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一、一般说明 (一)基本原理对于截面高度有变化的梁,其弯曲强度存在两个问题。即所选取的截面尺寸及混凝土标号是否足够,以及纵向受力钢筋面积由哪个截面控制并需要多少。为简明计,以双坡屋面梁为例予以说明。图1a为一双坡梁,截面尺寸及混凝土标号为已定。图1b中的实线(_x)_(max)为该梁的最大抗弯强度图,虚线M_x为已计入安全系数的外荷弯矩图。在图1b中两图已相交,故在梁的一定范围内,外荷弯矩大于梁的最大抗弯强度,这时纵筋A_g的数量再多也无济于事。必须加大梁的截面尺寸或提高混凝土标号,使(_a)_(max)图加大而位于M_x图之外,至少须两者相切。其相切的条件为:
I. General description (I) Basic principles There are two problems with the flexural strength of a beam with a varying section height. That is, whether the selected section size and concrete marking are sufficient, and which section control the longitudinal force-reinforced rebar area needs and how much. For the sake of simplicity, we will use a double-slope roof beam as an example. Figure 1a is a double-slope beam. The cross-sectional dimensions and concrete markings are fixed. The solid line (_x)_(max) in Fig. 1b is the maximum bending strength diagram of the beam, and the dotted line M_x is the outbound bending moment diagram that has been included in the safety factor. In Figure 1b, the two figures have intersected. Therefore, in a certain range of the beam, the external load bending moment is greater than the maximum bending strength of the beam. In this case, the number of longitudinal bars A_g is no more effective. It is necessary to increase the cross-sectional size of the beam or increase the concrete number so that the (_a)_(max) diagram is enlarged and located outside the M_x diagram, and at least both must be tangent. The tangent conditions are: