1. Bridges and Loads
Modified by Matthew Silbernagel

2. Great Stone Bridge in China
History of Bridge Development
100 B.C. Romans
700 A.D. Asia
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

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

4. Basic Concepts
Span - the distance between two bridge supports, whether they are columns, towers or the wall of a canyon.
Compression
Tension
Forces -
Engineers must design
bridges to withstand
these forces without
buckling or snapping.
The best way to do this
is to dissipate or transfer
the forces.

5. 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

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

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.
The weight of the bridge and any additional load is dissipated by the abutments of the arch through the semicircular design. The force of compression is pushed outward along the curve of the arch and into the abutments.

12. 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.

13. 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.

14. Osaka, Japan – Kansai Airport Sky Gate Bridge
Built in 1994, the longest beam bridge in the world (~2.5 mi) Around an airport, a suspension bridge’s tall supporting beams would interfere with airplanes.
Double-deck for automobile and rail travel
Survived a 6.7 earthquake four months after opening with minor damage only

15. Bridge Loads Three Main Types of Loads:
Dead loads include the weight of the bridge itself plus any other permanent object affixed to the bridge, such as toll booths, highway signs, guardrails, gates or a concrete road surface.
Live loads are temporary loads that act on a bridge, such as cars, trucks, trains or pedestrians.
Environmental loads are temporary loads that act on a bridge and that are due to weather or other environmental influences, such as wind from hurricanes, tornadoes or high gusts; snow; and earthquakes. Rainwater collecting might also be a factor if proper drainage is not provided.

16. Bridge Loads Load Combinations
The Uniform Building Code (UBC), the building code standard adopted by many states, defines five different load combinations. The load combination that produces the highest load or most critical effect is used for design planning.
The five UBC load combinations are:
Dead Load + Live Load + Snow Load
Dead Load + Live Load + Wind Load (or Earthquake Load)
Dead Load + Live Load + Wind Load + (Snow Load ÷ 2)
Dead Load + Live Load + Snow Load + (Wind Load ÷ 2)
Dead Load + Live Load + Snow Load + Earthquake Load

17. Bridge Loads Determining Member Size
After an engineer determines the highest or most critical load combination, the size of the members is calculated. A bridge member is any individual main piece of the bridge structure, such as columns (piers) or beams (girders). The strength of the material must be considered in these calculations.
Force acting on a column
Force acting on a beam