Load-Path Effects in Column Biaxial Bending with Axial Force

Abstract

Twelve identical cantilever-type column specimens were subjected to load paths of cyclic uniaxial or biaxial flexure with axial load, to provide data for the development and calibration of mathematical models of columns subjected to biaxial bending. The specimens had a shear span ratio of 6.0 and were overdesigned in shear, so that their behavior and damage were controlled by flexure. The only test variable was the load path. In 10 of the tests the axial load was essentially kept constant and in the rest it varied, either simultaneously or independently of the transverse loads and/or displacements. Very strong coupling was observed between the two directions of bending and between these two directions and the axial direction. The coupling between the two transverse directions significantly increased the hysteretic energy dissipation. This increase was related to the phase lag of the vector of biaxial deflections relative to that of biaxial transverse loads, and to the relative rotation of the biaxial load path with respect to that of biaxial deflections. Cycling of the deflections had two effects on axial deformations: (1) gradual permanent shortening for low-to-medium axial loads, or a gradual extension, which turned into shortening when failure was approaching, for very low axial loads; and (2) a recoverable axial extension, approximately proportional to the vector resultant of deflections. Cycling of the axial force caused a ratcheting increase of deflections under constant transverse force.