4结构设计原理

4 Philosophy of structural Design

结构设计原理

A structural engineering project can be divided into three phases: planning, design, and

construction、(一个结构工程项目可以分为三个阶段:规划、设计与施工)

Structural design involves determining the most suitable proportions of a structure and

dimensioning the structural elements and details of which it is composed、 (结构设计内容包括要确定结构最优的比例尺寸,并确定结构构件与细部的尺寸) 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、 (这就是整个结构工程项目中技术性最高、数学严谨性最强的阶段,但如果不能同规划与施工阶段完全协调、配合,就不能也不应该付诸实施) The successful designer is at all times fully conscious of the 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、 (一个成功的设计者总就是会全面考虑结构在初步规划中所涉及到的各种条件,以及以后的施工中会碰到的各种问题)

Specially, the structural design of any structure first involves the establishment of the

loading and other design conditions that must be resisted by the structure and therefore must

be considered in its design、 (在结构设计中,应首先确定结构所必须承受的荷载与其它设计条件,她们就是结构设计中的必要条件) 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, temperature, shrinkage,

creep, or other design conditions、 (然后分析在荷载、温度、收缩、徐变及其她设计条件下结构所产生的总内力(轴力、剪力、玩具、扭矩)、应力强度、应变、变形与反力)

Finally comes proportioning and selection of materials of the members and connections so

as to resist adequately the effects produced by the design conditions、 (最后确定各构件及连接的尺寸并选择其材料,以抵抗设计条件所产生的作用) The criteria used to judge

whether particular proportions will result in the desired behavior reflect accumulated

knowledge (theory, field and model test, and practical experience), intuition, and judgement、 (当判断某些特殊部位的设计就是否符合要求时,就要用到一些经验知识(理论、现场及模型试验、实际经验),与直观判别的方法) 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 stress intensities with those produced by

the service loadings and other design conditions、 (对于桥梁与房屋这些最常见的土木工程结构来说,过去最常用的就是将容许应力与使用荷载与其它设计条件下产生的应力进行比较,然后在此基础上进行设计) 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 material should not be

exceeded at the most highly stressed points of the structure、 (由于容许应力在选择时就是依据了如下概念:即结构的最大应力不得超过材料屈服点所对应的应力或应变,因此这种传统的设计基础被称为弹性设计) Of course, the selection of the allowable stresses

may also be modified by a consideration of the possibility of failure due to fatigue, buckling,

or brittle fracture or by consideration of the permissible deflections of the structure、 (当然,考虑到结构也可能由于疲劳、压曲、脆断或容许变形而引起失效,容许应力的选择也可随之调整)

Depending on the type of structure and the conditions involved, the stress intensities

computed bin 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 actual structure

by the actual conditions to which it is exposed、 (根据结构类型与所涉及到的条件,采用实际结构的分析模型,在假设的设计条件下计算出的应力强度,与实际结构在实际条件下所产生的应力强度,可能相符也可能不相符) The degree of correspondence is not

important, provided that the computed stress intensities can be interpreted in terms of

previous experience、 (只要计算出的应力强度能根据以往的经验来解释,吻合程度就不就是很重要) The selection of the service conditions and allowable stress intensities

provides a margin of safety against failure、 (使用条件与容许应力强度的选择为防止结构失效提供了安全储备) The selection of the magnitude of this margin depends on the

degree of uncertainty regarding loading, analysis, design, materials, and construction and on

the consequences of failure、 (储备量值的选择依赖于相关荷载、分析、设计、施工与失效后果的不确定性程度) For example, if an allowable tensile stress of 20000pis is selected for structural steel with a yield stress of 33000psi, the margin of safety (or factor of

safety) provided against tensile yielding is 33000/20000, or 1、65、 (比如:对于结构钢,屈服应力为33000,而容许应力为20000,那么有屈服拉应力提供的安全储备(安全系数)为33000/20000,或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、 (容许应力法有一个很大的缺点,就就是不能对各种类型的结构及其各部分提供相同的超载能力) As

a result, there is today a rapidly growing tendency to base the design on the ultimate

strength and serviceability of the structure, with older allowable-stress approach serving as