foundation system of burj al arab

Design of Pile Cap for Concentric Loading

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

=39 kip/pile

Total unfactored load= (170 + 160) kips

=330 kips
Pile number required, N =330/39

=8.46 or, 9 piles

Let’s arrange piles as per following pattern:

Pile arrangement in pile cap (9 piles)

Factored load = (1.2X160+1.6X170) kips

=464 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)

X(12+18) inches


Now, permissible shear around single pile, Vc= 4f’c b0d

=4X3000 X90.25X18

=355910 lb
=356 kips

Here shear force =Nominal reaction of pile=Pn

pile cap design-1


=68.88 kips <356 kips.

This means no pile will be punched when effective depth, d=18”

Now what can be punched through pile cap? One is pile itself; another is column or columns supported by pile cap. Should we check both cases?