Loads consist of: (1) concentrated and distributed forces ( direct actions) , (2) imposed deformations (indirect actions).

A load is assumed as a single load if it is not related to any other load or imposed deformation acting on the structure. In practice more than one single load acts on the structure, although it is convenient to consider each load separately.

Loads are random processes.?However, in order to match the?requirements of the methods of calculation actually used in most structural specifications ( allowable stresses and semi-probabilistic methods) , each load is also characterized by the parameters representative of the different computational methods.

The loads can be classified with respect to their effect on the structure (static or dynamic) or with respect to their variation of intensity. Loads can also be classified with respect to some particular aspect, such as limited or not limited, having long or short duration, dependent or not on human activities etc.

荷載包括：(1)集中力和分布力（直接作用），(2)強(qiáng)制性的變形（間接作用）。

如果一個(gè)荷載不與作用在結(jié)構(gòu)上的任何其他荷載或強(qiáng)制性變形發(fā)生關(guān)系，那么這個(gè)荷載就可以被假定為單一荷載。實(shí)際上，作用在結(jié)構(gòu)上的單一荷載通常不止一個(gè)，然而，獨(dú)立地分析每一個(gè)荷載總是比較方便的。

荷載是隨機(jī)過(guò)程。但是，為了與大多數(shù)結(jié)構(gòu)規(guī)范中實(shí)際使用的計(jì)算方法（容許應(yīng)力法和半概率法）相協(xié)調(diào)，每個(gè)荷載也用不同計(jì)算方法的參數(shù)表達(dá)式來(lái)表示。

荷載可以根據(jù)它們對(duì)結(jié)構(gòu)的影響（靜態(tài)或動(dòng)態(tài)）或者根據(jù)它們的集度變化進(jìn)行分類(lèi)。荷載也可以根據(jù)一些特定的方式進(jìn)行分類(lèi)，例如受約束的或不受約束的，長(zhǎng)期的或短期的，依賴于人類(lèi)活動(dòng)或不依賴于人類(lèi)活動(dòng)等。

1?Classification of loads with respect to the structural response

A distinction is made between two types of load according to the response of the structure:

(a) Static loads, which are applied to the structure without causing significant accelerations of the structure or of structural elements.

(b) Dynamic loads, which cause significant accelerations of the?structure.

The same load might be static or dynamic depending upon the structure to which it is applied. Generally loads can be considered static loads, provided that the dynamic effects are taken into account by an increment of the intensity of the loads. In other cases a dynamic analysis is necessary.

1. 根據(jù)結(jié)構(gòu)響應(yīng)對(duì)荷載進(jìn)行分類(lèi)

按照結(jié)構(gòu)的響應(yīng)可以區(qū)別兩種類(lèi)型的荷載：

(1)靜荷載，它作用到結(jié)構(gòu)上不會(huì)使結(jié)構(gòu)和結(jié)構(gòu)的構(gòu)件產(chǎn)生明顯的加速度。

(2）動(dòng)荷載，它會(huì)使結(jié)構(gòu)產(chǎn)生明顯的加速度。

同樣的荷載可以是靜態(tài)的，也可以是動(dòng)態(tài)的，這取決于它所作用的結(jié)構(gòu)。通常，倘若動(dòng)力影響可以用荷載集度的增量加以考慮，則這些荷載可以看成靜荷載。在其他情況下，需要進(jìn)行動(dòng)力分析。

2?Classification of the loads with respect to the variation in time of their intensity

If we indicate by??the life of the structure, which can be assumed equal to 50 years (unless there are different provisions) , the loads are divided in the following way:

(a) Dead loads act on the structure for the whole of its life with negligible variations of intensity. They include: the weight of the structure; the weight of each superstructure; forces caused by the pressure of the ground (except from the effects of moving loads applied on the ground); deformations imposed by the fabrication and erection processes; loads due to the displacements of supports; the actions due to prestressing.

(b) Live loads act on the structure with instantaneous values which can be noticeably different from each other. These actions can be further divided into loads of long and short duration:

(i) Variable long-duration loads act on the structure, and their duration is of the same order as the life of the structure . Examples of loads which fall into this category are: the weight of the non-structural elements which form the construction (floor, plaster, roof covering etc.); the weight of furniture, stocked merchandise, parked cars etc.; loads related to the fabrication or erection or with some of their phases.

(ii) Variable short-duration loads act on the structure for a period that is short compared with the life of the structure . Typical examples are: moving loads due to people, moving cars etc.; the action of the wind; ice formation; earthquakes in seismic areas.

(iii) Some actions can be considered of short or long duration depending upon the particular case. Among these are: snow loads; the effects due to variation of water level in a tank; the effects due to variation of temperature.

(c) Exceptional loads are those loads which are very unlikely to act on the structure, such as those due to: collisions, explosions, fires, earthquakes in nonseismic areas.

2. 根據(jù)荷載集度隨時(shí)間的變化對(duì)荷載進(jìn)行分類(lèi)

如果我們用代表結(jié)構(gòu)的壽命，假定它為50年（除非另有規(guī)定），那么，可以按下述方式劃分荷載：

(1)恒荷載，建筑物使用壽命期間，它始終作用在結(jié)構(gòu)上，而且荷載集度的變化可以忽略不計(jì)。恒荷載包括：結(jié)構(gòu)的自重，各上層建筑物的重量，由地面壓力引起的力（施加在地面上的移動(dòng)荷載的影響除外），由制作和安裝過(guò)程引起的變形，由支座位移引起的荷載，由預(yù)加應(yīng)力引起的作用力。

(2)活荷載，它作用在結(jié)構(gòu)上的數(shù)值在不同的瞬時(shí)可以發(fā)生很大變化。它可以進(jìn)一步分為長(zhǎng)期荷載和短期荷載：

①作用在結(jié)構(gòu)上的可變長(zhǎng)期荷載，其持續(xù)時(shí)間和結(jié)構(gòu)的使用壽命一樣長(zhǎng)。屬于這一類(lèi)荷載的例子有：組成建筑物的非結(jié)構(gòu)構(gòu)件（樓板、灰泥、屋面覆蓋層等）的重量；設(shè)備、儲(chǔ)存的貨物、停放的車(chē)輛等的重量；與制作或安裝有關(guān)的荷載。

②作用在結(jié)構(gòu)上的可變短期荷載，其持續(xù)的時(shí)間與結(jié)構(gòu)的使用壽命相比是短暫的。典型的例子有：人群荷載、車(chē)輛荷載等移動(dòng)荷載，風(fēng)荷載、冰荷載，地震區(qū)的地震。

③某些作用既可以認(rèn)為是短期荷載，也可以認(rèn)為是長(zhǎng)期荷載。需要依特定的情況而定。這些作用有：雪荷載；由于容器中液面變化產(chǎn)生的作用力；由于溫度變化產(chǎn)生的作用力。

(3)偶然荷載是指不太可能作用在結(jié)構(gòu)上的那些荷載。諸如由下列事件引起的荷載：碰撞，爆炸，火災(zāi)，非地震區(qū)的地震。

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3?Classification of loads with respect to their variation of intensity along the structure

We distinguish throughout the structure between:

(a) Fixed loads with a known spatial distribution, such that the magnitude and position of the load is known on all of the structure when it is known at a given point.

(b) Nonfixed loads, which can have ( within certain bounds) any distribution throughout the structure.

Many loads can be considered to consist of a fixed and a nonfixed portion. It is often necessary to separate a load into a fixed portion and other portions acting in a random way on the structure. The analysis of nonfixed loads implies consideration of different loading conditions, each of them being defined when the magnitude and position of all the nonfixed loads are given.

3. 根據(jù)作用在結(jié)構(gòu)上荷載集度的變化對(duì)荷載進(jìn)行分類(lèi)

我們把作用在結(jié)構(gòu)上的荷載區(qū)分為：

(1)具有已知空間分布的固定荷載，當(dāng)空間結(jié)構(gòu)上某一點(diǎn)的荷載大小和位置為已知時(shí)，結(jié)構(gòu)上所有構(gòu)件的荷載大小和位置都是已知的。

(2)非固定荷載，它是在結(jié)構(gòu)的某一范圍內(nèi)任意分布的荷載。

許多荷載可以看成是由固定荷載部分和非固定荷載部分組成的。往往需要把作用在結(jié)構(gòu)上的荷載分成固定荷載部分和以隨機(jī)方式作用在結(jié)構(gòu)上的其他荷載部分。對(duì)非固定荷載的分析意味著需要考慮各種荷載狀態(tài)，當(dāng)所有非固定荷載的大小和位置為已知時(shí)，各種荷載條件就可以被確定。