Wind actions w

(1) The characteristic values of wind actions should be assessed in accordance with the methods defined in ENV 1991-2-4 and on the basis of the selected return period.

Note 1 : Attention should be paid to the fact that on internal walls which will be an internal part of the structure after completion, during construction the external pressure coefficients cpe for free-standing walls may have to be applied.

Note 2 : In some cases, wind loads may be considered as accidental actions in combinations during execution, e.g. in cyclonic zones.

(2) Where relevant, the loaded area should include the areas of cranes, falsework, etc., that contribute to the action effects under consideration.

(3) For hoisting or launching operations or other construction stages lasting only a few hours, the project specification should define the maximum wind velocity to be considered in the design. Unless otherwise specified, this wind velocity should not be less than 20 m/s at any height.

(4) Where relevant, the critical wind velocity should be determined for cross wind vibrations, galloping and flutter to ensure that these phenomena will not govern the design during the execution stages.

Note : Rain-wind induced vibrations of slender elements of circular cross-section may need to be considered where relevant.

(5) Unless otherwise specified, the characteristic value of wind forces to be considered for the design of bridges during execution should be assessed on the basis of a nominal return period equa to:

To] years for a nominal duration of the transient situation of one year;

years, as for persistent design situations, for a duration greater than 1 year.

(6) Where static equilibrium needs to be considered during the execution phase of a bridge, the values of the characteristic wind pressures on the favourable parts should be reduced by (see 10.11 in ENV 1991-2-4).

Note : Attention should be paid to the fact that a static analysis may be insufficient in some cases and more refined models should be adopted.

(1) Snow loads should be assessed in accordance with ENV 1991-2-3, depending on the local situation of the site and on the basis of the selected return period. However, unless otherwise specified, the load to be considered as the characteristic value during execution stages should not be less than 50 % of the characteristic value determined for persistent design situations.

(2)P For bridges, if daily snow removal (weekends and holidays included) is assured and provisions compatible with the safety requirements for its removal are defined in the project specification, the snow load on the areas from which snow is removed may be reduced to

30 % of the representative value specified in ENV 1991-2-3.

(3) For bridges, when justified by climatic conditions and by the duration of the execution process, the characteristic snow load, represented by a uniformly distributed load, for the verification of static equilibrium during execution should be considered as composed of a nonsymmetrical distribution of snow located in the most unfavourable position. For this verification the snow loads should be assumed to be 25 % of the characteristic snow load on the ground determined for persistent situations.

(1) In general, actions due to water (free or ground water) should be represented as:

- static pressures;

- if relevant, hydrodynamic effects.

Note : In general, phenomena covered by hydrodynamic effects are:

- the hydrodynamic force due to currents on immersed obstacles;

- forces due to wave actions;

- water effects caused by an earthquake (tsunamis).

(2) For the sake of simplicity, actions due to water generally may be taken into account in combinations as permanent actions: the variability of water pressure or of water level should be taken into account by the means of the design situations defined in the project specification. Where actions due to water are to be considered as variable, design values should be directly specified.

(3) Water actions exerted by currents on immersed structures are perpendicular to the contact surfaces, and may induce dynamic effects. They should be determined for the relevant current speed, water depth and shape of the structure, defined in the project specification and depending on the design situation for the construction period under consideration.

(4) The magnitude of the horizontal force exerted by currents on the vertical surface of an immersed object (Fig. 4.2) may be determined by the following expression:

Fwa=kPwahbvvsa

where:

vwa is themean speed of the water, averaged over the depth, in m/s; pwa is the density of water in kg/m3;

h is the water depth, but not including, where relevant, local scour depth in meters; b is the width of the object in meters; k is the shape factor: k = 0,72 for an object of square or rectangular horizontal cross-section, k = 0,35 for an object of circular horizontal cross-section.

General__

scour depthj^"

Current pressure

Local scour depth

Figure 4.2: Pressure and force due to currents on cofferdams and bridge piers

General__

scour depthj^"

Current pressure

Local scour depth

Figure 4.2: Pressure and force due to currents on cofferdams and bridge piers

Note . The total effect of water includes both static and dynamic pressures on each surface.

(5) Actions from rainwater should be considered when either the failure of dewatering devices has to be taken into account or ponding effects may occur either due to imperfections of the surface or deflections.

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