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1、 Tension members are found in bridge and roof trusses , towers, bracing systems, and in situation where they are used as tie rods（连杆）. The selection of a section to be used as a tension member is one of the simplest problems encountered in design. As there is no danger of buckling, the designer need
2、s only to compute the factored force（分解力）to be carried by the member and divide that force by a design stress to determine the effective cross-sectional area required1. Tension Membersn. Then it is necessary to select a steel section that provides the required area. Though these introductory calcula
3、tions for tension members are quite simple, they do serve（完 成）the important tasks of getting students started with design ideas and getting their “feet wet” regarding（涉足于） the massive LRFD Manual.受拉构件在桥梁和屋架、塔、支撑 系统以及用作连杆时被见到。用作受拉 构件的截面选择是设计中遇到的最简单 的问题之一。因为没有屈曲的危险，设 计者只需要计算由该构件承受的分解力 ，并将该力除以设计应力以确定所需
4、的 有效横截面面积。然后有必要选择一个 钢截面，它提供了需要的面积。尽管这 些关于受拉构件的介绍性的计算非常简 单，但它们确实完成了这样一个重要的 目标，即使学生开始拥有设计的概念， 并使他们涉足于大量的LRFD手册。nLoad Resistance Factor Design(LRFD)n 钢结构设计规范(荷载抗力系数设计 )nAllowable Stress Design (ASD) One of the simplest forms of tension members is the circular rod, but there is some difficulty in connec
5、ting it to many structures. The rod has been used frequently in the past, but has only occasional uses today in bracing systems, light trusses, and in timber construction. One important reason rods are not popular with designers is that they have been used improperly so often in the past that they h
6、ave a bad name; however, if designed and installed correctly, they are satisfactory for many situations.受拉构件最简单的形式之一是圆钢，但是将其与很多结构连 接有一些困难。过去常常采用圆钢，但是如今只偶然用于支撑系 统、轻质桁架以及木结构。圆钢不受设计者欢迎的一个重要原因 是过去常常被不恰当地使用，以至于落得坏名声；但是如果正确 地设计和安装，它们在很多情况下是令人满意的。The average size rod has very little stiffness and may quite
7、 easily sag under its own weight, injuring the appearance of the structure. The threaded rods formerly used in bridges often worked loose and rattled（发出嘎嘎声）. Another disadvantage of rods is the difficulty of fabricating them with the exact lengths required and the consequent difficulties of installa
8、tion.通常尺寸的圆钢几乎没有刚度，并且在自重下很容易下垂，因而破坏了结构的 外观。以前用于桥梁的有螺纹的圆钢常常 松掉，并发出嘎嘎声。圆钢的另一个缺点 是难以按需要的精确长度来制作，随之发 生的是安装的困难。When rods are used in wind bracing, it is a good practice to produce initial tension in them, as this will tighten up the structure and reduce rattling and swaying. To obtain initial tension the
9、 member may be detailed（设计）shorter than their required lengths, a method that gives the steel fabricator very little trouble.nA common rule of thumb（单凭经验的方 法）used is to detail the rods about 1/16 in. short for each 20 ft. of length. Another very satisfactory method involves tightening the rods with
10、some sort of sleeve nut（轴套螺母 ）or turnbuckle（套筒螺母）. Part 8 of the LRFD Manual provides detailed information for these devices.当圆钢被用作抗风支撑时，一个很好的 做法是使它们产生初张力，因为这将拉紧 结构，并减少发出嘎嘎声和摇晃。为获得 初张力，杆件可以设计得比它们需要的长 度短，该法对钢材的制作者来说几乎没什 么麻烦。一个常用的经验法是设计该圆钢 时每20英尺的长度缩短约1/16英寸, 另一个 非常令人满意的方法包括采用某种套筒螺 母拉紧圆钢。LRFD手册的第八部分提供
11、了 有关这些方法的详细信息。The preceding discussion on rods should illustrate why rolled shapes（轧制型钢）such as angles have supplanted rods for most applications. In the early days of steel structures, tension members consisted of rods, bars, and perhaps cables. Today, although the use of cables is increasing for s
12、uspended-roof（ 悬索屋顶）structures, tension members usually consist of single angles, double angles, tees, channels（槽钢）, W sections（型钢）, or sections built up from plates（薄钢板）or rolled shapes. nThese members look better than the old ones, are stiffer, and are easier to connect. Another type of tension se
13、ction often used is the welded tension plate or flat bar（扁钢）, which is very satisfactory for use in transmission towers（发射塔）, signs（广 告牌）, foot bridges（人行桥）, and similar structures.前面关于圆钢的讨论应该阐明为什么诸 如角钢的轧制型钢已在多数应用中代替了圆 钢。在早期的钢结构中，受拉构件包括圆钢 、型钢以及可能是索。今天尽管索在悬索屋 顶结构中的使用不断增加，受拉构件通常包括单角钢、双角钢、T形钢、槽钢、W型钢 ，或
14、由薄钢板或轧制型钢组合成的型钢。这 些构件看上去比老的更好、更结实、更容易 连接。另一种常用的受拉构件是焊接的受拉 薄钢板或者是扁钢，它在用于发射塔、广告 牌、人行桥以及类似的结构中是非常令人满 意的。The tension members of steel roof trusses may consist of single angles as small as 21/2 2 1/4 for minor members. A more satisfactory member is made from two angles placed back to back with sufficient
15、 space between them to permit the insertion of plates(called gusset plates 节点板) for connection purposes. nWhere steel sections（型钢）are used back- to-back in this manner, they should be connected every 4 or 5 ft. to prevent rattling, particularly in bridge trusses. Single angles and double angles are
16、probably the most common types of tension members in use. Structural tees make very satisfactory chord members for welded trusses because web members can conveniently be connected to them.钢屋架的受拉构件由单角钢组成，对 次要构件其尺寸可小至2 1/2 2 1/4 。一种较满意的构件是由两根背对背放 置、中间有足够的间距允许插入金属板 （称为节点板）以用于连接的角钢制成 。在型钢采用背对背的方式处，它们应 该每隔4或5英尺连接一下以免发出嘎嘎 声，特别是在桥梁桁架中。单角钢和双 角钢可能是受拉构件中最常用的类型。 对焊接桁架用结构T形钢可得到满意的 弦杆，因为腹杆能方便地与它们连接起 来。For bridges and large roof trusses tension members may consist of channels, W or S shapes, or even
