1、 附录 Philosophy of Structural Design A structural engineering project can be divided into three phases: planning, design, and construction. Structural design involves determining the most suitable proportions of a structure and dimensioning the structural elements and details of which it is composed.
2、 This is the most highly technical and mathematical phase of a structural engineering project, but it cannot-and certainly should not-be conducted without being fully coordinated with the planning and construction phases of the project. The successful designer is at all times fully conscious of the
3、various considerations that were involved in the preliminary planning for the structure and, likewise, of the various problems that may later be encountered in its construction. Specially, the structural design of any structure first involves the establishment of the loading and other design conditi
4、ons that must be resisted by the structure and therefore must be considered in its design. Then comes the analysis (or computation ) of the internal gross forces (thrust, shears, bending moments, and twisting moments), stress intensities, strains, deflections, and reactions produced by the loads, te
5、mperature, shrinkage, creep, or other design conditions. Finally comes the proportioning and selection of materials of the members and connections so as to resist adequately the effects produced by the design conditions. The criteria used to judge whether particular proportions will result in the de
6、sired behavior reflect accumulated knowledge (theory, field and model tests, and practical experience), intuition, and judgment. For most common civil engineering structures such as bridges and buildings, the usual practice in the past has been to design on the basis of a comparison of allowable str
7、ess intensities with those produced by the service loadings and other design conditions. This traditional basis for design is called elastic design because the allowable stress intensities are chosen in accordance with the concept that the stress or strain corresponding to the yield point of the mat
8、erial should not be exceeded at the most highly stressed points of the structure. Of course, the selection of the allowable stresses may also be modified by a consideration of the permissible deflections of the structure. Depending on the type of structure and the conditions involved, the stress int
9、ensities computed in the analytical model of the actual structure for the assumed design conditions may or may not be in close agreement with the stress intensities produced in the actual structure by the actual conditions to which it is exposed. The degree of correspondence is not important, provid
10、ed that the computed stress intensities can be interpreted in terms of previous experience. The selection of the service conditions and the allowable stress intensities provides a margin of safety against failure. The selection of the magnitude of this margin depends on the degree of uncertainty reg
11、arding loading, analysis, design, materials, and construction and on the consequences of failure. For example, if an allowable tensile stress of 20000 psi is selected for structural steel with a yield stress of 33000 psi, the margin of safety (or factor of safety) provided against tensile yielding i
12、s 33000/20000, or 1.65. The allowable-stress approach has an important disadvantage in that it does not provide a uniform overload capacity for all parts and all types of structures. As a result, there is today a rapidly growing tendency to base the design on the ultimate strength and serviceability
13、 of the structure, with the older allowable-stress approach serving as an alternative basis for design. The newer approach currently goes under the name of strength design in reinforce-concrete design literature and plastic design in steel-design literature. When proportioning is done on the strengt
14、h basis, the anticipated service loading is first multiplied by a suitable load factor (greater than 1), the magnitude of which depends upon the uncertainty of the loading, the possibility of its changing during the life of the structure, and, for a combination of loadings, the likelihood, frequency
15、, and duration of the particular combination. In this approach for reinforced-concrete design, the theoretical capacity of a structural element is reduced by a capacity-reduction factor to provide for small adverse variations in material strengths, workmanship, and dimensions. The structure is then
16、proportioned so that, depending on the governing conditions, the increased load would (1) cause a fatigue or a buckling or a brittle-fracture failure or (2) just produce yielding at one internal section (or simultaneous yielding at several sections) or (3) cause elastic-plastic displacement of the s
17、tructure or (4) cause the entire structure to be on the point of collapse. Proponents of this latter approach argue that it results in a more realistic design with a more accurately provided margin of strength over the anticipated service conditions. These improvements result from the fact that none
18、lastic and nonlinear effects that become significant in the vicinity of ultimate behavior of the structure can be accounted for. In recent decades, there has been a growing concern among many prominent engineers that not only is the term “factor of safety” improper and unrealistic, but worse still a
19、 structural design philosophy based on this concept leads in most cases to an unduly conservative and therefore uneconomical design, and in some cases to an unconservative design with too high a probability of failure. They argue that there is no such thing as certainty, either of failure or of safe
20、ty of a structure but only a probability of failure or a probability of safety. They feel, therefore, that the variations of the load effects and the variations of the structural resistance should be studied in a statistical manner and the probability of survival or the probability of serviceability
21、 of a structure estimated. It may not yet be practical to apply this approach to the design of each individual structure. However, it is believed to be practical to do so in framing design rules and regulations. It is highly desirable that building codes and specifications plainly state the factors
22、and corresponding probabilities that they imply. If a good alignment requires a curved bridge-over a part or the total length then all external longitudinal lines or edges of the structure should be parallel to the curved axis, thereby following again the guideline of good order. The transverse axis
23、 of piers or groups of columns should be rectangular (radial) to the curved axis, unless skew crossings over roads or rivers enforce other directions. The requirements of traffic design result occasionally in very acute angles or in level branching which cause difficulties for the bridge engineer to
24、 find pleasing solutions for the bridges. 结构设计原理 一个结构设计工程可以被分为三个阶段：计划、设计、施工。 结构设计包含确定结构最合适的比例并且测量单元体的尺寸及其包含的细部。这是一项结构工程中技术性和数学性最强的一个阶段，但是如果不能全面的与计划和施工阶段相协调的话，它是不能被进行的。成功的设计者在任何时候都能全面地考虑到结构初步设计中包含的各种因素，同时还充分考虑到以后施工中可能遇到的各种问题。 尤其，任何一个结构的结构设计首先包括结构所必须抵抗的荷载及其它设计因素的确定，因此，在设计中必须考虑到。然后开始分析（或计算）由荷载、收缩、徐变或其它设计因素引起的总内力（推力、剪力、弯矩和扭矩），应力强度、应变、变 形及反力等。最后是比例的确定和选择构件和连结件的材料，用来充分
