Diaphragm Forces Due to Seismic Load |
  
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Diaphragm Forces Due to Seismic Load |
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The more elaborate form of the question is as follows: When calculating diaphragm design forces in accordance with equation (12.10-1) of ASCE 7-05, the “w” terms refer to weight tributary to diaphragms. It is the design philosophy of some users that these terms should NOT be penalized by the weights of heavy walls in the structure, because if those walls align all the way to the foundation, they never tax the diaphragm with any transfer forces. However, ENERCALC SEL is hard-coded to take the values of the “w” terms from the Base Shear & Vertical Distribution tab, which refers to the total effective weight of the structure, and therefore they include the weights of the walls. Isn’t this unfairly increasing the required diaphragm design forces determined by the Diaphragm Forces calculation according to equation (12.10-1)?
To address this concern, let’s start by rearranging ASCE 7-05 equation (12.10-1) slightly so it takes the following form:
Fpx = 
* [wpx /
]
The first thing to recognize is that 
is a value that is pretty clear. It is the sum of the design forces at all the levels from the level of interest and above. It is probably calculated in accordance with ASCE 7-05 section 12.8 for Equivalent Lateral Force Procedure, and that section refers back to section 12.7.2 for the definition of effective seismic weight. Section 12.7.2 is clear that the effective seismic weight includes the “total dead loads” plus the potential for some other contributors. But the point to be made here is that the first term in the rearranged form of equation (12.10-1) must include the weights of diaphragms and all walls. It is important to establish this fact first, and then move on to the discussion about the second term.
Now let’s move on to that second term above, as shown in the brackets. And for the purposes of this discussion, we will make one simplifying assumption, which is that neither the weight of the floors nor the weight of the walls varies significantly from one floor to another. This is probably a pretty reasonable assumption for the typical structure to which ENERCALC is applied.
The term in brackets could be rewritten as shown below:
[(wdiaphragm x + wwalls x) / 
]
Next we will assume that the weight of the walls equals some percentage of the weight of the typical diaphragm. Let’s say that the weight of the walls equals “x” times the weight of a typical diaphragm, and see what that means to the value of the term in brackets.
[(1+x)(wdiaphragm x) / 
]
Finally, we could make one more modification to bring the factor of (1+x) outside of the summation operator in the denominator, and the expression would look like this:
[(1+x)(wdiaphragm x) / (1+x)
]
Now it is pretty easy to see that with the basic assumptions made above, the value of the ratio in the brackets turns out to be numerically the same whether the ratio is calculated with or without the weight of the walls included. And since the first part of this discussion was to establish that the value of 
is already pretty clearly defined, we have now proven that the value of Fpx is unaffected by the inclusion or omission of the weight of the walls in a typical ENERCALC SEL project.
Having said all of the above, the remaining items to consider are the calculated minimum and maximum limits on the value of the force, Fpx. Here it is clear that including the weights of walls would skew the results of the calculation. So as of build 6.13.1.22, caution is warranted when applying these limits if the values of wi include significant contributions of weight from heavy walls. As a future enhancement, we hope to provide additional control over the weight values that are read from the Base Shear & Vertical Distribution tab and ported into the Diaphragm Forces tab. Once this happens, it will be easier to let the system calculate and apply the minimum and maximum limits on Fpx. But for now, the user should be keenly aware of how these limits are calculated, and why they may need adjustment for specific building types.
