【正文】 降低。 Beams with central concentrated loadsA simply supported beam with a central concentrated load Q acting at a distance above the centroidal axis of the beam is shown in Fig. . When the beam buckles by deflecting laterally and twisting, the line of action of the load moves with the central crosssection, but remains vertical, as shown in Fig. . The case when the load acts above the centroid is more dangerous than that of centroidal loading because of the additional torque which increases the twisting of the beam and decreases its resistance to buckling.，一個承受跨中集中荷載Q的簡支梁，Q作用位置距離梁中軸線為。 Beams with unequal end momentsA simply supported beam with unequal major axis end moments M and as shown in Fig. . It is shown in section that the value of the end Jimoment Mab at elastic flexuraltorsional buckling can be expressed in the .form of （）in which the moment modification factor which accounts for the effect of the nonuniform distribution of the major axis bending moment can be closely approximated byor by 端彎矩不等的簡支梁，一簡支梁承受端彎矩M和，彎扭屈曲梁端彎矩公式為： （）表明強軸彎矩不均勻分配作用影響的修正系數(shù)近似表示為：或者These approximations form the basis of a very simple method of predicting the buckling of the segments of a beam which is loaded only by concentrated . loads applied through transverse members preventing local lateral deflection and twist rotation. In this case, each segment between load points may be treated as a beam with unequal end moments, and its elastic buckling moment may be estimated by using equation and either equation or and by taking L as the segment length. Each buckling moment so calculated corresponds to a particular buckling load parameter for the plete load set, and the lowest of these parameters gives a conservative approximation of the actual buckling load parameter. This simple method ignores any buckling interactions between the segments. A more accurate method which accounts for these interactions is discussed in section .這種近似是一種預測集中荷載作用下梁段的屈曲情況簡單方法，是為了防止截面發(fā)生側向位移或扭轉。這種修正，在大多數(shù)梁截面設計中是可以忽略的，柱的設計中則不然。梁端簡支側向彎曲和扭轉不會發(fā)生，因為端部可以在平面內自由轉動從而不限制梁端轉角。In this chapter, the behaviour and design of beams which fail by lateral buckling and yielding are discussed. It is assumed that local buckling of the pression flange or of the web (which is dealt with in Chapter 4) does not occur. The behaviour and design of beams bent about both principal axes, and of beams with axial loads, are discussed in Chapter 7.在本章，將講述由側向屈曲和屈服引起破壞的梁的性能和設計方法。這兩種變形是相互聯(lián)系的：當梁側向傾斜時，所承受的彎矩會對側向梁軸產生扭矩并引起梁扭轉。s inplane load carrying capacity, as indicated in Fig. . 說明在第五章關于梁的平面內性能的討論中，假定梁按剛性主平面放置時，梁僅在該平面內傾斜。如果梁沒有足夠的側向剛度或側面支撐，梁會發(fā)生平面外屈曲。這種特性，對于抵抗側向彎曲和扭轉能力差的無限制I形梁來說很重要，被成為彎扭屈曲。假設第四章中討論的局部屈曲不會發(fā)生。當側移和扭轉達到平衡時，在作用下梁會彎曲。它把梁的和當作零處理，使得梁的彈性屈曲彎矩接近無窮大。在這種情況下，集中荷載作用點之間的梁可以被當作承受不相等梁端彎矩的梁，L為集中荷載作用點之間的梁段長度。當梁發(fā)生側向偏移和扭轉時，荷載作用線隨著跨中截面的的旋轉而移動，但仍保持垂直。當梁的上下翼緣相等時，適用于公式：where is the distance between flange centroids. The variation of the buckling load with lyt^jds is shown by the solid lines in Fig. 6,6, and it can be seen that the differences between top and bottom flange loading increase with the beam parameter K. This effect is therefore more important for deep beamtype sections of short span than for shallow column type sections of long span. Approximate expressions for the variations of the moment modification factor am with the beam parameter K which account for the dimensionless load heightfor equal flanged Ibeams are given in [2]. Alternatively, the maximum moment at elastic buckling may be approximated by usingand , in which 表示翼緣形心間的距離，屈曲彎矩隨著的變化而變化。或者，梁的最大屈曲彎矩近似適用于公式：當時， Other loading conditionsThe effect of the distribution of the applied load along the length of a simply supported beam on its elastic buckling strength has been investigated numerically by many methods, including those discussed in [35]. A particularly powerful puter method is the finite element method [610], while the finite integral method [11, 12], which allows accurate numerical solutions of the coupled minor axis bending and torsion equations to be obtained, has been used extensively. Many particular cases have been studied [13 16], and tabulations of elastic buckling loads are available [2, 3, 5, 13, 15, 17], as is a userfriendly puter program [18] for analysing elastic flexuraltorsional buckling. 其他荷載情況承受沿直線分布荷載的簡支梁的荷載分配情況對彈性屈曲長度的影響有很多種方法考慮，包括【35】中討論的。For other beam loadings than those shown in Fig. , the moment modification factor may be approximated by usingin which Mm is the maximum moment,，the moments at the quarter points, and the moment at the midpoint of the beam.，彎矩修正系數(shù)適用于公式:為最大彎矩，是1/4位置的彎矩，為梁跨中彎矩?！?6，1921】是幾種使用的數(shù)學方法。The simple loaddeformation relationships of equations and are of the same forms as those of equations and for pression members with sinusoidal initial curvature. It follows that the Southwell plot technique for extrapolating the elastic buckling loads of pr