Seismic capacity of reinforced concrete frames with insufficient development length of joint reinforcement

Gravity loads acting on reinforced concrete frame structures produce negative bending moments (tensile stresses at the top of the beam) at the ends of the beams in the frame. Since the bottom reinforcing steel in these beams is not required to carry the negative moments, the ACI code permits minimum embedment of the steel into the supporting column. This small embedment severely limits the lateral load carrying capacity of the frame since lateral loads produce positive bending moments at one end of the beams in the frame. Many older structure were designed with minimal seismic requirements and therefore have details using the minimum embedment lengths. The National Earthquake Hazard Reduction Program (NEHRP) requires that these structures be reviewed for seismic loading. This paper outlines an analysis procedure that can be used to treat such beam connections.