Sunday, August 11, 2013

Reinforced concrete footings - Design

Few important things to remember about design of reinforced concrete footings:
(1)The thickness at the edge of a reinforced concrete footing resting on soil shall not be less than 15    cm.
(2) The thickness at the edge of a reinforced concrete footing on piles shall not be less than 30 cm.
(3) A foundation supporting of all the columns of a structure is called a raft footing.
(4) A common footing provided for two columns is called a combined footing.
(5) In the case of a common footing provided for two columns, where the projections beyond the edge column parallel to the length of the footing is restricted we must provide a rectangular combined footing.
(6) In case of a common footing provided for two columns, where the projection beyond the columns  parallel to the length of the footing are both restricted, we must provide a trapezoidal combined footing.
(7) A common foundation provided for a numbers of columns in a row is called a strip footing.
(8) For determining the shear force and bending moment for a footing the dead load of the footing is not considered when the footing rests on the soil.
(9) Weight of the footing may be assumed to be 10% of the column loads.
(10) For driving a pile into the ground a heavy hammer with a small drop should be used.
(11) For piles of length up to 30 times their least width the area of longitudinal reinforcement is not    less than 1.25% of the gross area.
(12) Piles of length more than 20 metres should be at least 45 cm* 45 cm.
(13) When a pile of length l is lifted at two points the distance of each support from the respective end should be  0.207*l.
(14)  A pile is lifted by two point suspension for the condition that the maximum bending moment is as small as possible. This maximum bending moment equals approximately  (w*l^2)/47.
(15) When a long pile of length l is lifted at three points, i.e. with one supporting point at the center and the other supporting points at a distance a from the respective end then, a=0.15*l.
(16) A pile of length l is lifted by three point suspension for the condition that the maximum bending  moment is as small as possible. This maximum bending moment is approximately  (w*l^2)/90.
(17) A pile of length l lying on the ground has to be hoisted to the vertical position. For the condition    that the maximum bending moment is as small as possible, the pile should be lifted at a point 0.293*l from the upper end.
(18) While calculating the ultimate bearing capacity of a pile by static formula the tip reaction may be taken equal to 1/5 of the ultimate load.
(19) A pile consisting of a vertical shaft of plain concrete with a bulb enlargement at the bottom is called a  pedestal pile.
(20)  A pile is driven into the soil by using a drop hammer. The best drop of the hammer is 120 cm.

Reference: Hand book of Civil Engineering by S.Ramamrutham.

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