Introduction

EC2(1) considers all loads as variables in time and space and applies statistical principles to arrive at the loads for design. There is an underlying assumption that the basic loads themselves are described in statistical terms. Thus, when variable loads of different origins, for example superimposed loads on floors and wind loads on the faces of buildings, have to be considered acting together in a load case, the probability of both loads not being at their full characteristic values is allowed for by multipliers called ^ factors. These factors too are derived statistically and values are given in EC1(20) and the NAD to EC2(1).

Thus when a number of variable loads have to be considered simultaneously in any load case, each load is treated in turn as the primary load and others are considered secondary. The primary load is applied at its characteristic value multiplied by the partial safety factor. All secondary loads are applied at their characteristic values multiplied by the partial safety factor and further multiplied by a ^ factor. These 4> factors vary depending upon the limit state and the type of loading being considered.

Mathematically the design load for ultimate limit state may be represented as:

primary load secondary load

While the above procedure is the general approach, EC2 also provides simplified rules:

(a) where only one variable load occurs the design load

(b) when more than one variable load occurs the design load

It is important to note that this Code permits the use of either approach although in some circumstances the general method may result in higher loading.

In practice the simplified procedure will be perfectly satisfactory for most situations and could be used.

The following examples are given to illustrate the thinking behind the general approach and indicate where the general approach may be required.

Usually, when dead loads produce a favourable effect, 7G can be taken as unity. However, if the variation of the magnitude of the dead load is likely to prove sensitive then yG should be taken as 0.9.

For the particular case of continuous beams without cantilevers, the Code permits the use of yQ = 1.35 for all the spans.

When calculating the loads on vertical elements of multi-storey structures the vertical loads may be based on either:

(a) loads from beams multiplied by suitable ^ and yf values; or

(b) loads on beams multiplied by 7f values and a global reduction in loading applied using the procedure given in BS 6399(21). This is the approach in the NAD.

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