1、Unit 11 第十一单元 Steel Members 钢构件,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
2、 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,1. Tension Members,. Then it is necessary to select a steel section that provides the required area. T
3、hough these introductory calculations 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
4、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。,Load Resistance Factor Design(LRFD)钢结构设计规范(荷载抗力系数设计 ) Allowable Stress Design (ASD),One of the simplest forms of tension members is the circular rod, but there is some d
5、ifficulty 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 designers is that they have been used improperly so often in t
6、he past that they have 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 stiffne
7、ss and may quite 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 difficul
8、ties 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, 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 initi
9、al 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. Another very satisfactory method involves tightening t
10、he rods with 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
11、八部分提供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
12、for suspended-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.,These members look better than the old ones, are stiffer, and are easier to connect. Another type of tension
13、 section 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，或由薄
14、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 space
15、 between them to permit the insertion of plates(called gusset plates节点板) for connection purposes.,Where 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 probably
16、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
17、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 sections built up from some combination of angles, channels, and plates. Single channels are frequently used, as they have lit
18、tle eccentricity and are conveniently connected. Although, for the same weight, W sections are stiffer than S sections, they may have a connection disadvantage in their varying depths. For instance, the W12 79, W12 72, and W12 65 all have slightly different depths (12.38 in., 12.25 in., and 12.12 in
19、., respectively) while the S sections of a certain nominal size all have the same depths. For instance, the S12 50, the S12 40.8 and the S12 35 all have 12.00 in. depths.,对桥梁和大型屋架，受拉构件可以由槽钢、W型钢或S型钢，或者甚至由角钢、槽钢和薄钢板的某种组合而成的型钢组成。单槽钢常常被采用，因为它们几乎没有偏心，且容易连接。尽管相同重量的W型钢比S型钢更结实，但W型钢由于其变化的厚度可能存在连接上的不利条件。例如W12
20、79、 W12 72以及 W12 65都有略微不同的厚度（分别为12.38 英寸、 12.25 英寸以及 12.12 英寸），而有着某个名义尺寸的S型钢都有相同的厚度。例如，S12 50、S12 40.8 以及 S12 35，其厚度都为12英寸。,Although single structural shapes are a little more economical than built-up sections, the latter are occasionally used when the designer is unable to obtain sufficient area or
21、 rigidity from single shapes. Where built-up sections are used it is important to remember that field（现场）connections will have to be made and paint applied; therefore, sufficient space must be available to accomplish these things.尽管单个结构型钢比组合的型钢稍微更经济一些，但是当设计者采用单个型钢不能够得到足够的面积或刚度时，偶然也会采用后者。在采用组合型钢处，要记住
22、很重要的一点是将不得不进行现场连接，并进行涂漆；因此，必须获得足够的空间来完成这些事情。,Members consisting of more than one section need to be tied together. Tie plates(also called tie bars) located at various intervals or perforated cover plates serve to hold the various pieces in their correct positions. These plates serve to correct any u
23、nequal distribution of loads between the various parts. They also keep the slenderness ratios (to be discussed) of the individual parts within limitation and they may permit easier handling of the built-up members.,Long individual members such as angles may be inconvenient to handle due to flexibili
24、ty, but when four angles are laced together into one member, as shown in Fig. 11-1, the member has considerable stiffness. None of the intermittent tie plates may be considered to increase the effective areas of the sections. As they do not theoretically carry portions of the force in the main secti
25、ons, their sizes are usually governed by specifications and perhaps by some judgment on the designers part. Perforated cover plates are an exception to this rule, as part of their areas can be considered as being effective in resisting axial load.,由几个截面组成的构件需要连接在一起。以不同的间隔设置的连接板（也称为连杆）或多孔盖板用以保持不同的构件均
26、在其恰当的位置上。这些板用以调整不同部分之间的荷载分配的不平衡。它们也使单个部分的长细比（将要讨论）保持在限制的范围内，这样他们可以允许较容易地处理组合构件。长长的单个构件如角钢，由于易弯性可能处理时有困难，但是当四根角钢绑扎成一根构件时，如图11-1所显示的，该构件具有相当大的刚度。可以不考虑间断的连接板对截面的有效面积的增加。因为他们理论上不承受主截面中的部分力，它们的尺寸通常由规范以及可能根据设计者方面的某些判断来决定。多孔盖板则是该规则的例外，因为它们的部分面积在抵抗轴向荷载时可认为是有效的。,A few of the various types of tension members
27、in general use are illustrated in Fig. 11-1. In this figure the dotted lines represent the intermittent tie plates or bars used to connect the shapes.常用的不同类型的一些受拉构件被举例在图11-1中。在该图中虚线代表了用以连接型钢的间断的连接板或连杆。,Steel cables are made with special steel alloy wire ropes（特种合金钢丝绳） which are cold-drawn to the des
28、ired diameter. The resulting wires with strengths of about 200,000 to 250,000 psi can be economically used for suspension bridges, cable supported（悬索）roofs, ski lifts（运送滑雪者上山坡的吊索设备）, and other similar applications.钢索是由冷拉至需要直径的特种合金钢丝绳制成的。形成的具有强度约为200,000到250,000 psi的钢丝能经济地用于悬索桥、悬索屋顶、运送滑雪者上山坡的吊索设备以及其他
29、类似的应用。,Normally, to select a cable tension member the designer uses a manufacturers catalog. From the catalog the yield stress of the steel and the cable size required for the design force are determined. It is also possible to select clevises（马蹄钩）or other devices to use for connectors（连接器）at the ca
30、ble ends. 通常，为选择索的受拉构件，设计者采用制造商的产品目录。从产品目录中可以确定钢材的屈服应力和设计力所需要的索的尺寸。也有可能选择马蹄钩或其他的装置用作索端的连接器。,2.Axially Loaded Compression MembersThere are several types of compression members, the column being the best known. Among the other types are the top chords of trusses, bracing members, the compression flang
31、es of rolled beams and built-up beam sections, 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 than their thicknesses. Short vertical members subjected to compress
32、ive loads are often called struts（支柱）or simply compression members; however, the terms column and compression member will be used interchangeably in the pages that follow.,有多种受压构件，其中柱子是最知名的。在其他的类型中有桁架的上弦杆、支撑杆件、轧制梁和组合梁截面的受压翼缘以及同时承受弯曲荷载和压力荷载的构件。柱子通常被认为是直的竖向构件，其长度比它的厚度大得多。承受压力的短的竖向构件常常称为支柱或简单地称为受压构件；但是
33、术语柱子和受压构件在下面的页面中将被交替地使用。,There are three general modes by which axially loaded columns can fail. These are flexural buckling, local buckling and torsional buckling. These modes of buckling are briefly defined below. (1)Flexural buckling (also called Euler buckling欧拉屈曲) is the primary type of bucklin
34、g. Members are subject to flexure or bending when they become unstable. (2)Local buckling occurs when some part or parts of the cross section of a column are so thin that they buckle locally in compression before the other modes of buckling can occur. The susceptibility（敏感性）of a column to local buck
35、ling is measured by the width-thickness ratios of the parts of its cross section. (3)Torsional buckling may occur in columns that have certain cross-sectional configurations（外形）. These columns fail by twisting(torsion) or by a combination of torsional and flexural buckling.,轴向受力的柱子发生失效有三种一般的模式。它们是弯曲
36、屈曲、局部屈曲和扭转屈曲。这些屈曲模式简单地定义如下： （1）弯曲屈曲（也称为欧拉屈曲）是主要的屈曲类型。当构件变得不稳定时，它们易于弯曲。 （2）局部屈曲发生在柱子某部分的横截面太薄了以至于在其他屈曲模式发生之前它们便受压局部屈曲。柱子对局部屈曲的敏感性可以通过该部分的横截面的宽厚比来测得。 （3）扭转屈曲可以发生在具有某种横截面外形的柱子中。这些柱子由于扭曲（扭转）或扭转屈曲与弯曲屈曲的组合而失效。,The longer a column becomes for the same cross section the greater becomes its tendency to buckl
37、e and the smaller becomes the load it will support. The tendency of a member to buckle is usually measured by its slenderness ratio, which has previously been defined as the ratio of the length of the member to its least radius of gyration（回转半径）. The tendency to buckle is also affected by such facto
38、rs as the types of end connections, eccentricity of load application, imperfection（缺陷）of column material, initial crookedness（弯曲）of column, residual stresses from manufacture, etc. 相同横截面的柱子越长，则屈曲的趋势越大，它能承受的荷载将越小。构件屈曲的趋势通常根据其长细比来测定，它在先前已定义为构件的长度与它最小的回转半径之比。屈曲的趋势也受到诸如端部连接的类型、荷载施加的偏心距、柱子材料的缺陷、柱子的初始弯曲、制
39、造引起的残余应力等因素的影响。,The loads supported by a building column are applied by the column section above（在上方）and by the connections of other members directly to the column. The ideal situation is for the loads to be applied uniformly across the column, with the center of gravity（重心）of the loads coinciding w
40、ith（与一致）the center of gravity of the column. Furthermore, it is desirable（希望）for the column to have no flaws（缺陷）, to consist of a homogeneous（均匀的）material, and to be perfectly straight, but these situations are obviously impossible to achieve. 建筑物中的柱子承受的荷载是由其上方的柱子部分和直接与柱子相连的其他构件施加的。理想的情况是该荷载均匀地施加在柱子
41、上，荷载的重心与柱子的重心一致。而且希望柱子没有缺陷、由均匀的材料组成以及是完全笔直的，但是这些情形显然不可能达到。,Loads that are exactly centered（居中）over a column are referred to as axial or concentric（同轴的）loads. The dead loads may or may not be concentrically placed over an interior building column and the live loads may never be centered. For an outsi
42、de column the load situation is probably even more eccentric（偏心）, as the center of gravity of the loads will usually fall well on（恰好落在）the inner side of the column. In other words, it is doubtful that a perfect axially loaded column will ever be encountered in practice. 在柱子上精确居中的荷载称为轴向或同轴荷载。恒载可能或不一定
43、同轴地置于建筑屋内部的柱子上，而活载可能永远也不会居中。外部柱子的荷载情形甚至可能更偏心，因为荷载的重心通常恰好落在柱子的内侧。换句话说，在实际中曾遇到一个完全轴向受荷的柱子是令人怀疑的。,The other desirable situations are also impossible to achieve because of the following: imperfections of cross-sectional dimensions, residual stresses, holes punched for bolts, erection stresses, and trans
44、verse loads. It is difficult to take into account all of these imperfections in a formula.由于下列的原因，其他希望的情形也是不可能达到：横截面尺寸的缺陷；残余应力；螺栓冲孔；安装应力；横向荷载。很难在一个公式中考虑所有的这些缺陷。,Slight（微小的）imperfections in tension members and beams can be safely disregarded（忽略）as they are of little consequence（后果）. On the other hand
45、（相反）, slight defects（缺陷）in columns may be of major significance（重大意义）. A column that is slightly bent at the time it is put in place（就位）may have significant（很大的）bending moments equal to the column load times the initial lateral deflection. 受拉构件和梁中微小的缺陷可以安全地忽略掉，因为它们几乎没有什么后果。相反，柱子中微小的缺陷可能意义重大。柱子就位时的轻微
46、弯曲可能产生很大的弯距，它等于柱子上的荷载乘以初始的横向挠度。,Obviously, a column is a more critical（关键的）member in a structure than is a beam or tension member because minor（较小的） imperfections in materials and dimensions mean a great deal. The fact can be illustrated by a bridge truss that has some of its members damaged by a tr
47、uck. The bending of tension members probably will not be serious as the tensile loads will tend to straighten（使伸直）those members; but the bending of any compression members is a serious matter（问题）, as compressive loads will tend to magnify（扩大）the bending in those members.显然，比起梁或受拉构件，结构中的柱子是较为关键的构件，因为
48、材料和尺寸中较小的缺陷意味着很多。该事实可通过一个桥梁桁架来说明，其有一些被卡车损坏的构件。受拉构件的弯曲可能不是严重的问题，因为拉力往往使这些构件伸直；但是任何受压构件的弯曲则是一个严重的问题。因为压力往往扩大了这些构件中的弯曲。,The preceding（前述的）discussion should clearly show that column imperfections cause them to bend and the designer must consider stresses due to those moments as well as due to axial load
49、s.前述的讨论应该清楚地表明是柱子的缺陷使它们弯曲，并且设计者必须考虑由于这些弯距和轴向力引起的应力。,The spacing（间距）of columns in plan establishes（确定）what is called a bay（开间）. For instance, if the columns are 20 ft. on center in one direction and 25 ft. in the other direction the bay size is 20 ft 25 ft Larger bay sizes increase the users flexibil
50、ity（灵活性）in space planning. As to economy, a detailed study by John Ruddy indicates that when shallow spread footings（扩展基础）are used, bays with length-to-width ratios of about 1.25 to 1.75 and areas of about 1000 sq ft. are the most cost efficient（费用最经济）. When deep foundations are used, his study shows that larger bay areas are more economical.,
