International Concrete Abstracts Portal

Title: Full-Scale Testing of a New Double-Beam Coupling Beam (DBCB) with A Simplistic Reinforcing Layout

Author(s): Kyoungsub Park and Shih-Ho (Simon) Chao

Publication: CRC

Volume:

Issue:

Appears on pages(s): 350

Keywords:

DOI:

Date: 8/5/2025

Abstract:
Tests were carried out on a series of double-beam coupling beams (DBCBs), including two full-scale DBCBs and one half-scale DBCB. The beams had aspect ratios (beam span/beam depth, ln/h) of either 2.4 or 3.2 and were tested with or without a slab. These specimens were tested to study the seismic behaviors and performance of double-beam coupling beams (DBCBs) using the design procedure proposed by Choi and Chao (Choi et al., 2020). In recent half-scale tests using Gr. 60 rebars, double-beam coupling beams (DBCBs) have been demonstrated to be a promising alternative to diagonally reinforced concrete coupling beams (DCBs). These tests, conducted by Choi et al. (2018, 2020), showed that the seismic performance of DBCBs is equivalent to that of DCBs, even without the use of diagonal reinforcements. This has the potential to significantly reduce reinforcement congestion and construction difficulties. This study aimed to investigate various aspects of DBCBs under fully reversed cycling loading. The objectives were: (1) to verify a proposed DBCB design procedure, (2) to examine the size effect on the spacing of transverse reinforcement, (3) to evaluate the performance of DBCBs made with high-strength rebars (ASTM A706 Gr. 80) and determine their required development length, (4) to assess the size and location of utility duct openings, and (5) to examine the effect of the slab on the performance of the DBCBs. According to the experimental results, all three specimens that were reinforced with high-strength rebars (ASTM A706 Gr. 80) showed excellent ductility and stable performance until their shear strength decreased significantly. Specifically, the half-scale DBCB specimen with a slab demonstrated a stable hysteresis loop up to a chord rotation of 8%, with only a slight loss of strength. The two full-scale DBCB specimens also exhibited stable hysteresis loops up to a chord rotation of 6% without any significant loss of strength. No noticeable size effect associated with hoop spacing was observed in full-scale specimens. However, the larger longitudinal rebars used in the full-scale specimens experienced inelastic buckling after a 6% chord rotation. This was not observed in the half-scale specimens. While the full-scale 2.4 aspect ratio DBCB specimen without middle layer longitudinal reinforcement performed satisfactorily, it is highly recommended to include a middle layer of longitudinal reinforcement in each individual beam to improve the confinement of the concrete core and impede the crack propagation. The longitudinal high-strength reinforcement (Gr. 80), which was anchored in the end concrete blocks representing wall piers, experienced higher strains compared to the previous half-scale specimens that used conventional reinforcement (Gr. 60) (Choi et al, 2018). However, the strain distributions in the Gr. 80 reinforcement were very similar to those of the Gr. 60 reinforcement. This implies that the development length defined as 60% of that required by ACI18.8.5.3 is valid for Gr. 60 conventional steel, as well as with Gr. 80 high-strength steel. The full-scale specimens were tested with horizontally inserted circular PVC pipes for utility ducts, and the results indicate that it is possible to place four 3-inch-diameter circular penetrations simultaneously at both ends of a DBCB, as well as at the midspan of the upper and lower beams, without compromising its shear strength, stiffness, or ductility. The addition of a slab changes the elastic neutral axis and the location of the maximum horizontal shear in the gross section, which is above the unreinforced concrete strip (UCS) located at the beam's mid-height (Naaman and Chao, 2022). This causes a slight delay in the concentration of diagonal shear cracks within mid-height UCS. However, this delay has no effect on the separation of the UCS at larger rotations. On the other hand, the slab provides additional confinement to the coupling beam, reducing the width of diagonal shear cracks. A comparison of half-scale specimens with and without a slab reveals that the slab improves the shear strength and energy dissipation capacity, resulting in a "fuller" hysteresis loop for DBCBs. No evidence suggests that Gr. 80 rebars have any negative impact on shear strength and ductility compared to Gr. 60 rebars. The test results show that both full-scale DBCB specimens with aspect ratios of 2.4 and 3.2 reached an ultimate chord rotation of 6%, which is approximately equivalent to the rotational demand of an MCE level earthquake, before their strengths decreased below 80% of the peak shear force. Additionally, both specimens achieved a peak shear stress of around 10√f_c' psi (0.83√f_c' MPa). The experimental results support the conclusions of the previous half-scale DBCB tests and suggest that a coupling beam with an aspect ratio between 2.4 and 3.2 can employ non-diagonal reinforcement layouts and up to four circular openings (two at the beam ends and two at the mid-span of the beam) without compromising its seismic performance. Lastly, an updated design flowchart and a comprehensive design example based on research findings in this study are included.