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| 3.21: Making Bridges (page 178-179) Pg. 11 Understanding Concepts: Read page 178-179 and answer the following questions: 1) Describe the differences between: a) A Beam Bridge and a Suspension Bridge A Beam Bridge: Consists of one or more solid units (beams) supported along the length by columns. It is used for short spans. A Suspension Bridge: Consists of cables strung between each end of the bridge and over towers. Suspension cables support the load. Suspension bridges are usually used to cross wide spans. b) An Arch Bridge and a Cantilever Bridge An Arch Bridge: Consists of one or more arches that support a roadway on top, and is often used where support columns would be difficult to place (e.g., spanning a deep ravine). A Cantilever bridge: Is like two seesaws connected by a span. The mass on either end supports the mass of the span in the middle. This type of bridge can withstand a significant load. c) A Truss Bridge and a Cantilever Bridge A Truss Bridge: Consists of a triangulated frame (triangular supports are called trusses), usually made from steel. The load is supported by this system of trusses along the length of the bridge. It is often used to support very heavy loads. Longer truss bridges are usually built in the cantilever style of overall construction 2) Beam Bridges are often used for short distances because they�re more economical than other types. Explain why a beam structure might not be the best choice of a bridge that needs a long span. The limitation includes: the length of beam (too much deflection in a longer beam) and the number of supports needed to adequately support the load. If supports cannot be used (poor soil conditions under the foundation), a truss or arch bridge is a better choice. Grade 7 � Unit 3 Quiz # 1 Structural Strength and Stability Fill in the blanks: Fill in the blanks using the word box below, and then press "Check" to check your answers. Lowering Widening balanced base distribution forces gravity mass material pyramid A structure is stable, if it remains on it's base, undamaged, when exposed to forces that are expected. When designing a structure, you must consider the centre of gravity, or, the point of the object where mass is concentrated. An object that is supported at the centre of gravity is balanced. Stability is dependant on three things: - the material that the object is made from - the arrangement of the material - the distribution of the mass Widening the base of a structure will make it more stable, as in the example of a pyramid. Lowering the centre of gravity will also make a structure more stable. That's why a crouched football player is much harder to knock down! |