Multi-way bridges are located throughout the world, though they are rare. Some are as small as a
footbridge, while others are multi-lane roadways. Three-way bridges are often referred to as "T-bridges" or "Y-bridges", due to their shape when viewed from above. Three cities in
Michigan each have a three-way bridge named "Tridge", combining "tri" and "bridge": and the pilots of
Enola Gay aimed for
Hiroshima's T-shaped
Aioi Bridge when they dropped the atom bomb. While designing the Tripartite Bridge in 1846—a Y-bridge proposed to span the
Allegheny River and
Monongahela River in
Pittsburgh, Pennsylvania—engineer
John A. Roebling identified one of the challenges of erecting a three-way
suspension-type bridge. Suspension-bridge cables on two-way bridges support heavy loads and are anchored solidly at either end, while on a three-way bridge the cables of each of the three spans need to anchor at a central pier in the water, where cable forces from each span would have to balance each another: "the intersection of the cables at the top of the center pier...would have created enormous horizontal forces, and the stone arches connecting the three towers could hardly have resisted the tensions imposed by the cables radiating from their tops." The bridge was never built. ==Three-way bridges==