Rigid Diaphragm with Chevron Bracing – Collectors & ASCE 7 Section Overstrength Requirements

[Futzin]

Consider if you will the 6-bay frame arrangement (attached) utilizing chevron bracing as the LFRS elements along with the following parameters/assumptions:

1) The structure is SDC C.
2) R = 3 for structures not specifically detailed for seismic resistance (i.e., AISC 341 requirements are not invoked)
3) There are no structural irregularities as defined by ASCE 7.
4) The frame supports a reinforced concrete slab-on-metal-deck which acts as the roof diaphragm. Contribution of the decking to the diaphragm strength/stiffness is neglected.
5) The diaphragm can be idealized as rigid per Section 12.3.1.2 of ASC 7-16 and a 3D structural analysis utilizing semi-rigid plate elements suggests the behavior of the diaphragm is practically rigid.
6) The slab has sufficient strength and rigidity and is provided with sufficient reinforcement such that diaphragm shears can be transmitted over the length of the chevron beams only as opposed to treating the non-chevron beams as collectors for transmission of diaphragm shears to the braced frames.
7) The headed studs are sufficient to transmit the diaphragm shears over the length of the chevron beams only.

Some background. I had modeled the structure utilizing plate elements to represent the diaphragm. I acknowledge that I could have modeled the diaphragm utilizing rigid nodal restraints and gotten approximately the same force distribution to the braced frames, but the use of plates was advantageous for my load application methodology.

I had initially/incorrectly assumed that axial diaphragm forces would manifest in the beams adjacent to/in between the braced bays (i.e. these beams would act as collectors) from the analysis. After reviewing the results, I realized this wasn’t the case. A majority of the force was concentrating and being transmitted directly to the node at the intersection point of the braces and plate elements. Upon reflection, I realized this made sense due to the stiffness of the plate elements and that if I were to instead represent the diaphragm utilizing a rigid nodal restraint that I should expect zero axial force to manifest in these beams (no stress/no strain, etc.). Clearly, however, the diaphragm shears cannot all be transferred through an infinitesimally discreet point. So I decided to take the force delivered to the braced frames and consider them evenly distributed along the entire column line for the purposes of collector connection design (therefore all beams along the line would be provided with shear studs for force transfer) like I had initially the model would act. Sure, I was applying the traditional flexible diaphragm force distribution method to a rigid diaphragm, but no harm/no foul right?

Enter ASCE 7-16 Section 12.10.2.1 which requires collectors and their connections be designed for loads considering seismic overstrength (omega factor). I was getting fairly massive connections and beginning to wonder if these “collectors” were actually necessary. If the diaphragm had the strength to transfer the loads over a shorter distance (say, the length of the chevron beams themselves), then why not neglect putting studs on the beams in-between and adjacent to the frames and not have collectors at all? The following paper seemed support my conclusion:

https://datacenterhub.org/resources/12268/download/10NCEE-000933.pdf

Up to this point, I feel comfortable with the direction I’m going in. I welcome any disagreements or comment on/with my thought process above. I had a few lingering questions though:
- As far as ASCE 7 is concerned, where does a collector end and the SFRS begin?
- Is the chevron beam itself a “collector”?
- If so, does the gusset plate to beam weld need to consider overstrength forces? Does the bolted connection between the gusset plate and a brace need to consider overstrength forces?
- If that’s the case, then why aren’t the column base brace connections required to be designed for overstrength forces? (It’s acknowledged that ACI 318 anchorage ductility requirements require consideration of overstrength in certain situations.) It doesn’t seem to make much sense for one end connection of a brace to have 3x the capacity of the other end.

TIA for your input and thanks to all the regular contributors through the years who have provided such valuable insights.

 

[driftLimiter]

My thinking is simply based on the AISC 341 detailing requirements for ordinary concentric braced frames.

The required strength of diagonal brace connections shall be determined using the over strength load

Section 5c also requires beams to be designed for overstrength, in the OP's case the ASCE 7 collector requirement is triggering the same requirement so that's taken care of.

 

[driftLimiter]

-I'm strongly considering going to a seismically-detailed ordinary concentrically braced frame. The slight R-value increase and significant Omega-naught decrease would both be beneficial. The additional analysis and detailing requirements don't appear too onerous. Plus I would no longer feel like I was designing collectors to overstrength for no appreciable benefit...

This is what I think I would look at doing Futzin. Simply because I prefer a system detailed for ductility rather than a dubious distributed ductility (R=3).

This Article Is a cool discussion on this topic. It suggests that if your interested in gaining economy of weight to 'Aim High' or in otherwords if your trying to reduce weight of steel and foundations try for SCBF or ICBF to minimize those values.

The writers of the article do not seem to share my view regarding being concerned with the level of ductility in an R=3 system (which goes along with what @sandman21 is saying).