Page 223 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.3

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.3

where R a = required strength (ASD)
R n = nominal strength (as given in the Specification)
Ω= safety factor
R n /Ω = allowable strength

5.1.3 Analysis Methods
Elastic, inelastic, or plastic analysis is permitted. However, members designed on the basis of plas-
tic hinging are limited to those with steels having a specified minimum yield stresses not exceeding
65 ksi (450 MPa), and are subject to certain other provisions.
Beams and girders composed of compact sections, and satisfying unbraced length requirements,
including composite members, may be proportioned for nine-tenths of the negative moments at points
of support, produced by the gravity loading and computed by an elastic analysis, provided that the
maximum positive moment is increased by one-tenth of the average negative moments. This reduction,
as detailed in App. 1 of the Specification, is not permitted for moments produced by loading on
cantilevers. If the negative moment is resisted by a column rigidly framed to the beam or girder, the
one-tenth reduction may be used in proportioning the column for combined axial force and flexure,
provided that the axial force does not exceed the following: 0.15φ c A g F y for LRFD or 0.15φ c A g F y / Ω c
for ASD, where
2
2
A g = gross area (total cross-sectional area), in (mm )
F y = specified minimum yield stress, ksi (MPa)
φ c = resistance factor for compression = 0.90 (LRFD)
Ω c = safety factor for compression = 1.67 (ASD)

5.1.4 Classification of Connections

For design purposes, connections are classified as either simple connections or moment connec-
tions. A simple connection is one that transmits negligible moment. Unrestrained rotation (pinned
condition) is assumed in design. However, it is important to make sure that details are such that they
can withstand the rotation that will develop. Inelastic rotation is permitted.
Moment connections can be one of two types: FR (fully restrained) or PR (partially restrained).
A FR connection transfers moment with negligible rotation. It must have sufficient strength and stiff-
ness to maintain the angle between connected members, such as in a rigid frame. A PR connection
transfers moment, but the rotation is not negligible. The moment–rotation relationship, generally non-
linear, must be documented in the technical literature or established by analytical or experimental means,
and the connection components must have adequate strength, stiffness, and deformation capacity at the
limit states.

5.1.5 Classification of Sections for Local Buckling

Steel sections subjected to compression from direct forces or flexure may be classified as compact,
noncompact, or slender-element sections. For a section (such as an I or a box section) to qualify as
compact, the flanges must be continuously connected to the web or webs and the width–thickness
ratios of compression elements must not exceed a certain limiting width–thickness ratio, λ p . If the width–
thickness ratio of any compression element exceeds λ p , but does not exceed the limit λ r , the section
is termed noncompact. Further, if the width–thickness ratio of any element exceeds λ r , the section is
referred to as a slender-element compression section. Values of λ p and λ r for numerous cases are given
in Table 5.1. Further discussion of local buckling, particularly as applied to cold-formed members, is
given in Chap. 9.

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