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.
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.
Continuous Span Beam Bridge
• several beam bridges can be linked
together, creating what is called a
continuous span.
Chesapeake Bay Bridge-Tunnel
Moveable Bridge
• has a deck that moves.
• A swing bridge has a deck that rotates around a
center point;
• a drawbridge has a deck that can be raised and
lowered;
• a bascule bridge deck is raised with
counterweights like a drawbridge;
• the deck of a lift bridge is raised vertically like a
massive elevator.
George P. Coleman Bridge – A Swing Beam Bridge
Truss Bridge
• consists of an assembly of triangles. Truss
bridges are commonly made from a series
of straight, steel bars.
Complex Truss Bridge - Cantilever bridge Firth of Forth Bridge, Scotland
Truss Bridge: Forces
• Every bar in this cantilever bridge
experiences either a pushing or pulling
force. The bars rarely bend. This is why
cantilever bridges can span farther than
beam bridges.
Arch Bridge
• has great natural strength. Thousands of
years ago, Romans built arches out of
stone. Today, most arch bridges are made
of steel or concrete.
Ancient Roman aqueduct
Segovia, Spain
Arch Bridge: 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.
Suspension Bridge
• 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.
Golden Gate Bridge San Francisco
Suspension Bridge: 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.
Forces on Suspension Bridge
Suspension: Cable-stayed bridge
• The cable-stayed bridge, like the
suspension bridge, supports the
roadway with massive steel cables, but in
a different way. The cables run directly
from the roadway up to a tower, forming a
unique "A" shape.
Sunshine Skyway Bridge, Florida
The Bridge Challenge
• Welcome to Craggy Rock!
You got here just in time. This growing
community needs four new bridges, and
they're very picky about what they want!
Bridge Challenge
• Test your engineering skills and try to
match the right bridge to the right location.
• You may use each bridge type only once -that's a direct order from the mayor of
Craggy Rock. Good luck!
What Types of Bridge can you
choose from?
• Beam
• Arch
• Truss
• Suspension
• Location 1: Multi-lane bridge for commuters and tourists
• Location 2: Footbridge across a stream
• Location 3: Highway bridge across a busy shipping port
• Location 4: Railroad bridge in a national park
Location 1:
Build a multi-lane bridge for
commuters and tourists
Span: 2,000 feet
Crossing: River
Connects: City and major highway
• Special Notes:
"I want a one-of-a-kind bridge that will span our
beautiful river and welcome visitors from all over
the world to our thriving city. Make sure the new
bridge leaves enough room for sailboats on the
river. Please don't build a bridge that looks like
the Golden Gate Bridge in San Francisco or the
Brooklyn Bridge in New York."
-- Mayor of Craggy Rock
What kind of bridge should you build?
(Select one)
Suspension
Beam
Drawbridge
Arch
Location 1: Drawbridge
• A drawbridge is the best choice for this
location. Drawbridges, like the Tower
Bridge in London, England, can be quite
majestic. They're also practical because
they open to allow the passage of boats
and ships. The mayor is delighted with
your selection!
But there is a
problem!!!
Convenience & Beauty!!
• In one of the rowdiest town meetings ever,
the residents of Craggy Rock spoke out
against the new drawbridge. "It's ugly,"
one angry resident said, "and there's
always traffic on the bridge. They open it
for boats, but what about us? We have to
get to work, too!"
Which bridge will you choose?
• The mayor is begging you to build a new bridge.
The local steelworkers are on strike, so she
advises you to build a bridge with as little steel
as possible.
Cantilever Bridge
Combination: Arch and Beam
Cable-Stayed Bridge
Cable-Stayed Bridge
• Great Choice! It is one of the most popular
bridge types in recent years. The roadway
hangs from cables like a suspension
bridge, forming a unique "A" shape. The
mayor is happy because the bridge is made
mostly of concrete, a relatively cheap
material. The residents are thrilled because
ships can pass beneath the bridge without
causing traffic jams!
Location 2:
Build a bridge for rollerbladers and
bikers
Span: 100 feet
Crossing: Stream
Connects: Two bike paths
• Special Notes:
"We're tired of getting bumped off the road by
angry drivers! We want a simple bridge -- one
that will connect the bike paths in East and West
Craggy Rock over the city stream. We don't
have much money, so we're not asking for
much. We just want to get to work every day
without risking our lives!"
-- Craggy Rock Bike-n-Blade Coalition
What kind of bridge should you build?
(Select one)
Suspension
Beam
Drawbridge
Arch
Location 2: Beam Bridge
• This is an excellent spot for a beam
bridge. Beam bridges typically span up to
250 feet, and they're also the cheapest
and easiest bridges to build. Congrats -you've just made a lot of rollerbladers and
bikers very happy!
But there is a
problem!!!
Flash Flood!
• When it rains, the city stream rises. A
really strong storm could cause flash
floods, turning the stream into a raging
river. Piles of rocks and a wall of gooey
mud could push on the piers of the
concrete beam bridge and cause it to
topple. Luckily, you can fix it before that
happens!
How will you build a new, floodproof bridge?
Build a higher deck?
Build deeper piers?
Use stronger material?
Deeper Piers
• Good Idea!! One of the best ways to
strengthen a bridge in a flood zone is to
set the piers deeper into the ground. The
deeper the piers are lodged into the
ground, the harder it is to knock them over.
The next time floodwaters pound on the
piers of your new bridge, it probably won't
budge!
Location 3:
Build a highway bridge across a
busy shipping port
Span: 5,000 feet
Crossing: Ocean bay
Connects: Island and mainland
• Special Notes:
"Our records indicate that more than 500
ships pass through Craggy Rock Harbor
each day. Please build a bridge that does
not block the flow of water traffic. We do
not want angry sailors or congestion of
any kind in our harbor. Thank you."
-- The United States Coast Guard
What kind of bridge should you build?
(Select one)
Suspension
Beam
Drawbridge
Arch
Location 3: Suspension Bridge
• This is an ideal location for a suspension
bridge. Suspension bridges, like the
Akashi Kaikyo Bridge in Japan, can span
great distances and still leave room for
passing ships and boats. The U.S. Coast
Guard is thrilled with your selection!
But there is
another
problem!!!
Hurricane Winds
• Your new bridge is moving up and down a
lot in the wind! This isn't a big problem
under normal conditions, but what if a
hurricane were to blow through town? It
could cause the bridge to move too much,
or even twist so violently that it would snap
in half!
How can you strengthen the
bridge to resist strong winds?
Stiffen the deck with a
solid steel girder?
Stiffen the deck with
an open truss?
Make the bridge narrower?
Open Truss
• Good Idea! This is a great solution! By adding
an open truss beneath the deck of the
bridge, you've designed a structure that is
less likely to move up and down a lot on a
gusty day, because the wind will blow right
through it! Since the Tacoma Narrows Bridge
disaster in 1940, many major suspension
bridges have been built with a stiffening
truss.
Tacoma Narrows Bridge disaster,
1940
• On the morning of November 7, 1940, the Tacoma
Narrows Bridge twisted violently in 42-mile-per-hour
winds and collapsed into the cold waters of the Puget
Sound. The disaster -- which luckily took no human lives
-- shook the engineering community and forever
changed the way bridges were built around the world.
• Engineer Leon Moisseiff had designed the ultimate in
slender bridges and strengthened his narrow bridge with
a solid steel girder beneath the roadway. But soon after
it opened, the Tacoma Narrows started behaving
strangely. Wind caused the bridge to sway back and
forth, and it also sent rippling waves along the deck. The
Tacoma Narrows tore itself apart only four months later.
Truss Support is Important!
• Years later, engineers found that the solid
girders actually blocked the wind and caused the
slender bridge to twist. The twisting bridge
fanned the steady wind into a swirling motion,
which caused the bridge to twist even more -and eventually snap in two. The Tacoma
Narrows Bridge was replaced in 1950 by a new
bridge stiffened with a truss. Rather than
blocking the wind, the open truss allowed the
wind to blow through the new bridge.
Tacoma Narrows Bridge
disaster,1940
• Thought you’d like to see some pictures of
what happened to this narrow suspension
bridge.
Location 4:
Build a railroad bridge in a national
park
Span: 500 feet
Crossing: Deep river gorge
Connects: Two rocky bluffs
• Special Notes:
"We do not want the new bridge to detract
from the natural beauty of Craggy Rock
National Park. Please build a simple and
elegant bridge -- one that does not have
towers or piers."
-- The National Park Service
What kind of bridge should you build?
(Select one)
Suspension
Beam
Drawbridge
Arch
Location 4: Arch Bridge
• This is the perfect spot for an arch bridge.
The rocky bluffs will support the
abutments, and no towers or piers are
needed because the arch shape can
support itself.
But, of course,
there is
another
problem!!!
Concrete Has Weathered!
• Twenty years have passed. You're an older,
wiser, and wealthier engineer. The phone
rings. It's the National Park Service. Chunks
of concrete are falling from the arch bridge
into the gorge. Twenty years of cold weather
have chewed away at the bridge. It's time to
replace the cracking concrete with a new
material.
Which new material will you
choose?
Steel?
Wood?
Cast Iron?
New Material: Steel
• In fact, the world's longest spanning arch
bridge, the New River Gorge Bridge in West
Virginia, is made entirely of steel. Steel is
strong in compression, the squeezing force
that all arch bridges experience. Steel can
also be arranged into a lightweight truss
pattern, like the Garabit Bridge in France.
You Did It!!!!!!
Well Done!!