Crunching the numbers on America's infrastructure...
Crunching the numbers on America's infrastructure...
Bridges built outward from piers using balanced projecting arms, ideal for spanning wide obstacles without falsework.
Cantilever bridges are built using structures that project horizontally from a fixed support point, like an arm reaching out into space. The cantilever principle allows bridges to be constructed outward from a pier without the need for temporary supports (falsework) below, making them ideal for deep valleys, busy waterways, and locations where temporary supports would be impractical or impossible.
The classic cantilever bridge design features two cantilever arms extending from opposite piers, meeting in the middle where a shorter suspended span connects them. This configuration was extremely popular for railroad bridges in the late 1800s and early 1900s because it could handle the heavy loads of rail traffic while being built without disrupting river navigation below.
Modern cantilever construction has evolved significantly. Balanced cantilever construction using post-tensioned concrete segments is now one of the most common methods for building medium to long-span bridges worldwide. Segments are cast or placed symmetrically on both sides of a pier, balanced like a seesaw, until they reach the next pier or meet a cantilever arm coming from the opposite direction.
A cantilever works by extending a structural member beyond its support point, with the overhanging portion resisting bending through its own structural depth. In a cantilever bridge, the arm near the pier is anchored by the weight of the structure on the opposite side or by a direct anchorage, creating a balanced system. The top chord of a cantilever arm is in tension (being pulled apart) and the bottom chord is in compression (being squeezed together) near the pier, which is the opposite of a simple beam. This reversal of forces is what allows the arm to project outward. The suspended span in the center, hung between the tips of two cantilever arms, behaves like a simple beam. In modern balanced cantilever construction with concrete, post-tensioning cables are added progressively as each segment is placed, creating the forces needed to support the growing cantilever.
The Forth Bridge in Scotland (1890) was one of the first major cantilever bridges and used 54,000 tons of steel. Its engineers famously demonstrated the cantilever principle for the press by having two men sit in chairs extending their arms while a third man sat between them, supported by their outstretched hands.
Commodore Barry Bridge (Pennsylvania/New Jersey)
Crescent City Connection (New Orleans, LA)
Tappan Zee Bridge (replaced, was a cantilever)
Forth Bridge (Scotland, iconic railroad cantilever)
Quebec Bridge (Canada, longest cantilever span at 549m)
Toledo-Puerta de Hierro Bridge (Madrid, Spain)
Iconic bridges with a deck hung from cables draped over tall towers, capable of spanning enormous distances.
Modern bridges where cables run directly from towers to the deck, creating a dramatic fan or harp pattern.
One of the oldest bridge forms, using a curved structure to transfer loads outward to abutments at each end.
Bridges built from interconnected triangular elements, combining strength and material efficiency.