonable to consider that there will be a 
contribution from the masonry or concrete wall alone.

Finally, it may be best to play safe and add the walls if the design is very 
close to allowable tolerances. The few extra bucks involved can save a 
fortune down the road.

Hope this helps,
Dennis Wish PE