The easiest approach is to use the acceptance criteria (plastic rotation
angles) found in Table 6-7 of FEMA 273, which explicitly reflect the level
of axial loading, transverse detailing, and level of shear demand (and
implicitly take cyclic response into account).
When developing my own moment-curvature relations I make a point of
considering the full range of axial load levels that I anticipate; the
curvature ductility capacity is strongly influenced by the presence of axial
loads. If you don't want to use the entire family of curves in your
analysis, the only single value that is guaranteed to be conservative is the
curvature ductility capacity at the maximum axial load. Based on
engineering judgement you might be able to rationalize a "typical maximum"
load that is some multiple (greater than unity) of the maximum gravity axial
load. For very regular frames you might be able to feel comfortable with
this level of approximation; for irregular systems, the results might vary
so widely that this approach would not be reasonable. The least effort
solution is probably to start with the curvature ductility capacity based on
the maximum axial load and then perform additional checks (with "correct"
demands) on those columns that don't pass.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
> -----Original Message-----
> From: satd(--nospam--at)techie.com [mailto:satd(--nospam--at)techie.com]
> Sent: Wednesday, March 15, 2000 8:55 AM
> To: SEAOC
> Subject: Performance/Ductility
> I am working on performance evaluation of a 10 story SMRF and am
> using non-linear dynamic analysis for this purpose. The analysis
> computes the curvature ductility demands at the ends of members
> based on an effective moment of inertia (.7Ig for columns and
> .5Ig for beams -FEMA273) and the P-M envelope for the designed
> section based on ACI stress block.
> For computing the curvature ductility capacities for a group of
> similar (same design) COLUMNS, what level of axial load should I
> assume for a meaningful comparison of the calculated demands.
> In reality, every column will exhibit maximum demands at
> different level of axial loads and hence, even if the designs are
> similar, their ductility capacitites will be different. Since, I
> have several columns in this structure, could there be a logical
> "mean" value of axial load that I may use for computing a
> somewhat "lower bound" of ductility capacity?
> Any thoughts in this regard will be very much acknowledged.
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