Engineering and Construction

It is always desirable to design and construct deep foundations so that the vertical load is concentric, i.e., the vertical load is applied at the center of gravity of the pile cap or the center of gravity of the pier. For a pile cap that carries more than one vertical load, the pile cap should be designed and constructed so that the vertical loads are symmetric.

There may be design situations where the pile cap is subjected to a moment, such as where there is a fixed end connection between the building frame and the cap. This moment can be represented by a load Q that is offset a certain distance (known as the eccentricity) from the center of gravity of the pile cap. For other projects, there may be property line constraints and the load must be offset a certain distance (eccentricity) from the center of gravity of the pile cap.

One method commonly used to evaluate an eccentrically loaded pile cap is to use Eqs. 6.12 and 6.13.

It is assumed that the eccentrically loaded cap will cause the piles located under one side to carry a higher load than as compared to the piles located on the opposite side of the pile cap. The first step in the procedure is to ignore the piles, assume a rigid pile cap, and use a linear pressure distribution such as shown in Fig. 6.16. The values of q′ and q′′ are then calculated by using Eqs. 6.12 and 6.13. The final step is to proportion the pressure to an individual pile, such as shown in Fig. 6.16. A usual requirement is that the load Q must be located within the middle 1/3 of the pile cap and Eqs. 6.12 and 6.13 are only valid for this condition. The pile carrying the largest load (i.e., number 1 in Fig. 6.16) must not exceed its allowable load capacity. Equations 6.12 and 6.13 can be used for an eccentricity in one direction (i.e., across the width or length of the pile cap) or for an eccentricity in both pile cap directions.





Example Problem 6.7 A 4 by 4 pile group will support a pile cap that is subjected to a vertical load of 500 kips that is offset from the centerline of the pile cap by a distance of 2.0 ft (i.e., e = 2.0 ft in only one direction). The piles have a diameter of 1.0 ft (0.3 m), are spaced at four pile diameters (centerline to centerline) in both directions, and the pile cap is 16 ft by 16 ft (4.9 m by 4.9 m). Using Fig. 6.16, determine the load that each pile supports.

Solution The first step is to check that the eccentricity is within the middle one-third of the pile cap. The middle third of the pile cap is a distance of 2.7 ft (0.8 m) from both sides of the centerline of the pile cap. Thus the eccentricity is within the middle one-third of the pile cap.

Since there are four rows of piles, each row will carry a Q = 500/4 = 125 kips per row or 125,000 lb per row.