Peer-to-Peer Standards Exchange

Following the provisions of AISC 342-22 and referencing ASCE 41-17 where needed, I am trying to develop force-deformation relations for a modern RBS connection properly designed and detailed per AISC 341.  The curve should be Type 1 in AISC 342 Fig. C1.1 (below) or ASCE 41 Fig. 7-4 since the connection has highly ductile behavior.  I have several questions.

1. The chord rotation at yield determined by AISC 342 Eq. C2-2 does not seem appropriate since the hinge is formed at a non-trivial distance from the column face.  Should I modify the equation by including the shear amplification in the determination of MCE?  For example, MCE = Mp,RBS + VRBS*Sh?
2. Following from question #1, should I also include shear amplification when determining the yield strength, Qy?
3. Looking at charts from cyclic tests on RBS connections, the post-elastic slope seems much steeper than the 0.03 value recommended in AISC 342 Section C2.4a.1.b.  Have there been any studies suggesting a larger value specific for RBS connections?
4. Neither AISC 342 nor ASCE 41 provide much guidance on determining the slope between Points C and D.  A steep slope (e.g., 10:1) is conservative, but it can cause analysis errors and does not seem reasonable for a connection that exhibits gradual strength degradation behavior.  Is this left entirely to engineering judgment?  Could I justify a 1.5:1 slope for an RBS connection?  How about 1:1 instead?

#ASCE41
#Seismic

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Matthew Bosch-Willett P.E., S.E., M.ASCE
DIRECTOR OF STRUCTURAL ENGINEERING
Walnut Creek CA
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Mathew

The answers to your questions depend on how you configure your model.  If you configure your model such that there is a discrete hinge located at the center of the RBS, rather than the face of the column, the shear amplification will automatically be computed by the software.  However, if you model the discrete hinge at the face of the column ,the answer is yes, you must include the shear amplification effect in Qy and all other parameters.

The branch of the hysteretic backbone from point "C" to "D" represents the development of a fracture in the RBS connection.  Yes - even ductile moment connections will fracture due to low-cycle fatigue.  For RBS connections, tested per the protocol in AISC 341 this typically occurs somewhere between -.05 and 0.06 radians of total rotation, depending in part on the compactness of the beam section.  The reality is that this is is a brittle behavior,and should really be represented by a vertical branch.  Also , the dropoff typically occurs only in the direction of loading that puts the fractured flange in tension.  When the fractured flange is in compression, the connection retains almost all of its strength.  This behavior can be replicated using complex fiber models for the beam, but this is likely not necessary.  As to the slope of the line C-D, make it close to vertical, but provide sufficient deviation from vertical so that your software does not become unstable when the fracture develops.

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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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