Diaphragm transfer forces

[mferg318]

How is everyone handling transfer forces from discontinuous shear walls above a diaphragm, similar to the situation shown below? I'm specifically looking for how to incorporate the shear wall loads into the shear diagram for the diaphragm. Are you adding them as point loads, or distributing across the width of the diaphragm? There seems to be mixed opinions in the literature.

 

[RipPECity]

A similar question i have had on this topic -

When you are designing your transfer diaphragm, how do you handle the overstrengthed upper level shear force, in combination with the inertial forces of the diaphgram at that level for a diaphragm check? 12.10.1.1 states to combine the overstrength transfer force with the inertial force, but if you are combining a ELF shear force from the primary lateral system with a calculated diaphragm force from equation 12.10-1 or 12.10-2, is that not double counting some of the load? This has always confused me.

 

[Deker]

@RipPECity Read this thread and report back if you still have questions: https://www.eng-tips.com/threads/collector-design-force.512008/

 

[driftLimiter]

A similar question i have had on this topic -

When you are designing your transfer diaphragm, how do you handle the overstrengthed upper level shear force, in combination with the inertial forces of the diaphgram at that level for a diaphragm check? 12.10.1.1 states to combine the overstrength transfer force with the inertial force, but if you are combining a ELF shear force from the primary lateral system with a calculated diaphragm force from equation 12.10-1 or 12.10-2, is that not double counting some of the load? This has always confused me.

The Fpx is based on the inertial forces from that diaphragm level. The transfer forces don't show up in that as we calculate both Fx and Fpx by each diaphragm level and do not include any contribution for forces from above.

Basically the procedure is set up assuming that the inertial force is generated at the diaphragm level, and then flows into the MSFRS. Once the load is in the MSFRS it doesn't come back out into the diaphragm. This is why we have to specifically include the effect of transfer forces in addition to Fx or Fpx.

 

[RipPECity]

@RipPECity Read this thread and report back if you still have questions: https://www.eng-tips.com/threads/collector-design-force.512008/

Thank you, that helps a lot. I think the part that had confused me was with a building w/ relatively equal floor tributary weights where the ELF distribution per floor decreased down the height of the building, being combined with the diaphragm design force, which would generate relatively equal diaphragm design forces per floor. In my hypothetical scenereo I have been thinking about/sketched out, checking a lowest floor diaphgram taking a transfer force from the entire building above would be higher than the total base shear of the building (combining ELF for all floors other than lowest & dia. design for lowest). What you said makes total sense though...I copied it below.

A reference stating that it's required, or a reference showing how to perform the analysis? It's true that the ELF force distribution doesn't necessarily occur at the same time as the peak diaphragm response at a given level, but there's no way to account for that in an ELF based design. When a transfer diaphragm experiences its max inertial response it also clearly experiences some level of transfer force from the VLFRS above. See the code snip below that requires the two load cases to be considered concurrently. If you felt that was too conservative, you could perform a nonlinear time history analysis (which is obviously way beyond what you would do for a standard building).