25 January 2005
Combined Axial and Bending Resistance of Structural Steel.
The response of an element under combined axial force and bending moment
can be force controlled (i.e. non-ductile) or deformation controlled (i.e. ductile). The
response is determined by the magnitude of the axial force, cross sectional properties,
magnitude/direction of moments, and the slenderness of the element. If the element is
sufficiently braced to prevent buckling and the ratio of applied axial force to the axial
force at yield (Pu/Py where Py = AgFy) is less than 0.15, the member can be treated as
deformation controlled with no reduction in plastic moment capacity, i.e. as a flexural
member in accordance with Section 5-3.1.1. For all other cases, treat the element as a
beam-column and make the determination of whether the element is deformation or
force controlled in accordance with the provisions of FEMA 356 Chapter 5.
If the controlling action for the element is force controlled, evaluate the
strength of the element using the interaction equations in Chapter H of AISC LRFD
2003, incorporating the appropriate strength reduction factors Φ and the over-strength
factor Ω. Remove the element from the model when the acceptability criteria is violated
and redistribute the loads associated with the element per Section 3-2.4.3.
If the controlling action for the element is deformation controlled, the element
can be modeled for inelastic action using the modeling parameters for nonlinear
procedures in Table 5-6 in FEMA 356. The linear static and dynamic procedures
specified in FEMA 356 are not consistent with the analysis approach of this UFC;
however, the nonlinear modeling parameters provided in FEMA 356 can be utilized to
determine the equivalent plastic hinge properties (see 5-220.127.116.11) for use in the linear
static analysis procedure of this UFC. In linear analyses, take the force deformation
characteristics of the elements as bilinear (elastic perfectly plastic), ignoring the
degrading portion of the relationship specified in FEMA 356. The modeling of plastic
hinges for beam-columns in linear static analyses must include a reduction in the
moment capacity due to the effect of the axial force (see FEMA 356 Equation 5-4).
For nonlinear analysis, the modeling of elements, panel zones, or connections must
follow the guidelines in FEMA 356. Nonlinear analyses must utilize coupled (P-M-M)
hinges that yield based on the interaction of axial force and bending moment. In no
cases shall the deformation limits established in FEMA 356 exceed the deformation
limits established in Table 5-3 of this UFC.
The acceptability criteria for shear of structural steel is based on the nominal
shear strength of the cross-section, per AISC LRFD 2003, multiplied by the strength
reduction factor Φ and the over-strength factor Ω. If the element violates the shear
criteria, remove the element and redistribute the loads associated with the element per
All connections must meet the requirements of AISC LRFD 2003; employ the
appropriate strength reduction factor Φ for each limit state and over-strength factor Ω.