Transcript Truss Bridges

Characteristics of
Three Bridge Types
Important Terms:
Squeezing (Compression)
Compression is a force that squeezes a
material together. When a material is in
compression, it tends to become shorter.
Stretching (Tension)
Tension is a force that stretches a
material apart. When a material is in
tension, it tends to become longer.
See a demonstration of these forces!
1. Truss Bridges
• Consist of vertical, lower horizontal &
diagonal members
• Typically composed of triangular
units that are connected at joints
Characteristics of
Truss Bridges
• The “truss” is usually a triangular
unit.
• It creates a very rigid structure & one
that transfers the load from a single
point to a much wider area
• Truss bridges are usually made from
a series of straight, steel bars.
Forces Acting on
Truss Bridges
The larger the
height is compared
to the span, the
greater its strength
Every bar in this bridge experiences
either a pushing or pulling force.
The bars rarely bend.
Pros & Cons of
Truss Bridges
• Economical to construct
• Spans longer distances than
beam bridges
• Ability to support weight relies on
the strength of the joints
2. Beam Bridges
• Also known as a “girder”
bridge
•Simplest type of bridge
Characteristics of
Beam Bridges
• Made up 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.
• These bridges rarely span more than
250 feet.
Under load, the beam's top surface is
pushed down or compressed while the
bottom edge is stretched or placed
under tension
When something pushes down on
the beam the beam bends. Its top
edge is pushed together, and its
bottom edge is pulled apart.
Pros & Cons of
Beam Bridges
• Needs to resist twisting and
bending under load
• Simple design: Less expensive and
requires little maintenance
• Only suitable for short spans (about
thirty to six hundred feet)
3. Suspension Bridges
• Cables are hooked on vertical
suspenders that support the
load
Characteristics of
Suspension Bridges
• These bridges can span 2,000 to 7,000
feet -- much farther than any other
type of bridge!
• Most suspension bridges have a truss
system beneath the roadway to resist
bending and twisting.
“Cable-stayed” suspension
• 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.
• 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.
Early suspension bridges
did not account for wind
Pros & Cons of
Suspension Bridges
• Allows for longer spans than other bridge types
• Requires stable ground for the “anchorages” at
either end
• May withstand earthquakes better than other
bridge types
• Aesthetically more beautiful than other two
types of bridge
• Lots of bridge photos on the quiz link