10/23/2014 Bridges and Applications Bridges and Applications • Bridges are used to span across distances that are difficult to otherwise pass through. • Rivers • Deep gorges • Other roadways Bridges and Applications Arch Bridges • There are four basic types of bridges – Arch – Beam – Suspension – Cable‐stayed • Each type has different design and is therefore better suited to different applications 1 10/23/2014 Arch Bridges Arch Bridges • Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end. Abutments, carry • These supports, called the abutments, carry the load the load and keep the ends of the bridge from spreading outward. and keep the ends of the bridge from spreading outward Arch Bridges Arch Bridges • When supporting its own weight and the • Today, materials like steel and pre‐stressed weight of crossing traffic, every part of the concrete have made it possible to build longer arch is under compression. and more elegant arches. • For this reason, arch bridges must be made of materials that are strong under compression. New River Gorge, – Rock West Virginia. – Concrete 2 10/23/2014 Arch Bridges Arch Bridges • Usually arch bridges employ vertical supports • Typically, arch bridges span between 200 and called spandrels to distribute the weight of 800 feet. the roadway to the arch below. Arch Bridges One of the most revolutionary arch bridges in recent years is the Natchez Trace Parkway Bridge in Franklin, Tennessee, which was opened to traffic in 1994. It's the first American arch bridge to be constructed from segments of precast concrete, a highly economical material. Note it was designed without spandrels to create a more open and aesthetically pleasing appearance. 3 10/23/2014 Beam Bridges Beam Bridges • A beam or "girder" bridge is the simplest and most inexpensive kind of bridge. • According to Craig Finley of Finley/McNary Engineering, "they're basically the vanillas of the bridge world." Beam Bridges Beam Bridges • In its most basic form, a beam bridge consists • Pre‐stressed concrete of a horizontal beam that is supported at each is an ideal material for end by piers. beam bridge • The weight of the beam pushes straight down construction. on the piers. • High strength steel cables are stretched and encased in concrete. 4 10/23/2014 Beam Bridges Beam Bridges • The concrete withstands the forces of compression well, and the steel rods embedded within resist the forces of tension. • Prestressed concrete also tends to be one of the least expensive materials in construction. Beam Bridges Beam Bridges • The beam bridge's biggest limitation is its • This doesn't mean length. beam bridges aren't used to cross great • The farther apart its supports, the weaker a distances; it only beam bridge gets. means that they • Individual beam‐bridge girders rarely stretch must be “daisy‐ more than 250 feet. chained” together. • This creates what's known as a continuous span. 5 10/23/2014 Beam Bridges Beam Bridges The world's longest bridge is a continuous‐span Although impressive, continuous beam bridge. span bridges have one major Almost 24 miles long, the Lake Pontchartrain drawback – Causeway’s Southbound Lane comprises 2,243 they are not well separate spans. suited for locations that require clearance below. Suspension Bridges Suspension Bridges • Pleasing to look at, light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet—far longer than any other kind of bridge. • They also tend to be the most expensive to build. 6 10/23/2014 Suspension Bridges Suspension Bridges • True to its name, a suspension bridge • These cables rest atop high towers and are suspends the roadway from huge main secured at each end by anchorages. cables, which extend from one end of the bridge to the other. Suspension Bridges Suspension Bridges • The towers enable engineers to stretch the Anchorages main cables over long distances. • The cables carry most of the bridge’s weight to the anchorages, which are embedded in either solid rock or massive concrete blocks. • Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free. Towers 7 10/23/2014 Suspension Bridges Suspension Bridges • In the early 19th century, engineers began • Currently, the Humber Bridge in England using iron chains for cables. has the world's longest center span— • Today, the cables are made of thousands of measuring 4,624 feet. individual steel wires bound tightly together. • A single steel wire only 0.1‐inch thick can support over half a ton without breaking. Suspension Bridges Suspension Bridges To keep the structure stable, engineers have added pendulum‐like devices on the towers to keep them from swaying and a stabilizing fin beneath the center deck to resist typhoon‐ strength winds. The Akashi Kaikyo Bridge links the islands of Honshu and Shikoku in Japan. The bridge's center section stretches a staggering 6,527 feet. 8 10/23/2014 Suspension Bridges Suspension Bridges • Because suspension bridges are light and • At the time it opened for traffic in 1940, flexible, wind is always a serious concern. the Tacoma (WA) Narrows Bridge was the third‐longest suspension bridge in the world. • It was promptly nicknamed "Galloping Gertie," due to its behavior in wind. • Not only did the deck sway sideways, but vertical undulations also appeared in quite moderate winds. Suspension Bridges Suspension Bridges • Only four months after it opened, the Tacoma Narrows Bridge collapsed in a wind of 42 mph—even though engineers had supposedly designed the structure to withstand winds of up to 120 mph. • Most experts agree the collapse had something to do with a complex phenomenon called resonance, the same force that can cause a soprano's voice to shatter a glass. 9 10/23/2014 Cable‐Stayed Bridges Cable‐Stayed Bridges • The difference between suspension and cable‐stayed bridges is in how the cables connect to the towers. • In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end. • In cable‐stayed bridges, the cables are attached to the towers, which alone bear the load. Cable‐Stayed Bridges Cable‐Stayed Bridges • The cables can be attached to the roadway in either of two main ways. Parallel Radial • In a radial pattern, cables extend from several points on the road to a single point at the top of the tower. • In a parallel pattern, cables are attached at different heights along the tower, running parallel to one another. 10 10/23/2014 Cable‐Stayed Bridges Cable‐Stayed Bridges • The first known sketch of a cable‐stayed • For medium‐length spans—those between bridge appears in a book called Machinae Novae published in 1595, but it wasn't until 500 and 2,800 feet—cable‐stayeds are the this century that engineers began to use bridge of choice. them. • Compared to suspension bridges, cable‐ • In post‐World War II Europe, where steel was scarce, the design was perfect for rebuilding stayeds require less cable, can be bombed‐out bridges that still had standing constructed out of identical precast foundations. concrete sections, and are faster to build. • Cable‐stayed bridges have gone up in the United States only recently, but the response • The result is a cost‐effective bridge that is has been passionate. undeniably beautiful. Cable‐Stayed Bridges Painted yellow to contrast with its marine surroundings, the Sunshine Skyway bridge in Tampa, Florida was one of the first cable‐stayed bridges to attach cables to the center of its roadway as opposed to the outer edges, allowing commuters an unobstructed view of the magnificent bay. 11.