Dynamic Analysis of Portal Frame Railway Bridges - Case Studies Investigating the Structural Response

Sammanfattning: As the world continues to see an increase in carbon dioxide emissions, actions aiming towards reducing man's negative impact on the environment are paramount. To be able to compete with the air travel industry, the Swedish Transport Administration suggests a vast expansion of the existing railway system. This includes the construction of high speed railroads connecting the largest urban areas of Sweden. With such a development comes numerous railway bridges, whom will have trains operating at speeds exceeding 200 km/h and thus demanding a dynamic evaluation. An immense amount of aspects require consideration when conducting such analyses. However, simple and effective methods are crucial in order to facilitate the design process for the engineer. In this thesis, three statically designed portal frame bridges of span lengths ranging from 6.5--18 m were analysed with regards to their dynamic behaviour. The commercial Finite Element Analysis software BRIGADE/Plus was used to evaluate their response. In attempting to map out the structural behaviour of the assessed railway bridges, while at the same time elevating the authors' general knowledge within the field of railway bridge dynamics, investigations were conducted to verify the viability of the methods suggested in Eurocode. Furthermore, previous research indicates that the use of a higher soil stiffness makes for a better representation of reality when conducting dynamic analyses. Therefore, parametric studies were performed in order to verify said proclamation with regards to two calculation methods. Based on acquired results, the study concludes that the estimative procedure, proposed in Eurocode, meant to determine the dynamic responses through magnification of each of the statically retrieved reactions is somewhat flawed. In utilisation of specific modeling concepts, certain configurations generate highly disproportionate magnifiers, and thus, non-viable approximations. Generally, this behaviour is amplified with decreasing span length and declining soil stiffness. Indications are that shorter bridges are more susceptible to influence from a certain eigenmode, which causes the established behaviour. When approaching the dimensions of the largest bridge investigated, this becomes a non-issue; the same goes for the introduction of soil parameters which are vastly stiffer than those proposed by Trafikverket. Furthermore, with the introduction of larger soil stiffness parameters, a tendency to lessen the vertical acceleration response is found. Again, however, this alteration has a diminishing influence on portal frame bridges with larger dimensions.