Page 232 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.12

CRITERIA FOR BUILDING DESIGN

5.12 CHAPTER FIVE

b eff = 2t + 0.63, in (b eff = 2t + 16, mm) but not more than actual distance from edge of hole to
edge of part, measured normal to the force
d = pin diameter, in (mm)
t = plate thickness, in (mm)
Certain dimensional requirements apply. The pin hole should be located midway between the lon-
gitudinal edges of the member. When the pin is expected to provide for relative movement between
1
connected parts while under full load, the diameter of the pin hole should not be more than / 32 in
(1 mm) greater than the diameter of the pin. The width of the plate beyond the pin hole should not be
less than 2b eff + d and the minimum extension a beyond the bearing end of the pin hole, parallel to
the axis of the member, should not be less than 1.33b eff . Plate corners beyond the pin hole can be cut
at 45° to the axis of the member, provided the net area beyond the pin hole, on a plane perpendicu-
lar to the cut, is not less than that required beyond the pin hole parallel to the axis of the member.
The AISC Specification also gives provisions for the design of eyebars.

5.4 DESIGN OF COMPRESSION MEMBERS

The design compressive strength φP n and the allowable compressive strength P n /Ω are determined
using φ c = 0.90 (LRFD) and Ω c = 1.67 (ASD) for all cases. The nominal compressive strength P n is
determined as the least value calculated for the limit states of flexural buckling, torsional buckling,
and flexural-torsional buckling, as applicable. Flexural buckling is applicable for doubly symmetric
and singly symmetric members. For singly symmetric and unsymmetric members, and certain doubly
symmetric members, such as cruciform or built-up columns, the limit states of torsional or flexural-
torsional buckling are also applicable.
An important factor in the design of compression members is the slenderness ratio, KL/r, where
L = laterally unbraced length of member, in (mm), r = governing radius of gyration, in (mm), and
K = effective length factor. Although the AISC Specification imposes no maximum slenderness limit,
it is sometimes considered good practice to limit KL/r to 200 for members designed on the basis of
compression.
The buckling coefficient K is the ratio of the effective column length to the unbraced length L.
(Also see Art. 5.2.) Values of K depend on the support conditions of the column to be designed. The
AISC Specification indicates that K should be taken as unity for columns in braced frames unless
analysis indicates that a smaller value is justified. Analysis is required for determination of K for
unbraced frames, but K should not be less than unity. Design values for K recommended by the
Structural Stability Research Council for use with six idealized conditions of rotation and translation
at column supports are illustrated in Fig. 5.1.
The following articles give the compressive strength of members without slender elements.
Provisions for the latter may be found in Art. E7 of the AISC Specification.

5.4.1 Compressive Strength for Flexural Buckling
The nominal compressive strength P n for the limit state of flexural buckling is
(5.19)
P n = A g F cr
where F cr is the buckling stress, determined according to Eqs. (5.20) and (5.21).
When (KL/r) ≤ 4.71 EF/ or F e ≥ 0.44 F y ,
y
y /
F = (.658 FF e F ) y (5.20)
0
cr
When (KL/r) > 4.71 EF/ y or F e < 0.44F y ,
(5.21)
F cr = 0.877F e

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