Bridges

Bridge Engineering:Lessons from Rome to Tacoma

Clear Lake MS Engineering 03-27-2006

700 A.D. Asia700 A.D. Asia

1,304 years 1,304 years agoago

100 B.C. Romans100 B.C. Romans2,104 years ago2,104 years ago

Clapper Bridge

Tree trunkStone

Arch design evenly distributesstressesNatural concrete made from mud and straw

Roman Arch Bridge

History of Bridge DevelopmentHistory of Bridge Development

Great Stone Bridge in China

Low bridgeShallow archAllows boatsand water to passthrough

History of Bridge DevelopmentHistory of Bridge Development

Truss Bridges

Mechanics of DesignWood

Suspension Bridges

Use of steel in suspending cables

1900 1900

1920 1920

Prestressed ConcreteSteel

2000 2000

Compression Tension

Basic Concepts Basic Concepts

Span - the distance between two bridge supports, whether they are columns, towers or the wall of a canyon.

Compression –

Tension -

Force -

Concrete has good compressive strength, but extremely weak tensile strength. What about steel cables?

Basic Concepts Basic Concepts

Beam - a rigid, usually horizontal, structural element

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

Beam

Pier

Load - weight on a structure

The type of bridge used depends on the obstacle. The main feature that controls the bridge type is the size of the obstacle.

Types of BridgesTypes of BridgesBasic Types:

•Truss Bridge•Beam Bridge•Arch Bridge•Suspension Bridge•Floating Bridge

Truss Beam ArchSuspension

Floating

Truss Bridge

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 BridgesTypes of Bridges

Beam BridgeBeam 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.

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 BridgesTypes of Bridges

Beam BridgeBeam Bridge

Arch BridgesArch 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 BridgesTypes of 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 BridgesTypes of Bridges

Arch BridgesArch Bridges

Suspension BridgesSuspension 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 BridgesTypes of 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 BridgesTypes of Bridges

Suspension BridgesSuspension Bridges

•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.

Types of BridgesTypes of Bridges

Floating BridgeFloating Bridge

Floating Bridges

Retractable!

But high maintenance!

How do the following affect your structure?Ground below bridgeLoadsMaterialsShapes

Bridge Bridge Engineering Engineering

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

SummarySummary

Bridge EngineeringBridge Engineering

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: Case Study: Tacoma Narrows Tacoma Narrows FailureFailure

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!?