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Seismic loading on connections between floor diaphragm and bracing frame?

Seismic loading on connections between floor diaphragm and bracing frame?

(OP)
Hello people, in terms of AS/NZS code, if there is a concrete diaphragm transferring forces to a steel brace frame via epoxy anchored bolts, should the load on the bolts be determined via Parts and Components 1170.5 Section 8 OR just derived from Cd(T) for mu=1.25? In my case, the difference in loading is by a factor of 4 which is huge.

A colleague is telling me that Parts and Components is what the loading should be for diaphragm loads into braced frames.... i think this is inappropriate because Parts and Components should be used for as it is described (machines and other appendages attached to the main building structure).

Thoughts?

RE: Seismic loading on connections between floor diaphragm and bracing frame?

I would agree with you, we always have used diaphragm load for connection design.

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RE: Seismic loading on connections between floor diaphragm and bracing frame?

As a new Zealand designer I am assuming seismic loads govern, if the (steel?) braced frame is being designed for a ductility of 1.25 then if it's a Concentrically Braced Frame with tension only Bracing then you should be doing a capacity design for a Category 3 tension only Bracing system. If this is the case then you will should be designing for either the lesser of the upper limit design actions (eg mu=1 with Sp=0.9), or the overstrength actions from the brace.

This basically means there is little benefit taking mu=1.25 for these systems, but everyone does it. We do a lot of peer reviews and see that see done all the time, completely ignoring almost all of section 12 of NZS3404. We even see people designing for a ductility of 3 or higher without a scrap of capacity designed being done. So don't get caught out!

Additionally refer to section 17.6 of NZS3101, you need to choose one of the seismic design philosophies relating to the design of fixings. If you are relying on a 'brittle' failure mechanism for example the concrete failing first vs the drilled in bolts yielding first (which is almost always the case) then you should be designing the fixings for the elastic loads as noted (i.e. For a ductility of 1.00) The additional 0.75 factor noted effectively modifies the ductility 1.25 actions to ductility 1.00 actions if you compare seismology coefficients. Depending on how critical the connection is I would even try to design it for the overstrength as earthquakes in reality can be larger than the design earthquake.

Hopefully this helps!

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