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Steel Designers' Manual - 6th Edition (2003)
29.1.3.3 Loading
In most cases the structural loads are principally vertical, with relatively minor horizontal
components. Small horizontal loads are commonly resisted by passive soil
resistance on the side of pile caps and beams rather than by bending in the pile
itself, because the structural movements are very small. Where the lateral loading
is larger, it is better resisted by the inclusion of raking piles which provide greatly
improved stiffness in the horizontal plane. Any induced uplift forces resulting from
the use of raking piles can be resisted by the weight of the superstructure or by
tension in the remaining associated piles in a group (see Fig. 29.5).
29.1.3.4 Axial capacity and load transfer
A pile subjected to a load parallel to its longitudinal axis will support that load partly
by shear generated over its shaft length, due to the soil–pile skin friction or adhesion,
and partly by normal stresses generated at the base or tip of the pile, due to
end bearing resistance of the soil (see Fig. 29.6).
The basic relationship is given in BS 800412 for the ultimate capacity Rc of the
pile. This relationship assumes that the ultimate capacity Rc is equal to the sum of
the shaft friction capacity Rs and base capacity Rb, i.e.
Rc = Rs + Rb = qsAs + qbAb where q s is the unit shaft friction value*
A s is the surface area of the pile in contact with the soil †
q b is the unit base resistance value
horizontal and
vertical foundation
loads
pile cap
compression pile
Fig. 29.5 Raking pile foundation
t
tension pile
Bearing piles 873