Fall Asleep While Learning About Bridges

Welcome to the I Can't Sleep podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster.

Today's episode is from a Wikipedia article titled,

Bridge.

A bridge is a structure built to span a physical obstacle,

Such as a body of water,

Valley,

Road,

Or railway,

Without blocking the path underneath.

It is constructed for the purpose of providing passage over the obstacle,

Which is usually something that is otherwise difficult or impossible to cross.

There are many different designs of bridges,

Each serving a particular purpose and applicable to different situations.

Designs of bridges vary depending on factors such as the function of the bridge,

The nature of the terrain where the bridge is constructed and anchored,

The material used to make it,

And the funds available to build it.

The earliest bridges were likely made with fallen trees and stepping stones.

The Neolithic people built boardwalk bridges across marshland.

The Arcadico Bridge,

Dating from the 13th century BC in the Peloponnese,

Is one of the oldest arch bridges in existence and used.

The Oxford English Dictionary traces the origin of the word bridge to an old English word b-r-y-c-g of the same meaning.

The Oxford English Dictionary also notes that there is some suggestion that the word can be traced directly back to Proto-Indo-European beru.

However,

They also note that this poses semantic problems.

The origin of the word for the card game of the same name is unknown.

The simplest and earliest types of bridges were stepping stones.

Neolithic people also built a form of boardwalk across marshes.

Examples of such bridges include the Sweet Track and the Post Track in England,

Approximately 6,

000 years old.

Ancient people would also use log bridges consisting of logs that fell naturally or were intentionally felled or placed across streams.

Some of the first human-made bridges with significant span were probably intentionally felled trees.

Among the oldest timber bridges is the Holtzbrücke-Rapperswil-Herden bridge.

It crossed Upper Lake Zürich in Switzerland.

Prehistoric timber pilings discovered to the west of the Sedum Causeway date back to 1523 BC.

The first wooden footbridge there led across Lake Zürich.

It was reconstructed several times through the late 2nd century AD,

When the Roman Empire built a 6-meter-wide 20-foot wooden bridge to carry transport across the lake.

Between 1358 and 1360,

Rudolf IV,

Duke of Austria,

Built a new wooden bridge across the lake that was used until 1878.

It was approximately 1,

450 meters,

4,

760 feet long,

And 4 meters,

Or 13 feet wide.

On April 6,

2001,

A reconstruction of the original wooden footbridge was opened.

It is also the longest wooden bridge in Switzerland.

The Archideko Bridge is one of four Mycenaean corbel arch bridges,

Part of a former network of roads designed to accommodate chariots,

Between the fort of Tiryns and town of Epidaurus in the Peloponnese in southern Greece.

Dating to the Greek Bronze Age,

13th century BC,

It is one of the oldest arch bridges still in existence and use.

Several intact arched stone bridges from the Hellenistic era can be found in the Peloponnese.

The greatest bridge builders of antiquity were the ancient Romans.

The Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs.

Some stand today.

An example is the Alcântara Bridge,

Built over the river Tagus in Spain.

The Romans also used cement,

Which reduced the variation of strength found in natural stone.

One type of cement,

Called pozzolana,

Consisted of water,

Lime,

Sand,

And volcanic rock.

Brick and mortar bridges were built after the Roman era,

As the technology for cement was lost and later rediscovered.

In India,

The Arthashastra treatise by Kautilya mentions the construction of dams and bridges.

A Mauryan bridge near Gurnar was surveyed by James Princip.

The bridge was swept away during a flood and later repaired by Paspa Gupta,

The chief architect of Emperor Chandragupta I.

The use of stronger bridges using plated bamboo and iron chain was visible in India by about the 4th century.

A number of bridges,

Both for military and commercial purposes,

Were constructed by the Mughal administration in India.

Although large bridges of wooden construction existed in China at the time of the Warring States period,

The oldest surviving stone bridge in China is the Sao Tsao Bridge,

Built from 595 to 605 AD during the Sui dynasty.

This bridge is also historically significant as it is the world's oldest open spandrel stone segment arch bridge.

European segmental arch bridges date back to at least the Alcanader Bridge,

Approximately 2nd century AD,

While the enormous Roman-era Trajan's Bridge,

105 AD,

Featured open spandrel segmental arches in wooden construction.

Rope bridges,

A simple type of suspension bridge,

Were used by the Inca civilization in the Andes mountains of South America,

Just prior to European colonization in the 16th century.

The Ashanti built bridges over streams and rivers.

They were constructed by pounding four large forked tree trunks into the stream bed,

Placing beams along these forked pillars,

Then positioning cross beams that were finally covered with four to six inches of dirt.

During the 18th century,

There were many innovations in the design of timber bridges by Hans-Ulrich Grubenmann,

Johannes Grubenmann,

As well as others.

The first book on bridge engineering was written by Hubert Gauthier in 1716.

A major breakthrough in bridge technology came with the erection of the Iron Bridge in Shropshire,

England in 1779.

It used cast iron for the first time as arches to cross the River Severn.

With the Industrial Revolution in the 19th century,

Truss systems of wrought iron were developed for larger bridges,

But iron does not have the tensile strength to support large loads.

With the advent of steel,

Which has a high tensile strength,

Much larger bridges were built,

Many using the ideas of Gustav Eiffel.

In Canada and the United States,

Numerous timber covered bridges were built in the late 1700s to the late 1800s,

Reminiscent of earlier designs in Germany and Switzerland.

Some covered bridges were also built in Asia.

In later years,

Some were partly made of stone or metal,

But the trusses were usually still made of wood.

In the United States,

There were three styles of trusses,

The Queen Post,

The Burr Arch,

And the Town Lattice.

Hundreds of these structures still stand in North America.

They were brought to the attention of the general public in the 1990s by the novel,

Movie,

And play,

The Bridges of Madison County.

In 1927,

Welding pioneer Stefan Brela designed the first welded road bridge in the world,

The Morzaitz Bridge,

Which was later built across the River Sludwia at Morzaitz near Lowicz,

Poland,

In 1929.

In 1995,

The American Welding Society presented the Historic Welded Structure Award for the bridge to Poland.

Bridges can be categorized in several different ways.

Common categories include the type of structural elements used,

By what they carry,

Whether they are fixed or movable,

And by the materials used.

Bridges may be classified by how the actions of tension,

Compression,

Bending,

Torsion,

And shear are distributed through their structure.

Most bridges will employ all of these to some degree,

But only a few will predominate.

The separation of forces and moments may be quite clear.

In a suspension or cable state bridge,

The elements and tension are distinct in shape and placement.

In other cases,

The forces may be distributed among a large number of members,

As in a truss.

Beam bridge.

Beam bridges are horizontal beams supported at each end by substructure units,

And can be either simply supported when the beams only connect across a single span,

Or continuous when the beams are connected across two or more spans.

When there are multiple spans,

The intermediate supports are known as piers.

The earliest beam bridges were simple logs that sat across streams and similar simple structures.

In modern times,

Beam bridges can range from small wooden beams to large steel boxes.

The vertical force on the bridge becomes a shear and flexural load on the beam,

Which is transferred down its length to the substructures on either side.

They are typically made of steel,

Concrete,

Or brick.

They are typically made of steel,

Concrete,

Or wood.

Girder bridges and plate girder bridges,

Usually made from steel,

Are types of beam bridges.

Box girder bridges,

Made from steel,

Concrete,

Or both,

Are also beam bridges.

Beam bridge spans rarely exceed 250 feet long,

As the flexural stresses increase proportionally to the square of the length,

And deflection increases proportionally to the force power of the length.

However,

The main span of the Rio Niteroi Bridge,

A box girder bridge,

Is 300 meters,

Or 980 feet.

The world's longest beam bridge is Lake Pontchartrain Causeway on southern Louisiana in the United States,

At 23.

83 miles,

With individual spans of 56 feet.

Beam bridges are the simplest and oldest type of bridge in use today,

And are a popular type.

Truss Bridge A truss bridge is a bridge whose load-bearing superstructure is composed of a truss.

The truss is a structure of connected elements forming triangular units.

The connected elements,

Typically straight,

May be stressed from tension,

Compression,

Or sometimes both in response to dynamic loads.

Truss bridges are one of the oldest types of modern bridges.

The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by 19th and early 20th century engineers.

A truss bridge is economical to construct owing to its efficient use of materials.

Cantilever Bridge Cantilever bridges are built using cantilevers,

Horizontal beams supported on only one end.

Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses.

Cantilever bridges are constructed using much the same materials and techniques as beam bridges.

The difference comes in the action of the forces through the bridge.

Some cantilever bridges also have a smaller beam connecting the two cantilevers for extra strength.

The largest cantilever bridge is the 549-meter Quebec Bridge in Quebec,

Canada.

Arch Bridges Arch bridges have abutments at each end.

The weight of the bridge is thrust into the abutments at either side.

The earliest known arch bridges were built by the Greeks and include the Archideco Bridge.

With a span of 220 meters,

The Sulcan Bridge over the Soca River at Sulcan in Slovenia is the second largest stone bridge in the world and the longest railroad stone bridge.

It was completed in 1905.

Its arch,

Which was constructed from over 5,

000 tons,

4,

900 long tons,

5,

500 short tons of stone blocks in just 18 days,

Is the second largest stone arch in the world,

Surpassed only by the Friedrichsbrücke in Plauen and the largest railroad stone arch.

The arch of the Friedrichsbrücke,

Which was built in the same year,

Has the span of 90 meters and crosses the valley of the Syrabach River.

The difference between the two is that the Sulcan Bridge was built from stone blocks,

Whereas the Friedrichsbrücke was built from a mixture of crushed stone and cement mortar.

The world's largest arch bridge is the Cheochenmen Bridge over the Yangtze River with a length of 1,

741 meters and a span of 552 meters.

The bridge was opened April 29,

2009 in Chongqing,

China.

Tide Arch Bridge Tide arch bridges have an arch-shaped superstructure but differ from conventional arch bridges.

Instead of transferring the weight of the bridge and traffic loads into thrust forces into the abutments,

The ends of the arches are restrained by tension in the bottom chord of the structure.

They are also called bowstring arches.

Suspension Bridge Suspension bridges are suspended from cables.

The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo.

In modern bridges,

The cables hang from towers that are attached to caissons or cofferdams.

The caissons or cofferdams are implanted deep into the bed of the lake,

River,

Or sea.

Subtypes include the simple suspension bridge,

The stressed ribbon bridge,

The underspanned suspension bridge,

The suspended deck suspension bridge,

And the self-anchored suspension bridge.

There is also what is sometimes called a semi-suspension bridge,

Of which the ferry bridge in Burdon-upon-Trent is the only one of its kind in Europe.

The longest suspension bridge in the world is 4,

608 meters,

1950 Senekal Bridge in Turkey.

Cable-stayed Bridge Cable-stayed bridges like suspension bridges are held up by cables.

However,

In a cable-stayed bridge,

Less cable is required,

And the towers holding the cables are proportionately higher.

The first known cable-stayed bridge was designed in 1784 by C.

T.

Or C.

J.

Lusher.

The longest cable-stayed bridge since 2012 is the 1,

104-meter Rusky Bridge in Vladivostok,

Russia.

Forward Bridge A forward bridge is designed to allow the flow of water over the way.

Some engineers subdivide beam bridges into slab,

Beam and slab,

And box girder on the basis of their cross-section.

A slab can be solid or voided,

But this is no longer favored for inspectability reasons,

While beam and slab consists of concrete or steel girders connected by a concrete slab.

A box girder cross-section consists of a single-cell or multicellular box.

In recent years,

Integral bridge construction has also become popular.

Most bridges are fixed bridges,

Meaning they have no moving parts and stay in one place until they fail or are demolished.

Temporary bridges,

Such as bailey bridges,

Are designed to be assembled,

Taken apart,

Transported to a different site,

And reused.

They are important in military engineering and are also used to carry traffic while an old bridge is being rebuilt.

Movable bridges are designed to move out of the way of boats or other kinds of traffic,

Which would otherwise be too tall to fit.

These are generally electrically powered.

A tank bridge transporter,

TBT,

Has the same cross-country performance as a tank,

Even when fully loaded.

It can deploy,

Drop off,

And load bridges independently,

But it cannot recover them.

Double-decked or double-decker bridges have two levels,

Such as the George Washington Bridge,

Connecting New York to Bergen County,

New Jersey,

U.

S.

As the world's busiest bridge,

Carrying 102 million vehicles annually.

Tress work between the roadway levels provided stiffness to the roadways and reduced movement of the upper level when the lower level was installed three decades after the upper level.

The Tsing Ma Bridge and the Kap Shui Mun Bridge in Hong Kong have six lanes on their upper decks,

And on their lower decks there are two lanes and a pair of tracks for MTR metro trains.

Some double-decked bridges only use one level for street traffic.

The Washington Avenue Bridge in Minneapolis reserves its lower level for automobile and light rail traffic and its upper level for pedestrian and bicycle traffic,

Predominantly students at the University of Minnesota.

Likewise,

In Toronto,

The Prince Edward Viaduct has five lanes of motor traffic,

Bicycle lanes,

And sidewalks on its upper deck,

And a pair of tracks for the Bloor-Danforth subway line on its lower deck.

The western span of the San Francisco-Oakland Bay Bridge also has two levels.

Robert Stevenson's high-level bridge across the River Tyne in Newcastle-upon-Tyne completed in 1849 is an early example of a double-decked bridge.

The upper level carries a railway and the lower level is used for road traffic.

Other examples include Britannia Bridge over the Menai Strait and Craighaven Bridge in Derry,

Northern Ireland.

The Orison Bridge between Copenhagen and Malmo consists of a four-lane highway on the upper level and a pair of railway tracks at the lower level.

Tower Bridge in London is a different example of a double-decked bridge,

With a central section consisting of a low-level bascule span and a high-level footbridge.

A viaduct is made up of multiple bridges connected into one longer structure.

The longest and some of the highest bridges are viaducts,

Such as the Lake Pontchartrain Causeway and Millau Viaduct.

A multi-way bridge has three or more separate spans which meet near the centre of the bridge.

Multi-way bridges with only three spans appear as a T or Y when viewed from above.

Multi-way bridges are extremely rare.

The Tridge,

Margaret Bridge and Zanesville Y Bridge are examples.

A bridge can be categorised by what it is designed to carry,

Such as trains.

Pedestrian or road traffic,

Road bridge.

A pipeline,

Pipe bridge.

Or waterway for water transport or barge traffic.

An aqueduct is a bridge that carries water,

Resembling a viaduct,

Which is a bridge that connects points of equal height.

A road-rail bridge carries both road and rail traffic.

Overway is a term for a bridge that separates incompatible intersecting traffic,

Especially road and rail.

A bridge can carry overhead power lines,

As does the Sturtström Bridge.

Some bridges accommodate other purposes,

Such as the Tower of Navimost Bridge in Bratislava,

Which features a restaurant or a bridge restaurant,

Which is a bridge built to serve as a restaurant.

Other suspension bridge towers carry transmission antennas.

Conservationists use wildlife overpasses to reduce habitat fragmentation and animal-vehicle collisions.

The first animal bridges sprung up in France in the 1950s,

And these types of bridges are now used worldwide to protect both large and small wildlife.

Bridges are subject to unplanned uses as well.

The areas underneath some bridges have become makeshift shelters and homes to homeless people,

And the under-timbers of bridges all around the world are spots of prevalent graffiti.

The materials used to build the structure are also used to categorize bridges.

Until the end of the 18th century,

Bridges were made out of timber,

Stone,

And masonry.

Modern bridges are currently built in concrete,

Steel,

Fiber-reinforced polymers,

FRP,

Stainless steel,

Or combinations of those materials.

Living bridges have been constructed of live plants,

Such as Ficus elastica tree roots in India and wisteria vines in Japan.

For small-foot bridges,

The cantilevers may be simple beams.

However,

Large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel,

Or box girders built from pre-stressed concrete.

The cables are usually made of steel cables galvanized with zinc,

And are used to support the bridge.

The cables are usually made of steel cables galvanized with zinc along with most of the bridge,

But some bridges are still made with steel-reinforced concrete.

Stone,

Brick,

And other such materials that are strong in compression and somewhat so in shear.

Beam bridges can use pre-stressed concrete,

An inexpensive building material,

Which is then embedded with rebar.

The resulting bridge can resist both compression and tension forces.

Truss.

The triangular pieces of truss bridges are manufactured from straight and steel bars,

According to the truss bridge designs.

Unlike buildings whose design is led by architects,

Bridges are usually designed by engineers.

This follows from the importance of the engineering requirements,

Namely spanning the obstacle and having the durability to survive,

The minimal maintenance,

In an aggressive outdoor environment.

Bridges are first analyzed.

The bending moment and shear force distributions are calculated due to the applied loads.

For this,

The finite element method is the most popular.

The analysis can be one,

Two,

Or three-dimensional.

For the majority of bridges,

A two-dimensional plate model,

Often with stiffening beams,

Is sufficient or an upstand finite element model.

On completion of the analysis,

The bridge is designed to resist the applied bending moments and shear forces.

Section sizes are selected with sufficient capacity to resist the stresses.

Many bridges are made of pre-stressed concrete,

Which has good durability properties,

Either by pre-tensioning of beams prior to installation or post-tensioning on-site.

In most countries,

Bridges,

Like other structures,

Are designed according to the load and resistance factor design,

LRFD principles.

In simple terms,

This means that the load is factored up by a factor greater than unity.

The effect of the factored load,

Stress,

Bending moment,

Should be less than the factored resistance to that effect.

Both of these factors allow for uncertainty and are greater when the uncertainty is greater.

Most bridges are utilitarian in appearance,

But in some cases the appearance of the bridge can have great importance.

Often,

This is the case with a large bridge that serves as an entrance to a city or crosses over a main harbor entrance.

These are sometimes known as signature bridges.

Designers of bridges in parks and along parkways often place more important on aesthetics as well.

Examples include the stone-faced bridges along the Taconic State Parkway in New York.

Bridges are typically more aesthetically pleasing if they are simple in shape.

The deck is thinner in proportion to its span,

The lines of the structure are continuous,

And the shapes of the structural elements reflect the forces acting on them.

To create a beautiful image,

Some bridges are built much taller than necessary.

This type often found in East Asian-style gardens is called a moon bridge,

Evoking a rising full moon.

Other garden bridges may cross only a dry bed of stream-washed pebbles,

Intended only to convey an impression of a stream.

Often in palaces,

A bridge will be built over an artificial waterway as symbolic of a passage to an important place or state of mind.

A set of five bridges cross a sinuous waterway in an important courtyard of the Forbidden City in Beijing,

China.

The central bridge was reserved exclusively for the use of the emperor and empress with their attendance.

The estimated life of bridges varies between 25 and 80 years depending on location and material.

Bridges may age 100 years with proper maintenance and rehabilitation.

Bridge maintenance consists of a combination of structural health,

Monitoring,

And testing.

This is regulated in country-specific engineer standards and includes an ongoing monitoring every three to six months,

A simple test or inspection every two to three years,

And a major inspection every six to ten years.

In Europe,

The cost of maintenance is considerable and is higher in some countries than spending on new bridges.

A lifetime of welded steel bridges can be significantly extended by after-treatment of the weld transitions.

This results in a potential high benefit using existing bridges far beyond the planned lifetime.

While the response of a bridge to the applied loading is well understood,

The applied traffic loading itself is still the subject of research.

This is a statistical problem as loading is highly variable,

Particularly for road bridges.

Load effects in bridges,

Stresses,

Bending moments,

Are designed for using the principles of load and resistance factor design.

Before factoring to allow for uncertainty,

The load effect is generally considered to be the maximum characteristic value in a specified return period.

Notably,

In Europe,

It is the maximum value expected in 1,

000 years.

Bridge standards generally include a load model deemed to represent the characteristic maximum load to be expected in the return period.

In the past,

These load models were agreed by standard drafting committees of experts,

But today this situation is changing.

It is now possible to measure the components of bridge traffic load to weigh trucks using weigh and motion,

WIM,

Technologies.

With extensive WIM databases,

It is possible to calculate the maximum expected load effect in the specified return period.

This is an active area of research addressing issues of opposing direction lanes,

Side-by-side,

Same direction lanes,

Traffic growth,

Permit,

Non-permit vehicles,

And long-span bridges.

Rather than repeat this complex process every time a bridge is to be designed,

Standards authorities specify simplified notional load models,

Notably HL93,

Intended to give the same load effects as the characteristic maximum values.

The Eurocode is an example of a standard for bridge traffic loading that was developed in this way.