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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

= +

=