Cyclic Inelastic Buckling of Tubular Steel Braces

摘要

Experimental results are presented from tests on six individual brace members subjected to severe inelastic cyclic loading. The tubular brace specimens considered are one-sixth scale models of braces of the type used for offshore platforms. Each brace specimen is alternatively subjected to cycles of compressive inelastic buckling followed by tensile stretching. Examined are the effects resulting from different end fixity conditions (pinned vs. fixed), and from different diameter-to-wall thickness ratios (33 vs. 48). Also, the behaviors of heat treated braces are compared to the behavior of braces made from tubing as received from the manufacturer. The inelastic responses of the braces are presented and interpreted, including axial load-axial displacement hysteretic loops; axial load-midspan lateral deflection hysteretic loops; brace deflected shapes; brace energy dissipation; and inelastic axial strains, curvatures and rotations in plastic regions. Special attention is directed to the deterioration of buckling load with inelastic cycling. Design code buckling formulas are for columns or braces that have not been previously yielded and have initial cambers within the code allowances. Cyclic inelastic loadings cause changes in the mechanical material properties of the braces which subsequently reduces the buckling load. A method of predicting the reduction in buckling load is presented. The changes in material properties are identified for a 'critical section' within the plastic hinge of a brace and the reduced buckling loads under repeated cyclic loadings are calculated.