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 5-3.1.1.2) 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

Section 3-2.4.3.

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.

5-8

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