Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 305

endanger the whole structure. However, continuous truss bridges do not
experience the tipping forces that a cantilever bridge must resist, because
the main span of a continuous truss bridge is supported at both ends
(Kulicke 2000).
10.2.3 truss arch bridges
A truss arch bridge, such as Francis Scott Key Bridge (I-695) in Baltimore,
Maryland, as shown in Figure 10.9, combines the elements of the truss
bridge and the arch bridge. The actual resolution of forces will depend
on the design. As long as the horizontal movement of the top chord is
restrained, like a regular arch bridge, horizontal thrusting force will
be generated and therefore the top chord of a truss arch bridge will be
under compression. When the top chords are free to move horizontally,
no horizontal thrusting forces will be generated and this arch-shaped
truss bridge works essentially as a bent beam. If horizontal thrust is gen-
erated but the apex of the arch is a pin joint, it is termed a three - hinged
arch . If no hinge exists at the apex, it will normally be a two-hinged
arch. A tied-arch bridge is an arch bridge in which the outward-directed
horizontal forces of the arch, or the top chord, are borne as tension by
the bottom chord (either tie rods or the deck itself) rather than by the
ground or the bridge foundations. Deck loads including live loads are
transferred, as tension, by vertical ties of the deck to the curved top
chord, tending to flatten it and thereby to push its tips outward into the
abutments, like other arch bridges. However in a tied-arch or bowstring
bridge, these movements are restrained not by the abutments but by the
Figure 10.9 Francis Scott Key Bridge (I-695) in Baltimore, Maryland.