Overstrength vs Redundancy

[TJW]

Searching through previous posts on these subjects, I found a lot of great discussion and useful information. Thank you all for your insight. I have maybe a slightly different twist on this subject that has resulted in some (rather strong) differences of opinion in my firm.
Assume you have a structure, in seismic design category D, and we are using Table 15.4-1 "Seismic Coefficients for Non-Building Structures Similar to Buildings". From that Table, assume we are using Building Frame Systems with unlimited height and detailing per AISC 360, which results in R=1.5 and Omega=1.0. Further assume that, due to the configuration of the structure, Rho=1.3.
Granted, with detailing per AISC 360, there are not a lot of requirements for Omega, but, where Omega is required (say anchorage), the formulas with Omega would result in lower seismic loads than the corresponding equations with Rho. Would you then use the equations with Rho because they result in more conservative loads or use the Omega equations? My feeling is that, with this system, we are using such a low R value resulting in higher seismic loads, I would use the Omega equations when specified. We received a (unofficial) interpretation from ASCE indicating we should use the rho equations since it results in higher loads. This seems overly conservative to me and I was wondering if anyone out there had a similar situation and how you handled it.

Thank you in advance for your consideration,
Tom W

 

[sandman21]

ASCE 12.4.2.3 has two separate equations one with rho and overstrength. Overstregnth typical governs as it is normally greater than 1.3 but when it is not and rho is 1.3 it would control.

 

[BAGW]

Would use both equations omega and rho. As rho is greater than omgea, rho controls. The building ends up being designed for R close to 1.0 fully elastic

 

[Coty]

An R of 1.5 means the members remain essentially elastic, so there are not any inelastic demands on the connections, hence the omega = 1.

The Rho/redundancy equations are there to hedge bets in case overstress occurs in a frame due to structural irregularities. Overstrength and redundancy serve overlapping purposes when focusing in on connections (make them stronger), so they are not applied concurrently. However, when omega is 1 you essentially do not have overstength anymore and the clauses in section 12.3.4.1 where rho is 1.0 no longer apply, so those members and connections that you could design without considering rho NOW must be design for rho.