Bridges

Some Local Bridges

Beam Bridges

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

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

Ancient Roman aqueduct Segovia, Spain 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

Golden Gate Bridge - San Francisco 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

The Bridge Challenge!

The Bridge Challenge Welcome to Craggy Rock! You got here just in time. This growing community needs four new bridges, and they're very picky about what they want!

Bridge Challenge Test your engineering skills and try to match the right bridge to the right location. You may use each bridge type only once -- that's a direct order from the mayor of Craggy Rock. Good luck!

What Types of Bridge can you choose from? Beam Arch Truss Suspension

Location 1: Multi-lane bridge for commuters and tourists Location 2: Footbridge across a stream Location 3: Highway bridge across a busy shipping port Location 4: Railroad bridge in a national park

Location 1: Build a multi-lane bridge for commuters and tourists Span: 2,000 feet Crossing: River Connects: City and major highway Special Notes: "I want a one-of-a-kind bridge that will span our beautiful river and welcome visitors from all over the world to our thriving city. Make sure the new bridge leaves enough room for sailboats on the river. Please don't build a bridge that looks like the Golden Gate Bridge in San Francisco or the Brooklyn Bridge in New York." -- Mayor of Craggy Rock

What kind of bridge should you build? (Select one) Beam Suspension Drawbridge Arch

Location 1: Drawbridge A drawbridge is the best choice for this location. Drawbridges, like the Tower Bridge in London, England, can be quite majestic. They're also practical because they open to allow the passage of boats and ships. The mayor is delighted with your selection!

But there is a problem!!!

Convenience & Beauty!! In one of the rowdiest town meetings ever, the residents of Craggy Rock spoke out against the new drawbridge. "It's ugly," one angry resident said, "and there's always traffic on the bridge. They open it for boats, but what about us? We have to get to work, too!"

Which bridge will you choose? The mayor is begging you to build a new bridge. The local steelworkers are on strike, so she advises you to build a bridge with as little steel as possible. Cantilever Bridge Combination: Arch and Beam Cable-Stayed Bridge

Cable-Stayed Bridge Great Choice! It is one of the most popular bridge types in recent years. The roadway hangs from cables like a suspension bridge, forming a unique "A" shape. The mayor is happy because the bridge is made mostly of concrete, a relatively cheap material. The residents are thrilled because ships can pass beneath the bridge without causing traffic jams!

Location 2: Build a bridge for rollerbladers and bikers Span: 100 feet Crossing: Stream Connects: Two bike paths Special Notes: "We're tired of getting bumped off the road by angry drivers! We want a simple bridge -- one that will connect the bike paths in East and West Craggy Rock over the city stream. We don't have much money, so we're not asking for much. We just want to get to work every day without risking our lives!" -- Craggy Rock Bike-n-Blade Coalition

What kind of bridge should you build? (Select one) Beam Suspension Drawbridge Arch

Location 2: Beam Bridge This is an excellent spot for a beam bridge. Beam bridges typically span up to 250 feet, and they're also the cheapest and easiest bridges to build. Congrats -- you've just made a lot of rollerbladers and bikers very happy!

But there is a problem!!!

Flash Flood! When it rains, the city stream rises. A really strong storm could cause flash floods, turning the stream into a raging river. Piles of rocks and a wall of gooey mud could push on the piers of the concrete beam bridge and cause it to topple. Luckily, you can fix it before that happens!

How will you build a new, flood-proof bridge? Build a higher deck? Build deeper piers? Use stronger material?

Deeper Piers Good Idea!! One of the best ways to strengthen a bridge in a flood zone is to set the piers deeper into the ground. The deeper the piers are lodged into the ground, the harder it is to knock them over. The next time floodwaters pound on the piers of your new bridge, it probably won't budge!

Location 3: Build a highway bridge across a busy shipping port Span: 5,000 feet Crossing: Ocean bay Connects: Island and mainland Special Notes: "Our records indicate that more than 500 ships pass through Craggy Rock Harbor each day. Please build a bridge that does not block the flow of water traffic. We do not want angry sailors or congestion of any kind in our harbor. Thank you." -- The United States Coast Guard

What kind of bridge should you build? (Select one) Beam Suspension Drawbridge Arch

Location 3: Suspension Bridge This is an ideal location for a suspension bridge. Suspension bridges, like the Akashi Kaikyo Bridge in Japan, can span great distances and still leave room for passing ships and boats. The U.S. Coast Guard is thrilled with your selection!

But there is another problem!!!

Hurricane Winds Your new bridge is moving up and down a lot in the wind! This isn't a big problem under normal conditions, but what if a hurricane were to blow through town? It could cause the bridge to move too much, or even twist so violently that it would snap in half!

How can you strengthen the bridge to resist strong winds? Stiffen the deck with a solid steel girder? Stiffen the deck with an open truss? Make the bridge narrower?

Open Truss Good Idea! This is a great solution! By adding an open truss beneath the deck of the bridge, you've designed a structure that is less likely to move up and down a lot on a gusty day, because the wind will blow right through it! Since the Tacoma Narrows Bridge disaster in 1940, many major suspension bridges have been built with a stiffening truss.

Tacoma Narrows Bridge disaster, 1940 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.

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.

Back to our bridge project. The mayor is getting impatient!

Location 4: Build a railroad bridge in a national park Span: 500 feet Crossing: Deep river gorge Connects: Two rocky bluffs Special Notes: "We do not want the new bridge to detract from the natural beauty of Craggy Rock National Park. Please build a simple and elegant bridge -- one that does not have towers or piers." -- The National Park Service

What kind of bridge should you build? (Select one) Beam Suspension Drawbridge Arch

Location 4: Arch Bridge This is the perfect spot for an arch bridge. The rocky bluffs will support the abutments, and no towers or piers are needed because the arch shape can support itself.

But, of course, there is another problem!!!

Concrete Has Weathered! Twenty years have passed. You're an older, wiser, and wealthier engineer. The phone rings. It's the National Park Service. Chunks of concrete are falling from the arch bridge into the gorge. Twenty years of cold weather have chewed away at the bridge. It's time to replace the cracking concrete with a new material.

Which new material will you choose? Steel? Wood? Cast Iron?

New Material: Steel In fact, the world's longest spanning arch bridge, the New River Gorge Bridge in West Virginia, is made entirely of steel. Steel is strong in compression, the squeezing force that all arch bridges experience. Steel can also be arranged into a lightweight truss pattern, like the Garabit Bridge in France.

You Did It!!!!!!

Well Done!!