Behavior of Hollow, Rectangular Concrete Piers Subjected to Biaxial Bending

Hollow, rectangular concrete piers have been used to support cable stay and long-span balanced cantilever bridges for the last forty years. Compared with solid columns, hollow piers offer the advantages of high bending and torsional stiffness, significant reductions in the volume of materials, and large reductions in dead load. Earlier investigations concluded that no reduction in strength should occur for cross sections subjected to combined axial load and uniaxial bending with wall slenderness ratios, defined as the unsupported length of the cross section divided by the wall thickness of the slender walls, less than 15. However, the response of hollow, rectangular piers subjected to simultaneous axial load and biaxial bending has not been studied.

Five rectangular, hollow concrete columns, with wall slenderness ratios between 6 and 14 and designed in accordance to the AASHTO LRFD Bridge vi Design Specifications, were tested under quasi-static, monotonic simultaneous axial load and biaxial bending. Neither cyclic loading nor horizontal or transverse loads were considered. A fiber model of the cross section and two material models for confined concrete were used to perform sectional analysis of each specimen. A finite element model was used to calculate the behavior of the test specimens. Also, the current design procedures for hollow, rectangular concrete piers were re-evaluated and found to produce safe estimates of the strength of such piers.

In hollow, rectangular concrete piers the value of the strength ratio, defined as the measured axial strength divided by the axial strength calculated using a rectangular stress block model of concrete, decreases as the wall slenderness ratio increases. The main parameter that controls those variations is the wall slenderness ratio. The current approximate design procedures for hollow, rectangular concrete piers with wall slenderness ratios 15, are valid for piers subjected to axial compression and biaxial bending.

Material models for confined concrete provided accurate estimates of the axial capacity and moment-curvature response of the hollow piers tested in this investigation. Future areas of research are recommended.