Behavior of Concrete Columns Under Controlled Lateral Deformation

Abstract

This paper reports tests of nine model cantilever columns. The objectives of this phase of a study of the deformation capacity of columns in structural systems were to experimentally determine and analytically predict the complete axial load-moment-curvature (PM-4) relationships for columns with large axial loads and minimal ties. The controlled lateral deformation loading method used maintained stability after the maximum moment resistance was reached and allowed the descending branch of the P-M-+ relationships to be measured. This bracing against instability is similar to the lateral resistance which stronger members in an unbraced frame can provide. The measured P-M-+ relationships were not as brittle as previously indicated, but were nearly flat-topped and exhibited significant ductility. An analytical procedure was developed which closely agreed with the relationships observed. Concrete crushing strains ranged between 0.0096 and 0.0160 in./in. Predictions of an ultimate ltrain equation indicated that the values greater than 0.003 in./in. ,here due to the appreciable moment gradients. The results indicate ;,at the ACI Building Code limiting strain criteria can be safely inn-eased to 0.004 in./in. for columns with moment gradients which produce reversal of moments in the column length. Such an increase could have substantial benefits by allowing economical usage of higher strength reinforcement (f, of 80 to 90 ksi) in many columns.