عنوان مقاله [English]
Analysis of elastic buckling of Monosymmetric I–Section (MIS) is rather difficult, and it requires special attention at various stages. Investigations on elastic buckling of I-section of bridges, are mostly concentrated on symmetrical coupled I-sections, and on lesser extent on MIS. When buckling occurs in a beam with MIS, cross–bending stresses and twist bending occur about rotating axis of the member. This bending twist causes changes in twisting stiffness of the member.In this paper, theory of side–twist buckling of beams with I-sections together with MIS and important parameters affecting buckling capacity of continues beams are studied. Eleven two – span beams which were divided into three groups are chosen. Most of the beams had thick and short flange and thin webs, therefore twisting occurs in web, and buckling in flange similar to stiff members. In compression flanges three modes of buckling: twisting, lateral and vertical buckling may occur. Considering lateral buckling of compression flange, only rotating strength of sections and also column reaction of compression flanges resist lateral buckling. In general, rotating resistance of sections and column reaction of compression flanges act together. Here, the shear buckling resulted from shear stresses, which is very important, is also considered. Thin plates of web, can stand tensile stresses, but they are weak in compression stresses, and buckle under stresses less than yield stresses of plate.Furthermore, for analysis of the beams, a nonlinear computer program NISA II was used, all the beams had the same mesh, boundary conditions, and two equal spans (6m) except for these beams in which the effect of different spans was required (L1= 5, 6m L2= 2, 3, 4 and 5m).In order to have confidence in the results, the beams were analyzed under static loads. For this purpose maximum deflection that occur under the loading points, were obtained from the program, and then the results were compared with theoretical results. The results indicate that the chosen model has yielded good results.To study the effect of monosymmetric (0.1, 0.3, 0.5, 0.7 and 0.9), in the first 5 beams, the width of top flange was changed while the rest of parameters, remain constant. In other six beams only the thickness of top flange was changed. Also, to study the effects of loading conditions on buckling strength, load on first span kept constant, then load on second span changed from zero up to the load of the first span (P2/P1=0.25, 0.5, 0.75 and 1.00). The other important parameter is the effect of span ratio, therefore buckling load with respect to different span ratios was obtained for L2/L1=0.4, 0.5, 0.67, 0.8 and 1.00. Similarly the effects of span ratio was obtained for uniform distributed load. The effect of biaxial stresses on MIS was obtained and the results are plotted on different figures. Finally the figures of critical loads versus each of the considered parameters are prepared based on the obtained results. It was found that buckling load of such beams is increased by increasing the width of top flange, provided the length of spans being equal and the other geometric characteristics of section and boundary conditions remain constant. Furthermore, if the ratio of concentrated applied loads in mid-spans are about 0.8, the maximum buckling capacity of section will be obtained.