Overall Stability Research Of Braced Steel Frame Beams

The overall stability of steel beams is one of the most important steps in steelstructure design.It is easy for steel frame beams to fail by lateral-torsional buckling ifthe lateral supports are unable to be provided. In the current design code for steelstructures, the calculations for the overall stability of steel beams are limited tosimply supported beams subjected to end moments, uniformly distributed load orcentral concentrated load alone. Due to the complicated end restrains and bothpositive and negative moment diagram of the beams, the overall stability of steelframe beams is quite different from that of simply supported beams.But the currentdesign code is lack of relevant provisions.Because of the complicated end restrains, a new model is proposed as follows:Major axis end-restraint provided by the adjacent members is simulated by endmoment.Minor axis end-restraint is fixed.End twisting and end warping areprevented.A suitable sub-frame is chosen from the rigid-jointed and braced steelframe in this thesis. And formulas for internal forces of beams considering in-planerestraints provided by the adjacent members are deduced by rotational displacementmethod.Based on formulas for internal forces of steel frame beams and energy method,the total potential energy equation for the flexural-torsional buckling of steel framebeams with doubly symmetric I-setion is derived. Then a theoretical formula forelastic critical moment of steel frame beams under uniformly distributed load orcentral concentrated load is obtained by Rayleigh-Ritz method.By using the above-mentioned theoretical formula, the influence of the axialpressure in the columns, stiffness of adjacent beams and columns, steel frame beamparameter on the overall stability of steel frame beams is analysed, and the theoreticalresults are compared with the data given by Trahair. Agreement between two resultsthrough two different methods is satisfactory. Therefore, it is shown that thetheoretical formula deduced in this thesis has certain applicability. According to theresults of parameter analysis, simplified formulas for equivalent moment coefficientof steel frame beams under uniformly distributed load or central concentrated loadapplied at the top flange are proposed through regression analysis by using statisticalsoftware SPSS.Then the calculational formula for the overall stability of steel framebeams under transverse loads is obtained and verified through the data given by Trahair.It can provide references for further improvement of the design code andengineering application in the overall stability of braced steel frame beams.