Transcript Document 7331044

Columns and Other Compression Members
• The strength of steel compression
members is usually limited by their
tendency to buckle.
• The load at which a compression member
becomes unstable is the buckling load.
• The buckling load depends on the length,
cross-section, and end conditions of the
column and the stiffness of the material.
Buckling Load
Pcr 
 2 EI
(KL) 2
• Pcr is the load at which the compression
member becomes unstable
• E is modulus of elasticity of steel = 29,000 ksi
• I is moment of inertia of the cross section
• L is the length of the compression member
• K is the effective length factor
Effective Length Factors
for Braced Frames
L
K=1
K=
K=
p. 16.1 – 189 LRFD
Strength of Steel Compression Members
• Design Equations p. 16.1 – 27
• Tabulated in Table 4-2, p.4-21→ 4-151
• Strengths given in table are for buckling about the y-axis
z
z
P
P
y
x
y
x
1st Column Design Example p. 63 notes
A column in a braced frame building is 22
ft tall, with no intermediate lateral support
in any direction. The construction is such
that is may be considered pinned at each
end, in all directions. Select the lightest
possible A992 steel column to support an
axial live load is 225 kips and dead load of
220 kips.
2nd Column Design Example p. 63 notes
In order to reduce the size of the column in
the previous example, it is to be supported
against weak axis buckling at mid-height.
z
z
x
y