Different Types of Bridges

  The different types and shapes of bridges range from beam bridges to triangle and square bridges. When constructing a bridge, looking into what bridges are the strongest should be the first step.

   The first of three common bridge types is the beam bridge. The first beam bridges were most likely constructed with tree trunks or logs; this assumption was made based upon the origin of the wood. If using piers, we should decide how tall they should be and take into account what is going to move below the bridge. For the span of the bridge, the strength, weight, and the length of the beams used will control how long the span of the bride is. The thicker the material, the greater weight the bridge will hold as well as the longer the bridge will span. However, the larger the beam is, the heavier the bridge is.  So one aspect must be compromised for another.

    A different kind of long spanning bridge is a cantilever. This type of bridge is similar to the lever bridge. In a cantilever bridge, the two ends of a bridge are secured to the ground.  They would act similar to two connected diving boards.  In a cantilever bridge, the inner, center beam would be laid across these diving boards with the ends supporting it and keeping it aloft. These bridges are designed to cross large bodies of water.

   The second type of bridge is the suspension bridge. These bridges are able to use tension to stay together and can span the longest of all bridges. Unlike most bridges, the suspension bridges pull in a sense instead of pushing. These bridges are very pliable and often sway. This is okay as long as it is not in sync with the wind. The swaying could leave the bridge breaking if the wind picks up. 

http://www.sciencetech.technomuses.ca/english/schoolzone/Info_structures.cfm

http://science.howstuffworks.com/engineering/civil/bridge7.htm

Tips When Building Bridges

When building a balsa wood bridge it is important to keep a few things in mind. In order to get the best results out of the bridge, we should follow these simple tips. They will make the bridge stronger and work with the light balsa wood.

The glue used to keep the bridge together is very important. Balsa wood is porous, and due to this, glue that stretches out when its drying is the best that can be used. Gorilla Glue and Probond meet the criteria. However, they will both increase the weight of your bridge more than other glues. When building a bridge, it should be remembered that a lot of glue is unnecessary. It just needs to hold the pieces of the bridge together, such as a joint.

Every piece of wood has a grain, and balsa wood has two very different types. On some balsa wood, the grain is thinner and almost reaches from one end of the cut to the other. This is favorable if there will be a lot of tension on the bridge. The second type of balsa wood has very short grain lines. It is best to use this when we are dealing with compression. This type of balsa wood is stiffer than the first.

Another tip is to keep the bridge in as dry of an environment as possible. The humidity can make your bridge heavier. When using the glue, try not to overload it onto the bridge; a small, simple amount will do the trick. All the while, make sure our hands are relatively clear. The oil on our hands can ruin the glue job, especially on the joints.

If we have enough balsa wood, try to make more than one bridge. With more bridges, we can see the weak points in each and relate that to our final bridge. Also, make sure to add lateral bracing when planning out our bridge. Lateral bracing prevents the bridge from turning, or twisting.

These tips are simply pointers and are not completely necessary. Although, they should be taken into consideration when we are building a bridge, because they will certainly help you make a stronger one.

http://www.garrettsbridges.com/building/25-bridge-building-tips/

http://www.garrettsbridges.com/building/balsa-wood/

The Basics In Building A Bridge

  When building a bridge, we should consider all the possible types we can make. There are different spans, trusses, and girders we can incorporate into our design, and each has their own strengths and weakness, some more than others. The first step before considering designs, however, should be to identify what the bridge will have to hold, or the use of the bridge, and in what circumstances.

  The four principal terms employed when defining a normal bridge are span, material, the position of the roadway regarding the construction of the bridge, and form. Each term has subtopics that leave us many options with the bridge.

  The first term is span; there are three main types of spans, and these can be made with many different things. The three things are beams, girders, or trusses.  When dealing with an arched bridge, a continuos hinged or simple hinged span will work the best. A span is the distance between the legs of the bridge.  Spans are horizontal and when the term span is used, it could also be relating back to the construction of the bridge itself.

  The second main part of a bridge is a truss. The truss is the roadway of the bridge. The main types of trusses are deck, pony, and through trusses. A deck truss is used when the bridge is a main street, with traffic and many cars. The pony truss is employed if the builder wants the traffic to travel in the middle of two superstructures. Pony trusses, however, are not crossed like and “X” on the top of the bridge.  The last truss is the through truss. With this truss, the traffic would travel underneath the superstructure. Unlike pony trusses, through trusses use cross-bracing.

  In a bridge, the girder is the framework made to hold up the weight. What makes horizontal girders so strong is their stiffness. With this they are able to brace the vertical weight on the bridge. A girder is usually very thick, and is sometimes made up of more than one beam.  

         To strengthen the girder, haunches can be involved in its planning. In a haunched girders are not straight, but rather arched towards the end of the bridge. The constraint of these bridges is their size. On most bridges, smaller beams are used, because they are easier to move around. With this, the smaller pieces have to be joined together, usually by splice plates. 

http://pghbridges.com/basics.htm

http://pghbridges.com/termsBrg.htm