MadSci Network: Engineering |
Amy I'm pleased to find a young woman interested in Engineering. First: Every structure is made up of members, which support loads in two basic ways. In tension (by being pulled at) or by compression (being pushed on). Tension is what happens in a rope. The stress in the material is simply the weight divided by the area of the rope. Exceed the stress and the rope breaks. Compression is more complex. The load that a compression member, such as a post or column, can support is governed by several factors. The longer a column is, the more flexible it will be, and the less it can support. Increasing a column’s cross-section -- Making it thicker -- gives it more stiffness, and increases the load it can carry. The members making up a structure are assembled into frames. Here’s an experiment for you. Cut strips from the cardboard back of a pad of paper. Make 2 strips four inches long, two five inches, and one six inches. Punch holes in the strips a half inch from each end. Now use brads to pin the four inch and five inch strips together into a rectangular frame. Lay the frame flat on a desk, and push gently down on the middle of the longer strip. Notice that although the frame will stay rectangular if the force is applied at right angles to the strip, it will fold up as soon as any sideways force is applied. Now insert the longest strip diagonally between the corners. The frame is now a pair of triangles with the two long sides adjacent. But it has become more rigid. That's why almost all structures have triangles in their design. Buildings have diagonal bracing in some floors to resist the wind. Truss bridges are a series of triangles that transfer the loads from mid-span to the ends. A geodesic dome is a ball made up of triangles connected at the corners. Wind turbine towers are triangles made up of three or four legs in a pyramid shape. A triangle is the only shape that is naturally rigid. Of course it's hard to live inside a triangle -- spent any time in an attic or a tent? It's not too efficient with space. Also, a pyramid-shaped frame will have to have longer members to enclose the same space as a box. Because they're longer, the compression members in the pyramid would have to have a larger cross-section to support the same weight than shorter, vertical columns. When an engineer designs a building, bridge, or whatnot, she's trying to place the structural elements in the optimum location where they can carry the loads. So, while a frame made of triangles will bear loads most efficiently, most structures are constructed in a pattern of columns and beams (with diagonal cross bracing) to make the best use of space. Here's a building using triangles: http://www.greatbuildings.com/cgi-">http://www.greatbuildings.com/cgi-">http://www.greatbuildings.com/cgi- bin/gbi.cgi/John_Hancock_Center.html/cid_john_hancock_002.gbi And here's more than you probably wanted to know about bridges: http://www.brantacan.co.uk/trus s.htm And a page about geodesic domes: http://www.insit e.com.br/rodrigo/bucky/geodome.html And here's a bridge that uses triangles in a unique way. My family drove under it last Sunday: http://www.aisc.org /msc/0007_05_oldplankroad.pdf
Try the links in the MadSci Library for more information on Engineering.