ACI STRUCTURAL JOURNAL TECHNICAL PAPER Title no. 89-S57 Behavior of Interior Narrow and Wide Beams by Egor P. Popov, Julie Mark Cohen, Kosonike Koso-Thomas, and Kazuhiko Kasai Narrow as well as wide beams are used in moment-resisting reinforced concrete frames in seismic zones; typically the longitudinal beam bars in such frames are placed inside the column. Experimental investigations into the behavior of reinforced concrete beam-column-slab cruciform interior joint subassemblages under simulated seismic loadings are described in this paper. The subassemblages studied include narrow and wide beams with some of the top longitudinal reinforcing bars placed outside the columns, thereby reducing the bars in the columns. This detailing is not permitted in U.S. seismic codes. However, test assessment indicates that placement of some of the longitudinal bars outside of the columns is permissible, leading to lesser congestion of reinforcement. Keywords: beams (supports); beam-column frame: bond (concrete to reinforcement); columns (supports); cyclic behavior; earthquake-resistant structures; framing systems; hysteresis; interstory drift; joints (junctions); reinforced concrete; seismic design; serv- iceability; structural design. Cost minimization of construction is usually the control- ling criterion for structural system design. One of the ways to optimize the design of a building is to provide a larger number of stories within a given height. As a result, structural floor systems are becoming progressively shallower, even in seismic zones. For example, in California, post-tensioned flat slab floors are being used in conjunction with shearwalls as a dual seismic-resistant system, in which the shearwalls provide the primary lateral resistance and the slab-column joints are relied upon as a secondary or back-up system. For each floor type used in seismic zones, the structural beam- column and slab-column joints must be designed to have sustained strength, and be required to dissipate energy through deformation reversals into the inelastic range. A framing system which offers a compromise in stiffness between those with beams narrower than columns and those with flat slabs is a system with beams which are wider than the columns. However, there is little information available for cyclic behavior of wide beams at interior joints, and consequently, no guidelines are provided for wide beams in the U.S. codes. For example, ACI requirements 1 and recom- mendations2 apply only to (1) joints in which the column width is equal to or greater than the beam width, and (2) narrow beams whose longitudinal reinforcing is inside both the column and beams. recommendations, 2 which are based on a review of all pertinent research up to 1983, state that "all of the beam longitudinal reinforcement (is) to be ACI Structural Journal I November-December 1992 located within the column longitudinal reinforcement. Infor- mation is needed on the behavior of connections where beams are wider than columns, and beam longitudinal reinforcement cannot be placed within the column longitudinal reinforce- ment." There is concern about the confinement and cyclic bond behavior and anchorage of the longitudinal beam bars that pass outside of the columns. Also, there are issues that need to be addressed about the inherent flexibility of a fram- ing system with shallow, wide beams, and the damage that can be caused to nonstructural components from excessive inelastic rotations at the beam-column joints and magnitude of interstory drifts. Several examples can be noted in which wide beams have been used. In the examples cited here, some of the longitu- dinal beam bars pass outside of the column bars and are not confined by the concrete mass of the columns. A common use of wide beams can be found in such structures as parking garages in which the floors are two-way waffle slab systems. Often, shallow beams are formed by filling in the waffle slabs between columns, which results in the beams being wider than the columns. Another shallow floor system, widely used in Mexico, consists of cast-in-place slabs and ribs infilled with hollow ceramic tiles, or reinforced concrete. 3 A variation on the use of wide beams can be found in parking garages, warehouses, commercial buildings, office buildings, etc. In these structures, flat slabs may be used with dropped panels at columns or dropped panels forming shal- low beams between columns. A special case using wide beams can be found in the San Jose State Office Building in California. *.4 A reinforced con- crete ductile moment resisting frame with a two-way waffle slab system was chosen over a ductile moment steel frame for its fire rating. In a typical bay, 30 in. diameter columns were located at approximately third points along the girders' lengths for architectural and technical reasons, rather than *Architect: ELS Design Group, Berkeley, CA. Structural Engineer: E. G. Hirsch and Associates, San Francisco, CA. ACI Structural Journal, V. 89, No.6, November-December 1992. Received Sept. 6, 1991, and reviewed under Institute publications policies. Copyright © 1992, American Concrete Institute. All rights reserved, including the making of copies, unless permission is obtained from the copyright proprietors. Pertinent discus- sion will be published in the September-October 1993 ACI Structura!Journal if received by May I, 1993. 607