Rigid latticed steel frames: two large-scale tests and failure predictions

Abstract

This work is concerned with the predictions of failure under static load for two identical free-standing latticed cantilever frames. Each structure was 3 m high and consisted of three storeys with panel points 1 m apart; in plan it was 1 m2; its diagonals were at 45° in the form of St. Andrew's cross; all joints were welded and were considered rigid. The four bases were bolted to the laboratory floor and the static load was applied at the top as two compressive horizontal forces parallel to the sides of the structure. The individual members were all solid mild steel round rods; the verticals were of uniform cross section of 25.4 mm in diameter with a slenderness of 157; the horizontal cross-arms were of uniform cross section of 19 mm in diameter; and the diagonals were of uniform cross section of 15.9 mm in diameter. The diagonals were cut at their mid-lengths and welded to give a flush joint.The total horizontal loads that caused failure of the two structures were 79.6 and 79.2 kN. In both cases failure occurred by the sudden buckling of one of the bottom-storey vertical legs, inwards towards the centre of the structure. Also upon failure, the diagonals in the bottom storey of the compression face buckled inwards and the compression diagonal of the side contiguous to the buckled leg also buckled.Three elastic stiffness matrices were used to predict the failure loads: (1) linear with yield criterion, 116 kN; (2) stability with change in sign of the total potential energy, 66 kN; and (3) bowing with the criterion of maximum curvature of the load–deflection curve, 80 kN. The failure loads of the two structures were thus accurately predicted by the bowing matrix. Key words: latticed rigid frames, large-scale tests, bowing matrix, steel.