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Full-scale testing confirms seismic performance
Composite moment frame systems, comprising steel beams and reinforced concrete columns, were introduced in the U.S. and Japan about 25 years ago. In the U.S., these systems were developed as an alternative to steel moment frames used in high-rise construction, primarily to reduce material costs.1 In Japan, similar systems were developed as an alternative to reinforced concrete moment frames used in low-rise construction, primarily to improve seismic performance by using long-span steel beams as ductile components.2 Ensuing research and building code development over the past 15 years have helped establish composite frames as viable alternatives to either conventional steel or reinforced concrete construction. The most recent phase of this research has culminated in a full-scale frame test to validate the performance of composite frames for seismic design.
Innovative construction staging and connection detailing add to the attractiveness of composite frame construction. Two common types of construction are shown in Fig. 1. In the cast-in-place approach (Fig. 1(a)), small steel erection columns support continuous steel beams at the upper levels, as reinforcement and concrete are placed in the columns below. This approach is best suited for mid-rise and high-rise buildings, where the operations can be staged over several floors. A variation of this approach was developed in Japan and uses special reinforcing bar cages in place of the steel erection columns. In the precast approach (Fig. 1(b)), precast columns incorporating short steel beam segments are erected. The columns are connected to previously-placed columns using grouted splice sleeves, and infill beams are field-bolted to the steel beam segments.
BEAM-COLUMN CONNECTION DESIGN
Composite frames can be detailed to provide tough and reliable beam-column connections that are well-suited for seismic applications. A typical connection is shown in Fig. 2. Because the steel beam is continuous through the column, splices can be avoided at points of maximum bending moment. Compared to conventional steel frame construction, this detail helps to avoid fracture problems such as those encountered in welded frames during the 1994 Northridge earthquake. Compared to reinforced concrete construction, this detail reduces problems associated with reinforcing bar anchorage and congestion in seismically conforming joints.
Tests have demonstrated that composite connection details can provide excellent deformation capacity and that the strength of a properly detailed...