Overall Stability Research Of Unbraced Steel Frame Beams With The Same Terminal Restraint

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. The calculation the current design codefor steel structures uses is an approximate method which simplelies the steel framebeams as the simplely surported beams under the internal force of steel frame.However, because of the complicated end restrains and both positive and negativemoment diagram of the beams, the overall stability of steel frame beams is quitedifferent from that of simply supported beams. This thesis conducts the research onoverall stability of unbraced steel frame beams with the same terminal restraint. Thecontent is as follows:(1) Due to the complicated end restrains, a new model is proposed as follows:Major axis end-restraint provided by the adjacent members is simulated by springhinge. Minor axis end-restraint is fixed. End twisting and end warping are prevented.In this thesis, a suitable sub-frame is chosen from braced and unbraced steel framewhich is rigid-jointed. The restraint considering in-plane restraints provided by theadjacent members are simplified as a sping hinge at each end of the beam. And therotational stiffness of each spring hing can be calculated by the rotational stiffness ofthe adjacent rods.(2) Based on formulas for internal forces of steel frame beams and energymethod, the total potential energy equation for the flexural-torsional buckling ofunbraced steel frame beams with doubly symmetric I-setion is derived. Then atheoretical formula for elastic critical moment of unbraced steel frame beams with thesame terminal restaint under uniformly distributed load or central concentrated load isobtained by Rayleigh-Ritz method.(3) A certain number of unbraced steel frame models are made by the finiteelement software ANSYS, and the correctness of these models is verified.(4) By comparing the results of the above-mentioned theoretical formula with theresults of finite element software, agreement between the results through two differentmethods is satisfactory. Therefore, it is shown that the theoretical formula deduced inthis thesis has certain applicability. The influence of the axial pressure in the columns,stiffness of adjacent beams and columns, steel frame beam parameter on the overallstability of steel frame beams is analysed.(5) The theoretical results are compared with the results of revised U.S code. It is shown that the results of the revised U.S code are commonly smaller which means itwill cost more steel.