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Reinforced concrete coupled shear walls are effective systems for resisting lateral loads, often used in mid to high-rise buildings in earthquake-prone areas. These walls usually feature openings for doors and windows, dividing a solid wall into two separate piers. The strength of these walls comes not just from the sum of two individual piers, but from wall piers cross-section and the framing action between the wall piers through the coupling beams. In an earthquake, coupling beams serve as fuse elements, distributing seismic energy throughout the height of the building. This not only reduces the bending stress at the base of the shear walls but also improves their overall strength, stiffness, and resistance to lateral forces. Properly designed coupling beams, with sufficient longitudinal, diagonal, and confinement reinforcement, can effectively absorb energy while maintaining significant strength and stiffness, even under large deformations.The objective of this study was to develop, calibrate, and validate a new coupling beam model that integrates axial and lateral interactions under cyclic loading conditions. This model aims to reliably predict the elastic and inelastic responses of diagonally reinforced coupling beam elements. The proposed analytical model incorporates a fiber-based concrete cross-section, and diagonal trusses to account for axial interactions between the nonlinearity in the steel and concrete along the beam's length. This feature allows the model to capture additional axial force developed in the element due to the axial restraint from the wall piers, thereby increasing or decreasing the lateral strength of the beam. Additionally, the model includes the slip-extension behavior between the coupling beam and the supporting wall through zero-length fiber-based elements at both ends of the beam. Finally, with the development of the new analytical model and recent advancements in understanding the shear strength of RC shear walls, a new coupled/core wall design approach has been introduced to optimize the design of RC core walls. A variety of archetypes have been designed, based on both current design practices and the proposed approach. Detailed analytical models have been developed, and the efficiency of the proposed design has been evaluated through nonlinear static and dynamic analyses. To conduct the dynamic analysis, suites of ground motions were selected using the CMS approach and scaled to the MCER level of hazard. It has been demonstrated that the designed archetypes based on proposed procedure provide a more reliable shear responses under seismic loading compared to current design practices.
Coupled shear walls are a lateral load resisting system used in buildings to resist seismic and wind loads. In coupled walls, coupling beams span between adjacent shear walls and are typically located at floor level. Coupling beams are designed to yield and form plastic hinges before the wall piers. Damage patterns observed after the 2010-2011 Canterbury earthquake sequence in New Zealand showed instances in which coupled walls did not behave as intended in design, as plastic hinges formed at the base of the wall piers but not at the beam ends. The Canterbury Earthquakes Royal Commission suggested that this undesirable response may have been caused by coupling beam axial restraint from walls and floors increasing the strength of the coupling beams.To better understand the effect of axial restraint on coupling beam behavior, seven one-half-scale reinforced concrete coupling beams were designed using ACI 318-19 and were constructed and tested. The main test variables were span-to-depth ratio, reinforcement configuration (conventional or diagonal), primary reinforcement ratio and bar diameter, and level of axial restraint. Six beams consisted of three identical pairs, with the two beams in each pair tested at a different level of constant stiffness axial restraint.Test results indicated that axial restraint, which is not included in the ACI 318-19 equation for nominal shear strength of diagonally reinforced coupling beams, increased the beam strength. Axial restraint also influenced the load-displacement responses of the beams and the observed damage patterns. The conventionally reinforced beams were observed to yield in shear, while damage concentrated at the ends of the diagonally reinforced beams. The onset of significant strength degradation in the diagonally reinforced beams was associated with buckling of diagonal reinforcement rather than crushing of confined concrete, such that variation in axial compression on identical pairs of beam did not lead to a significant difference in deformation capacity. Test beams with #6 diagonal reinforcement had improved deformation capacity over those with #4 diagonal reinforcement, due to the influence of the ratio of transverse reinforcement spacing to diagonal bar diameter (s/db) on bar buckling.
Following the 2010/2011 Canterbury Earthquakes, an investigation by the Canterbury Earthquakes Royal Commission (CERC) considered the performance of a range of buildings in Christchurch. Several of the buildings investigated by the CERC included reinforced concrete coupled walls, which are comprised of two wall piers linked (or coupled) by a series of coupling beams at each floor level. Notably the coupled wall buildings investigated by the CERC were observed to have performed undesirably when compared to their design intent. It was found by the CERC that these coupled walls tended to display higher strengths and lower ductility capacity than was intended in design. The postulated reason for this behaviour was that interaction between structural components strengthened the coupling beams by restraining the tendency of the coupling beams to axially elongate. To better account for this interaction in design practice, it was recommended by the CERC that the behaviour of coupled walls be investigated further. In this study, structural interaction between coupling beams and floors was first considered using finite element software VecTor2. It was found that the floors tended to restrain the elongation of coupling beams and to cause large coupling beam strength increases. The extent of floor that was activated to restrain coupling beam elongation being found to be dependent upon the arrangement of the floor. Existing provisions of NZS 3101:2006 for upper bounds on floor effective widths were found to be valid for assessment of the maximum coupling beam strength amplification caused by floor interaction. Analysis of a series of seismically loaded coupled walls interacting with floors was undertaken using VecTor2 software. In agreement with the findings of the CERC, axial restraint of coupling beams was found to have a large impact on coupled wall performance. Coupling beam strengths were measured up to 300% of their design strength, which tended to change the strength hierarchy of the coupled wall. In particular it was found that many existing coupled walls would have behaved similarly to a single cantilever wall with penetrations because the coupling beams were too strong to yield. These coupled walls tended to display lower energy dissipation and higher wall pier damage than assumed in design. The coupled wall provisions proposed (at the time of writing) in the 2014/2015 NZS 3101:2006 Amendment were found to over-estimate the impact of the floor systems on restraining coupling beam elongation. However these provisions did not include the effect of the wall piers restraining coupling beam elongation, so overall coupled wall overstrength capacities tended to be under-predicted. As an approximate method of accounting for axial restraint in design of coupled walls, it was recommended that redistribution of design demands be used to reduce the coupling beam design capacity and to achieve a more desirable coupled wall behaviour.
The effects of axial restraint on the stiffness and ductility of diagonally reinforced concrete coupling beams are investigated analytically through the use of finite element analysis. To provide accurate result without undue computational expense, a mesh sensitivity study is performed to determine the optimal mesh of the analysis. To validate the modeling technique, an analysis was performed reproducing experimental testing performed at the University of Cincinnati with good agreement between the two. An analysis was performed reproducing the experimental testing performed by Dr. Fortney at the University of Cincinnati. Finally, non-linear analysis of a prototype building utilizing the hysteretic response obtained from the finite element analysis for each case of axial restraint was performed. External axial restraint in diagonally reinforced concrete coupling beams is shown to increase the initial stiffness of the coupled core wall lateral force resisting system, as well as improve the post-peak stiffness and energy dissipation.
Forty scientists working in 13 different countries detail in this work the most recent advances in seismic design and performance assessment of reinforced concrete buildings. It is a valuable contribution in the mitigation of natural disasters.
This book introduces practising engineers and post-graduate students to modern approaches to seismic design, with a particular focus on reinforced concrete structures, earthquake resistant design of new buildings and assessment, repair and strengthening of existing buildings.
Reinforced concrete structural walls provide an efficient lateral system for resisting seismic and wind loads. Coupling beams are commonly used to connect adjacent collinear structural walls to enhance building lateral strength and stiffness. Steel-Reinforced Concrete (SRC) coupling beams provide an alternative to reinforced concrete coupling beams, diagonally-reinforced for shorter spans and longitudinally-reinforced for longer spans, and offer potential advantages of reduced section depth, reduced congestion at the wall boundary region, improved degree of coupling for a given beam depth, and improved deformation capacity. Four large-scale, flexure-yielding, cantilever SRC coupling beams embedded into reinforced concrete structural walls were tested by applying quasi-static, reversed-cyclic loading to the coupling beam (shear) and the top of the wall (moment, shear, and constant axial load) to create cyclic tension and compression fields across the embedment region. The primary test variables were the structural steel section embedment length, beam span length (aspect ratio), quantities of wall boundary longitudinal and transverse reinforcement, and applied wall loading (moment, shear, and axial load). Based on test results, long embedment length, sufficient wall boundary reinforcement, and low-to-moderate wall demands across the embedment region are all associated with favorable coupling beam performance, characterized by minimal pinching and strength degradation in the load-deformation response and plastic hinge formation at the beam-wall interface with a lack of damage (plasticity) in the embedment region. The variation in aspect ratio was not found to significantly affect performance. Detailed design and modeling recommendations for steel reinforced concrete (SRC) coupling beams are provided for both code-based (prescriptive) design and alternative (non-prescriptive) design. For both code-based and alternative design, modeling a rigid beam for flexure and shear deformations with rotational springs at the beam-wall interfaces is recommended for stiffness, as test results indicate that the majority of the coupling beam deformations were associated with interface slip/extension. Alternative stiffness modeling recommendations are provided, in which an effective bending stiffness that accounts for the aspect ratio or beam length is used instead of interface rotational springs. A capacity design approach, in which the provided embedment strength exceeds the expected beam strength, is recommended for determining the required embedment length of the steel section into the structural wall. Recommendations for computing the nominal and expected (upper bound) flexure and shear strengths are provided. For alternative design, additional parameters are provided to define the strength and deformation capacity (to complete the backbone relations) and to address cyclic degradation for each of the test beams.