How can designers relieve the extreme congestion of intersecting rebars and hoop ties in conventional seismic beam-to-column connections? Battelle researchers believe they have found a good design answer and have at the same time produced a connection that reduces severe bond deterioration of main bars passing through columns in interior connections. The newly developed concrete connection, which uses steel fibrous concrete in the connection region, is not only seismic-resistant but also less expensive than other beam-column connections. Developed at Battelle's Pacific Northwest Laboratories, the connection has shown several advantages over conventional connections.
The large quantities of steel hoops and crossties that are used to confine the concrete in the connection increase fabrication costs and make concrete placement difficult. In tests, the new design reduced the congestion by replacing the hoops in the connection region with steel fibers. At the same time, by increasing the strength of the beam, the steel fibers force the plastic hinge in the beam to develop away from the face of the column, thus reducing the problem of bond deterioration of main bars passing through the connection. Easing that problem is the fact that steel fibrous concrete can increase the anchorage bond strength of deformed bars by 40 percent.
During the experimental program, tests were conducted on two full-size beam-to-column connections. One was a conventional connection using hoops in accordance with the seismic-resistant design provisions of ACI 318-71; the other was a modified connection using steel fibrous concrete instead of hoops in the connection region. Both types of connection were subjected to cyclic loading equivalent to two major earthquakes. The modified connection was stronger than the conventional one, developing a moment capacity of 57,700 pound feet, an increase of 25 percent over the conventional connection.