Civil Engineering Reference
In-Depth Information
From Figure 11.4,
(b) Is it a Slender Column?
9
12
12
u
10 ft
8.25 ft
k
u
r
(1.74)(12
8.25)
0.3
20
28.71
Maximum
k
u
r
for a short column in an unbraced
75
75
frame by ACI Equation 10- 7
34
12
22
It's a slender column
11.8
MAGNIFICATION OF COLUMN MOMENTS
IN NONSWAY FRAMES
When a column is subjected to moment along its unbraced length, it will be displaced lat-
erally in the plane of bending. The result will be an increased or secondary moment equal
to the axial load times the lateral displacement or eccentricity. In Figure 11.5 the load
P
causes the column moment to be increased by an amount
P
. This moment will cause
to
increase a little more, with the result that the
P
moment will increase, which in turn will
cause a further increase in
and so on until equilibrium is reached.
Figure 11.5
Moment magnification in a nonsway column.
