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Improvement of Fatigue Resistance Through Box Action for I-girder Composite Bridges
2016 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

The increased amount of traffic combined with higher traffic loads will lead to that many bridges of today are in need of strengthening or replacement. This because of the coming rules for fatigue and because of the old design codes that did not consider such high loads that we have today. When strengthening existing I-girder composite bridges (which are the majority of the steel bridge stock), one concept is to make the cross section act like a box section, by adding a horizontal truss between the bottom flanges. This means that the eccentric loads produce a torque that is transferred by shear forces around the section. The preferred type of truss is a K-truss, since other types will force the diagonals to take part in the global bending, which will make them sensible to buckling between the joints. This means a lot in the Ultimate Limit State (ULS), but even more in the Fatigue Limit State (FLS). In the FLS the fatigue is determined by the stress ranges in certain parts of the structure, for instance the welded details in an I-girder. If the girders can act like brothers or at least as “step-brothers”, sharing the load effects from eccentric loading, the stress ranges can be significantly reduced. This thesis presents a study that shows the effects by these horizontal trusses between lower flanges for bridges and how the fatigue resistance is improved. Reduced stress ranges and increased amount of tolerated load cycles will extend the lifetime of the details, and by so the lifetime for the bridge. Bergeforsen Bridge is chosen as a case study in the thesis to implement the method with horizontal trusses. The bridge is a multi-span bridge with three spans without curvature, which makes it perfect for the purpose of analysing the effects of box action introduced by K-trusses. The chosen dimensions for the trusses is180x100x8 mm, which corresponds to a cross section area of 4004 mm2 for a cold-formed rectangular hollow section. The additional weight is 1.6 % of the original steel weight and only 0.7 % of the total dead load for the bridge. The load distribution between the girders or as called in this thesis lane factor, LF is 0,74 (0,95 without a truss) which extends the lifetime of the bridge 3.5 times, with aspect to the most exposed detail for fatigue, on-site welded joints.

Place, publisher, year, edition, pages
2016. , 105 p.
Keyword [en]
Keyword [sv]
Teknik, Box action, Composite, Fatigue, Framework, Truss, I-girder, 3-D, Modelling, Steel, Strengthening
URN: urn:nbn:se:ltu:diva-47254Local ID: 4d13b0c0-618a-4dd7-9179-655272642cb0OAI: diva2:1020574
Subject / course
Student thesis, at least 30 credits
Educational program
Civil Engineering, master's level
Validerat; 20160225 (global_studentproject_submitter)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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