2.22-I

I.

FOUNDATIONS (Continued)

at least 16 inches. Even with these precautions, the uplift per pile should be limited to 4 tons. In Reference 7, the results of bond tests for timber piles embedded in concrete indicate values ranging from 21 psi for concrete placed in water to 96 psi for concrete placed in the dry; but longtime results may be considerably less due to shrinkage of the wood or softening of the wood if submerged in water.

Compact soil offers considerable resistance to lateral forces. However, clays under continuous loading may flow and permit deflection, causing excessive bending stresses in the piles. Reference 7 gives the following test results for timber and concrete piles driven in medium sand at Alton, Illinois:

Lateral Resistance

of Piles

*The load deflection diagram was a straight line up to these values.

J. Piles, when used, are assumed to take all the load, and no allowance is made for bearing of the structure on the earth between piles. On structures subjected to horizontal as well as vertical forces, the piling also has to transfer the horizontal forces to the foundation. These horizontal forces can be transferred either by bending in the piles, by the use of batter piles, or by a combination of the two. Probably the safest method for carrying large horizontal forces to the foundation material is by use of batter piles.

K. A method of analysis for pile foundations is given Reference 1, a brief explanation of which follows. The method is for foundations consisting of vertical piles and piles battered in one direction only, or piles battered in two directions with no vertical piles. It is based on the assumptions that the pile foundation has one plane of symmetry, that all piles are parallel to this plane, and that the resultant of all horizontal and vertical forces on the foundation acts in this plane. Also, it is assumed that the base of the structure is in one horizontal plane and that the piles are hinged at the base and at an additional point below the base. The distance between the hinges is assumed to be the same for all piles in the foundation. Simple instructions for the use of this method follow.

(1) Find the direction, location, and magnitude of the resultant forces at the base of the structure for the various loading conditions assumed.

Pile Founda

tions-General Considerations

Pile

Founda

tions-

Detailed

Analysis

2.22L

FOUNDATIONS (Continued)

(2) Break the resultant for each condition into components, one parallel to each pile group.

(3) With the bearing value of a pile known or assumed, divide each component by the bearing value to get the approximate number of piles in each group.

(4) Determine the elastic center, which is the point of intersection of the lines drawn through the center of gravity of the piles in each group and parallel to the piles in the respective groups.

(5) Arrange the piles in the foundation in such a manner that the elastic
center is as close as possible to all of the larger resultant forces
for the different cases.

(6) Compute the pile loads using the formulas outlined in Figure 15.
(7) After computing several cases, it may be found that it is necessary
to add piles or change the elastic center to keep within the desired
pile load. For small changes in the location of the elastic center,
this can be done without recomputing the entire foundation.

(8) Graphical methods and scaled dimensions are satisfactory for this
analysis as there are so many indeterminates that great refinement
in calculations is not necessary.

(9) If horizontal forces are so great that an excessive number of batter piles are required, it may be permissible (depending on soil condition) to take part of the horizontal force in bending in all of the piles. L. Reference is made to the following publications for further information on piles and pile driving.

1. Vetter, C. P., "Design of Pile Foundation"--Bureau
of Reclamation Technical Memorandum No. 531,
(Synopsis of above by C. P. Vetter, Proceedings
ASCE, February 1938, Volume 64, No. 2, page 311).

2. Terzaghi, "Theoretical Soil Mechanics"

See Pages 46 and 47 of Appendix I to Chapter 4, for an example of the use of sheet piling to increase the length of percolation path under a structure founded on porous material.

*In cases where compacted fills are used, the assumed weights should be determined by the engineer in responsible charge of building design.

Fig. 2a Par. 2.6C
Sheet 1

MINIMUM UNIFORMLY DISTRIBUTED LIVE LOADS FOR MAJOR
POWER PLANTS AND PUMPING PLANTS