Bridges_notes

Bridges are structures that connect one place to another. They use different materials and designs that vary depending on the function and place where they are constructed.

Bridges are commonly constructed with stone, wood, timber, bricks, steel, concrete, and sometimes aluminum. These materials have different qualities and textures that give the bridge properties that, depending on the context, can improve or perturb it work. Currently the most common material used is iron and cable on suspension bridges, because they resist seismic movements and are cheaper than others.

  ** What are the benefits of bridges? **  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.  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.

 TYPES: The type of bridge used depends on various features of the obstacle.  The main feature that controls the bridge type is the size of the obstacle. The biggest difference between the three is the distances they can cross in a single **span **. A span is the distance between two bridge supports, whether they are columns, towers or the wall of a canyon. 1. **__A beam bridge__** is basically a rigid horizontal structure that is resting on two piers, one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward. Many beam bridges that you find on __<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; color: #005288; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">[|highway overpasses] __ use concrete or __<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; color: #005288; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">[|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 lattice work, or a **<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">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. <span style="font-family: Arial,Helvetica,sans-serif; line-height: 18px;">Beam bridges come in dozens of different styles. The design, location and composition of the truss is what determines the type



2. <span style="font-family: Arial,Helvetica,sans-serif; line-height: 18px;">__**An arch bridge**__ is a semicircular structure with abutments on each end. The design of the arch, the semicircle, naturally diverts the weight from the bridge deck to the abutments. Arch bridges are always under compression. The __<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; color: #005288; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">[|force] __ of compression is pushed outward along the curve of the arch toward the abutments. The shape of the arch itself is all that is needed to effectively dissipate the __<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; color: #005288; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">[|weight] __from the center of the deck to the abutments.

3. <span style="font-family: Arial,Helvetica,sans-serif; line-height: 18px;">__**A suspension bridge**__ is one where __<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; color: #005288; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">[|cables] __ (or ropes or chains) are strung across the river (or whatever the obstacle happens to be) and the deck is suspended from these cables. The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the __<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; color: #005288; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">[|earth] __ where they are firmly entrenched. Almost all suspension bridges have, in addition to the cables, a supporting truss system beneath the bridge deck (a **<span style="border-bottom-width: 0px; border-color: initial; border-left-width: 0px; border-right-width: 0px; border-style: initial; border-top-width: 0px; font-family: Arial,Helvetica,sans-serif; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px; vertical-align: baseline;">deck truss **). This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.