OMF Moment Connection Design options

[E720]

Hello All,

In the Seismic Provisions 341-10 Section E1 it describes the requirements for the connection design for ordinary moment frames. Basically the connection (a)has to have a flexural strength that is 10% stronger than the flexural strength of the beam, (b)has to have a flexural and shear strength equal to the maximum moment and shear that can be delivered to the system from some other limit state (ie foundation uplift, flexural yielding of the column when it is less than the beam, etc.), or (c)has to have a connection designed similarly to those required by IMF and SMF. Is there an option that usually results in the simplest, cheapest to construct connection? It seems to me that the best option would be to check the required strength for option (a) and choose a few different limit states for option (b) and just design the connection for the lesser of those. Is there any reason to have to use a prequalified connection (RBS, etc.)? Just wondering if there are any issues with my train of thought. Thanks

 

[T_Bat]

That sounds right to me. The prequalified stuff often gets more onerous for the fabricator, erector, and even for your calcs. Typically for OMF's I use forces equivalent to an R=1 which satisfies item (b) you quote. Often for OMF frames the seismic demand is not that large and designs are based mostly on wind loads, drifts, or deflections. That usually means that the expected flexural strength of the beam will be much higher than the R=1 loads.

 

[JoshPlumSE]

For OMF, I believe the go to limit is usually to base it on a demand moment calculated using R=1.0. Either that or basing it on a moment that will cause a shear of 1.1Ry (assuming LRFD) in the panel zone.

Note: I was just looking through the 341-2016 commentary on this and it appears they're now allowing the overstrength load combinations. Meaning using Omega rather than R=1. So, an amplification of 3.0 instead of 3.5.