UFC 4-023-03
25 January 2005
that hinging of columns occurs last, the capacity of the structure to resist collapse can
be maximized. It is also likely that the "Effective Column Height" requirement in Section
2-2.1 will lead to stronger columns and less demand for inelastic action of the columns
in resisting progressive collapse. However, since in some cases beam-columns with
significant axial forces may be encountered, there should be provisions to model the
response of these elements.
B-4.5.1
Nonlinear Analyses of Beam-Columns.
FEMA 356 provides modeling parameters for nonlinear analyses of columns
with axial force and bending moment. The rotational parameters/limits are based on
cross sectional properties and the ratio of PUF/PCL (PCL would be taken as φPn for use
with this UFC; PUF will be taken as the axial force in the member computed in
accordance with the loading specified in 3-2.4.1 or 3-2.4.2 as appropriate). If PUF/PCL is
less than 0.5, the member is assumed to be deformation controlled. If the ratio is
greater than 0.5, the member is taken as force controlled. Since PCL takes into account
the potential limit states associated with column behavior and the parameters are
further determined based on cross sectional properties, FEMA 356 was judged the best
reference for determining modeling parameters for beam-columns in this UFC. FEMA
356 was also prepared as a consensus document, further warranting its use for this
purpose. At this time, it is not possible to determine how the parameters given in FEMA
356 would be adjusted for the type of response and loading associated with progressive
collapse compared to seismic loading and response. The parameters may be different
since the parameters used in FEMA 356 are based on backbone curves derived from
pushover curves. However, it is believed that the values are conservative and valid for
use in the context of this UFC until further research can be completed.
References that may be useful for modeling and determining the response of
beam-column elements in the inelastic range include Beedle 1958, Inelastic Behavior of
Load-Carrying Members by Smith and Sidebottom (Smith and Sidebottom 1965), and
the Applied Plastic Design in Steel by Disque (Disque 1971). The latter reference has
moment rotation curves and column design charts with axial-moment interaction for
columns with applied end moments in single or double curvature
B-4.5.2
Linear Analyses of Beam-Columns.
Since the linear procedures in this UFC include the insertion of equivalent
plastic hinges, they are not compatible with the linear procedures developed in FEMA
356. Therefore the acceptance criteria utilized in FEMA 356 for linear procedures are
not applicable for this UFC.
Also, since "expected" or "lower-bound" strengths are not used in this UFC,
both capacities required for using FEMA 356 are calculated using the material
properties specified in this UFC. Further, the appropriate strength reduction factors Φ
and the over-strength factor Ω should be utilized in accordance with this UFC when
using FEMA 356. References to AISC LRFD in FEMA 356 will be taken as the latest
edition when used in conjunction with this UFC.
B-16
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