Bridges • There are basically 4 different • types of bridges: • Arch bridge • Suspension bridge • Beam bridge • Cable-stayed bridge • An engineer would choose the correct bridge based on how far it must span from one support to the next. • Each bridge deals differently with tension and compression. • If part of the bridge cannot stand the compression it will buckle. • If part of the bridge cannot stand the tension it will snap. Ganter Bridge (Simplon Pass, Brig, Switzerland)
Bridges Engineers must take many variables into consideration. On November 7, 1940, at approximately 11:00 AM, the first Tacoma Narrows suspension bridge in Washington collapsed due to wind-induced vibrations. The bridge had only been open for traffic a few months.
Bridges Arch bridges are not stable during construction until the two sides meet. Therefore, scaffolding, or "centering,” must be assembled below, or cables must be anchored to hold it up. The latter allows the traffic below to continue until the bridge is complete. The Pont du Gard Aqueduct, France The Natchez Trace Bridge, Tennessee
Arch Bridges Using a rectangular strip of oak-tag (manila folder), bend it into a shape of an arch. Push down as shown in the diagram. What happens? Now try this: Using the same arch, use two piles of books as abutments. • An abutment is the part of a structure that • bears the weight of an arch, • supports the end of a bridge, • or anchors the cables of a suspension bridge.
Suspension Bridges Suspension Bridges use steel cables to support weight. Steel wire is very strong and can support large tensile forces. A single 0.1” thick wire can support more than half a ton without breaking! Humber Bridge, England Golden Gate Bridge, California Akashi Kaikyo Bridge, Japan
Suspension Bridges Using 2 textbooks of the same size and a piece of string, tightly tie the tops of each book to its corresponding end of string, then stand the books far enough apart so the string in-between hangs loosely. Press down on the string between the books. What happens?
Suspension Bridges Now try this: Stand two books about 10 inches apart. This time do not tie the string to the 2 standing textbooks. Put a stack of heavy books on each end of string to secure it to the table. Then pass the string over each book. Again let the string hang loosely between the standing books. Now press on the center of the string between the books. What happens? Notice how the stacks of books help to stabilize the bridge.
Beam Bridges Beam bridges are categorized by the design, location and composition of the trusses used. These are a few common types (notice the location of the roadway colored gray in each): Howe Kingpost Truss Deck Truss Warren Truss Pratt or Through Truss
Beam Bridges Beam Bridges consist of a horizontal beam supported at each end by piers. The weight of the bridge and traffic on it pushes straight down on the piers, so the beam itself must be strong. Rocks Village Bridge , Massachusetts Dump Road Bridge , Minnesota Sellwood Bridge, Oregon
Beam Bridges Take a thick sponge and cut a notch out of the top and bottom of the sponge as shown, then lay the sponge on top of two piles of books. Press down on the sponge. What happens to the notches? The notch on top closed up because the top of the sponge is in compression. The notch on the bottom stretched out because the bottom of the sponge is in tension.
Cable-Stayed Bridges Cable-stayed bridges differ from suspensions bridges in the way that the cables are connected to the towers. In cable-stayed bridges, the cables are attached to the towers, which bear the load alone.
Cable-Stayed Bridges The cables of a suspension bridges ride freely across the towers, transmitting the load to the anchorages at either end.
Cable-Stayed Bridges Cable-stayed bridges are distinguished by the number of spans, number of towers, girder type, number of cables and the arrangement of their cables. Typical towers include: Single Double Portal A-shaped Typical cable arrangements include: Mono Harp Fan Star
Cable-Stayed Bridges Get a 5 foot piece of rope and a partner. Hold your arms out and have your partner tie an end of the rope to each elbow with the middle of the rope laying on your head. Relax your arms. With your arms as the bridge and your head as the tower, the cable-stayeds hold your elbows up.
Cable-Stayed Bridges Now have your partner tie a second piece of rope (one foot longer then the first) to each wrist and lay the middle over your head. Relax your arms. Where do you feel a pushing force, or compression? Can you feel how the cable-stayeds transfer the load of the bridge (your arms) to the tower (your head)?