Beam Bridge• consists of a horizontal beam supported at

each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes.

Beam Bridge: Forces

• When something pushes down on the beam, the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.

Continuous Span Beam Bridge

• several beam bridges can be linked together, creating what is called a continuous span.

Chesapeake Bay Bridge-Tunnel

Moveable Bridge

Has a deck that moves. • A swing bridge has a deck that rotates around a

center point; • a drawbridge has a deck that can be raised and

lowered; • a bascule bridge deck is raised with

counterweights like a drawbridge; • the deck of a lift bridge is raised vertically like a

massive elevator.

George P. Coleman Bridge – A Swing Beam Bridge

Truss Bridge

• consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars.

Complex Truss Bridge - Cantilever bridge - Firth of Forth Bridge, Scotland

Truss Bridge: Forces

• Every bar in this cantilever bridge experiences either a pushing or pulling force. The bars rarely bend. This is why cantilever bridges can span farther than beam bridges.

Arch Bridge• has great natural strength. Thousands of

years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete.

Ancient Roman aqueduct Segovia, Spain

Arch Bridge: Forces

• The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.

Suspension Bridge• can span 2,000 to 7,000 feet -- way farther

than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.

Golden Gate Bridge -San Francisco

Suspension Bridge: Forces • In all suspension bridges, the roadway

hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge.

• The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.

Forces on Suspension Bridge

Suspension: Cable-stayed bridge • The cable-stayed bridge, like the

suspension bridge, supports the roadway with massive steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique "A" shape.

Sunshine Skyway Bridge, Florida

How can you strengthen the bridge to resist strong winds?

Stiffen the deck with an open truss?

Stiffen the deck with a solid steel girder?

Make the bridge narrower?

• On the morning of November 7, 1940, the Tacoma Narrows Bridge twisted violently in 42-mile-per-hour winds and collapsed into the cold waters of the Puget Sound. The disaster -- which luckily took no human lives -- shook the engineering community and forever changed the way bridges were built around the world.

• Engineer Leon Moisseiff had designed the ultimate in slender bridges and strengthened his narrow bridge with a solid steel girder beneath the roadway. But soon after it opened, the Tacoma Narrows started behaving strangely. Wind caused the bridge to sway back and forth, and it also sent rippling waves along the deck. The Tacoma Narrows tore itself apart only four months later.

Tacoma Narrows Bridge disaster, 1940

Truss Support is Important!

• Years later, engineers found that the solid girders actually blocked the wind and caused the slender bridge to twist. The twisting bridge fanned the steady wind into a swirling motion, which caused the bridge to twist even more -- and eventually snap in two. The Tacoma Narrows Bridge was replaced in 1950 by a new bridge stiffened with a truss. Rather than blocking the wind, the open truss allowed the wind to blow through the new bridge.

Tacoma Narrows Bridge disaster,1940

• Thought you’d like to see some pictures of what happened to this narrow suspension bridge.