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Steel Designers' Manual - 6th Edition (2003)
818 Foundations and holding-down systems
Fig. 27.2 Stability of foundation
27.1.2 Design of foundations
In order to assess the distribution of pressure under a foundation it is necessary to
make a reasonable estimate of the weight of the foundation. In addition to distributing
the forces to the ground the foundation block is also required to provide stability
in cases where overturning moments are present.
Referring to Fig. 27.2, loads P, H and M are factored as appropriate while W, the
foundation mass, is factored by 1.0, being a restoring moment. Moments about A
give
M HD PK WL
+ - - £ 0
2
From this a minimum value of W for stability is produced.
The minimum value for D for a mass concrete foundation is established by 45°
dispersal from the edge of the baseplate shown in Fig. 27.3. Shallower foundations
can be used if they are suitably reinforced.
The distribution of pressure under the foundation is then assessed as follows.
Case 1
See Fig. 27.4(a):
f
g =
P+ W ( M+ DH)6
±
2
LB BL
It is necessary for f gmax to be less than the stipulated ground bearing capacity for the
foundation to be satisfactory.
For f gmin to be zero (Fig. 27.4(b)):
P+ W ( M+ DH)
6
-
= 0
2
LB BL
Replacing the forces by the resultant acting at eccentricity x: