Confined Boundary Elements Research Articles

Behavior under lateral cyclical load of thin reinforced concrete walls of the industrialized system

Reinforced concrete wall buildings of the industrialized system widely used in Colombia for housing construction in recent years present particular characteristics that make them a research subject. The reduced thicknesses of their walls that give rise to high slenderness ratios, the use of nonductile reinforcement arranged in a single curtain, and the absence or low level of confinement at the ends of the walls are some of the most relevant characteristics. This article presents the results of five quasi-static cyclical tests of thin-reinforced concrete walls (TRCW) on a full-scale, which are representative of the industrialized system. Wall properties were selected from a database of 136 existing buildings. These specimens were built with different cross-sections, lengths, and reinforcement contents at the ends. The experimental results confirm the importance of the properties and amount of web reinforcement and the effect of the use of properly confined boundary elements. The observed response and analysis of the experimental database of TRCW suggest that some walls could present a limited displacement capacity compared to the maximum drifts allowed by the design code.

Experimental study on seismic behavior of precast concrete sandwich shear walls under high axial compression ratio

To shed light on the seismic behavior of precast concrete sandwich shear walls (PCSSWs) in high-rise buildings, reversed cyclic loading test were conducted on eight full-scale shear wall specimens under high axial compression ratio. Totally four variables were considered in the test, i.e. type of shear wall (PCSSW, non-insulated precast concrete shear wall (PCSW), cast-in-place (CIP) shear wall), location of insulation layer (off-center or center in the section), confinement at boundary elements (with or without confined core), and details of grouted sleeve connection (double or single row of grouted sleeves). Note that in this paper the PCSSWs with off-centered insulation layer had confined cores at boundary elements while steel stirrups for additional confinement were not used in the PCSSWs with centered insulation. Test results revealed that the PCSSWs showed failure mode, hysteresis loops, loading capacities and displacement ductility similar to PCSW and CIP specimens. The loading capacities of PCSSWs and CIP specimen increased by 8.2 % ∼12.4 % because of the confined core at boundary elements. The displacement ductility of specimens with confined boundary elements varied from 2.35 to 2.65, which was higher than that of the specimens with unconfined boundary elements (1.75 ∼ 2.10). Besides, the number of rows of grouted sleeves had minor effect on hysteretic response of specimens. Based on the test results, a quadrilinear degradation restoring force model was proposed.

Moment–Curvature Relationship of RC Structural Walls with Confined Boundary Elements Using Pre-Fabricated Rectangular Continuous Hoops

현 설계기준에서 제시한 단부횡보강 상세는 시공성과 생산품질이 현저히 떨어지므로 내진성능과 시공성을 확보할 수 있는 RC 구조벽체의 단부횡보강 상세 개발이 필요하다. 최근 선재 제작기술의 발전으로 다양한 선조립철근의 제작이 가능해지고 있으며 특히 다양한 연속횡보강 철근 상세의 제작이 가능하게 되었다. 본 연구에서는 사각 연속횡보강 선조립철근 단부횡보강 상세를 적용한 RC 구조벽체의 모멘트-곡률 관계에 대한 분석을 진행하였다. 비선형 단면해석에 의하면 사각 연속횡보강의 상세를 적용한 RC 구조벽체는 사각 연속횡보강 영역의 면적이 증가할수록 내진성능을 확보할 수 있다. 이러한 연구결과에 근거하여 사각 연속횡보강 선조립 철근의 상세 적용시, 단부횡보강 영역의 면적만큼 사각 연속횡보강 영역의 면적을 확보해야 한다.

Effective flexural stiffness for reinforced concrete shear walls having confined boundary elements

In the design of reinforced concrete (RC) shear walls strength, ductility and effective stiffness of the elements must be taken into account and are important parameters in terms of structural safety. Accurate estimation of the ductility and effective stiffnesses of RC members has always been an attractive subject of study as it provides a reliable estimate of the capacity of buildings under seismic loads. In this study, RC shear wall models with different concrete strength, longitudinal and transverse reinforcement ratios were designed to investigate effective section stiffness and coefficients. The effective stiffness of the cracked section in the RC shear walls designed in different parameters were analytically obtained. Analytically investigated parameters were calculated from TBEC (2018), ACI318 (2014), ASCE/SEI41 (2017) and Eurocode8 (2004, 2005) regulations and nonlinear behaviors. The results obtained according to different design parameters were compared and examined. In the relations suggested for the effective section stiffness coefficient, the confining effect is not taken into account as in the regulations. Therefore, it means neglecting the effects of parameters such as concrete strength, confining effect and axial load levels acting on the section. This situation can lead to unrealistic results in the design and evaluation of RC elements. For this reason, determining the moment-curvature relationship in the design and evaluation of RC elements and obtaining effective section stiffness values are of great importance in order to obtain more realistic results.

Experimental Investigation of Axial Load and Detailing Effects on the Inelastic Response of Reinforced-Concrete Masonry Structural Walls with Boundary Elements

In typical wall load-bearing reinforced-masonry (RM) buildings, the lateral and vertical forces are resisted by rectangular shear walls. Thus, the walls are subjected to high vertical forces from gravity loads that are expected to limit the displacement and energy dissipation capacities. Moreover, the rectangular RM shear walls have limited lateral stability because of the single vertical reinforcement layer. The intent of this study is to investigate the inelastic cyclic response of RM structural walls subjected to axial compressive stress that results in precompression ratios, P/Agfm′, higher than 10%. The main objective is to propose practical component-level seismic detailing recommendations to enhance the overall structural performance. In this respect, three half-scale, fully grouted RM shear walls were tested under constant axial load, in-plane fully reversed cyclic loading, and top moment. The tested specimens are flexural dominant to simulate the response of mid and high-rise RM shear walls under strong seismic actions. The walls were designed to have enlarged boundary elements built using C-shaped blocks to evaluate the ability of end zone detailing and confinement to alleviate the impact of the high axial load. The test results demonstrated an overall enhanced structural performance for the three walls. The three specimens attained high ductility levels, high energy dissipation capacity, and failure in the ductile flexural mode. The presence of the well-detailed and confined boundary elements was effective in mitigating the impact of the high axial compression load. Thus, utilizing this type of masonry shear wall increases the competitiveness of masonry buildings as an alternative construction method.

Thin slender concrete rectangular walls in moderate seismic regions with a single reinforcement layer

Some Latin-American countries, including Colombia, Peru, Panamá and the Dominican Republic, have adopted an industrialized system for the construction of buildings using thin slender reinforced concrete walls. The main advantage of this system is that it can increase the construction speed and reduce the use of nonstructural walls, as all architectonical spaces are defined by the structural walls. Additionally, designers tend to use thin structural walls with low steel reinforcement ratios, which is reflected in a reduction of the construction cost. The typical wall section for 6 to 10-story buildings is characterized by a thickness of around 100 mm and a single layer of welded wire mesh acting as longitudinal and transverse reinforcement. Additional reinforcing bars may be placed at the wall edges to increase moment capacity, but in most cases, there are no confined boundary elements along the edges. Despite the system's popularity, experimental data for these types of walls is still scarse. In addition to this, structural walls of low thickness and high aspect ratio with unconfined or poorly confined boundary elements have shown structural deficiencies in the 2010 Central Valley Chile earthquake. In this paper, existing and new experimental data from representative thin slender walls, used in moderate seismic regions, was analyzed to evaluate the structural system under lateral loads. Two unconfined reinforced concrete walls with typical section detailing were tested. Additionally, these tests were complemented with an experimental database of 28 rectangular wall units of thickness less than 100 mm, as reported in the literature. This data was used to analyze the behavior of rectangular thin slender walls in terms of axial load ratio, boundary elements conditions, plastic hinge length, and maximum drift capacity. The experimental data shows a significant reduction in drift capacity as axial load, clear interstory height to wall thickness ratio, or wall length increases. It is also evident that plasticity is concentrated at the base of the walls, mainly due to the low vertical reinforcement ratios. Finally, a capacity vs. demand stochastic analysis was carried out to evaluate the performance of buildings up to 10 stories in a moderate seismic zone. These analyses show that for moderate seismic regions the probability of reaching a severe damage limit state is low for buildings configured with rectangular walls having a single layer of reinforcement.

Seismic performance and height limits of ductile reinforced masonry shear wall buildings with boundary elements

The National Building Code of Canada, NBCC-15 has recently added a new Seismic Force Resisting System (SFRS) category, ductile shear walls, for Reinforced Masonry (RM) buildings. Although it was assigned a higher ductility-related force modification factor compared to that of moderately ductile walls, NBCC-15 assigned the same building height limits for the ductile and moderately ductile RM walls. This study aims to assess (i.e. numerically) the seismic performance and collapse capacity of ductile RM buildings, having heights exceeding the code limit, built using ductile RM shear walls with boundary elements as the SFRS. The main objective is to propose height limits based on solid and objective seismic performance acceptance criteria. In this regard, six archetype buildings with varying heights are designed according to CSA S304-14 with ductile RM structural walls having confined boundary elements. The reference buildings are located in two regions representing the high and moderate seismicity levels of NBCC-15. The seismic performance is evaluated using nonlinear pseudo-static pushover and Incremental Dynamic Analyses (IDA). The quantification of seismic performance and collapse capacity is executed using the procedure outlined in FEMA P695. This research study contributes to the understanding of the seismic performance and collapse capacity of ductile RM shear wall buildings with boundary elements. It provides practical design recommendations to enhance the overall system performance. In addition, the study proposes a simple, yet efficient nonlinear numerical macro-modeling approach for RM shear walls. Finally, based on the findings of this study, it is suggested to assign a 70 m limit for the height of buildings in moderate seismicity regions and a 50 m limit for the buildings in regions with high seismic hazard.

Seismic Behavior of Precast Concrete Shear Walls with Different Confined Boundary Elements

To ensure the mechanical property of lapped spliced reinforcement and concrete confinement of boundary elements for precast concrete shears, two kinds of constraints, local constraint by additional spiral stirrups placed around each reinforcement splicing and overall constraint by lapped welding closed stirrups replacing traditional stirrups and ties, were proposed in this paper. Low-cyclic reversed loading tests were conducted on three full scale specimens, including one reference cast-in-situ specimen, one precast concrete specimen with local constraint and one precast concrete specimen with overall constraint. By comparing the aspects of strength, stiffness, ductility and energy-dissipation capacity with that of cast-in-situ specimen, it is found that local constraint and overall constraint can be considered equivalent. However, due to the over restraint by local constraint detail, the specimen with local constraint exhibited earlier yielding, poorer energy-dissipation capacity and better ductility than that with overall constraint. Coupled with the consideration of steel usage and construction simplicity, overall constraint detail should be preferred in practical engineering.

Cyclic loading test for precast concrete composite shear walls

Shear wall systems as horizontal load resisting system are used widely in high-risen building. An innovative shear wall, named Precast Concrete (PC) composite shear wall with confined boundary elements, is proposed in this paper. The new-style structural wall is composed of PC panels, in-filled concrete and confined boundary members. Three full-scale specimens were subjected to cyclic lateral load under axial load. These structural characteristics were investigated and analysed including bearing capacity, hysteretic loops, skeleton curves, ductility, and energy dissipation under cyclic load. The specimens exhibited good ductility and underwent stable seismic behavior. Specially, the precast shear wall with joint motor show sufficient ductility, deformation capacity, stiffness and energy dissipation. The specimens experience the failure in the sequence of concrete crushing and the propagation fractures at the boundary of the wall. The experimental results indicate that the confined boundary elements have a great remarkable effect on the precast shear wall.

Experimental study on seismic behavior of steel tube confined high-strength concrete shear walls

The high strength concrete shear walls are characterized by its high bearing capacity, high stiffness but a poor deformation capacity. In order to improve the deformation capacity of these members, specimens of high-strength concrete shear wall with steel tube confined boundary elements were designed and the quasi-static tests were conducted. The failure mode, failure mechanism, deformation capacity, hysteretic behavior, stiffness degradation and energy dissipation capacity of specimens were researched. The results indicate that the steel tube boundary elements can provide an effective confinement to the wall pier and the horizontal bearing capacity of specimens dropped slowly. Under a high axial force, the vertical bearing capacity of specimens can be maintained, and the deformation and energy dissipation capacity could be obviously enhanced due to the use of steel tubes in the boundary elements. The ultimate displacement and the energy dissipation of steel tube confined high-strength concrete shear walls were increased by 27 % and 81 % respectively compared to high-strength concrete shear walls with the same axial load ratio. According to the experimental results, the calculation formula is addressed for the cross-section bearing capacity of steel tube confined high-strength concrete shear wall. As a result, it is a good practice that the steel tube confined boundary elements can be utilized at the bottom strengthening zone so as to improve the seismic performance of high-strength concrete shear walls.

Seismic Design Parameters for Special Masonry Structural Walls Detailed with Confined Boundary Elements

AbstractIn this paper, the results from a test program focusing on the behavior of eleven fully grouted reinforced masonry (RM) structural walls containing confined boundary elements are analyzed and presented according to force, displacement, and performance-based seismic design considerations. To be consistent with current force-based seismic design codes, analysis is presented to predict the strength, stiffness, displacements, and ductility-related force modification factors using the eleven test specimens. A new estimate of the plastic hinge region for displacement calculations is also proposed which considers the inherent planes of weakness in masonry mortar joints as well as the angle of inclination of shear cracks. Finally, within the context of performance-based design methodologies, the occurrence of specific damage states is identified. A methodology is proposed to estimate crack width in the walls based on curvature measurements which is also verified using digital image correlation analysis. T...

Web Crushing Capacity of High-Strength-Concrete Structural Walls: Experimental Study

This paper discusses the relationship between concrete strength and web crushing capacity based on results from large-scale tests of thin-webbed structural walls with confined boundary elements. Eight walls with concrete strengths ranging from 39 to 138 MPa (5.6 to 20 ksi) were tested to web crushing failure under cyclic and monotonic loading. These tests clearly demonstrated differences between elastic and inelastic web crushing behavior and their dependence on concrete strength. Walls with higher concrete strengths reached higher levels of displacement ductility due to an increase in web crushing capacity. Evidence with respect to monotonic tests showed that degradation of the diagonal compression struts from cyclic loading increases with concrete strength, thus limiting the inelastic deformation capacity gains. Thus, concrete compressive strength does not linearly increase web crushing strength as implied by rational web crushing models; rather, the relationship is nonlinear, with a decreasing limit as concrete strength increases. The ACI (American Concrete Institute) shear stress limit considerably underestimated the web crushing capacity of the walls. Test results and observations are reported with the intent of providing physical insight into the web crushing failure mechanism and the inherent limits of thin-webbed concrete members in shear.

Seismic assessment of existing RC shear walls non-compliant with current code provisions

This work forms part of a research programme to assess and strengthen existing reinforced concrete (RC) walls that were designed using older seismic codes. The main object of the present paper is to examine the behaviour of concrete shear walls that do not comply with modern seismic codes (Eurocode 2 (EC2) and Eurocode 8 (EC8)) and compare their experimental performance with the provisions for assessment of RC members included in EC8 Part 3. For this purpose, a series of six shear walls (height h = 1·40 m, length l = 0·74 m, thickness b = 0·10 m) was designed and tested under static cyclic loading. The wall specimens were characterised by various types of reinforcement arrangement, focusing on different amounts of shear reinforcement. The issues investigated are the failure modes, ductility level, the absence of confined boundary elements, the low amount of shear reinforcement, the influence of diagonal reinforcement and the influence of axial load. The experimental results are compared with the provisions included in EC8 Part 3, which predict the shear strength, the chord rotation ductility and the effective stiffness of RC shear walls.

Performance of reinforced concrete buildings during the 27 February 2010 Maule (Chile) earthquake

The paper presents observed damage in reinforced concrete buildings during the 27 February 2010 Maule earthquake in Chile. Performance of concrete frame and shear wall buildings are discussed with emphasis on seismic deficiencies in design and construction practices. It is shown that the majority of structural damage in multistorey and high-rise buildings can be attributed to poor performance of slender shear walls, without confined boundary elements, suffering from crushing of concrete and buckling of vertical wall reinforcement. Use of irregular buildings, lack of seismic detailing, and the interference of nonstructural elements were commonly observed seismic deficiencies. A comparison is made between Chilean and Canadian design practices with references made to the applicable code clauses. Lessons are drawn from the observed structural performance.

Seismic Performance Quantification of Reinforced Masonry Structural Walls with Boundary Elements

The design of reinforced masonry (RM) structural walls subject to high inelastic curvature demands can often be hindered by prescriptive requirements in design codes regarding the ratio between compression zone depth (c) and wall length (ℓw). A possible solution to this would be the detailing of a confined boundary element at the critical wall toes to increase the curvature ductility capacity of the plastic hinge region. The Masonry Standards Joint Committee (MSJC) code currently prescribes the possible use of such boundary elements for the special reinforced masonry shear walls category. However, the MSJC provides no relevant prescriptive detailing guidance, leaving the onus on the designer to specify appropriate detailing to ensure the desired performance. The results of an experimental program designed to develop a confinement scheme based on conventional masonry construction practices are reported in this paper. Five half-scale RM walls, detailed with confined boundary elements containing a double layer of vertical reinforcement enclosed by stirrups, were tested under reversed cycles of quasi-static loading. The walls, detailed to represent typical low- to medium-rise construction, varied by their height, length, and aspect and vertical reinforcement ratios. Test results are reported in terms of force-based seismic design parameters, such as the wall strength, stiffness and ductility characteristics, as well as displacement and performance-based seismic design parameters for the next-generation of seismic codes in North America. Overall, the performance of the walls indicates that the selected boundary element detailing is a designer-friendly means of addressing the need for lower c/ℓw ratios and higher curvature levels as indicated by the MSJC.

단부 횡보강된 구조벽의 모멘트-곡률 관계

For performance-based design using nonlinear static analysis, it is required to predict the inelastic behavior of structural members accurately. In the present study, a nonlinear numerical analysis was peformed to develop the method describing the moment-curvature relationship of structural wall with boundary confinement. Through the numerical analysis, variations of behavioral characteristics and failure mechanism with the arrangement of vertical reinforcement and the length of boundary confinement were studied. According to the analysis, the maximum moment-carrying capacity of structural walls with adequately confined boundary elements is developed at the moment the unconfined concrete reaches the ultimate compressive strain. Walls with flexural re-bars concentrated on the boundaries fails in a brittle manner. As vortical re-bars in the web increases, the brittle failure is prevented and a ductile failure occurs. Based on the findings, moment-curvature curves for walls with a variety of re-bar arrangement were developed. According to the proposed relationships, deformability of the structural walls wth boundary confinement increases as the compressive strength of the confined concrete increases compared to the applied compressive force.

Ductility and Detailing Requirements of Bearing Wall Buildings

An analytical procedure to determine the need for concrete confinement at the boundaries of structural walls in a building subjected to earthquakes is described. The procedure is based on comparing directly the expected displacement capacities and displacement demands for the building. For bearing wall buildings, relatively simple expressions are derived that enable the need for confined boundary elements to be evaluated. The expressions are verified using observations from the laboratory and from postearthquake response studies. The primary variables that determine the need for confined boundary elements are found to be the ratio of wall cross‐sectional area to the floor‐plan area, the wall aspect ratio and configuration, the wall axial load, and the wall reinforcement ratio. The study concludes that concrete confinement in walls of bearing wall buildings may be necessary at the extremities of walls having T, L, or other similarly shaped cross sections. Confinement is typically not required for symmetrically reinforced, rectangular wall cross sections. Current U.S. code requirements for confinement do not properly discern those cases where concrete confinement is or is not required, and are generally quite conservative.