Bridge Engineering:
Lessons from
Rome to Tacoma
Clear Lake MS Engineering
03-27-2006
History of Bridge Development
100 B.C. Romans
2,104 years ago
700 A.D. Asia
Clapper Bridge
1,304 years
Tree trunk ago
Stone
Roman Arch Bridge
Arch design
evenly distributes
stresses Great Stone Bridge in China
Natural concrete Low bridge
made from mud Shallow arch
and straw 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
Beam
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
•Floating Bridge
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
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!
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!?