RCS Frame Systems
Composite moment frames evolved in the United States during the late 1970’s and 1980’s as a variation of conventional steel moment frames in mid- to high-rise buildings. Later in the 1980’s, similar systems were being developed for low-rise construction in Japan. The composite frames typically resemble conventional steel frame construction except that the steel columns are replaced by high strength reinforced concrete. The primary motivation for this is a 10 to 1 cost advantage of concrete over structural steel for resisting compressive column loads (Griffis 1992). Innovative construction staging operations also reduce the cost and construction time. As shown in Fig. 1a, a typical construction sequence utilizes small steel erection columns to advance steel framing several floors ahead of placing reinforced concrete columns. Shown in Fig. 1b is an alternative precast construction method that has been applied to low-rise buildings in Japan. In this scheme, the steel beam is cast integral with the column and field spliced a short distance away from the column face. Variations to these methods, such as utilizing the column reinforcing bar cage as the erection column have also been developed.
Figure 1. Alternative construction methods for composite framing systems (a) cast-in-place with steel erection columns, (b) precast.
Shown in Fig. 2 is a typical beam-column connection for composite frames where the steel beam passes continuous through the joint, thereby avoiding interruption of the beam at the column face. This eliminates the need for welding or bolting the beam at the point of maximum moment and, thereby, avoids fracture problems encountered in welded frames during the 1994 Northridge and 1995 Hanshin earthquakes. The concrete column reinforcement runs continuous through the joint and is spliced near column mid-height. One of the challenges in research and design has been to develop simple yet effective details to transfer large shears and moments between the beams and columns.
Figure 2. Connection between Steel Beam and Reinforced Concrete Column
Tests have demonstrated that composite connection details of the type shown in Fig. 2 can provide excellent strength and deformation capacity. For example, plotted in Fig. 3 are load-deflection curves of two composite beam-column subassemblies that exhibit stable hysteretic behavior (Kanno and Deierlein 1998). The plot in Fig. 3a is for a test where inelastic effects are confined to steel beam hinging outside the column, and the one in Fig. 3b is for a case where the beam is oversized and remains elastic with all inelastic deformations occurring in the joint. While the behavior shown in Fig. 3a (beam hinging) is preferred for seismic design, the stable response in Fig. 3b demonstrates the inherent robustness of these connections, even when the beam is stronger than the joint. Design guidelines for the connections are available (ASCE 1994, AIJ 1994) that incorporate force-transfer mechanisms similar to those commonly employed for reinforced concrete and/or steel connections