Page 126 - Handbook of Structural Steel Connection Design and Details

Design of Connections for Axial, Moment, and Shear Forces

Design of Connections for Axial, Moment, and Shear Forces 111

for erection, which is much more stringent than the OSHA load, is a
1-kip working load, applied at the top of the column in any horizontal
direction. If the column is, say, 40 ft high, this 1-kip force at a lever arm
of 40 ft will cause a significant couple at the base plate and anchor
bolts. The base plate, anchor bolts, and column-to-base plate weld should
be checked for this construction load condition. The paper by Murray
(1983) gives some yield-line methods that can be used for doing this.
Figure 2.26 shows four anchor rods. This is an OSHA erection require-
ment for all columns except minor posts.

2.2.5 Splices—columns and truss chords
Section J1.4 of the AISC Specification (2005) says that finished-to-bear
compression splices in columns need be designed only to hold the parts
“securely in place.” For this reason, the AISC provides a series of “standard”
column splices in the AISC Manual of Steel Construction. These splices are
nominal in the sense that they are designed for no particular loads. Section
J1.4 also requires that splices in trusses be designed for at least 50% of the
design load (required compression strength), or for the moment and shear
resulting from a transverse load equal to 2% of the required compressive
strength of the member, whichever is less severe. The difference between
columns and “other compression members,” such as compression chords of
trusses, is that for columns, splices are usually near lateral support points,
such as floors, whereas trusses can have their splices at mid-panel points
where there is no lateral support. Either the 50% requirement or the 2%
requirement can be used to address this situation.

Column splices. Figure 2.27 shows a standard AISC column splice for
a W14 99 to a W14 109. If the column load remains compression,
the strong-axis column shear can be carried by friction. The coefficient
of static friction of steel to steel is on the order of 0.5 to 0.7, so quite high
shears can be carried by friction. Suppose the compression load on this
column is 700 kips. How much major axis bending moment can this
splice carry? Even though these splices are nominal, they can carry
quite significant bending moment. The flange area of the W14 99 is
2
A 0.780 14.565 11.4 in . Thus, the compression load per flange is
f
700 11.4/29.1 274 kips. In order for a bending moment to cause a
tension in the column flange, this load of 274 kips must first be unloaded.
Assuming that the flange force acts at the flange centroid, the moment
in the column can be represented as:
M T(d t ) T(14.16 0.780) 13.38T
f
If T 274 kips, one flange will be unloaded, and M 13.38 274
3666 kips-in 306 kips-ft. The design strength in bending for this
column (assuming sufficient lateral support) is M 647 kips-ft. Thus,
p
because of the compression load, the nominal AISC splice, while still
seeing no load, can carry almost 50% of the column’s bending capacity.

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