能依赖于横截面的尺寸和形状，这种情况下的构件选择会变得比较困难。暂时我们将采用许用应力法，似乎它只取决于材料的强度和安全系数。
A trial member will be acceptable（合格）when the stress is equal to, or less than, the allowable stress, that is, if For design purposes this inequality is more useful in the form
In the usual design process the maximum bending moment is taken from（取自于）the bending moment diagram（弯矩图） and the allowable stress is determined (quite frequently in accordance with（根据）the rules of some legally constituted code) from standard strength tests in combination with（与..结合）a safety factor. The right-hand side of (4-6) is then known, and it remains（仍然是） to select or design a member that will satisfy the inequality. When a standard section is to be used, the tables（表格）could be searched until a member is found such that the combination of I and c satisfies (4-6). This takes more time than is really necessary, since the tables also provide the value of I/c for each member under the heading（标题）S, the section modulus（截面模量）.
当试算构件的应力等于或小于许用应力时，也就是说，如果
在通常的设计过程中，最大的弯距从弯距图上取得，而许用应力通过标准强度试验并考虑安全系数后确定（往往是根据一些法规的规则）。这样，已知式（4-6）右手边的值，则仍然是选择或设计构件以满足该不等式。当使用标准截面时可以查找表格，直至找到的构件其I和c值的组合能满足式（4-6）。这样花费的时间比实际需要的多，因为表格中在截面模量S的标题下也提供了每一个构件的I/c的值。
试算构件即为合格。根据设计的需要，（上述）不等式以下列形式出现更有用，即
That is, the section modulus is defined as（定义为）
To select a member, the S column（列） is consulted（查阅） and any member that satisfies (4-8) could be used. The members with very high values of S will obviously be understressed（应力不足的）and wasteful of material. The best design, if there are no other constraints, will be that which satisfies (4-8) with the minimum amount of material.
With tabulated values of S available it is much more convenient to use (4-6) in the form
The smallest acceptable S does not necessarily coincide with（符合）the most economical member. To select the lightest and most economical standard section, the listed values of mass should be examined to find the lightest member with an acceptable S. The problem becomes much more complex if built-up（组合）member is being designed because its cost will depend upon the combined costs of web plate, angles and cover plates as well as fabrication（装配）costs so that the lightest member is not necessarily the most economical.
截面模量定义为
为选择构件而查阅S这一列，则任何满足式（4-8）的构件都可采用。显而易见，对S值很高的构件，其应力是不足的，并浪费了材料。如果没有其他的限制，最好的设计将是以最少的材料满足式（4-8）。能接受的最小的S值不必是最经济的构件。为了选择最轻和最经济的标准截面，应检查列出的质量值，以找到能接受的S值下的最轻构件。如果在设计一个组合构件时，则问题变得复杂得多，因为它的费用将依赖于腹板、角钢和盖板的费用以及装配的费用，因此，最轻的构件未必是最经济的构件。
根据表格中得到的值S，将式（4-6）以下列形式使用要方便得多，即
Deflections Due to Bending 弯曲挠度
The main purpose of this chapter（本节） was to develop（提出）the flexure（屈曲）formulas, and to provide some experience in applying them. Statically indeterminate（超静定）cases were encountered and some insight（认识） gained as to（就..）the difficulty and importance of this category of problem.
本节的主要目的是提出屈曲公式，并在运用公式时提供一些经验。当遇到超静定的情况时，就此类问题的难点和重点获得一些认识。
Superposition（叠加法）was presented（提出） as the preferred（优先的）method for solving certain problems. However, becoming familiar with（熟悉）superposition was more important than finding solutions to the problems（问题的答案） because superposition has application in many areas of stress analysis and will be used frequently in our future studies. 为解决某些问题，叠加法作为优先的方法被提出。但是，熟悉叠加法远比找到问题的答案重要，因为叠加法已经用于应力分析的很多领域，而且，在我们今后的研究中还会经常使用。
Moment-area（弯距图面积法）was found to be a convenient method for solving various problems. It is a method that becomes quite complicated and requires further development（展开） when more advanced structures are encountered. At the present stage it is sufficient for you to be acquainted with（了解）the fundamentals（基本原理）of the method. Deflection of long-radius（长半径） curved beams was introduced（引入）to illustrate the power of the principles underlying（构成..的基础）the moment-area method and so that you would appreciate（知道）the differences between straight and curved beams.