Bridges_+notes

The word bridge means a link or connection between two objects - usually places either side of an obstacle, such as a river, chasm, or estuary.

Think of the bridges you remember:  Across gulfs and rivers, between peoples and countries, bridges break down separation and foster connectedness.
 * What are they made of?
 * What are they connecting?
 * How is the span between their piers or towers?


 * As far back as we can see in history, human beings have used new technology to solve problems and ease their physical burdens. The distinctiveness of humans as a species is defined by their use of tools, and bridges are technological tools that aim to solve the problem of crossing an obstacle in such a way as to cut down the effort and time needed to do so. The better a bridge is, the less attention the user will need to pay it.

What are the benefits of bridges?
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 This means that, in economic terms, the cost of travel and trade falls and the financial benefits of increased social cohesion and sharing resources rise. Other longer-term payoffs from easier travel, which is crucially dependent on good bridges, come as a result of increased opportunities to share ideas – intellectual, political and religious. Today bridges allow easy travel across major rivers and estuaries, over the new obstacles of motorways and railway lines, and between neighbouring islands. International trade and travel depend on shipping and air routes, but efficient distribution networks depend on bridges. ||  || =BATS: The Basics of Bridge Design = If you're going to build a bridge, you'll need some help from **BATS** -- not the furry, winged mammals that so often live beneath bridges, but the key structural components of bridge construction:**beams**, **arches**, **trusses** and**suspensions**. Various combinations of these four technologies allow for numerous [|bridge designs], ranging from simple **beam bridges**, **arch bridges**, **truss bridges**and **suspension bridges** to more complex variations, such as the pictured**side-spar cable-stayed bridge**. For all its 21st century complexity, the side-spar design is based on suspension principles first used some two centuries earlier. The key differences between these four bridge types comes down to the lengths they can cross in a single**span**, which is the distance between two **bridge supports**, the physical braces that connect the bridge to the surface below. Bridge supports may take the form of columns, towers or even the walls of a [|canyon]. Modern beam bridges, for instance, are likely to span up to 200 feet (60 meters), while modern arch bridges can safely cross 800-1,000 feet (240-300 meters). Suspension bridges are capable of extending from 2,000-7,000 feet (610-2,134 meters). Regardless of the structure, every bridge must stand strong under the two important forces.
 * || || Some of the benefits of bridges are obvious: supplies of food and traded goods can get across an obstacle or through difficult terrain in a shorter time

=Tension and Compression: Two Forces Every Bridge Knows Well = What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension. **Tension**: What happens to a rope during a game of tug-of-war? Correct, it undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress. **Compression**: What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you shorten its length. Compressional stress, therefore, is the opposite of tensional stress. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">**Buckling** occurs when compression overcomes an object's ability to endure that force. **Snapping** is what happens when tension surpasses an object's ability to handle the lengthening force. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it. It's the difference in, say, eating one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">In transferring force, a design moves stress from an area of weakness to an area of strength. As we'll dig into on the upcoming pages, different bridges prefer to handle these stressors in different ways.

=<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif; font-size: 16px;">The Beam Bridge = <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;"> [|Bridge] building doesn't get any simpler than this. In order to build a beam bridge (also known as a **girder bridge**), all you need is a rigid horizontal structure (a beam) and two supports, one at each end, to rest it on. These components directly support the downward weight of the bridge and any traffic traveling over it. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">However, in supporting weight, the bream bridge endures both compressional and tensional stress. In order to understand these forces, let's use a simple model. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">If you were to take a two-by-four and lay it across two empty milk crates, you'd have yourself a crude beam bridge. Now if you were to place a heavy weight in the middle of it, the two-by-four would bend. The top side would bend in under the force of compression, and the bottom side would bend out under the force of tension. Add enough weight and the two-by-four would eventually break. The top side would buckle and the bottom side would snap. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Many beam bridges use concrete or [|steel] beams to handle the load. The size of the beam, and in particular the height of the beam, controls the distance that the beam can span. By increasing the height of the beam, the beam has more material to dissipate the tension. To create very tall beams, bridge designers add supporting **latticework**, or a **truss**, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force spreads through the truss. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Yet even with a truss, a beam bridge is only good for a limited distance. To reach across a greater length, you have to build a bigger truss until you eventually reach the point at which the truss can't support the bridge's own weight.

=<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif; font-size: 16px;">Truss Bridges: Beam Bridges With Braces = <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Travel around the world, and you'll encounter dozens of variations on your standard beam bridge. The key differences, however, all come down to the design, location and composition of the truss. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">During the early Industrial Revolution, beam bridge construction in the United States was rapidly developing. Engineers gave many different truss designs a whirl in an attempt to perfect it. Their efforts weren't for naught. Wooden bridges were soon replaced by [|iron] models or wood-and-iron combinations. <span style="background-color: #ffffff; color: #333333; display: block; font-family: arial,helvetica,clean,sans-serif;"> <span style="background-color: #ffffff; color: #333333; display: block; font-family: arial,helvetica,clean,sans-serif;"> <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">All these different truss patterns also factored into how beam bridges were being built. Some takes featured a **through truss** above the bridge, while others boasted a **deck truss** beneath the bridge. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">A single beam spanning any distance undergoes compression and tension. The very top of the beam gets the most compression, and the very bottom of the beam experiences the most tension. The middle of the beam experiences very little compression or tension. This is why we have I-beams, which provide more material on the tops and bottoms of beams to better handle the forces of compression and tension. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">And there's another reason why a truss is more rigid than a single beam: A truss has the ability to dissipate a load through the truss work. The design of a truss, which is usually a variant of a triangle, creates both a very rigid structure and one that transfers the load from a single point to a considerably wider area. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">While truss bridges are largely a product of the [|Industrial Revolution], our next example, the arch, dates back much further in time.

=<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif; font-size: 16px;">The Arch Bridge = <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">After more than 2,000 years of architectural use, the arch continues to feature prominently in bridge designs and with good reason: Its semicircular structure elegantly distributes compression through its entire form and diverts weight onto its two **abutments**, the components of the bridge that directly take on pressure. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Tensional force in arch [|bridges], on the other hand is virtually negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">But as with beams and trusses, even the mighty arch can't outrun physics forever. The greater the degree of curvature (the larger the semicircle of the arch), the greater the effects of tension on the underside of the bridge. Build a big enough arch, and tension will eventually overtake the support structure's natural strength. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">While there's a fair amount of cosmetic variety in arch bridge construction, the basic structure doesn't change. There are, for example, [|Roman], Baroque and Renaissance arches, all of which are architecturally different but structurally the same. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">It is the arch itself that gives its namesake bridge its strength. In fact, an arch made of stone doesn't even need mortar. The ancient Romans built arch bridges and aqueducts that are still standing today. The tricky part, however is building the arch, as the two converging parts of the structure have no structural integrity until they meet in the middle. As such, additional scaffolding or support systems are typically needed. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Modern materials such as steel and prestressed concrete allow us to build far larger arches than the ancient Romans did. Modern arches typically span between 200 and 800 feet (61 and 244 meters), but West Virginia's New River Gorge Bridge measures an impressive 1,700 feet (518 meters)

<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Tructural types of bridges:
 * [[image:http://en.structurae.de/images/espaceur.gif width="9" height="4" align="middle"]] ||
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 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1003"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=6237"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1002"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1085"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1006"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1090"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=3036"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1051"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1049"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1046"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1001"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1086"]] ||
 * [[image:http://en.structurae.de/images/reddiamond.gif width="9" height="9" align="middle" caption="*" link="http://en.structurae.de/structures/stype/index.cfm?id=1004"]]

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The effect of wind on bridges

Severe winds of many types with associated water penetration and water impacts are responsible for the largest component of insured losses in the U.S. Yet the insured losses are only a part of the total physical losses. Hurricane losses can be large and examples are Hurricane Andrew that generated more than $36 billion in losses and Hurricane Katrina that generated more than $100 billion in losses. Although individual Tornado events may not reach these totals, there are numerous events each year that result in a cumulative loss of comparable magnitude. Many thousands of homes and businesses are destroyed or rendered unusable each year. During the period from 2003 to early 2008 there have been 10 damaging hurricanes that have struck the U.S. and there have been more than 1000 tornadoes each year, many of them causing damage. In addition to physical and economic losses many lives have been lost. The reduction of losses and impacts is complex due to the existence of a large inventory of structures and infrastructure that were constructed before we understood how to construct them to provide acceptable resistance with a large reduction of potential losses. For new construction the research carried out on wind loadings and resistance some years ago has provided a basis of reducing losses if applied. However, the level of research on wind engineering has fallen to a level and many important areas of knowledge building to support loss reduction are not being pursued today. This is particularly true with respect to existing construction. The level of losses being sustained is not necessary and could be substantially reduced through increased support of wind engineering. Unfortunately the long-term recovery and economic impacts from wind storms are not uniform and those who least can afford to be impacted are also those who are at the lower end of the recovery cycle. Average yearly economic and life losses in the United States due to wind has far exceeded that from Earthquakes and only floods have the demonstrated potential to cause greater yearly losses.

Types of bridges

The main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever bridges and suspension bridges. You will have noticed that this list does not include truss bridges. These are usually arches, beams or girders, or cantilevers, or they may be parts of bridges, for example the suspended span of a cantilever bridge, or the deck of a cable-stayed bridge or a suspension bridge. The phrase "truss bridge", however, is sometimes reserved for those which act primarily as beams, while the others are discussed under the heading of the bridges of which they form a part. You could say that a truss, like a box-girder or a pre-stressed span, is more a type of construction than a type of structure.

Arch Bridges

The essence of an arch is that ideally there should be no tendency for it to bend, except under live loads. It should be purely in compression, and for that reason it can be made of materials such as, masonry, cast iron and concrete, that perform poorly in tension. Of course, in a trussed arch there will be some tension members, but the main ones are always in compression. These main members are always much thicker than than the cross-members.

On the other hand, in a deck-stiffened arch, the deck is much thicker than the arch, because the deck is resisting any tendency to bend or buckle, leaving the arch chord to resist pure compression. In such a bridge, the deck can be very much thinner than a simple beam across the gap, because its weight is supported by the arch, and the arch can be very much thinner than a simple arch, because it is stiffened by the beam.

These two types of arch are shown below.



In any structure, except a simple pier or column, it is impossible to have compression without tension. In the case of an arch, the tension is in the ground, which is therefore a member that costs nothing. If we take this argument further, it can prove that arch spans can be made longer than beam spans. Although the ground under an arch is in tension, the ground just outside the abutments is compressed by the thrust of the arch. Between the regions of tension and compression, the ground is subject to complicated mixtures of tension, compression and shear stresses.

Although an arch is generally not under stress to make it bend, it has curvature designed in, because it is in a gravitational field. The amount of curvature at any point is designed so that the whole thing is perfectly balanced, neither tending to increase the curvature or to decrease it. The ideal shape is called the [|funicular], the exact shape of which depends on the weight distribution, so the funicular is not necessarily a simple mathematical curve such as a circle or a parabola. The arch and the suspension bridge are generally closer to the funicular, or natural curve, than any other type. In this they imitate the path of projectiles, which also follow curved natural paths, and even light, which curves in a gravitational field. Nevertheless, although the cause, gravity, is the same for both arches and projectiles, the detailed reasons for the curvature are different. We must always beware of making false analogies, though similarities have on occasion been valuable in science and mathematics in finding solutions to problems.

Why must an arch be curved? If we consider any section of an arch, the forces comprise two distinct kinds - those pulling down (the weight of the section pulling down, and the load, if any) - and the forces from the sections on either side. In order to balance the downward forces, the forces from the side must not be exactly in line: the angle between them, repeated throughout the arch, is the reason for the curvature. A beam, because it is straight, cannot work like this - it has to balance the downward forces by means of shear stress. Cable-Stayed Bridges The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the centre. A big difference between cantilever bridges and cable-stayed bridges is that the former usually have a suspended span, and the latter do not. A cable stayed-bridge lacks the great rigidity of a trussed cantilever, and the continuous beam compensates for this to some extent. Indeed, while a long cable-stayed span is under construction, there may be great concern about possible oscillations, until the cantilevers are joined. For the Pont de Normandie, there was even thought of using active correctors if things threatened to get out of hand. In fact, the construction went smoothly. Cantilever Bridges In the arch and beam we saw that the bridges were supported at two places - the ends. In fact, if you want to hold any object in position in two dimensions, you always need two points of attachment. In three dimensions you need three points. Two-legged animals need feet in order to achieve stability in the three dimensions. This statement is not strictly true, because animals can balance using their muscles to change their position in response to perceived movement. Nevertheless, feet make the process easier. <span style="background-color: #ffcccc; font-family: Verdana; font-size: medium; text-align: -webkit-left;">A cantilever differs from the arch and the beam in that the attachment points are not necessarily at opposite ends. The cantilever is rather like a bracket, projecting out into space. The two forces almost always act in opposite directions. In the lower half of the first photograph, the oscillation in the wind is revealed by the longer exposure. Whenever there is both mass and elasticity, there are natural resonant frequencies. The second photograph shows a vertical cantilever deflecting in a wind, with oscillation in the right hand half of the picture.

<span style="background-color: #ffcccc; font-family: Verdana; font-size: medium; text-align: -webkit-left;">Beam bridges

**Beam bridges** are the most simple of [|structural] forms being supported by an [|abutment] at each end of the [|deck]. No [|moments] are transferred throughout the support hence their structural type is known as // [|simply supported] //.

The simplest beam [|bridge] could be a [|slab] of [|stone], or a [|plank] of [|wood] laid across a stream. Bridges designed for modern [|infrastructure] will usually be constructed of [|steel] or [|reinforced concrete], or a combination of both. The [|concrete] used can either be [|reinforced], [|prestressed] or [|post-tensioned].

Types of construction could include having many [|beams] side by side with a deck across the top of them, to a main beam either side supporting a deck between them. The main beams could be [|I-beams], [|trusses] , or [|box girders]. They could be [|half-through], or braced across the top to create a [|through bridge].

Truss bridge

A ** truss bridge ** is a [|bridge] composed of connected elements (typically straight) which 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 nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials.