What is a pile cap and the General guidelines for the design of the pile caps ?
Pile caps are structural elements that tie a group of piles together.
Pile caps may support bearing walls, isolated columns and/ or groups of several columns.
Pile caps are used to transmit the forces from the columns or walls to the piles.
There are two approaches for the design of pile caps involving two or more piles.
In the first approach, we consider the pile caps as a be and in between the piles.
The second approach, assumes that the pile cap acts as a Truss or as a space frame, children
forces act as tension members and concrete acts as a compression member.
The plan dimensions of a Pile cap depend on the closest allowable spacing of the piles which
is generally 1 m.
Its depth is based on the shear and/or development length of the Column bars.
Small Pile caps may be designed as a reinforced concrete Truss as shown in the figure.
The steel acts as a tension chord and the concrete as diagonal struts.
General guidelines for the design of the pile caps are as follows
1. The Pile cap along with the column pedestal should be deep enough to allow for the necessary anchorage of the column and pile reinforcement.
2. The clear overhang of the pile cap beyond the outermost pile in the group should be within 10 to 15 cm.
3. Levelling course of mass concrete of about 8 cm thickness may be provided under the pile cap.
4. Reinforcement from Pile must be properly tied to the pile cap.
5. Clear cover to the main reinforcement from the bottom of the cap should not be less than 60 mm.
6. Computations of moments and shears may be based on the assumption that the reaction from any pile is concentrated at the centre of the pile.
7. In computing external there are many sections c-c putting supported on piles as shown in figure, to the entire reaction from any pile of diameter Dp whose centre x1 is located at 0.5 Dp or more outside the section will be assumed as producing shear on the section. The force will be zero at a section c'-c', due to a reaction from AP whose centre x2 is at 0.5 Dp or more inside the section. Intermediate positions of the pile centre, the portion of the pile reaction to being assumed as producing shear on the section is based on straight line interpolation between full value when the pile Centre is at x1 = 0.5 Dp outside the section c-c, and 0 value when the file Centre is at x2 = 0.5 Dp inside the section c'-c'.
8. Minimum thickness of pile caps at the edges should not be less than 30 cm
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