Page 41 - Dynamic Loading and Design of Structures

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1998; European Standard, 2000). In operational codes, the values adopted are usually based
on historical values linked to successful experience, and are often simplified in order to limit
the different values that a designer needs to consider for all the different load cases.

Frequent value ( 1 Q )
k
The frequent value is used for the dominating variable action in combinations at the ultimate
limit state involving accidental actions. It is also used in reversible serviceability limit states.
As in the previous case, it can be expressed as a fraction of the principal characteristic value
through a factor, 1 (< 1). Typically, the frequent value may be estimated from the point-in-
time (or instantaneous) distribution of the action, i.e.

where q=0.01 is suggested for buildings (European Standard, 2000). The criterion may also
be expressed as a return period; for example, for road bridges the frequent value of the traffic
load is determined as having a return period of 1 week.

Quasi-permanent value ( 2Qk)
The quasi-permanent value is used for the non-dominating variable action in combinations at
the ultimate limit state involving accidental actions. It is also used in reversible serviceability
limit states and in the calculation of long term effects in serviceability limit states.
The quasi-permanent value may be regarded as a special case of the frequent value with
q=0.5. Thus,

It can also be defined as the mean value of the instantaneous probability distribution. In
certain cases (e.g. wind or road traffic) the value of 2 is so low that it is set equal to zero
(European Standard, 2000).
Table 1.5 summarizes some of the factors as given in the Eurocode (European Standard,
2000). More cases are covered therein, and each code has its own system of factors, broadly
based on the principles outlined above. However, as already

Table 1 .5 Typical factors for buildings according to Eurocode 1 .

Action 0 1 2

Imposed load (domestic, residential, office) 0.7 0.5 0.3
Snow load (Scandinavia and Rest of Europe for altitude >1,000 m) 0.7 0.5 0.2
Snow load (rest of Europe for altitude <1 ,000 m) 0.5 0.2 0.0
Wind load 0.6 0.2 0.0