Analysis and Design of Cable-Stayed Steel Columns Using the Stiffness Probe Method

The stiffness probe method (SPM) is a new numerical procedure that calculates buckling loads. SPM probes the local stiffness of a given structure at the point of application of a small transverse perturbation force as the applied load is increased. The local stiffness degrades from a maximum for an unloaded structure to zero at the buckling load. An artifice spring is added to the original structure that eventually absorbs the full perturbation force at a prescribed small deflection, thereby keeping structural deformations small as the buckling load is approached. As a result, using an indicator that approaches zero at buckling rather than having to rely on increasingly larger deflections at buckling as in conventional P-Delta methods, SPM ensures an accurate numerical result for the critical load. We use SPM herein to study the behavior of one and two cross-arm cable-stayed columns under applied load. A formula is given to calculate the minimum slenderness that justifies converting a tube into a cable-stayed column. Various factors such as cable prestrain, cable cross-sectional areas, and tiers of cross-arms affecting column strength are examined for a series of cable-stayed columns. We find that cable-stayed columns may buckle either in a one-lobe symmetrical mode or two-lobe anti-symmetrical mode, the latter case being contrary to conventional thinking. A design example for a given cable-stayed column using the AISC Specification is presented. The effect of optimum cable prestrain to enhance column buckling strengths is discussed. A strength enhancement ratio (SER) is defined that evaluates the additional column strength gained after transforming a given steel tube into a cable-stayed column.