The Next Generation of Coupling Beams

A collection of experimental, parametric, and analytical studies was carried out to further the understanding of the behavior and constructability of coupled core wall (CCW) systems. Issues related to the design of wall piers in CCW systems and the beams used to couple the wall piers, structural response of the overall system, and construction difficulties arising from reinforcing steel congestion were explored. The research presented in this document focuses on a design approach of coupled core wall systems which concentrates on minimizing steel congestion while maintaining satisfactory structural behavior. Several different configurations of coupled core wall systems were examined. The experimental portion of this research entailed reverse cyclic testing of representative subassemblies consisting of a coupling beam connecting two wall piers. The subassemblies are considered to be approximately 1⁄2 scale of a typical coupled core wall structure and the four different types of beams tested are: (1) a diagonally-reinforced concrete coupling beam (DBCB), (2) an unencased steel beam (SCB), (3) an unencased steel beam with a steel fuse link located at mid span of the beam (FCB1 and FCB2) and, (4) a composite steel/concrete beam utilizing a vertical web shear plate with headed shear studs (SPCB). Specimens (1) and (2) are types of coupling beams common to CCW structures. Specimens (3) and (4) are new proposed types of coupling beams. Testing was performed by installing the test specimen within two load frames provided with actuators used to apply load to the specimen. One wall pier was “clamped” to a reinforced concrete pedestal while the coupling beam and other wall pier was cantilevered beneath the load frames’ actuators. Constant axial forces were imparted to the wall piers by virtue of pre-tensioned rods. These forces resisted the overturning forces during testing. The cantilevered wall pier was then loaded/displaced in cycles of increasing loads and/or displacements to impart shear force to the coupling beam. For these tests, effects of horizontal shear in the wall piers was neglected. The SCB test was performed as a control by which the other specimens would be measured. The main test goals for the DBCB specimen were (a) measuring behavior of a diagonally-reinforced concrete coupling beam with a practical length-to-depth ratio (in this case equal to 2.5) and (b) effect of lack of confinement steel around the diagonal bar groups (a minimum number of stirrups were provided as needed to hold the bars in place during construction). The main test goals of the FCB1 and FCB2 specimens were to investigate the potential for developing localized damage in the fuse to minimize postdamage repair and/or replacement of the coupling beam. The main goal for the SPCB was to evaluate the proposed design methodology for the new section, and its potential as a viable coupling beam alternative. The steel coupling beam (SCB) demonstrated superior stiffness and energy dissipation characteristics over all the other coupling beams. The confined core of the DBCB remained intact through 3% rotation, suggesting that at rotational demands of 3% or less it is not necessary to provide ACI-compliant confining steel to the diagonal bar groups. The fuse coupling beam demonstrated stable hysteretic behavior similar to that of the SCB specimen, and yielding of the fuse web was achieved while the main section of the beam remained elastic. The SPCB test failed to achieve sufficient composite action between the steel web plate and surrounding concrete. Further work is necessary to better engage the steel plate. Parametric analytical studies were also conducted to supplement the experimental studies. A parametric study pertaining to practical designs of diagonally-reinforced concrete coupling beams shows that achieving a practical diagonally-reinforced coupling beam is difficult, if not impossible. For most combinations of realistic span-to-depth ratios and service shear stress levels, the required area of steel needed for the diagonal bar groups either creates significant construction difficulties or is simply impossible to achieve. A parametric study pertaining to the design of wall piers shows that the current methods for determining the need for special boundary elements permitted by ACI 31802 are not applicable to wall piers in coupled core wall systems. An alternative method for determining the need for special boundary elements in walls piers in coupled core wall systems is proposed. The proposed method recommends that the need for special boundary elements be determined based on maximum concrete compressive strains expected at design loads. It is demonstrated that for a family of coupled core wall configurations special boundary elements are not required. To investigate the overall structural behavior of coupled core wall systems with wall piers and coupling beams designed based on the recommendations of this research, nonlinear time-history analyses were performed on two prototype structures. Both structures had the same plan dimensions, number of stories, slab thickness, wall thickness, and floor-to-floor heights. The first structure was designed using a steel coupling beam, and the second was designed using a diagonally-reinforced concrete coupling beam. The need for special boundary elements in the wall piers was based on maximum concrete compressive strains at design loads. Properties of the coupling beams used in the analyses of the two structures were based on measured behavior obtained during the experimental phase of the research. The analyses show that the steel coupling beam is superior to the diagonally reinforced coupling beam in terms of energy dissipation and drift control. However, the DBCB building satisfies code requirements. It should be noted that analyses did not consider the effects of shear on the axial and flexural load-carrying capacities of wall piers with no special boundary elements. Additional studies are, therefore, needed before the presented conclusions can be generalized.