Traffic loads on footbridges
5.3. Static load models for vertical loads – characteristic values 1 Uniformly distributed loads
Traffic actions to be taken into account for the design of bridges supporting footways or cycle tracksare represented by a uniformly distributed load; its recommended characteristic value is equal to qfk¼ 5 kN/m2(Fig. 5.1).
Loads due to cycle traffic are generally much lower than those due to pedestrian traffic, but it has been assumed that a frequent or occasional accumulation of pedestrians on cycle lanes may occur. Moreover, pedestrian loads on road or railway bridges give generally small effects compared to those due to road or railway traffic. Nevertheless, the Eurocode mentions that special consideration may need to be given to loads due to horses or cattle for individual projects. cl. 5.3.1(2): EN 1991-2 Note 1 to cl. 5.1(2): EN 1991-2 cl. 5.2.3(1): EN 1991-2 cl. 5.2.3(2): EN 1991-2 cl. 5.3.2.1: EN 1991-2 cl. 5.2.1(1): EN 1991-2 qfk
The characteristic value qfk¼ 5 kN/m2represents a physical maximum load including a
limited dynamic amplification (five heavy persons per square metre).
For the design of footbridges, the model for the assessment of general effects consists of a uniformly distributed load qfk applicable to the unfavourable parts of the influence
surface, longitudinally and transversally. The Eurocode leaves the choice of the character- istic value for the National Annex or for the individual project, but gives the following recommendations:
. Where the footbridge may carry (regularly or not) a continuous dense crowd (e.g. near
the exit of a stadium or an exhibition hall), a characteristic value qfk¼ 5 kN/m2may be specified.
. Where such a risk does not exist, it is possible to adopt a reduced value for long-span
footbridges. The recommended value for qfkis:
qfk¼ 2:0 þ
120
Lþ 30kN=m
2
qfk 2:5 kN=m2; qfk 5:0 kN=m2
where L is the loaded length in metres. This function is represented in Fig. 5.2.
5.3.2. Concentrated loads
The consideration of concentrated loads is required in order to check the resistance of a footbridge to local effects. In general, loads on footbridges may differ depending on their location and on the possible traffic flow of some vehicles. Three cases are envisaged by the Eurocode: Note 1 to cl. 5.3.2.1(1): EN 1991-2 Note 2 to cl. 5.3.2.1(1): EN 1991-2 cl. 5.3.2.2: EN 1991-2 Background documentation
Background information on loads due to concentration of people on building floors is rather poor. Tests have been performed in the past with people dancing on a dynamometric platform. Depending on the type of music, the loads varied from 2.9 to 5 kN/m2. With fast music, a magnitude of 5 kN/m2 was reached approximately twice per second. The load corresponding to a concentrated crowd was about 5.5 kN/m2and a maximum dynamic load density of 8 kN/m2 has been reached by several people jumping simultaneously. Experimental studies were performed for the design of the Stade de France. Dynamic tests were performed in the higher grandstand of Charlety Stadium in Paris, with a density of three people per square metre, but their purpose was to adjust the design in order to limit vertical accelerations and to avoid natural frequencies of the structure below or equal to 5 Hz. The reader should also refer to the TTL Designers’ Guide to EN 1991: Buildings.3
210 0 10 50 100 150 200 Loaded length L 6 5 4 3 2.5 2 1 0 qfk (kN/m 2)
. First case. Permanent provisions are made to prevent access of all vehicles to the
footbridge.
. Second case. The presence of a ‘heavy’ vehicle on the footbridge is not normally foresee-
able but no permanent obstacle prevents this presence: the Eurocode recommends strongly to take into account the accidental presence (accidental design situation) of a vehicle on the bridge deck.
. Third case. A ‘heavy’ vehicle is foreseen to be driven onto the footbridge deck: it may be a
vehicle for maintenance, emergencies (e.g. ambulance, fire) or other services.
In the first case a concentrated load is to be taken into account to check the resistance as regards local effects due, for example, to small equipment for maintenance of the footbridge. The recommended characteristic value of the concentrated load Qfwk is equal to 10 kN,
acting on a square surface of sides 0.10 m. All figures may be adjusted in the National Annex. The concentrated load does not act simultaneously with the uniformly distributed load.
In the second case, the Eurocode defines a load model to be taken into account to represent the accidental presence (accidental design situation) of a vehicle on the bridge deck, consisting of a two-axle load group of 80 and 40 kN, separated by a wheel base of 3 m (Fig. 5.3), with a track (wheel-centre to wheel-centre) of 1.3 m and square contact areas of side 0.2 m at coating level. This model may be adjusted in the National Annex or for the individual project.
In the third case, a service vehicle Qserv is defined. Its characteristics (axle weight and spacing, contact area of wheels, etc.), the dynamic amplification and all other appropriate loading rules may be defined for the individual project or in the National Annex. If no infor- mation is available, the vehicle previously defined for accidental design situations (second case) may be used as the service vehicle (characteristic load). Of course, the concentrated load Qfwk does not act simultaneously with this load model. Where relevant, several
service vehicles, mutually exclusive, may have to be taken into account and may be defined for the individual project.