A pile cap have to be supported a 18”X18” column which is subjected to live load=170 kips and dead load=160 kips under service loading. The column is reinforced with longitudinal bars of 12 No. 7 bars. Consider fy=60 Ksi and f’C =3 Ksi. The diameter of pile is 12”. The ultimate pile capacity=70 kip/pile and service load capacity=42 kip/pile as conformed by testing.
Complete pile cap design will be illustrated here; except superimposed load weight of pile cap also exerts some load on piles. Lets consider cap weight exerts 3 kips load on each pile.
Thus net service load = (42-3) kips/pile
Total unfactored load= (170 + 160) kips
Pile number required, N =330/39
=8.46 or, 9 piles
Factored load = (1.2X160+1.6X170) kips
i.e. Ultimate load on pile cap, Wu = 464 kips
Ultimate load on pile, Pu = 464/9 kips/pile
=51.56 kips/pile <70 kips (ultimate pile capacity).
Let’s try to check punching shear failure; at first consider punching shear due to single pile:
Assume an effective depth of cap=18”
Thus punching perimeter, bo = π X (pile diameter + effective depth of cap)
Now, permissible shear around single pile, Vc= 4√f’c b0d