The seismic behaviour of small reinforced concrete beam-column knee joints

Technical Abstract

Most research into beam-column knee joints has been under monotonic loading and many of these joints failed to reach their member moment capacity especially under opening moments. A few cyclic knee joint tests have been completed in the United States in the last five years. This report describes the cyclic testing of 11 small knee joints designed to the 1995 Concrete Standard, including three joints with anchorage plates on the principal reinforcing bars. Two joints designed and detailed to the 1965 Code were also tested.

Joints with U-bar anchorages performed better than joints with standard 90 degree hook details on beam and column bars. The current Standard (NZS1301:1995) designs were able to reach nominal moment capacity in both directions up to and including ductility 4 displacements, but subsequent strengths fell off at higher ductilities. Joints with extra diagonal bars across the inner corner were able to sustain their nominal members strengths to higher ductility levels. A nominal horizontal joint shear stress of 0.1fc' (Mpa) for knee joints, in ductile frame buildings is recommended. This limit is only half of the current NZS3101:1995 Standard recommendation. The 1960s designed joints behaved poorly, as expected, with joint shear and anchorage failures occurring, in both directions, at strength levels below the beam's nominal strength.

The two joints with anchorage plates attached to the ends of the "standard hook" anchorage failed to reach their respective nominal strengths in either direction, while the joint with an anchorage plate welded to each main U-bar sustained moments greater than nominal in both directions prior to ductility 2, but subsequently lost strength under closing moments during higher ductility cycles.