图10-1中的应力-应变曲线对一般的延性结构钢来说是典型的，并假定构件在受拉和受压时是相同的。（受压构件必须是粗短的，因为细长的受压构件承受压力时往往会侧向弯曲，且它们的性能大大地受这样形成的弯矩的影响。）图的形状随加荷的速度、钢材的类型和温度而变化。这样的一个变化通过一根标明屈服上限的虚线显示在图中。该应力-应变曲线的形状是低碳钢快速加载的结果，而屈服下限是缓慢加载的情况。

UNIT11
1. Tension Members 受拉构件
        Tension members are found in  bridge and roof trusses （屋架）, 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 needs 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 required. Then it is necessary to select a steel section（截面）that provides the required area. Though these introductory（介绍性的）calculations for tension members are quite simple, they do serve（完成）the important tasks（目标） of getting students started with design ideas（概念）and getting their “feet wet” regarding（涉足于） the massive（大量的）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。
        One of the simplest forms（形式）of tension members is the circular rod（圆钢）, but there is some difficulty in connecting 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（受..欢迎）designers is that they have been used improperly（不恰当地）so often in the past that they have a bad name（落得坏名声）; 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（通常）size rod has very little（几乎没有） stiffness and may quite easily sag（下垂）under its own weight（自重）, injuring（破坏）the appearance of the structure. The threaded rods formerly used in bridges often worked loose（松掉）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（随之发生的）difficulties of installation.
            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（抗风支撑） it is a good practice（作法）to produce initial tension（初张力）in them, as this will tighten up（拉紧）the structure and reduce rattling and swaying. To obtain initial tension the member may be detailed（设计）shorter than their required lengths, a method that gives the steel fabricator very little trouble. A 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. (Approximate stress f = εE = [1/16(12)(20)](29×106)= 7550 psi.) Another very satisfactory method involves tightening the rods with some sort of sleeve nut（套筒螺母）or turnbuckle（套筒螺母）. Part 8 of the LRFD Manual provides detailed information for these devices（方法）.
        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。（近似应力f = εE = [1/16(12)(20)](29×106)= 7550 psi.）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了有关这些方法的详细信息。
2.Axially Loaded Compression Members
轴压构件
            There are several types of compression members, the column being the best known（知明的）. Among the other types are the top chords（上弦）of trusses, bracing members, the compression flanges of rolled beams and built-up beam sections（截面）, and members that are subjected simultaneously to bending and compressive loads. Columns are usually thought of as being straight vertical members whose lengths are considerably greater（大得多）than their thicknesses. Short vertical members subjected to compressive loads are often called struts（支柱）or simply compression members; however, the terms column and compression member will be used interchangeably（交替地）in the pages that follow.