AXIAL COMPRESSIVE STRENGTH CALCULATION BASED ON THE ‘UNIFIED THEORY’ FOR SPECIAL-SHAPED CFT COLUMNS WITH MULTIPLE CAVITIES

Based on the unified theory for concrete filled steel tubes (CFT) and the test results, the variation of the concrete confinement effect when the cross-sectional shapes change from triangle, square, regular polygon to circular is investigated. The concrete is divided into effective and ineffective confined regions. A cross-sectional efficiency coefficient is introduced to discount the confinement effect. The unified circular cross-sectional formula is applied to calculate the axial compressive strength. By regression analysis, the relationship between the cross-sectional interior angle and the confinement effect is found. Taking quadrangle sections for example, the relationship between the regularity and the confinement effect is investigated, and a calculation method is provided. By splitting a special-shaped cross-section with multiple cavities into separate polygons and considering the influence of the shape efficiency and regularity, the strength of each cavity CFT is calculated and summated to obtain the total axial compressive strength. The results show that in regular polygon steel tube confined concrete, a liner relationship is exhibited between the interior angle and the initial tangent line angle of the second-order curved boundary between the effective and ineffective confined concrete regions. The proposed axial compressive strength calculation method for special-shaped CFT columns with multiple cavities can well reflect the real confinement features. The calculation results match the test results well and the method can fulfil the engineering design requirements.