Page 236 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.16

CRITERIA FOR BUILDING DESIGN

5.16 CHAPTER FIVE

where (KL/r) o = slenderness ratio of built-up member acting as a unit in the buckling direction
being considered
a = distance between connectors, in (mm)
r i = minimum radius of gyration of individual component, in (mm)
r ib = radius of gyration of individual component relative to its centroidal axis parallel
to member axis of buckling, in (mm)
α= separation ratio = h/2r ib
h = distance between centroids of individual components perpendicular to the member
axis of buckling, in (mm)

The following dimensional requirements apply for built-up members. Individual components of
compression members composed of two or more shapes should be connected to one another at inter-
vals a such that the effective slenderness ratio Ka/r i of each of the component shapes, between the
connectors, does not exceed three-fourths of the governing slenderness ratio of the built-up member.
Use the least radius of gyration r i to compute the slenderness ratio of each component part. The end
connection must be welded or fully tensioned bolted with Class A or B faying surfaces. The end con-
nection may be designed for the full compressive load in a bearing-type bolted connection, but the
bolts must be fully tightened (see Art. 5.9.5).
At the ends of built-up compression members bearing on base plates or milled surfaces, all com-
ponents in contact with one another must be connected by a weld having a length not less than the
maximum width of the member, or by bolts spaced longitudinally not more than four diameters apart
1
for a distance equal to 1 / 2 times the maximum width of the member.
Along the length of built-up compression members between the end connections, longitudinal
spacing for intermittent welds or bolts should be adequate to transfer required forces. Where a com-
ponent of a built-up compression member consists of an outside plate, and intermittent welds are pro-
vided along the edges of the components or bolts are provided on all gage lines at each section, the
maximum spacing should not exceed the thickness of the thinner outside plate times 075. / EF , nor
y
12 in (305 mm). When fasteners are staggered, the maximum spacing on each gage line should not
exceed the thickness of the thinner outside plate times 112. / EF y nor 18 in (460 mm).
Open sides of compression members built up from plates or shapes should be provided with
continuous cover plates perforated with a succession of access holes. According to the AISC
Specification, the unsupported width of such plates at access holes contributes to the design strength
provided the following requirements are met: (1) the width–thickness ratio conforms to the limita-
tions of Art. 5.1.5, (2) the ratio of hole length in the direction of stress to hole width of hole does not
exceed 2, (3) the clear distance between holes in the direction of stress is not less than the transverse
distance between the nearest lines of connecting fasteners or welds, and (4) the periphery of the holes
1
has a radius no less than of 1 / 2 in (38 mm).
As an alternative to perforated cover plates, lacing can be used with tie plates at each end and at
points where the lacing is interrupted. In members providing design strength, the end tie plates
should have a length of not less than the distance between the lines of fasteners or welds connecting
them to the components of the member. Intermediate tie plates should have a length not less than
1
one-half this distance. The thickness of tie plates should be not less than / 50th the distance between
lines of welds or fasteners connecting them to the segments of the members. In welded construction,
the welding on each line connecting a tie plate should total at least one-third the length of the plate.
In bolted construction, the spacing in the direction of stress in tie plates should be not more than six
diameters and the tie plates should be connected to each segment by at least three fasteners.
Lacing, including flat bars, angles, channels, or other shapes employed as lacing, must be so
spaced that L/r of the flange included between their connections does not exceed three-fourths of the
governing slenderness ratio for the member as a whole. Lacing must be proportioned to provide a
shearing strength normal to the axis of the member equal to 2% of the compressive design strength
of the member. The L/r ratio for lacing bars must not exceed 140 for single lacing systems, or 200
for double lacing systems. Join double lacing bars where they intersect. For single lacing bars in
compression, take L as the unsupported length of the lacing bar between welds or fasteners con-
necting it to the components of the built-up member. For double lacing, take L as 70% of that

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