| Reinforcement for bored piles
Piles loaded
in compression need reinforcement of the shaft to a depth of about 2 m
greater than the depth of temporary casing. This is to prevent the tendency
of concrete lifting, when pulling the casing. Piles subject to tension
or lateral forces and eccentric loading, require suitable reinforcement
designed to accommodate these forces. Nominal reinforcement for piles
in compression only would be four 12 mm bars for a 400 mm pile to five
16 mm bars for a 550 mm pile. A special cage of 5 mm steel, or hoops of
flat steel, are employed as lateral ties. The assembled cage should be
sufficiently strong to sustain lifting and lowering into the pile bore
without permanent distortion or displacement of bars. Bars should not
be assembled too densely that concrete aggregate cannot pass freely between
them.
It is frequently
necessary to overlap reinforcement bars in long piles. However, in this
case aggregates can assemble at the joints and prevent concrete from flowing
towards the borehole walls and result in a defective pile shaft. Overdesigning
of steel reinforcement in a bored pile can lead to practical difficulties
in constructing the pile shaft. Where large steel cross-sections are unavoidable,
consideration can be given to staggering the joints, although this can
be difficult to arrange in prefabricated cages. If bar connectors are
used, staggering is not a major problem as the lengths involved are shorter.
However, these are designed for use either in tension or alternatively
in compression, and it is not generally possible to predetermine the orientation
of the cage in the pile bore. Mild steel bars can be butt-joined by welding.
Provided high-quality welds are used, this can be an effective solution.
Provided
the cage can be oriented, maximum steel need only be placed over that
part of the pile subjected to maximum stress, and a reduced density can
be used in the plane of the neutral axis.
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