Pile Design and Construction Practice, Fifth edition

characteristic strength of 40 N/mm^2 has been used with a characteristic strength for the
reinforcement of 250 N/mm^2 , and 40 mm cover to the link steel.
It is desirable to limit the steel stresses to 250 N/mm^2 when determining ultimate
resistance moments. This will ensure that a 4 bar reinforcement arrangement complies
with Table 3.30 of BS8110: Part 1 regarding maximum spacing requirements, that is,
300 mm assuming no redistribution. If a characteristic strength of 460 N/mm^2 were to be
used, the maximum allowable bar spacing becomes 160 mm, thus requiring an 8 bar
arrangement.
However, the situation can arise in which a moment is induced in a pile due to the pile being
driven just within the permitted tolerance of 1 in 75 to the vertical. In such a case the pile
can be checked as a column with an axial load and moment, by use of the design charts in
BS8110: Part 3. These charts relate only to high-yield steel with a characteristic strength of
460 N/mm^2. It is therefore advisable to use high-yield steel for pile reinforcement, but to limit
the characteristic strength to 250 N/mm^2 when designing for lifting from the casting bed.
The charts can be used for a concrete having a higher characteristic strength but the lifting
lengths will be somewhat conservative. The concrete strength is usually governed by the
strength required to resist driving stresses (see Section 2.2.2), or to give durability in an
aggressive environment (see Section 10.3 and Clauses 4 and 7 of EC2).
The length and cross-sectional dimensions of the pile are obtained from considerations of
the resistance of the soil or rock to axial or lateral loading (Chapters 4 and 6). Then for the
given length and cross-section the pick-up point is selected, having regard to the type of
piling plant and craneage to be employed, and the economies which may be achieved by
lifting the piles from points other than at the ends or the centre. The bending moment due to

Structural design of piles and pile groups 377

Lifting rope Lifting points

(d) (e) (f)

(g) (h)

L

2

L

2

L

5

L

4

L

4

3L 10

L 3

L 4 L 5

L

5

(a) (b) (c)

Figure 7.1Methods of lifting reinforced concrete piles.