Effect of geometrical factors on stability characteristics of portal frames

This paper is concerned with the effect of slanting the columns and changing the ratio of the member lengths on the buckling and post-buckling behaviour of portal frames. Two types of structures are considered: hinged- and rigid-base frames, each type subjected to two loading patterns. In the first pattern (perfect loading), equal vertical loads are applied to the nodes. The second pattern (imperfect loading) consists of the same vertical loads and, additionally, of a horizontal nodal force. Numerical results are presented that show the beneficial effect of slanting the columns by negative angles (i.e. clockwise for the left-hand column and anticlockwise for the right-hand one), and the degrading effect of slanting the columns by positive angles. The effect has been observed both in perfect frames and in the frames subjected to imperfect loading. The paper considers also the effect of the frame geometry on the structure's sensitivity to imperfections. The imperfection sensitivity analysis of the frames is carried out according to a definition of sensitivity which differs somewhat from that commonly used.