Bridges_notes


 Bridges break down separation and foster connectedness. 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.

Some of the benefits of bridges are: supplies of food and traded goods can get across an obstacle or through difficult terrain in a shorter time. Bridges allow easy travel across major rivers and estuaries, over the new obstacles of motorways and railway lines, and between neighboring islands, so efficient distribution networks depend on bridges. 

**There are three major types of bridges:**
-The beam bridge

-The arch bridge 

-The suspension bridge

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. 

This is the earliest and simplest kind of bridge: a fallen tree lying over a stream. The design is as simple as a single rigid 'beam', resting on supports at either end and unsupported in the middle. The weight of the beam, and of any traffic on it, is carried directly to the ground by the supports, often called 'piers' in the trade.
 * The beam bridge **

**What are the disadvantages of a beam bridge?**
 All beams tend to 'sag' between the piers and 'hog' over the piers themselves. This results from the downward forces of the load and the upward forces at the pier supports. The greater the span or the load, the greater the tendency towards sagging and hogging. The longer a beam is, the weaker it becomes. The greater length gives more weight and more leverage for that weight - increasing the 'bending moment'.

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 force of compression is pushed outward along the curve of the arch toward the abutments. One disadvantage of the arch is that it needs firm support from the sides. If there are no abutments or banks to build against, the arch will spread and collapse. This is why large cathedrals need 'buttresses' to provide the horizontal support force necessary to counteract the horizontal dissipation of the load from the roof and walls.
 * The arch bridge **

A suspension bridge is one where 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. Modern suspension bridges have two tall towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight.
 * The suspension bridge **

**Are there any disadvantages to the suspension bridge?**
Yes, there are, there is a disadvantage to the suspension bridge design – namely its lack of the rigidity implicit in both beam and arch designs. This makes suspension bridges susceptible to the phenomenon of 'resonance', where vibrations build in magnitude due to some regular energy input. <span style="color: #ff6600; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">