bottoms are set on the pier sites at ground water level and pumped
full of compressed air while men enter them and excavating the soil,
undermine them, so they sink, until they land on the rock and are
filled solid with concrete to form the bases of the foundation piers.

On the average the formation should have a resisting power of two tons
to the square foot, dead load. By dead load is meant the weight of the
steelwork, floors and walls, as distinguished from the office furniture
and occupants which come under the head of living load. Some engineers
take into consideration the pressure of both dead and live loads gauging
the strength of the foundation, but the dead load pressure of 2 tons
to the square foot will do for the reckoning, for as a live load only
exerts a pressure of 60 lbs. to the square foot it may be included in
the former.

The columns carry the entire weights including dead and live loads and
the wind pressure, into the footings, these again distributing the
loads on the soil. The aim is to have an equal pressure per square
foot of soil at the same time, for all footings, thus insuring an even
settlement. The skeleton construction now almost wholly consists of
wrought steel. At first cast-iron and wrought-iron were used but it
was found they corroded too quickly.

There are two classes of steel construction, the cage and the skeleton.
In the cage construction the frame is strengthened for wind stresses
and the walls act as curtains. In the skeleton, the frame carries only
the vertical loads and depends upon the walls for its wind bracing.
It has been found that the wind pressure is about 30 lbs. for every
square foot of exposed surface.