The purpose of this experiment is to find out what is the effect of bridge type on how much weight it can hold. Many studies have been done about the relation between bridge type and the weight it can hold. This includes the two experiments performed by Munim Deen and Erik J. Deen’s investigative was to determine which type of arch bridge could support the most weight. His research concluded that the Single Above-Arch Bridge sustained the most weight. Munim Deens’ conclusion showed that every small part of the bridge makes a difference as to how much weight it can hold. (Deen) Erick J’s purpose was to determine which overall bridge design was stronger. His investigation is very similar to this one. By reading through Erik’s lab report, it was realized that the truss and arch bridge are the two strongest. Out of those, the truss bridge is the strongest bridge because it uses a pattern of triangles which push against each other, hence, strengthening the bridge. (J) It is hypothesized that if different types of bridges’ weight capacity are measured, then the truss bridge will be able to hold the most weight, because its triangle patterns push together, therefore strengthening the bridge.
Throughout the investigation, the experimenter will be manipulating the type of bridge. A bridge is defined as “A structure forming or carrying a road over a river, a ravine, etc., or affording passage between two points at a height above the ground.” (Oxford University Press) There are four manipulated variable levels: the truss bridge, the arch bridge, the span bridge, and the suspension bridge. While experimenting, all the bridges will be made out of a consistent type of wood. Many major differences between the different bridge types exist.
Truss bridges are used for a variety of reasons since they are inexpensive to build. The combination of their low cost and strength makes truss bridges one of the most common modern day bridges. Due to their strength, they are used for spanning over long distances. Truss bridges are based off of patterns of triangles. (J) Referring to Truss Bridge 1, the red represents tension, blue represents compression, and green is no load. As you can see the weight is equally spread out and the load is quite a bit less on the ends of the top and bottom chord than in the middle. Notice that the two middle pieces have no load. (Boon)
Arch bridges have been used all the way since the time monasteries were still around. Arch bridges are used in any large gap with sturdy walls. (J) The Romans perfected the arch bridge. Arch bridges using only a single arch must have spandrel columns. These columns help to transfer weight to the arch as well as to act as a connector between the bridge deck and the arch. Almost everything that is meant to withstand great pressure is curved. (Deen)
Span bridges are okay for short distance uses such as freeway passes. Due to its lack of supports, span bridges are weak. It’s like standing on a slat ruler. They work by using brute force, all it does is take the weight and do the best it can with the weight it is given. (J) Today, nearly 120,000 cars cross the Golden Gate Bridge each day. These bridges need to be built well and be able to withstand tremendous amounts of pressure.
Last but not least is the suspension bridge. Originally, suspension bridges were made out of rope and wood. Nowadays, they use a box section roadway supported by high tensile strength cables. A cable is essentially thousands of steel wires that are tightly bound together. Suspensions bridges can span distances from 2,000 to 7,000 feet, longer than any other type of bridge. These bridges tend to be the most expensive to build but are usually made for their looks. The Tacoma Narrows Bridge, refer to Tacoma Narrows Suspension Bridge 1, was built in 1940. It was the third largest bridge in the world. It later became known as “Galloping Gertie,” due to the fact that it would move up and down as well as side to side. In approximately four months, the bridge collapsed in a wind of 42 MPH. The bridge was designed to withstand winds up to 120 MPH. (Design Technology Department)
Building a bridge involves remarkable amounts of math and physics. Most bridges today are crossovers between two types of bridges. (J)
Boon, Garrett. Truss Design. n.d. 15 December 2010 .
Deen, Munim. Which Type of Arch Bridge Supports The Most Weight? n.d. 14 December 2010 .
Design Technology Department. Suspension Bridges – Design Technology. n.d. 15 December 2010 .
J, Erik. Which Bridge Design is Stronger? n.d. 14 December 2010 .
Oxford University Press. bridge, n.1. n.d. 14 December 2010 .
