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、荷載

作用在結(jié)構(gòu)上的荷載通常分為恒載和活載。恒載在結(jié)構(gòu)整個使用壽命期間的位置是固定的，其大小是不變的。通常，結(jié)構(gòu)的自重是恒載的最重要部分。它可以根據(jù)結(jié)構(gòu)的尺寸和材料的單位重量進行精確計算?；炷恋拿芏仁亲兓模瑢τ谳p質(zhì)混凝土大約從90至120 pct (14至19 kN/m3)，對于標準混凝土大約為 145 pcf (23 kN/m3)。在計算結(jié)構(gòu)混凝土的恒載時，考慮到鋼筋的存在，通常除了混凝土的重量以外還計入5 pcf (1 kN/m3)的增加量。

活載就是諸如居住、雪、風和車輛荷載或地震力等荷載。它們可能全部或部分地出現(xiàn)，或者根本不出現(xiàn)。這些荷載的位置是變化的。

計算恒載是工程師的職責，然而活載通常由當?shù)氐?、地區(qū)的或國家的規(guī)范和準則所規(guī)定。標準的來源是美國國家標準學會、美國州際公路與運輸工作者協(xié)會主辦的刊物，對于風荷載采用美國土木工程學會風力專題委員會的建議。

規(guī)定活載一般標明某些容許的超載，并可以明確地或隱含地計入動態(tài)影響作用?；钶d可以采用在樓板或橋梁標明最大荷載那樣的措施在某種程度上加以控制，但是也不能肯定這些荷載不會被超過。將規(guī)定荷載和所謂特征荷載區(qū)別開來往往是很重要的，也就是說，后者是正常使用情況下實際起作用的荷載，它可能很小。例如在計算結(jié)構(gòu)的長期撓度時，重要的是特征荷載，而不是規(guī)定荷載。

計算得到的恒載和規(guī)定活載的總和稱為使用荷載，因為這是在結(jié)構(gòu)使用壽命期間可預料到要作用的最大荷載。使用荷載乘以一個系數(shù)就是計算荷載，即破壞荷載；它就是結(jié)構(gòu)必須恰好能承受的荷載。

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、強 度

結(jié)構(gòu)的強度取決于建造它的材料的強度。材料的最小強度都以一些標準的方式來規(guī)定。美國混凝土學會（ACI）對混凝土的性質(zhì)及其成分、滿足質(zhì)量要求的拌和、澆筑和養(yǎng)護方法以及試驗方法均作了規(guī)定。在同一文件中，作為參考也列入了美國材料試驗協(xié)會(ASTM) 關(guān)于普通鋼筋、預應力鋼筋和混凝土的標準。

強度也取決于結(jié)構(gòu)施工的精心程度。構(gòu)件的大小可能與規(guī)定的尺寸有所不同，鋼筋可能移位，或者由于混凝土澆筑的不好可能會造成空洞。工程師工作的重要職責是要保證應有的施工監(jiān)督。工程師的失職曾經(jīng)不止一次地產(chǎn)生了造成巨大損失的后果。

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、結(jié)構(gòu)安全度

安全度要求結(jié)構(gòu)的強度足以承受可以預料到的，作用在結(jié)構(gòu)上的全部荷載。如果強度能夠精確地預先計算而且荷載也同樣確切地知道的話，則所提供的強度只要稍微超過荷載的要求就能保證安全。可是存在著許多因素會導致在荷載的估算以及分析、設計和施工等方面的不確定性。這些不確定因素就要求具有安全儲備。

近些年來，工程師們已經(jīng)開始認識到結(jié)構(gòu)安全問題實質(zhì)上就是概率統(tǒng)計問題，因此許多現(xiàn)行規(guī)范的安全規(guī)定都反映了這一觀點。

荷載和強度分別加以考慮。將大于1的荷載系數(shù)乘以所計算的恒載和估定或規(guī)定的使用活載，可以得到構(gòu)件在開始破壞時恰好能承受的計算荷載。對于不同的荷載類型，荷載系數(shù)是不相同的，它取決于各種不同荷載和不同荷載可能同時出現(xiàn)的不確定程度。