Actions for accidental design situations

This clause deals with:

. _{vehicle collision with bridge piers, soffit of bridge or decks} . _{the presence of heavy wheels or vehicle on footways}

. _{vehicle collision with kerbs, vehicle parapets and structural components.}

For collision forces from vehicles under the bridge, covering impact forces on piers and other supporting members, and impact on decks (Fig. 4.22), EN 1991-2 gives only recommenda- tions or recommended values. This is due to the fact that EN 1991-2 was developed before EN 1991-1-7 (Accidental actions). Therefore, the questions related to impact from vehicles

under the bridge are treated in Chapter 7 of this Designers’ Guide. Hereafter actions from vehicles on the bridge are only evoked.

4.7.1. Vehicle on footways and cycle tracks on road bridges

The presence of heavy wheels or vehicles on footways is an accidental design situation and needs to be taken into account for all bridges where footways are not protected by a rigid road restraint system.

The accidental action is due to one axle load from the Tandem System corresponding to notional lane No. 2, i.e. Q2Q2k¼ 200Q2(see Section 4.3.5 of this Designers’ Guide), to be

applied and oriented on the unprotected parts of the deck so as to give the most adverse

cl. 4.7.3.1(2): EN 1991-2 Time Reservoir method (a) (b) (c) σ ∆σ1 ∆σ ∆σ1 ∆σ2 ∆σ3 ∆σ3 ∆σ4 ∆σ2 ∆σ4 n4 Total cycles in design life n3 n2 n1

Fig. 4.21. Counting method of stress cycles: (a) Stress history at detail; (b) Cycle counting; (c) Stress- range spectrum

effect. The design situations to be taken into account are defined by the designer in agreement with the client. Figure 4.23, that derives from Fig. 4.9 of EN 1991-2, shows two examples of accidental design situations.

4.7.2. Collision forces on kerbs

The collision force is a horizontal force of 100 kN, perpendicular to the kerb and acting on a line 0.5 m long at a depth of 0.05 m below the top of the kerb. Where unfavourable, a vertical traffic load may be taken into account simultaneously, equal to 0:75_Q1Q_1k¼ 225Q1kN.

These forces are represented in Fig. 4.24 which derives from Fig. 4.10 of EN 1991-2.

Fig. 4.9: EN 1991-2 cl. 4.7.3.2: EN 1991-2 Fig. 4.10: EN 1991-2 1 2 1 2 3 0.40 0.50 2.00 2.00 0.40 αQ2Q2k 3

Fig. 4.23. Examples showing locations of loads from vehicles on footways and cycle tracks of road bridges (EN 1991-2, Figure 4.9)

2 1 45° 45° 0.50 m (1) Footway (2) Kerb 0.05 m 100 kN 0.75αQ1Q1k

The vehicle collision forces on kerbs have been introduced in the Eurocode to give a rule for the design of structural members supporting kerbs. And in rigid (concrete) members the angle of dispersal of the load may be taken equal to 458 as shown in Fig. 4.24.

4.7.3. Collision forces on vehicle restraint systems

For the detailed design of a bridge, precise rules have to be defined concerning the connection between the road restraint system and the relevant structural member of the bridge. However, in fact, in the British standard BS EN 1317, only performance classes are defined in its Part 2, and the performance is only defined by the containment level.

For the design of the connection, the Eurocode recommends four classes of values for the transferred horizontal force defined in Table 4.15. Of course, these recommended values may be replaced by more refined values in the National Annex, depending on test results obtained with commercial systems or devices.

These values globally cover the results of measurements during collision tests on real vehicle restraint systems used for bridges. The Eurocode mentions that there is no direct correlation between these values and the performance classes of vehicle restraint systems. The proposed values depend rather on the stiffness of the connection between the vehicle restraint system and the relevant structural member of the deck. Class D corresponds to a very strong connection, for example in the case of rigid steel road restraint systems. For the containment of heavy vehicles, the normal performance class of road restraint systems is performance class H. The most common performance classes are H2 and H3. Class C for the horizontal force may be associated with these performance classes. In that case, EN 1991-2 recommends applying the horizontal force, acting transversely, 100 mm below the top of the selected vehicle restraint system or 1.0 m above the level of the carriageway or footway, whichever is the lower, and on a line 0.5 m long. The recommended value of the vertical force acting simultaneously with the horizontal force is equal to 0:75Q1Q1k

(see Fig. 4.25).

Of course, it is desirable to prevent deterioration of the structure in case of impact of a heavy vehicle on a vehicle parapet. For this reason, the Eurocode recommends designing the structure supporting the vehicle parapet to sustain locally an accidental load effect corresponding to at least 1.25 times the characteristic local resistance of the vehicle parapet (e.g. resistance of the connection of the parapet to the structure) without combination with any other variable load. More accurate values may be given in national annexes, based on real tests.

4.7.4. Collision forces on structural members

Of course, the vehicle collision forces on unprotected structural members above or beside the carriageway levels need to be taken into account; this is the case, for example, for bridges with lateral lattice girders (Fig. 4.26). The Eurocode recommends taking into account the same impact force as for piers, acting 1.25 m above the carriageway level. However, when additional protective measures between the carriageway and these members are provided, this force may be reduced for the individual project.

This force is an accidental action and, of course, should not be combined with any other variable load for the verifications (Fig. 4.27).

cl. 4.7.3.3: EN 1991-2 cl. 4.7.3.3(2): EN 1991-2 cl. 4.7.3.4: EN 1991-2

Table 4.15. Recommended classes for the horizontal force transferred by vehicle restraint systems (Data taken from EN 1991-2, Table 4.9(n)) Recommended class Horizontal force (kN)

A 100

B 200

C 400

500 mm 110 500 mm 435 $150 Horizontal impact force Definition of the level of application 350 300 1000 300 100 mm or 1000 mm whichever is the lower Carriageway level Vertical force 0.75αQ1Q1k = 225αQ1 (kN)

Fig. 4.25. Representation of the design forces to be applied to a vehicle parapet for heavy vehicles