土木工程专业英语4课课件.ppt

4 Stress Limits in Design ;How large can we permit the stresses to be? Or conversely: How large must a part be to withstand a given set of loads? What are the overall conditions or limits that will determine the size and material for a part? 我们可以允许多大应力？或者反过来说，构件尺寸该多大才能抵抗给定的一组荷载？决定构件尺寸和材料的总体条件和限值是什么？;Design limits are based on avoiding failure of the part to perform its desired function. 设计限值是以构件不发生破坏而执行其要求的功能为基础的。 Because different parts must satisfy different functional requirements, the conditions which limit load-carrying ability may be quite different for different elements. 因为不同构件必须满足不同的功能要求，不同构件界定承载能力的条件可能差别很大。;As an example, compare the design limits for the floor of a house with those for the wing of an airplane. 举例来说，可以将住宅楼板和机翼的设计限值比较一下。 ;If we were to determine the size of the wooden beams in a home such that they simply did not break, we would not be very happy with them; they would be too “springy”. Walking across the room would be like walking out on a diving board. 如果我们要这样来确定木梁的尺寸，只要它不断裂就行，那我们可能就不会很喜欢这些木梁了，这些木梁可能弹性变形太大了。人们穿过房间就会像从跳水板下来一样。;Obviously, we should be concerned with the maximum “deflection” that we, as individuals, find acceptable. This level will be rather subjective, and different people will give different answers. 显然，我们应该关心我们作为个体能够接受的最大挠度，这个水平的主观性相当大，不同人将给出不同的答案。;In fact, the same people may give different answers depending on whether they are paying for the floor or not! 事实上，同一群人也会给出不同的回答，这取决于他们是否支付楼板的费用。 ;An airplane wing structure is clearly different. If you look out an airplane window and watch the wing during turbulent weather, you will see large deflections; in fact you may wish that they were smaller. However, you know that the important issue is that of “structural integrity”, not deflection. 一个机翼结构显然不同，你如从机窗向外观看恶劣天气中的机翼，你就会看到很大的变形，而事实上你会希望这种变形更小些才好。然而，你知道重要的是结构的完整性，而不是变形。;We want to be assured that the wing will remain intact. We want to be assured that no matter what the pilot and the weather do, that wing will continue to