TUBE MILL FOUNDATIONS
In placing of tube mill foundations the engineer must consider stresses due not only to static forces but also to dynamic forces, as the dynamic forces or those due to impact and vibration are more important than those due to steady compression or tension. The dynamic stresses being far in excess of those due to the weight of the machinery alone, instead of allowing a factor of safety of 3 to 6 as in ordinary structural work we must increase this many times to obtain a foundation that will stand the vibratory movements due to the shifting of the load within the mill.
The greatest compressive stress on the base of a tube mill of average weight is not over 10 Ib. per square inch, while the usual mixture of concrete used in such construction will stand a compressive stress of 2,400 Ib. per square inch or 240 times that due to the weight of the mill alone. This may in a sense be considered a factor of safety of 240. The Boston building laws allow pressures on concrete structures of not more than 55 Ib. per square inch while the City of New York allows 200 Ib. per square inch. If we take the Boston limit we have then a factor of safety of 5^ above that allowed in that city. Kent says that when the stresses are of a complex character and of uncertain amount, a very high factor is necessary, possibly even as high as 40. He gives us to understand that unless the strength of the material is known and the forces it must withstand are known, whatever factor of safety is assumed is a " factor of ignorance." So with concrete mixtures that must withstand dynamic stresses our factor of safety is a factor of ignorance because we have no standard tests for concrete, reinforced or otherwise, that will give an idea of the capabilities of the material for withstanding vibratory forces.
Without the advantage of dynamic tests on concrete specimens I would not apply any rigid rule, but it appears reasonable from the nature of the movements within a tube mill to place this factor at 100, for the concrete itself, but a foundation may fail from not being anchored to the bedrock or from not having the base cover enough area to prevent the structure from tumbling over or " creeping." Fig. 58 illustrates three forms of concrete foundation for a 5 by 22-ft. tire-mounted tube mill. A is a tubemill foundation erected in 1915 which was most unsatisfactory, the whole pillar moving and crumbling from the vibrations communicated to it. The designer, probably to save a little concrete, lessened the base area with a hole through the center. What was required was an extended base area with the pillar firmly anchored to the bedrock as in illustration B. The repair actually made was as shown in C, the concrete being extended from pillar to pillar, forming one solid base and well anchored with numerous rods. If the foundation had been made as shown in B, it is probable that no movement would have taken place, the joining of the two pillars into one block being unnecessary.
Concrete plays such an important part in tube mill work that a few words on the subject may not be out of place, for whether we have a foundation to erect or silex blocks or pebbles to cement in a mill the nature of cement and its mixture with rock and sand to form a concrete should be understood.