Acicolumn Capacity Calculator

A short reinforced concrete column loaded concentrically fails when the concrete and the longitudinal bars both reach their limiting stress, so the pure axial squash load is simply the sum of the two contributions: 0.85 fc times the net concrete area (Ag minus Ast) plus fy times Ast. The 0.85 coefficient reflects that concrete in a slender cast member is weaker than a moist-cured 150 mm test cylinder because of casting height, bleeding and sustained loading. No real column is loaded perfectly on centre, so ACI 318 caps the usable value at 0.80 of that squash load for tied columns and 0.85 for spirally reinforced ones, then applies phi of 0.65 or 0.75 respectively. Metric practice uses fc of 20 to 50 MPa and Grade 420 bars at fy = 420 MPa. The longitudinal steel ratio must lie between 1 and 8 percent of Ag, though anything above about 4 percent creates bar congestion at lap splices and is usually better solved by enlarging the section. Slenderness is checked against kLu/r, with r near 0.29 times the least dimension for a rectangle; below roughly 22 the column is treated as short, above it second-order moments must be handled by moment magnification or a P-delta analysis. The result here is pure axial capacity only. Real columns carry moment as well, so the governing check is a point on the axial-moment interaction diagram, and tie size and spacing, cover, splice length and seismic detailing are separate requirements.

Rational column design in the United States rests on the ACI-sponsored Reinforced Concrete Column Investigation carried out between about 1929 and 1933, with the principal test programme run by Frank E. Richart at the University of Illinois and companion work at Lehigh University. Those hundreds of column tests established that useful concrete strength in a column is close to 0.85 of the cylinder strength, and that longitudinal steel and concrete can be added directly at their respective yield and limiting stresses. Richart, Brandtzaeg and Brown had already published their landmark 1928 study on triaxially confined concrete, which explained why spirals give a column ductility and post-peak capacity that rectangular ties do not. Early codes handled accidental eccentricity explicitly, requiring designers to assume a minimum eccentricity of about 0.1 times the section depth for tied columns and 0.05 for spiral columns. ACI 318-77 replaced that awkward requirement with the flat 0.80 and 0.85 multipliers still used today, a simplification that gives essentially the same answer with far less arithmetic. The strength reduction factors of 0.65 for tied and 0.75 for spiral columns encode the same distinction the 1928 confinement research uncovered, and modern seismic provisions extend it further through confinement requirements for special moment frame columns.