Applying diaphragm force Fpx in conjunction w/ story force Fx? 1

[IrvStrucNewbie]

Hi all,

I've tried using the search function here but did not find any useful results. I am curious about the common practice at your office regarding applying the diaphragm force Fpx.

When doing a diaphragm analysis for a multi-story building, do you apply Fx on all the levels and replace the level of interest Fx with Fpx； or do you only apply the Fpx at that level because it is already amplified for higher mode effects? See the attached diagram below for an illustration.

I have got different opinions from different offices but just want to see what other people use.

 

[JAE]

The values of Fx are used to design the main force resisting system of the structure.
The value of Fpx is ONLY used to design the diaphragm itself.



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[IrvStrucNewbie]

JAE:

Thanks for the quick reply. Yes, I do understand this part. I guess I need to clarify my question a little bit.

Say I am designing the diaphragm itself, but to get the demand in the diaphragm, should I apply the Fx at all other levels AND replace the Fx at which level I am designing with Fpx (in my 3D model), or should I JUST apply the Fpx force at that level. These two methods would result in different demands (I am assuming due to the force transfers between the different levels.)

 

[JAE]

My interpretation of ASCE 7 (using 7-10 here) is that you determine diaphragm forces in your building from the prescribed forces (your Fx values).
This is then compared with the Fpx value from 12.10 of ASCE 7-10. The larger of these two will govern.

Read 12.10.1 and it is pretty clear that you design for stresses resulting from "design forces" (i.e. Fx)
Then it says in 12.10.1.1 that you also shall design diaphragms for forces not less than equation 12.10-1.
No combination of the two.

Later it states the fact that there might be some overall transfer across a diaphragm from offsets in brace locations or from the fact that brace stiffnesses may induce cross-diaphragm forces.
These cross-diaphragm (or transfer) forces should be added to 12.10-1 forces.




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[EDub24]

I too looked hard into this. You need to apply the story forces for the story forces above the floor in question as well as the diaphragm force for that particular floor when designing the diaphragm. In other words your diagram on the left. This is because there is a redistribution of story forces that you need to include with the diaphragm force. A common practice is to have a load case for each individual floor/diaphragm. One basic load case would be the story forces for all floors and then an additional BLC would be the difference between the story force and diaphragm forces for the level in question.

Look at the SEAOC Seismic Design Guide volume 3 for concrete buildings. They have an example for a concrete diaphragm in a multi-story building.

 

[structSU10]

Transfer force will likely need to be amplified by the overstrength factor when applied to the diaphragm. If that is the case, Fx with overstrength would be applied while using the Fpx for the level being designed.

 

[JAE]

EDub24's concept of using the Fx values ABOVE your subject diaphragm and the Fpx force at the diaphragm only would successfully capture any transfer forces through the diaphragm (if any).



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[IrvStrucNewbie]

bump. Wanna see if there's more idea out there

 

[Agent666]

I watched an AISC webinar once that took a slightly different approach, basically it said for everything to be in horizontal equilibrium under the global analysis, the higher Fpx diaphragm force accounting for diaphragm inertia is applied at the level being looked at, but the storey force below is the same as your global storey shear. Therefore the force from the storeys above is reduced to whatever produces the horizontal equilibrium of the system, then base the diaphragm design on this set of forces entering and exiting the diaphragm. I'll try find it as I'd never thought to do it this way before, and interested what others might have to say about it. These higher diaphragm forces are only within the diaphragm and do not transfer to the levels below in an additive fashion, and I thought this was a good way to bound the analysis of the diaphragm.

EDIT I believe it was this webinar.
Link