ASCE 7-16 10.1.1 Transfer Diaphgram Overstrength 1

[RipPECity]

Hi all,

I know there has been discussion on this topic previously, but I am seeing lots of varying opinions, both online and in my own office, on the use of overstrength loads for transfer diaphragm design.
To me - 12.10.1.1 is very clear in stating "for structures that have a horizontal structural irregularity Type 4, the transfer forces from the vertical seismic force resisting elements above the diaphragm to other vertical seismic force resisting elements below the diaphragm shall be increased by the overstrength factor before being added to the diaphragm inertial forces". This seems to be cut and dry correct? You have to design the diaphragm (as we are in the diaphragm design force section)for the overstrength load from seismic transfer forces.

What are other's thoughts on this?

Both of the below references take a different approach:

https://www.structuremag.org/?p=17288

https://www.skghoshassociates.com/blog/about-that-25-increase-in-diaphragm-connection-forces/

Even the SEAOC 2018 VOL-1 book makes a similar statement. They recommend using overstrength, but it isn't specifically required. Refer to Design example 24 (attached).

 

[Deker]

Yes, the diaphragm absolutely needs to be designed with the transfer force amplified by the overstrength factor when an HT4 irregularity occurs. The amplification applies to the transfer force only, not the inertial force. The transfer and inertial forces occur simultaneously and should be combined when designing the diaphragm.

What’s ambiguous is whether or not transfer forces should be amplified by the overstrength factor when an HT4 irregularity does not occur. See this link for more discussion: Link.

 

[RipPECity]

Deker,
That's what I thought too...and the code seems pretty clear. Do you know what the logic is behind that SEAOC example?

 

[Deker]

That SEAOC example with the same language appeared in previous versions of the Seismic Design Manual prior to ASCE 7-16 being adopted by the IBC. My guess is that it was written in reference to ASCE 7-10, which did not require overstrength on the transfer forces, and then never updated. It appears to be an oversight.