Evaluation of shear lag in standard H-/I-sections
Author
Sørensen, Ruben Krogh
Term
4. term
Education
Publication year
2013
Submitted on
2013-06-20
Pages
64
Abstract
This thesis investigates shear lag in standard H-/I-sections, which causes a non-uniform normal stress distribution in flanges under transverse loading. Shear lag is commonly accounted for by reducing flange width to an effective width, yet Eurocode 3 is unclear about the need to include this effect for hot-rolled I-sections. The work compares three approaches: the classical theoretical solution described by von Karman and cited by Timoshenko and Goodier, finite element models with shell elements (Ansys), and the simplified method in EC3, including beam-element analyses in ROSAP/STRECH. Effective width is computed using both theory and FEM and used to assess shear lag in simple examples and in a representative offshore substructure. The results show that high shear stresses near supports reduce effective width, that it depends on the material's shear stiffness, and that the EC3 procedure gives conservative predictions relative to shell finite element models. In addition, beams fully attached to adjacent members exhibit less shear lag propagation than simply supported beams. The study provides a basis for judging when shear lag should be considered in standard I-sections.
Dette speciale undersøger shear lag (forskydningsdeformationer) i standard H-/I-profiler, som kan give en uens normalspændingsfordeling i flanger under tværlast. Shear lag håndteres ofte ved at reducere flangebredden til en effektiv bredde, og der er usikkerhed i EC3 om behovet for at medtage effekten i varmvalsede I-profiler. Arbejdet sammenligner tre tilgange: den klassiske teoretiske løsning beskrevet af von Karman og refereret af Timoshenko og Goodier, numeriske finite element-modeller med skallelementer (Ansys), samt den forenklede metode i EC3, herunder beregninger med bjælkeelementer i ROSAP/STRECH. Effektiv bredde er beregnet med både teori og FEM og er brugt til at vurdere shear lag i enkle eksempler og i en repræsentativ offshore-delstruktur. Resultaterne viser, at store forskydningsspændinger nær understøtninger reducerer den effektive bredde, at den afhænger af materialets forskydningsstivhed, og at EC3-metoden giver konservative resultater sammenlignet med skallelement-baserede FE-modeller. Desuden ses, at fuldt sammenkoblede bjælker giver mindre udbredelse af shear lag end simpelt understøttede bjælker. Arbejdet giver dermed et grundlag for at vurdere, hvornår shear lag bør medregnes i standard I-profiler.
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