Civil Engineering Reference
In-Depth Information
Deck
Span
Abutment
Girders
SUPERSTRUCTURE
SUBSTRUCTURE
Cap Beam
Bearings
Column
Footing
Piles
Figure 4.4. Girder bridge.
girder bridge, the deck-line coincides with its superstructure. The primary
stresses that loads put upon it are bending and shear stresses.
Though the girder does withstand some compressive and twisting forc-
es, its most distinctive structural role is to resist downward bending. Bending
is greatest at the middle of the span, where
moment
(the leveraging effect
of distance along the beam) is the greatest. Let us say we have in mind a
steel I-girder. The longer it is, the more that a truck in the middle is going
to make it bend down. For a longer girder span, the web has to be made
deeper and the two flanges need a larger cross-sectional area. (Remember
that the web is a metal plate that connects the upper and lower flanges
and resists shear stresses.)
Made ever longer with the same cross-section size, the girder would
eventually collapse under its own weight. If we proportionally increase the
cross-section size, the bridge would cost too much. It is this fact that limits
the lengths of girder bridges. A solution for reducing mass would be to put
perforations in the web material, but that brings us to another type of bridge,
the truss, to which we return later.
