Loads, Strength, and Structural Safety

A. Loads

Loads that act on structures are usually classified as dead loads and live loads. Dead loads are fixed in location and constant in magnitude throughout the life of the structure. Usually the self-weight of a structure is the most important part of the dead load. This can be calculated closely, based on the dimensions of the structure and the unit weight of the material. Concrete density varies from about 90 to 120 pcf (14 to 19 kN/) for lightweight concrete, and is about 145 pcf (23 kN/) for normal concrete. In calculating the dead load of structural concrete, usually a 5 pcf (1 kN/) increment is included with the weight of the concrete to account for the presence of the reinforcement.

Live loads are loads such as occupancy, snow, wind, or traffic loads, or seismic forces. They may be either fully or partially in place, or not present at all. They may also change in location.

Although it is the responsibility of the engineer to calculate dead loads, live loads are usually specified by local, regional, or national codes and specifications. Typical sources are the publications of the American National Standards Institute, the American Association of State Highway and Transportation Officials and, for wind loads, the recommendations of the ASCE Task Committee on Wind Forces.

Specified live loads usually include some allowance for overload, and may include dynamic effects, explicitly or implicitly. Live loads can be controlled to some extent by measures such as posting of maximum loads for floors or bridges, but there can be no certainty that such loads will not be exceeded. It is often important to distinguish between the specified load, and what is termed the characteristic load, that is, the load that actually is in effect under normal conditions of service, which may be significantly less. In estimating the long-term deflection of a structure, for example, it is the characteristic load that is important, not the specified load.

The sum of the calculated dead load and the specified live load is called the service load, because this is the maximum load which may reasonably be expected to act during the service life of the structure. The factored load, or failure load which a structure must just be capable of resisting is a multiple of the service load.

一、荷载

作用在结构上的荷载通常分为恒载和活载。恒载在结构整个使用寿命期间的位置是固定的，其大小是不变的。通常，结构的自重是恒载的最重要部分。它可以根据结构的尺寸和材料的单位重量进行精确计算。混凝土的密度是变化的，对于轻质混凝土大约从90至120 pct (14至19 kN/m3)，对于标准混凝土大约为 145 pcf (23 kN/m3)。在计算结构混凝土的恒载时，考虑到钢筋的存在，通常除了混凝土的重量以外还计入5 pcf (1 kN/m3)的增加量。

活载就是诸如居住、雪、风和车辆荷载或地震力等荷载。它们可能全部或部分地出现，或者根本不出现。这些荷载的位置是变化的。

计算恒载是工程师的职责，然而活载通常由当地的、地区的或国家的规范和准则所规定。标准的来源是美国国家标准学会、美国州际公路与运输工作者协会主办的刊物，对于风荷载采用美国土木工程学会风力专题委员会的建议。

规定活载一般标明某些容许的超载，并可以明确地或隐含地计入动态影响作用。活载可以采用在楼板或桥梁标明最大荷载那样的措施在某种程度上加以控制，但是也不能肯定这些荷载不会被超过。将规定荷载和所谓特征荷载区别开来往往是很重要的，也就是说，后者是正常使用情况下实际起作用的荷载，它可能很小。例如在计算结构的长期挠度时，重要的是特征荷载，而不是规定荷载。

计算得到的恒载和规定活载的总和称为使用荷载，因为这是在结构使用寿命期间可预料到要作用的最大荷载。使用荷载乘以一个系数就是计算荷载，即破坏荷载；它就是结构必须恰好能承受的荷载。

B. Strength

The strength of a structure depends on the strength of the materials from which it is made. Minimum material strengths are specified in certain, standardized ways. The properties of concrete and its components, the methods of mixing, placing, and curing to obtain the required quality, and the methods for testing, are specified by the American Concrete Institute (ACI). Included by reference in the same document are standards of the American Society for Testing Materials (ASTM) pertaining to reinforcing and prestressing steels and concrete.

Strength also depends on the care with which the structure is built. Member sizes may differ from specified dimensions, reinforcement may be out of position, or poor placement of concrete may result in voids. An important part of the job of the engineer is to provide proper supervision of construction. Slighting of this responsibility has had disastrous consequences in more than one instance.

二、强 度

结构的强度取决于建造它的材料的强度。材料的最小强度都以一些标准的方式来规定。美国混凝土学会（ACI）对混凝土的性质及其成分、满足质量要求的拌和、浇筑和养护方法以及试验方法均作了规定。在同一文件中，作为参考也列入了美国材料试验协会(ASTM) 关于普通钢筋、预应力钢筋和混凝土的标准。

强度也取决于结构施工的精心程度。构件的大小可能与规定的尺寸有所不同，钢筋可能移位，或者由于混凝土浇筑的不好可能会造成空洞。工程师工作的重要职责是要保证应有的施工监督。工程师的失职曾经不止一次地产生了造成巨大损失的后果。

C. Structural Safety

Safety requires that the strength of a structure be adequate for all loads that may conceivably act on it. If strength could be predicted accurately and if loads were known with equal certainty, then safety could be assured by providing strength just barely in excess of the requirements of the loads. But there are many sources of uncertainty in the estimation of loads as well as in analysis, design, and construction. These uncertainties require a safety margin.

In recent years engineers have come to realize that the matter of structural safety is probabilistic in nature, and the safety provisions of many current specifications reflect this view.

Separate consideration is given to loads and strength. Load factors, larger than unity, are applied to the calculated dead loads and estimated or specified service live loads, to obtain factored loads that the member must just be capable of sustaining at incipient failure. Load factors pertaining to different types of loads vary, depending on the degree of uncertainty associated with loads of various types, and with the likelihood of simultaneous occurrence of different loads.

三、结构安全度

安全度要求结构的强度足以承受可以预料到的，作用在结构上的全部荷载。如果强度能够精确地预先计算而且荷载也同样确切地知道的话，则所提供的强度只要稍微超过荷载的要求就能保证安全。可是存在着许多因素会导致在荷载的估算以及分析、设计和施工等方面的不确定性。这些不确定因素就要求具有安全储备。

近些年来，工程师们已经开始认识到结构安全问题实质上就是概率统计问题，因此许多现行规范的安全规定都反映了这一观点。

荷载和强度分别加以考虑。将大于1的荷载系数乘以所计算的恒载和估定或规定的使用活载，可以得到构件在开始破坏时恰好能承受的计算荷载。对于不同的荷载类型，荷载系数是不相同的，它取决于各种不同荷载和不同荷载可能同时出现的不确定程度。