EN 1990 Section 4 – Basic variables

Section 4 of EN 1990 covers the three sets of basic variables considered in structural design, viz: actions, material properties and geometry. Here the treatment of actions and material properties will be discussed.

2.5.1 ACTIONS

It is appropriate first to note the use of the term actions in this context. In the past the term loads has traditionally been used, and in fact it remains an entirely valid term in a Eurocode context. However, in the Eurocodes the term loads is used to refer to a set of forces applied to a structure or the ground (i.e. direct actions). The term action is used more generically to mean both loads and also imposed deformations or accelerations, such as those due to thermal movements or earthquakes (i.e. indirect actions). In many ways, the use of the term actions addresses an ambiguity in the way the term load has been used in the past.

Actions are classified by their variation in time as either (see clause 4.1.1(1)P):

o permanent actions (denoted G), e.g. self-weight of structures, road surfacing and indirect

actions such as uneven settlements;

o variable actions (denoted Q), e.g. traffic load, wind and thermal actions; or,

o accidental actions (denoted A), e.g. impact from vehicles.

It will be sensible for designers to become familiar with this terminology, rather than using the terms dead and live load that may have been used in the past. Likewise, it will be advisable to reserve the words persistent and transient for design situations. Referring to a transient load in a Eurocode context is potentially rather confusing since it mixes the terminology for actions and design situations. For permanent actions, EN 1990:2002,4.1.2(2)P explains that their characteristic value should either be taken as a single value, Gk, or if the variability of G cannot be considered as small, as the worst

case of an upper value, Gk,sup, or a lower value, Gk,inf. Further guidance is provided on where the

variability can be considered to be small and specifically, EN 1990:2002, 4.1.2(5) states that the self weight of the structure may be represented by a single value Gk based on mean density and nominal

dimensions.

In bridge design, important cases where the variability of G cannot be considered as small are loads due to surfacing and ballast (see EN 1991-1-1:2002, 5.2.3). When the variability in G cannot be considered as small, it is helpful to note that 4.1.2(2)P does not require upper and lower values of G to be applied to the adverse and relieving areas of the influence surface. Rather, whichever single value gives the worst case is taken throughout.

For variable actions, EN 1990:2002, 4.1.3 introduces another new concept for many bridge designers. This is the concept of the four representative values of a variable action, and it is the third key concept, as summarised in Section 2.9. As discussed later, these representative values are used in the different combinations of actions.

The four representative values have different probabilities of occurrence. They are called the characteristic, combination, frequent and quasi-permanent values. The characteristic value is the main representative value, and is the value generally specified in the various parts of EN 1991. It is a statistically extreme value: in the calibration of the basic highway traffic loading model, LM1, it is a 1000-year return period value (see EN 1991-2: 2003, Table 2.1); for wind and thermal actions it is generally a 50-year return period value.

The combination value is established by EN 1990:2002 to address the reduced likelihood that extreme values of more than one variable action will occur simultaneously. The frequent value of a variable action can be understood as the value that is exceeded ‘occasionally, but not too often’ – perhaps weekly or monthly. The calibration of the frequent value of LM1 is based on a one week return period. The use of the word frequent here sometimes causes some confusion, since it is essentially a relative term; here it is frequent in relation to the characteristic value. The quasi- permanent value is generally the value that is exceeded most of the time. For traffic loads on bridges and wind actions, the recommended quasi-permanent value is therefore zero.

The four representative values of a variable action are illustrated in Fig. 2.1. The combination, frequent and quasi-permanent values of a variable action are found by multiplying the characteristic value by ψ0, ψ1, and ψ2 respectively. For bridge design, recommended ψ-factors are given in EN

1990:2002, A2.2. The UK National Annex modifies the values for road bridges and footbridges.

Characteristic value Qk Combination value

ψ

ψ

ψ

Fig. 2.1. Illustration of four representative values of a variable action

2.5.2 MATERIAL AND PRODUCT PROPERTIES

EN 1990:2002, 4.2(1) explains that properties of materials (including soil and rock) should be represented by characteristic values. It also states that when a limit state verification is sensitive to the variability of a material property, upper and lower characteristic values of the material property should be taken into account (clause 4.2(2)). Although it is rare that an upper characteristic material property will govern a design, rather than the lower value that is generally used, there are some important cases in bridge design when it can do so. These include earth pressures applied to integral bridges and other buried structures, where an upper characteristic angle of shearing resistance of the soil can govern.

2.6 EN1990 Section 5 – Structural analysis and design