THE DESIGN OF PILE FOUNDATIONS
In this article, i discussed the various DESIGN STEPS OF PILE FOUNDATIONS
Following are the steps used to design the pile foundations-
- Effective length the of pile
- Reinforcement in piles
- Longitudinal reinforcement
- Cover
- Pile cap
- Grade beams
let's discuss each of these steps in detail
When a finished pile projects above ground level and is not secured against buckling adequate
bearing, its effective length will be governed by the end conditions imposed on it by the
structure it supports and by the nature of the ground into which it is driven.
In the good ground, the lowest point of contra flexure can be taken to be at a depth below ground level
of about one-tenth of the exposed length. If the top straightens is soft clay silty soil, the lower
point of contra flexure may be taken at about one-half the depth of penetration into this stratum
arm but not less than one-tenth of the exposed length of the pile.
If the ratio of the effective length to the least lateral dimension exceeds 12, define should be
designed as a long column. If the pile is fixed in position and direction at the top end, the upper
point of contra flexure may be taken at one-fourth of the exposed link below the top of the Pile. If
the pile is not so fixed, the end conditions should be taken into account in determining the
effective length.
Reinforcement in piles
As per IS 2911, it is mandatory to provide minimum Steel of Mild 4% of the sectional area of
the pile for the full length of the pile.
Extra reinforcement as may be necessary for lateral load
and moments and piles is also provided.
The maximum percentage of longitudinal Steel
reinforcement should be limited to 1.5 to 2% alarm easy placement of concrete.
The distance
between bars in cast-in-situ piles should be at least 100 mm for the full length of the cage for
easy flow of concrete.
The minimum reinforcement necessary for column action is 0.8%, the provision of 0.4% steel is
not considered as giving any strength in such piles.
The safe load carrying capacity is calculated
assuming the pile is under-reinforced and the maximum allowable stress in concrete is 0.25 fck.
Longitudinal reinforcement
The minimum longitudinal reinforcement in piles is given as follows
1. In piles whose length is less than 30 times the least lateral dimension, the minimum
reinforcement should be 1.25%.
2. In piles whose length is 30 to 40 times the least lateral dimension, then minimum
reinforcement should be 1.5%.
3. In piles whose length exceeds 40 times the least lateral dimension, the minimum
reinforcement should be 2%.
As the percentage of longitudinal Steel use is always more than 0.8% in precast piles, design
this piles as short columns .Where the piles of poly embedded in stable soils or Rock. The end
conditions for a column should also be satisfied.
Lateral reinforcement
The minimum lateral reinforcement in a pile should be as follows.
1. The diameter of lateral enforcement should not be less than 5 mm.
2. In the body of the pile, the minimum reinforcement should be 0.2% of the gross volume of
the pile.
3. At each end of a pile for a length of about three times the least lateral dimension, the
minimum reinforcement should be 0.6% of the gross value of the Pile
Cover
Longitudinal reinforcement should normally be provided with a clear cover of about 40 mm.
Pile cap
Grade beams
The beam interconnecting all the pile caps of a system of piles is referred to as a grade beam.
The minimum depth of the grade beam should be 15 cm and may not exceed 20 cm for usual spans of above three meters. If grade beams are supported during construction, the maximum bending moment is taken as
M = wl2/50
Where,
w = uniformly distributed load taken up a maximum of two storey
l = effective span
Grade beams are not supported during construction, the maximum bending moment is taken as
wl2/30.
The panel action is considered in the grade beam if the ratio of depth to span is 0.6 minimum
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