in general, be covered in this chapter, but a particular point to be noted is that whereas
BS8004 allows an increase in working stresses for temporary works, such increases are not
permitted by EC2. BSEN 12699 allows for an increase in compressive stress generated
during driving (see Sections 2.2 and 2.3).
7.2 Designing reinforced concrete piles for lifting after fabrication
The reinforcement of piles to withstand bending stresses caused by lifting has to be considered
only in the case of precast reinforced (including prestressed) concrete piles. Bending takes place
when the piles are lifted from their horizontal position on the casting bed for transportation to
the stacking area. The most severe stresses thus occur at the time when the concrete is immature.
Timber piles in commercially available lengths which have a cross-sectional area sufficiently
large to withstand driving stresses will not be overstressed if they are lifted at the normal
picking-up points. Splitting could occur if attempts were to be made to lift very long piles
fabricated by splicing together lengths of timber, but there is no difficulty in designing spliced
joints so that the units can be assembled and bolted together while the pile is standing vertically
in the leaders of the piling frame. Again, steel piles with a cross-sectional area capable of
withstanding driving stresses and of sufficient thickness to allow for corrosion losses will not
fracture when lifted in long lengths from the horizontal position in the fabrication yard.
Reinforced concrete and prestressed concrete piles have a comparatively low resistance to
bending, and the stresses caused during lifting may govern the amount of longitudinal
reinforcing steel needed. The static bending moments induced by lifting and pitching piles
at various points on their length are shown in Table 7.1. These considerations are principally
concerned with piles cast on the project site using the techniques described in Section 2.2.2.
In the UK, driven precast concrete piles usually consist of the proprietary jointed types
described in Section 2.2.3. These are factory-made with specially designed facilities for
handling and transport.
The design charts in Figure 7.2a to d show the bending moments due to self-weight which
are induced when square piles of various cross-sectional dimensions are lifted from the
head or centre (pick-up point Aas in Figure 7.1g and h), from a point one-third of the length
from the head (pick-up point Bas in Figure 7.1d), and from points one-fifth of the length from
the head and toe (pick-up point Cas Figure 7.1a).
Also shown on the charts are horizontal lines representing the ultimate moment of resist-
ance of each pile section using four main reinforcing bars of various sizes. Concrete with a
376 Structural design of piles and pile groups
Table 7.1Bending moments induced by lifting and pitching piles
Condition Maximum static bending moment
Lifting by two points at L/5from each end WL/40 (Figure 7.1a)
Lifting by two points at L/4from each end WL/32 (Figure 7.1b)
Pitching by one point 3L/10 from head WL/22 (Figure 7.1c)
Pitching by one point L/3 from head WL/18 (Figure 7.1d)
Pitching by one point L/4 from head WL/18 (Figure 7.1e)
Pitching by one point L/5 from head WL/14 (Figure 7.1f )
Pitching from head WL/8 (Figure 7.1g)
Lifting from centre WL/8 (Figure 7.1h)