Student thesis: Master Thesis and HD Thesis

• Rasmus Bødker
• Heidi Christensen
4. term, Structural and Civil Engineering, Master (Master Programme)
This project is the result of a Master's Thesis produced on the master's degree programme in Structural and Civil Engineering at Aalborg University. The project is written in the period from the 1st February until the 10th June 2010. The project is titled "Dynamic human loading and stochastic load models for estimating the dynamic response of footbridges". The project deals with the design of stochastic load models for pedestrians crossing footbridges, and the acceleration of the footbridges caused by this dynamic load.

The development of new technologies and materials with higher strength implies that footbridges are constructed with larger spans, which lead to increasingly daring structures. This fact has generated very slender structural footbridges and, as a consequence of this design, a considerable increase of structural vibration problems. Vibration problems occur when the lowest eigenfrequency of a structure is near the load frequency, by which resonance potential will arise.

A footbridge must carry both inactive and active people. Inactive people are for instance people who sit or stand, and active are people who walk, jump or dance etc. The load from people can be of either high- or low-intensity, which is described by jumping or dancing people and walking people respectively. High-intensity load can cause damage to the structure and is described by the ultimate limit state, and low-intensity load can cause discomfort to those walking across the footbridge, which is described by the serviceability limit state.

Because of the construction of more daring structures and the increasing vibration problems, the motivation for development of methods to give a better evaluation of the footbridge user's comfort and safety is increased. The methods given in the present norms and standards for estimating vibration of a structure are based on semi-empirical expressions, in which the load from pedestrians is determined deterministic.

In reality, the load from pedestrians is not deterministic but stochastic, because the load frequency, step length and weight of a person vary according to the person who is crossing the footbridge. By using a stochastic load model to identify the vibration of a footbridge, a probabilistic estimation of the dynamic response of the structure is determined. A cumulative distribution function of the maximum accelerations gives information about the probability of exceeding a given acceleration. This information is important when evaluating the serviceability of a structure.

In this project, pedestrians on footbridges are in focus, and it is analyzed how the load from pedestrians can be described and thereby give the most realistic evaluation of the vibration of a footbridge. It is chosen that three bridges are to be analyzed, which all are of simple structures, because the load modelling is the primary objective of the project.

In the opening of the study, one single pedestrian is in focus, and the load from the walking person is described by a periodic load depending on time. The parameters in the load model are studied and stochastic variables for these are expressed. The difference of using more advanced load and bridge models is analyzed, and the most important parameters in the load models are identified.

Some footbridges carry more than one person at a time, and in the end of the study, load models for groups of people and crowds crossing a footbridge are studied and developed.
Language Danish 9 Jun 2010 126 Aalborg Universitet