Bridge Engineering: Lessons from Rome to Tacoma Modified by Matthew Silbernagel Clear Lake MS Engineering 03-27-2006

Great Stone Bridge in China History of Bridge Development 100 B.C. Romans 2,104 years ago 700 A.D. Asia 1,304 years ago Clapper Bridge Roman Arch Bridge Tree trunk Stone Arch design evenly distributes stresses Natural concrete made from mud and straw Clapper Bridges employed all over the world, most notably in England. Originally, tree logs used, but they tended to rot. Stones were better, but difficult to maneuver. The Romans developed highways that connected the empire. Bridges helped them do this. Great Stone Bridge in China Low bridge Shallow arch Allows boats and water to pass through

History of Bridge Development 1900 1920 Truss Bridges Mechanics of Design Wood 2000 Suspension Bridges Use of steel in suspending cables Prestressed Concrete Steel

Basic Concepts Span - the distance between two bridge supports, whether they are columns, towers or the wall of a canyon. Force - Compression Tension Compression – Tension - Concrete has good compressive strength, but extremely weak tensile strength. What about steel cables?

Basic Concepts Beam - a rigid, usually horizontal, structural element Pier Pier - a vertical supporting structure, such as a pillar Cantilever - a projecting structure supported only at one end, like a shelf bracket or a diving board Load - weight on a structure

Types of Bridges Basic Types: Truss Bridge Beam Bridge Arch Bridge Suspension Bridge Floating Bridge Floating Truss Beam Arch Suspension The type of bridge used depends on the obstacle. The main feature that controls the bridge type is the size of the obstacle.

Truss Bridge Typical Span Lengths 40m - 500m World's Longest Pont de Quebec Total Length 863m Center Span 549m A Matsuo Example 2nd Mameyaki Bridge Typical 40m to 500m All beams in a truss bridge are straight. Trusses are comprised of many small beams that together can support a large amount of weight and span great distances.

Types of 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. This is why beam bridges rarely span more than 250 feet.

Types of Bridges 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.

Types of Bridges Arch Bridges The arch has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet.

Types of Bridges Arch Bridges 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.

Types of Bridges Suspension Bridges This kind of bridges 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.

Types of Bridges Suspension Bridges 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.

Types of Bridges Floating Bridge Pontoon bridges are supported by floating pontoons with sufficient buoyancy to support the bridge and dynamic loads. While pontoon bridges are usually temporary structures, some are used for long periods of time. Permanent floating bridges are useful for traversing features lacking strong bedrock for traditional piers. Such bridges can require a section that is elevated, or can be raised or removed, to allow ships to pass.

Floating Bridges Retractable! But high maintenance!

Ground below bridge Loads Materials Shapes Bridge Engineering How do the following affect your structure? Ground below bridge Loads Materials Shapes

Bridge Engineering Summary To design a bridge like you need to take into account all the forces acting on it: The friction of the earth on every part The strength of the ground pushing up the supports The resistance of the ground to the pull of the cables The dead weight and all vehicle loads Then there is the drag and lift produced by wind and water The turbulence as fluids pass the towers Need to use appropriate materials and structural shapes in the cheapest way, yet maintaining a certain degree of safety. To account for natural disasters, engineers design bridges with a factor of safety: usually around 3 or 4.

Case Study: Tacoma Narrows Failure The first Tacoma Narrows suspension bridge collapsed due to wind-induced vibrations on Nov. 7, 1940. The bridge over engineered it to withstand hurricane winds, but the wind that day was only 40 mph… what happened!?