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8.2 Determination of size of plate
For purpose of analysis and design, eccentrically loaded column base
plates will be separated, according to the degree of eccentricity, into three
different classifications
Class I will pertain to all base plates the moment on which is so small in
proportion to the direct load that there is compression over the entire area
between the bottom of the base and its foundation
Class II will pertain a comparitively small range of base plates which have
tension over a small portion - one - third or loss of the area
Class III will include those which are exposed to a comparatively large
moment and which therefore have tension over a large portion - more than one -
third of the area between the bottom of the base plate and its concrete footing.
The method of the treatment employed for the three classification will be entirely
different from one - another
8.2.1 Class I Column base plates
c
c 3
2
c
P Mf y
A I
P Mf y
A I
P Pe.H / 2f
A 1/12BH
6 PeP
BH BH
6 ePf 1
BH H
=
= +
= +
= +
= +
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Fig 8.1
Here H is known very closely from practical considerations. The depth of
the column is always known and the necessary additional length of base plate to
provide for the column - base plate connection can be pretty accurately predicted
it will be 120 to 150mm beyond each column face for riveted column - base plate
connections, and for welded connections some smaller value may be arbitrarily
chosen in advance. This leaves 'B' as the only unknown. Solving the preceding
equation for B results in ( )2
c
PB H 6e
H f= + in which
Me
P= where B and H are
the width and length, respectively of the base plate and fc is the allowable
compressive stress on the concrete foundation. The anchor bolts for this class of
base plate have no computable stress. However they should be capable of
holding the columns firmly in place during erection and for withstanding any
unforeseen erection stresses.
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Design Example 1:
Determine the size of the base plate for a column carrying axial force of
2000kn.m and a moment of 200kn.m at the base. Foundation concrete is of M15
grade
The column available is ISWB600
c
c 2
P 6ef 1
BH H
M 200e 100mm
P 2000
2000x1000 6 x 100f 1
900900
= +
= = =
= +
Providing300
150mm2
= projection on either side
2 2cf 4.115N /mm 4N /mm= >
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Let us provide 160mm projection on either side
( )
c2
2 2
2
2000 x 1000 6x100f 1
920920
3.904 N /mm 4N /mm
2000 x 1000920 600 898mm 920mm
920
= +
=