BRIDGE TYPES AND SITES - Bridges - page 42

ARCH BRIDGES
Arch bridges may be classified into two subtypes, one being the traditional
arch, which has its main structure below the deck line. Resting on the arch
ribs called spandrels, the deck and its live loads exert downward compres-
sive forces. Made of masonry or stone, the arched structure was the kind
that the Romans were so adept at building. Some of them have withstood
the ravages of traffic and weather for millennia. They have served so well
because rock in itself is durable, large amounts may be obtained at low cost
(no special metallurgy is required), and the material excels at withstand-
ing the compression. Modern arches may, however, just as well be built of
reinforced concrete or steel trusses.
The arch ribs transmit compressive forces downward and outward (fig-
ure 4.5) along the curvature of the arch itself. There should be very little
tensile stress. If the arch is not to split apart at its center, the outward thrust
must be met by a countervailing force. Rocky canyon walls fit the bill; they
act as natural abutments. Where they are absent, enormous constructed
buttresses will do the job, but at added cost.
Arch construction is an ancient art. In the traditional method, the
bottom layers of blocks were piled on each other, building up the spandrels.
As additional layers angled out over the span to be bridged, wooden scaffold-
ing (called centering or falsework) held them up. The scaffolding continued
to support the layers of blocks as they crept toward the center, until the
Keystone
Figure 4.5. A simple stone arch bridge.