Concrete Columns Under Biaxially Eccentric Thrust

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Title: Concrete Columns Under Biaxially Eccentric Thrust

Author(s): Richard W. Furlong

Publication: Journal Proceedings

Volume: 76

Issue: 10

Appears on pages(s): 1093-1118

Keywords: bending; biaxial loads; columns (supports); compression tests; compressive strength;eccentric loads; failure; live loads; loads (forces) slenderness ratio; stiffness; stress block;stress-strain relationships; structural analysis; structural design.

Date: 10/1/1979

Abstract:
Data from biaxially eccentric load tests of 9 specimens with a 5 x 9 in. (130 x 230 mm) rectangular cross section plus 14 specimens with a 5 x 11 in. (130 x 280 mm) rounded end cross section are reported as a supplement to data already available from others for tests on square columns. Cross section strength analyses derived with the traditional rectangular stress block to represent concrete were found to underestimate observed capacities. ImproveId correlation between analysis and observation required the use of a nonlinear stress-strain function for concrete that remained unspalled until maximum surface strains reached at least 0.4 percent. Slenderness effects from biaxial bending were governed largely by weak axis flexibility, and significant differences were observed between the skew angle of the neutral axis and the skew angle of the eccentric load. However, no twisting about the longitudinal axis was detected. Moment magnification factors if used for design in accordance with ACI Building Code recommendatio ns determined separately for each principal axis of bending would have produced safe results. The elliptical relationship of ratios between bending moment and bending capacities about each principle axis produced strength estimates that averaged 10 percent greater than measured strength. The Bresler reciprocal thrust formulation of capacity produced results always safe, but results that averaged 26 percent above measured capacity.