Clarification on wx (from 12.8-12) vs. wpx (from 12.10-1)

[MrEngineerUS]

Okay, is there any difference at all in the values that these variables represent? And, if so, what is it? I've searched and searched for some sort of example showing the calculation of these but every design example just assumes they are already calculated so I can't verify if there is any difference or not. And, if there isn't, ASCE should really change wpx to just wx to eliminate the confusion.

 

[JAE]

They are different values.

First - assuming you are referring to ASCE 7-10. It helps if you specify the code and version. This is an international site.

Equation 12.8-12 is dealing with how the overall base shear is allocated to each level of your structure.
As a result, it can be thought of as the total mass at that story. This would probably include these items:
1. Floor framing and deck mass
2. All perimeter walls around that story for the half story above and the half story below that level.
3. All interior partitions attached to that level (again - possibly half-stories above and below that level).
4. Mechanical and electrical elements attached to that level.

Equation 12.10-1 is dealing with the amount of lateral seismic demand used to design the actual floor diaphragm.
This would look very similar to what I've listed above except where any of the mass is not attached to and creating shears and moments in the diaphragm.
The best example of this would be the side wall masses. Say you have a diaphragm spanning from an east exterior wall to a west exterior wall resisting north-south seismic.
The walls on the north and south faces would be attached to the diaphragm and induce seismic loads on that diaphragm along its length.
But the east and west exterior walls are on the ends of the diaphragm and the seismic load path for that mass does not go through the diaphragm but directly down the walls.

So these two values are similar but not the same.


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[OldPaperMaker]

JAE-very good explanation!

For a detailed example see Example 3.10 on page 3.34 of Design of Wood Structures (sixth edition) by Donald E. Breyer, Kenneth J. Fridley, Kelly E. Cobeen, David G. Pollock

 

[MrEngineerUS]

Thank you for the clarifications. That example in the Design of Wood Structures summarizes JAE's excellent explanation as well. Also, it does intuitively make sense which always helps me buy into an explanation.