Civil Engineering Reference
In-Depth Information
Table 3.10
Guidance on the minimum reinforcing steel for bored cast-in-place piles
Pile diameter
(mm)
Main reinforcement
Secondary reinforcement
No
Bar
Bar
Pitch (mm)
400
4
H12
H6
200
450
5
H12
H6
225
500
6
H12
H6
250
600
6
H12
H8
300
750
6
H16
H8
300
900
8
H16
H10
300
1050
11
H16
H10
300
1200
12
H16
H10
300
1500
12
H20
H10
400
1800
12
H20
H10
400
2100
16
H20
H10
400
2400
14
H25
H10
500
'H' steel denotes yield stress (
f
y
) 500N/mm
2
.
Notes
(a) The above guidelines are for 'build-ability' only. They are not appropriate where:
(i) BS5400 or associated Highways Agency codes of practice are specified (UK).
(ii) piles are required to resist any applied tensile or bending forces - the reinforcement has to be designed
for the specific loading conditions.
(iii) piles are required to accommodate positional and verticality tolerances, or where they are constructed
through very soft alluvial deposits (
c
u
< 10 kN/m
2
). Specific reinforcement design is then necessary.
(b) Minimum depth of reinforcement is taken as 3m below cut-off for simple bearing only. Any lateral loads or
moments taken by the pile will require reinforcement to extend to some depth below the zone subjected to
bending forces. This zone may be determined from a plot of the bending moments with depth. Furthermore,
the reinforcement would normally extend at least 1 m below the depth of any temporary casing.
(c)
Even with the appropriate reinforcement care will still be required to prevent damage to piles by construction
activities especially during cutting-down or in the presence of site traffic.
(for shear) is not recommended at closer than 100-mm centres. Concrete cover to the
reinforcement periphery is generally a minimum of 75mm.
It is frequently necessary to lap bars in long piles, and it is here that trouble can arise,
with aggregate blocking at the laps and preventing concrete flowing to the borehole
walls. Such an occurrence results in a defective pile, but is sometimes not appreci-
ated by designers. To over-design the steel reinforcement in a bored pile can lead to
practical difficulties in formation of the pile shaft. Where large steel cross-sections are
unavoidable, consideration can be given to staggering the laps, although this can be
difficult to arrange in pre-fabricated cages. If bar connectors are used, staggering is
not such a problem as the lengths involved are shorter, but these are designed for use
either in tension or alternatively in compression, and it is not generally possible to
pre-determine the orientation of the cage in the pile bore. Steel bars can be butt joined
by welding. Provided high-quality welds are used and the welding process carried out
with strict quality control, taking into account the type of steel employed, etc., this
can be an effective solution. Finally, provided the cage can be oriented with certainty,
maximum steel need only be placed over that part of the pile subjected to maximum
