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or with pile bearings when the live load is comparatively great, on account of the tendency to rock the piers. Where the ends of the arms come
together it is necessary to insert a detail that will take up shear but not
moment; for, otherwise, there would be a sudden break in the grade that
might give serious trouble. In the author's opinion this type is never
suitable for carrying steam-railway loadings and is none too good for
electric-railway structures, although satisfactory enough for highway
bridges. In order to determine for any case the relative economy of
cantilever and ordinary types, estimates for both superstructure and substructure will have to be made.
Columns
Columns are generally square or rectangular in cross-section for constructive and aesthetic reasons. A round or octagonal column is really
a better structural member; and, if the lines of the bridge are worked out
in accordance with it, there should seldom be any difficulty about the matter
of appearance. A round column can be hooped or banded better than
any other type. Frequently, for the sake of appearance, the size of a
column must be made greater than that necessitated by theoretical requirements.
Footings
Footings may be either plain or reinforced; and the question as to
which style to adopt is one solely of economics, because, as they are buried
out of sight, the consideration of aesthetics will not apply. If the area of
the footing is but little larger than that of the column supported, plain
concrete will be the cheaper; while for a spread foundation the reinforced
type will nearly always be found more economical. If a footing has to
be poured under water, plain concrete should invariably be employed;
and in wet-excavation work in general it is preferable, because it is often
difficult to prepare the bottom of the pit properly, and to stop absolutely
the flow of water from below. Such a flow is liable to wash out the cement
from the lower part of the footing; and thus it would destroy most of the
value of the reinforcing.
Plain footings are made of 1 : 3 : 5 concrete or sometimes 1 : 3 : 6;
but the latter, in the author's opinion, is too weak. The use of 1 : 2 : 4
concrete permits thinner footings, but this is not of much importance
when plain-concrete bases are used.
Highway Girder Bridges
In respect to the economics of girder bridges resting on columns, the
following points must be considered:
First. The panel length, when cross-girders are employed.
Second. The number and spacing of the longitudinal girders.
Third. The number of columns per bent.
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