The peak seismic drifts of steel MRFs are calculated by amplifying the elastic deformations due to the design lateral forces with a deflection amplification factor (DAF). The European and the Canadian codes define the DAF to be equal to the response modification factor R while the ASCE 7-10 defines the DAF to be equal to 0.6875R. Any thoughts about why there is this variance between the seismic codes.
The DAF, Cd, is not 0.6875 for all SLRS being used. It varies depending on what type of framing system used - I believe the ratio you are referring to is for steel special moment frames where Cd = 5.5 = 0.6875R, where R = 8.
Cd is simply an approximation to determine inelastic drift as a some multiple of the elastic drift. ASCE uses Cd as less then R on various framing types and the magnitude of reduction correlates to the systems ability to accommodate expected drift levels based on how the system is detailed. For example, something detailed as an SMRF will be reduced more then an OMRF. The reduction in R is intended to give some relief where special detailing has been applied.
In a purely technical definition, Cd can be thought of as the ratio maximum deflection experienced (dmax) including yielding/non-linear material behavior to the deflection computed if material stayed elastic (delastic). I have attached a devent paper that discusses some of these terms.
I cannot comment on differences in design codes, but can say there is some push in the seismic community to make Cd = R, regardless of framing type. We will see if this happens in subsequent versions of ASCE 7.
Thank you for the contribution. Yes, the ASCE 7 assigns different values to DAF depending on the structural-system type. For steel special MRFs, the value of DAF specified by ASCE 7 is equal to 0.6875 R.
The point is that for the same structure, the European and the Canadian codes define the DAF to be equal to the response modification factor R . This means significant difference in designing steel frames between the ASCE 7 and other international codes.
R combines both the strength and deflection through the energy/toughness. Cd represents the deflection only. I personally attributes using smaller values for Cd than R to stability issues. Allowing Cd values close to R may be dangerous since the overall stability of the structure and surrounding elements of the seismic lateral system may be jeopardize and perhaps to control the second order internal forces.
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