In considering the elements of such buildings, I wish to devote a few
words to the question of foundations, because in the excessive loads
imposed by this class of buildings, and in the frequent necessity of
constructing them upon sites where alluvial drift or quicksands form
compressible foundations, there is afforded an opportunity for the
widest range of engineering skill in dealing with the problem. In such
instances, a settling of the building must be foreseen and provided
for, in order that it may be uniform under the whole structure. This
is generally accomplished by means of independent foundations under
the various points of pressure, arranged so as to give a uniform
intensity of pressure upon all parts of the foundation. It is
considered important to limit the load upon such foundations to two
tons a square foot, although loads frequently exceed this amount.
There is a large building in New York City which has recently been
reconstructed, and the foundations rearranged, where the load reached
to the enormous amount of six to ten tons per square foot. It was a
frequent occurrence in the class of high mills spoken of to impose
loads of so much greater intensity upon the wall foundation than upon
the piers under the columns of the mill, that the floors became much
lower at the walls than at the middle.
The stone for such foundations should be laid in cement rather than in
mortar, not merely because cement offers so much greater resistance to
crushing, but because its setting is due to chemical changes occurring
simultaneously throughout the mass. The hardening of mortar, on the
other hand, is due to the drying out of the water mechanically
contained with it, and its final setting is caused by the action of
the carbonic acid gas in the air.
Although quicksands are never to be desired, yet they will sustain
heavy loads if suitably confined. When inclined rock strata are met
with, all horizontal components of stress should be removed by cutting
steps so that the foundation stones shall lie upon horizontal beds.
Foundations are frequently impaired by the slow, insidious action of
springs or of water percolating from the canal which supplies the
water power for the mill; and the proper diversion of such streams
should be carefully provided for.
In the question of foundations, there is much of a general nature
which is applicable to all structures; but, at the same time, each
case requires independent consideration of the circumstances involved.