Page 252 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CRITERIA FOR BUILDING DESIGN

5.32 CHAPTER FIVE

For the limit state of buckling,
(5.117)
F n = F cr
where F cr is the buckling stress for the section as determined by analysis. The AISC Specification
permits constrained local yielding adjacent to areas that remain elastic.

5.8 DESIGN OF COMPOSITE MEMBERS

In composite construction, a steel section works together with concrete to resist column forces, flex-
ure, or both. Composite columns involve either an encased steel section or a hollow structural sec-
tion filled with concrete. Composite flexural members are typically comprised of a steel beam and
an overlying concrete slab, with shear connectors (studs) at the interface. In this case, the slab acts
as a cover plate and allows the use of a lighter steel section. Additionally, concrete-encased and
concrete-filled sections are sometimes used as flexural members. In addition to the provisions summa-
rized below, the AISC Specification gives design requirements for combined axial compression and
flexure in composite members.

5.8.1 General Provisions for Composite Members
The AISC Specification requires that, when determining load effects (forces, stresses, and deforma-
tions) in members and connections of a structure that includes composite members, consideration be
given to the effective sections at the time each increment of load is applied. Design, detailing, and
material properties pertaining to the concrete and reinforcing steel should comply with specifications
of the American Concrete Institute (ACI 318). Available strength of members can be determined by
either the plastic stress distribution method or the strain-compatibility method. The tensile strength
of the concrete is neglected.
For the plastic stress distribution method, the available strength is computed assuming that the
steel components have reached a stress of F y in either tension or compression, and the concrete com-
ponents in compression have reached a stress of 0.85f c ′. This is the method that would be typically
used for regular sections. Equivalent stress blocks are assumed for the stress distributions and
moments summed about the neutral axis to calculate the resisting moment. For round HSS filled with
concrete, a stress of 0.95f c ′may be used for concrete components in compression, to account for the
beneficial effects of concrete containment.
For the strain-compatibility method, a linear strain distribution across the section is assumed, and
a maximum concrete compressive strain of 0.003. The stress–strain relationship, obtained from tests
or published results, is used to determine the stresses over the cross section, and moments summed
about the neutral axis to calculate the resisting moment. This method should be used for irregular
sections and cases where the steel does not exhibit a typical elastoplastic response.
Certain material limitations apply. For the determination of available strength, concrete must have
a compressive strength f c ′ of not less than 3 ksi (21 MPa) nor more than 10 ksi (70 MPa) for normal-
weight concrete and not less than 3 ksi (28 MPa) nor more than 6 ksi (42 MPa) for lightweight con-
crete. Higher-strength concrete materials may be used for stiffness calculations but not for strength
calculations. The specified minimum yield stress of structural steel and reinforcing bars used in cal-
culating the strength of a composite column must not exceed 75 ksi (525 MPa), unless higher mate-
rial strengths are justified by testing and analysis.

5.8.2 Encased Composite Columns

To qualify for design as an encased composite column, three conditions must be met: (1) the cross-
sectional area of the steel core must comprise at least 1% of the total composite cross section, (2) the

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