BEARING POWER OF COLUMN PILES. It is seldom, if ever, necessary to consider foundation piles as acting as a column, since the surrounding earth prevents lateral deflection, at least to a con siderable degree; and hence the supporting power of such a pile will usually approximate the crushing strength of the timber. If a con siderable portion of a pile is exposed to the air, it is likely to be braced so as to have sufficient lateral stiffness to develop the sup porting power of the soil, and hence usually such piles need not be considered as columns. But if a pile is driven through a considerable depth of water, the supporting power of the pile may be limited by its strength as a column, particularly if the upper portion of the soil into which it is driven is soft and gives but little lateral support. Such a pile acts as a column fixed at the base and free at the top. The ultimate bearing power of such a column is in which P is the ultimate load, E the coefficient of elasticity, I the moment of inertia of the cross section, and 1 the unsupported length.
If the diameter is 12 inches or less, and the free length (the length from the top to the point where the pile is firmly held by the lateral support of the soil) is more than 25 or30 feet, the ultimate load by the above formula may be less than the load often placed upon piles. The safe buckling strength of a pile is only a fractional part of the value given by the above formula; and hence the conclusion is that if a pile is to have any considerable unsupported length, it should be tested by the above formula to determine its buckling strength.
The safe end bearing-power of wood varies from 1,200 to 1,600 lb. per sq. in., and hence the safe crushing strength of a pile in pounds per square inch varies from 900 to 1,250 multiplied by the square of the diameter of the pile in inches.