Torsion has always been treated superficially in structural analysis. The difficulty of the theoretical background deters engineers so that many strategies have been developed in order to avoid its effects. Open-sections, the widely used members in steel construction, are susceptible to torsion but only elastic proposal may be found in current codes. Then, by rejecting the possibility to achieve yielding in profiles class 1 and 2, this approach is too conservative and underestimates the cross-sectional strength. It is well known that standards treat plastically only the column-beam interaction. EN 1993-1-1 proposes an approximation that presumes the entire bending resistance even if an axial force is applied to the structure. However it does not give any rules for torque cases even if warping generates normal stresses as well as other internal forces. Therefore, torsional effects have to be rigorously considered. This thesis develops a new design method starting from the physical meaning of the neutral axis. This magnitude univocally marks the frontier between the cross-sectional area devoted to axial and bending response leaving clear the interaction between them. This strategy could be exploited to torsional problems. The member provides the required torsional resistance in the extreme part of its cross-section. This amount is being settled by introducing the c parameter, hence the remaining reduced area will be able to ensure the ultimate resistance for the other concurrent internal forces. A three-dimensional space chart reflects the design proposal. Obtained results will identify the interaction diagram in which each axis represents one of the three internal forces (axial load, bending and torque). The failure surface displayed will define the frontier where evaluating the ultimate limit state resistance of the cross-section. In conclusion, this new design strategy considers the interaction among axial force, bending moment and torque acting simultaneously, through a strong, simple and rigorous formulation. Finally, the current proposal can be understood as an amendment for the cross-sectional strength that may be helpful in the future codes development.
La torsione è sempre stata trattata superficialmente nel calcolo strutturale. La complessità della sua teoria dissuade gli ingegneri che nel tempo hanno sviluppato molte strategie con la finalità di evitarene gli effetti. Le sezioni aperte, tra le più usate nelle costruzioni metalliche, sono molto suscettibili alla torsione. Tuttavia i codici vigenti contemplano solo formulazioni elastiche che, respingendo la possibilità di raggiungere lo snervamento, per profili classe 1 e 2 risultano troppo conservative sottostimando la resistenza della sezione. Attualmente le norme internazionali considerano solo l’interazione flesso-assiale. EN 1993-1-1 propone un’approssimazione nella quale presume che la capacità flessionale si conserva inalterata in presenza di azione assiale. D’altro canto, non include specifiche circa la torsione e gli effetti dell’ingobbamento anche se questo genera importanti tensioni. La tesi sviluppa un nuovo metodo di progetto partendo dal significato fisico dell’asse neutro. Questa entità segna univocamente la frontiera tra l’area della sezione devota alla risposta assiale e flessionale. La stessa strategia può essere estesa anche alla torsione. Il profilo offre la resistenza torsionale nella parte estrema della sezione che viene definita e circoscritta introducendo il parametro c. L’area ridotta rimanente sarà quindi capace di assicurare la resistenza ultima per le altre azioni interne concomitanti. Questa proposta progettuale trova riscontro in un grafico tridimensionale. I risultati definiscono un diagramma d’interazione nel quale ogni asse rappresentata una delle tre forze interne (carico assiale, flessionale e torcente). La superficie di rottura identificherà la frontiera dove valutare la resistenza a stato limite ultimo della sezione. Concludendo, questa nuova metodologia considera l’interazione tra azione assiale, momento flettente e torcente applicati simultaneamente. Tutto questo grazie a una formulazione robusta, semplice e rigorosa. Questa proposta può essere intesa come un emendamento per la resistenza ultima delle sezioni che potrebbe essere utile allo sviluppo di futuri codici normativi.
Torsion influence on the response of steel beams
COLOMBO, ATTILIO
2012/2013
Abstract
Torsion has always been treated superficially in structural analysis. The difficulty of the theoretical background deters engineers so that many strategies have been developed in order to avoid its effects. Open-sections, the widely used members in steel construction, are susceptible to torsion but only elastic proposal may be found in current codes. Then, by rejecting the possibility to achieve yielding in profiles class 1 and 2, this approach is too conservative and underestimates the cross-sectional strength. It is well known that standards treat plastically only the column-beam interaction. EN 1993-1-1 proposes an approximation that presumes the entire bending resistance even if an axial force is applied to the structure. However it does not give any rules for torque cases even if warping generates normal stresses as well as other internal forces. Therefore, torsional effects have to be rigorously considered. This thesis develops a new design method starting from the physical meaning of the neutral axis. This magnitude univocally marks the frontier between the cross-sectional area devoted to axial and bending response leaving clear the interaction between them. This strategy could be exploited to torsional problems. The member provides the required torsional resistance in the extreme part of its cross-section. This amount is being settled by introducing the c parameter, hence the remaining reduced area will be able to ensure the ultimate resistance for the other concurrent internal forces. A three-dimensional space chart reflects the design proposal. Obtained results will identify the interaction diagram in which each axis represents one of the three internal forces (axial load, bending and torque). The failure surface displayed will define the frontier where evaluating the ultimate limit state resistance of the cross-section. In conclusion, this new design strategy considers the interaction among axial force, bending moment and torque acting simultaneously, through a strong, simple and rigorous formulation. Finally, the current proposal can be understood as an amendment for the cross-sectional strength that may be helpful in the future codes development.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/77023