Double-head studs as cross ties in concrete columns: Earthquake resistance

Experimental and analytical research was conducted to investigate the seismic behavior of concrete columns with double-head studs as alternative to conventional cross ties commonly used in practice. Double-head studs are steel rods with anchor heads at both ends. Their advantages over conventional cross ties include enhanced anchorage, avoidance of congestion by elimination of hooks, and ease of preparation and installation.; A special test setup has been designed and constructed at the Structures laboratory of the University of Calgary. The main purpose of the test setup is to apply a constant axial load while imposing incremental lateral displacement reversals on the free edge of column specimens to bend about their weak axis. Nine column specimens with a concrete strength of 25 MPa have been tested. The columns' dimensions are 250 x 500 x 1500 mm, and are built integrally with a strong over-designed base. The test variables include the type of cross ties (double-head studs or conventional cross ties), axial load level, volumetric ratio and spacing of transverse reinforcement, and distribution of longitudinal bars around the core perimeter and the resulting tie configuration.; While columns with either type of lateral reinforcement have attained almost the same strength, columns with double-head studs have exhibited superior performance in terms of ductility and energy dissipation. With axial load levels up to 30% of the column nominal axial capacity, columns with lateral reinforcement ratio 50% of the minimum permitted by the ACI and CSA Codes have exhibited satisfactory ductile behavior. A simple computer program is developed to calculate the moment-curvature relationships for column cross sections up to large inelastic deformations. The resulting analytical moment-curvature curves are compared with envelope curves of the experimental results.; The finite element method is used to simulate the behavior of laterally confined concrete columns under large inelastic deformations. Experimental data available in the literature as well as from the present research are used to verify the finite element model. It has been able to predict satisfactorily the peak load of the experimental results; but it has not been possible to predict the descending branch of the load-displacement graph.