The Eurocode dealing with the wind loading also provides general guidance. If any of the critical frequencies is lower than 1.5 hertz then there are additional checks to perform or to introduce some dumping devices or we may redesign the structure.Īpart from dynamic effects of pedestrians, in certain situations, we may need to investigate the dynamic effects of wind loading. It means there are no risk of lateral instability. We may assume that the structure is stable.

If there are no significant lateral modes with frequencies below 1.5 hertz. That helps us to assess the possible vibration issue and the other requirement regarding the natural frequencies. They are defined in a nation annex or other national application document. And if any of the natural frequencies of the structure is within this spectrum there is a risk of issues related to the vibration or stability so specific models and detail criteria how to fulfill it and how to achieve that the structure is stable. And if there’s a group of joggers crossing a bridge, that may induce forces with a frequency about three hertz so we can see that this range say from 0.5 to 3 hertz would be a critical spectrum. In the horizontal direction, the effects of walking pedestrians would have a frequency between 0.5 and 1.5 hertz. Dynamic effects of walking pedestrians can be represented by a periodic force and this force will have a typical frequency of between one and three hertz. The vibration from normal pedestrian traffic should not cause discomfort to users so that is why the code specifies a limit defined as an acceleration limit. The second key point is to pay attention to the vibration.

Because if any of the natural frequency are identical to the frequency due to dynamic forces applied to the structure, there is a risk of resonance which may lead to unacceptable vibration or even loss of stability.

The first key point is the natural frequency of the structure. There are two key points which we need to pay attention. Now, we will look at what differentiates footbridges most from normal road bridges of short and medium span bridge that has the application of dynamic loads. Figure 6: Equivalent static design force due to the impactĭynamic Loads can be found in section 5.7, EN 1991-2.

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