Loading Conditions

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.

荷载包括：(1)集中力和分布力（直接作用），(2)强制性的变形（间接作用）。

如果一个荷载不与作用在结构上的任何其他荷载或强制性变形发生关系，那么这个荷载就可以被假定为单一荷载。实际上，作用在结构上的单一荷载通常不止一个，然而，独立地分析每一个荷载总是比较方便的。

荷载是随机过程。但是，为了与大多数结构规范中实际使用的计算方法（容许应力法和半概率法）相协调，每个荷载也用不同计算方法的参数表达式来表示。

荷载可以根据它们对结构的影响（静态或动态）或者根据它们的集度变化进行分类。荷载也可以根据一些特定的方式进行分类，例如受约束的或不受约束的，长期的或短期的，依赖于人类活动或不依赖于人类活动等。

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. 根据结构响应对荷载进行分类

按照结构的响应可以区别两种类型的荷载：

(1)静荷载，它作用到结构上不会使结构和结构的构件产生明显的加速度。

(2）动荷载，它会使结构产生明显的加速度。

同样的荷载可以是静态的，也可以是动态的，这取决于它所作用的结构。通常，倘若动力影响可以用荷载集度的增量加以考虑，则这些荷载可以看成静荷载。在其他情况下，需要进行动力分析。

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. 根据荷载集度随时间的变化对荷载进行分类

如果我们用代表结构的寿命，假定它为50年（除非另有规定），那么，可以按下述方式划分荷载：

(1)恒荷载，建筑物使用寿命期间，它始终作用在结构上，而且荷载集度的变化可以忽略不计。恒荷载包括：结构的自重，各上层建筑物的重量，由地面压力引起的力（施加在地面上的移动荷载的影响除外），由制作和安装过程引起的变形，由支座位移引起的荷载，由预加应力引起的作用力。

(2)活荷载，它作用在结构上的数值在不同的瞬时可以发生很大变化。它可以进一步分为长期荷载和短期荷载：

①作用在结构上的可变长期荷载，其持续时间和结构的使用寿命一样长。属于这一类荷载的例子有：组成建筑物的非结构构件（楼板、灰泥、屋面覆盖层等）的重量；设备、储存的货物、停放的车辆等的重量；与制作或安装有关的荷载。

②作用在结构上的可变短期荷载，其持续的时间与结构的使用寿命相比是短暂的。典型的例子有：人群荷载、车辆荷载等移动荷载，风荷载、冰荷载，地震区的地震。

③某些作用既可以认为是短期荷载，也可以认为是长期荷载。需要依特定的情况而定。这些作用有：雪荷载；由于容器中液面变化产生的作用力；由于温度变化产生的作用力。

(3)偶然荷载是指不太可能作用在结构上的那些荷载。诸如由下列事件引起的荷载：碰撞，爆炸，火灾，非地震区的地震。

 

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. 根据作用在结构上荷载集度的变化对荷载进行分类

我们把作用在结构上的荷载区分为：

(1)具有已知空间分布的固定荷载，当空间结构上某一点的荷载大小和位置为已知时，结构上所有构件的荷载大小和位置都是已知的。

(2)非固定荷载，它是在结构的某一范围内任意分布的荷载。

许多荷载可以看成是由固定荷载部分和非固定荷载部分组成的。往往需要把作用在结构上的荷载分成固定荷载部分和以随机方式作用在结构上的其他荷载部分。对非固定荷载的分析意味着需要考虑各种荷载状态，当所有非固定荷载的大小和位置为已知时，各种荷载条件就可以被确定。