Vertical Rebar Research Articles

Investigation on low-cost friction-based isolation systems for masonry building structures: Experimental and numerical studies

Masonry structures do not exhibit adequate seismic performance but they are still the most commonly constructed buildings across the globe. To enhance their seismic capacity and to protect them from the damaging effects of earthquakes in seismic active regions, an innovative low-cost friction-based seismic isolation system is developed in this study. The proposed isolation system, installed between the foundation and superstructure is made up of a 50 mm thick isolation layer material having low lateral stiffness, which is sandwiched between two 4 mm thick Teflon sheets and is further reinforced by vertical rebars. To establish the mechanical properties of the isolation system, experimental trials were performed using weak mortar, asphalt cushion, steel balls enclosed in mortar, coarse sand mortar, and rubberized mortar as an isolation layer material, and the proposed isolation system is installed between two hollow concrete blocks for testing purpose. Cyclic load tests were performed on the specimens and force-deformation curves were achieved. Two numerical models, one with a fixed base and another with the proposed isolator were developed using finite element analysis approach and analyzed against five different ground motions input. The isolation layer material used in the numerical model was selected based on friction coefficient and cyclic loading tests. In case of low seismic excitation, the isolation system does not activate to ensure structural integrity, however, when the lateral forces exceed the permissible limit then the isolation system permit deformation in both horizontal directions to dissipate seismic energy through friction while the vertical reinforcement ensure the re-centering mechanism of the structure. It is observed that the proposed isolation system considerably reduces the transmission of earthquake energy to the superstructure over a variety of ground motions. Depending upon the type of ground motion, an overall 45% − 56% reduction was observed in the absolute acceleration response of the top roof level of the isolated structure.

Effect of boundary confinement on ductility of RC walls

This study addresses the effect of boundary confinement on the ductility of shear walls considering various rebar hoop details. Eight walls were tested under cyclic lateral loads. The major test parameters were the type of vertical reinforcement (seismic or non-seismic grade rebar) and the type and spacing of the confinement rebar details (special details (ACI 318-19) or simplified details). The test results showed that the use of confinement hoops increased the deformation capacity of the wall specimens. Although use of the simplified rebar details also increased deformation capacity, they were not as effective as the special details. Furthermore, as the concrete strength increased, the deformation capacity also increased. On the other hand, use of seismic grade rebar did not significantly affect deformation capacity because the walls failed due to concrete crushing in the compression zone before the vertical rebar experienced a large degree of inelastic deformation or tensile fracture. The axial strain capacity of concrete according to the confinement details was investigated based on existing and present test results.

In-plane cyclic response of high-rise reinforced concrete masonry structural walls with boundary elements

The majority of tested Reinforced Concrete Masonry (RCM) shear walls with boundary elements represented walls in low- to mid-rise buildings. In addition, these tested walls had continuous vertical reinforcement with no lap splices, which is impractical in multi-story masonry buildings. Therefore, this study aims to evaluate the structural performance of high-rise RCM structural walls with boundary elements under reversed cyclic loading simulating seismic actions. This is achieved by constructing and testing four half-scale fully grouted RCM shear walls with boundary elements under constant axial load and quasi-static reversed cyclic loading. The walls were designed and constructed with similar geometry and material properties and were tested under the same level of axial compressive stress. The studied parameters are the wall’s shear span-to-depth ratio, the type of boundary elements’ masonry blocks (stretcher or C-shaped), and the lap splicing of vertical reinforcement in the plastic hinge region. The objective is to quantify the cyclic response of ductile RCM shear walls and to provide experimental evidence of its reliable structural performance for higher aspect ratios. Furthermore, the present study investigates the impact of the presence of lap splices in the plastic hinge region on the ductile response of high-rise RCM walls with boundary elements. The tested walls had an enhanced cyclic response due to its end zone confinement. The testing results demonstrated that the shear span significantly influences the distribution and layout of cracks, the lateral stiffness and resistance, and the post-peak response. Using the C-shaped blocks instead of the regular stretcher blocks in constructing the boundary elements enhanced the construction and performance of the walls. Lap splicing of vertical rebars increased the initial lateral stiffness, resulted in a higher rate of stiffness and strength degradation, and slightly limited the ultimate displacement ductility. However, with proper detailing of the splice and confinement of the end zones, the premature tensile bond failure was prevented.

Shake table test of a masonry building retrofitted with shotcrete

This paper presents the results of a series of shake table tests carried out on a half-scale single-story unreinforced masonry building with asymmetric openings. First, the unretrofitted building is subjected to seven increasing stepsof bidirectional seismic excitation. The damaged building is then rehabilitated using steel mesh and shotcrete layer with two walls retrofitted from the exterior face and the other two from the interior face. Afterward, the shake table test is again conducted on the retrofitted specimen in nine increasing excitation levels. Three cases of interior-to-interior, interior-to-exterior, and exterior-to-exterior shotcrete connections are considered at the intersection of perpendicular walls, and for each case, the development of cracks is investigated. Moreover, the effects of fixity of the shotcrete vertical rebars in the foundation are investigated through releasing the rebar-foundation connection in the last three steps of the test on the rehabilitated specimen. The results indicate the adequacy of the retrofit method in creating a strong bond that leads to an acceptable composite action between the brick layer and shotcrete; though, concentrated cracks were observed at the connections of the perpendicular walls. Furthermore, the shotcrete layer has proven to be able to prevent further propagation of the previously developed cracks, which can provide a practical solution for rehabilitation of damaged masonry buildings.

A numerical investigation of seismic performance for an RC frame with latticed concrete infill walls

Expansive polystyrene granule cement (EPSC) latticed concrete wall can be used as an infill wall. After the expanded polystyrene granule cement form has been demolished, the part that remains is a latticed concrete infill wall comprising lattice beams and columns whose cross section is circular with a diameter of 0.12 m. The horizontal and vertical rebar are arranged within the beams and columns. The dynamic responses of a full-scale single-story reinforced concrete (RC) frame with latticed concrete infill walls subjected to earthquake-induced ground motion are simulated using the nonlinear finite element ABAQUS software. This study aims to investigate the effects of different types of earthquakes on the structure and the acceleration responses of the structure. The results obtained indicate that the structure has excellent seismic behavior, although the structural responses vary between earthquakes. This research may help in promoting the design and application of RC frames with latticed concrete infill walls.

Research on pseudo-static cyclic tests of precast concrete shear walls with vertical rebar lapping in grout-filled constrained hole

In this paper, the seismic behaviors of four precast shear wall specimens and one cast-in-place shear wall specimen are investigated by pseudo-static test method. For the precast shear wall specimens, a new type of assembled rebar lap splices with prepared grout-filled holes and constraining spiral hoops were applied. Different vertical rebar lap splice positions and different rebar lap lengths at the horizontal assembly joints of the precast specimens were designed and fabricated in order to compare their seismic behaviors with the cast-in-place specimen. The geometric size and rebars of four precast specimens are totally same as those of the cast-in-place specimen. By analysis of test results including the load-lateral drift hysteresis curves, crack propagation and rebar strains, the failure modes, bearing capacity, hysteretic characteristics, ductility, rigidity degeneration and energy dissipation of all specimens were obtained and compared. The test results show that the precast concrete shear walls with horizontal assembly joints by this kind of rebar connection have good seismic behaviors and could be equivalent to the cast-in-place shear walls on the conditions of specified construction details.

Experimental and Numerical Study on the Seismic Performance of Prefabricated Reinforced Masonry Shear Walls

The seismic performance of prefabricated reinforced concrete block masonry shear walls (PRMSWs) was studied. Five PRMSWs were tested under cyclic loading to evaluate the effect of the axial compression ratio and the distribution of the vertical rebar on the inelastic behavior. Based on the experimental results, the lateral load capacity, failure mode, lateral drift, ductility, stiffness degradation, energy dissipation, and the seismic performance stability of the specimens were analyzed. The finite element analysis of the specimens was conducted with ABAQUS, which agreed quite well with the laboratory findings. Relevant results showed that PRMSW exhibited favorable ductility and energy dissipation. The increase of the compression ratio led to stiffer, but more brittle, inelastic behavior of the specimens that had higher flexural strength. The shear walls that had concentrated vertical rebar at the sides exhibited relatively higher load capacity and less ductility compared to the walls that had evenly distributed rebar. The inelastic lateral drift limit of the PRMSW could be assigned 1/120. The equivalent viscous damping ratio of the PRMSW was 9–13% at ultimate load. These results provide a technical basis for the design and application of the PRMSW structures.

Experimental Investigation on the Seismic Behavior of Newly-Developed Precast Reinforced Concrete Block Masonry Shear Walls

Typically, a special type of concrete block with cleaning holes is used in the bottom layer of traditional reinforced masonry shear walls (RMSWs) for mortar cleaning and vertical rebar connection, which results in reduced integrity and weakened structural behavior. In this paper, a precast construction technology was introduced to overcome these shortcomings. The cleaning-hole blocks were eliminated in the newly-developed precast RMSWs. Quasi-static tests on two traditional and two precast fully grouted RMSWs were conducted. The results showed that the flexural capacity of precast walls exhibited about a 10% increase when compared to traditional RMSWs under the same axial compression. Precast RMSWs that failed in flexural mode showed favorable deformation capacity and the displacement ductility value corresponding to 15% strength degradation reached 4.9. The wall stiffness degraded rapidly to 50% of the initial stiffness, K0, at 0.2% drift and, at 0.5% drift, the corresponding stiffness decreased to about 21% K0 at a more gradual rate. Furthermore, precast RMSWs exhibited significant energy dissipation capacity. The experiment suggests that precast RMSWs have a satisfactory seismic performance.

Cyclic tensile-compressive tests on thin concrete boundary elements with a single layer of reinforcement prone to out-of-plane instability

The growing need for residential housing in Latin American countries has led to the construction of reinforced concrete buildings with wall thicknesses as low as 8–10 cm. Such walls have typically only a single layer of vertical rebars and are therefore particularly susceptible to out-of-plane failure. In order to investigate the response of the corresponding wall boundary elements, twelve reinforced concrete members with a single layer of vertical rebars were tested under tension–compression cycles. The objective was to gain insight into the parameters governing wall instability and out-of-plane failure, namely the thickness, reinforcement ratio, and eccentricity of the longitudinal rebars with respect to the member axis. This paper summarises the results of the test program, where the specimens' response is analysed also at the global and local levels. The results show that the crack pattern has an important influence on the out-of-plane behaviour and the conditions leading to out-of-plane failure are described. Furthermore, the differences between members with a single layer of vertical rebars and members with two layers are discussed. The influence of the parameters considered in the experimental program is addressed, showing that sections with small thickness and large reinforcement content are more prone to out-of-plane failures. Finally, predictions given by existing models are compared to the new experimental data. The entire data set is publicly available.

Steel beam–column joint with discontinuous vertical reinforcing bars

The authors have previously proposed steel beam–column connections for precast concrete frames. The steel–concrete composite frames combined the advantages of the fast assembly of steel and the low cost of concrete structures. However, when not enough space is available at column–beam joints, steel sections from beams cannot be connected with column brackets. To address this issue, this paper explores the strategy of disconnecting some vertical reinforcing bars at the joints by connecting vertical steel reinforcements to steel plates placed above and below column steels, to provide a load transfer path. Loads from re-bars are transferred to steel plates, column steels, and back to steel plates and re-bars below the column steels. This strategy provided space for beam–column joints of composite frames. Extensive experiments were performed to verify load transfer from re-bars to steel plates above joints and from the steel plates to re-bars below the joint. The flexural load-bearing capacity of a column with a total of 24 vertical re-bars was compared to that of columns with discontinuous re-bars at the joints; the number of discontinuous re-bars at the joint used in the column specimens tested was 0 (0.0%), 4 (16.7%), 12 (50.0%), and 20 (83.3%).

Shaking Table Test of a RC Frame with EPSC Latticed Concrete Infill Wall

The expansive polystyrene granule cement (EPSC) latticed concrete wall is a new type of energy-saving wall material with load-bearing, insulation, fireproof, and environmental protection characteristics. A series of shaking table tests were performed to investigate the seismic behavior of a full-scale reinforced concrete (RC) frame with EPSC latticed concrete infill wall, and data obtained from the shaking table test were analyzed. The experimental results indicate that the designed RC frame with EPSC latticed concrete infill wall has satisfactory seismic performance subjected to earthquakes, and the seismic responses of the model structure are more sensitive to input motions with more high frequency components and long duration. The EPSC latticed concrete infill wall provided high lateral stiffness so that the walls can be equivalent to a RC shear wall. The horizontal and vertical rebar, arranged in the concrete lattice beam and column, could effectively restrain the latticed concrete infill wall and RC frame. To achieve a more comprehensive evaluation on the performance of the RC frame with latticed concrete infill walls, further research on its seismic responses is expected by comparing with conventional infill walls and nonlinear analytical method.

SEISMIC PERFORMANCE EVALUATION OF ASSEMBLED O-STABLE PANEL WALLS

O-Stable Panel is a new type of assembled structural wall. To evaluate the performance properties of the O-Stable Panel system, three integral prefabricated panel specimens with different vertical connections at the bottom and one full cast-in-place panel specimen were designed and tested under the low cycle lateral reciprocating loading. The performance of the prefabricated O-Stable Panel systems with different vertical connection configurations was compared with those of the full cast-in-place panel. The investigation reveals that all prefabricated O-Stable panel specimens developed vertical cracks penetrating through thickness of the panels at the panel joint and at the positions where panel thickness variation occurs at the ultimate failure state. The O-Stable panels possess the likely symmetric and stable hysteretic curves and no pinching appearance in shape of the curves. The panel with grouting sleeve in vertical joint for steel rebar appears sharp degrading in seismic index as the lateral drift of the wall increases beyond yield of the panel. For the assembled panel with preinstalled rebar stretched into the foundation beam for the vertical rebar joints, it has very close values in the hysteretic energy dissipation as that in the full cast-in-place panels.

Steel Beam-Column Joint with Discontinuous Vertical Reinforcing Bars

The authors proposed steel beam-column connections for precast concrete frames in previous studies. The steel-concrete composite frames provided fast assembly time as steels with economy of concrete structures. However, when enough space is not available at column-beam joints steel sections from beams cannot be connected with column brackets. This paper suggests that some vertical reinforcing bars are disconnected at joints by connecting vertical steel reinforcements to steel plates placed above and below column steels to provide load transferring path. Loads from re-bars are transferred to steel plates, column steels and back to steel plates and re-bars below column steels. Re-bars connected to steel plates by bolts at above and below column steel are discontinued at joint to provide spaces for connections between column brackets and beam steels. Extensive experiments were performed to verify load transfer from re-bars to steel plates above joints and steel plates to re-bars below joint. The flexural load bearing capacity of a column with total of 24 vertical re-bars were compared to columns with discontinuous re-bars at joints. The number of discontinuous re-bars at joint used in column specimen was 0 (0.0%), 4 (16.7%), 12 (50.0%), and 20 (83.3%). The numbers in parenthesis are the percentages of discontinuous rebars to the total number of vertical re-bars of control column. Experiments showed how loads from vertical steel reinforcements that were cut off at joints were transferred to steel plate. The test results also demonstrated that a part of flexural capacities were reduced for specimen with discontinuous vertical re-bars. The reduction of 6.0 %, 13.7% and 54.0% of flexural capacities were observed for columns with 4 (16.7%), 12 (50.0%) and 20 (83.3%) discontinuous vertical rebars, respectively. The test results can be used to design vertical reinforcing bars and column joints that can provide space for column brackets to which steel members of beams are connected.

Alternatives to lifting concrete masonry blocks onto rebar: biomechanical and perceptual evaluations

This study examined the use of and barriers to H-block and high lift grouting, two alternatives to lifting concrete masonry blocks onto vertical rebar. Peak and cumulative shoulder motions were evaluated, as well as adoption barriers: H-block cost and stakeholder perceptions. Results indicated that using the alternatives significantly decreased peak shoulder flexion (p < 0.001). A case study indicated that building cost was higher with H-block, but the difference was less than 2% of the total cost. Contractors and specifiers reported important differences in perceptions, work norms, and material use and practices. For example, 48% of specifiers reported that use of high lift grouting was the contractor's choice, while 28% of contractors thought it must be specified. Use of H-block or high-lift grouting should be considered as methods to reduce awkward upper extremity postures. Cost and stakeholders’ other perceptions present barriers that are important considerations when developing diffusion strategies for these alternatives. Practitioner Summary: This study provides information from several perspectives about ergonomic controls for a high risk bricklaying task, which will benefit occupational safety experts, health professionals and ergonomists. It adds to the understanding of shoulder stresses, material cost and stakeholder perceptions that will contribute to developing effective diffusion strategies.

Behavior of steel tube-reinforced concrete composite walls subjected to high axial force and cyclic loading

This paper proposes an innovative composite shear wall, named the steel tube-reinforced concrete (ST-RC) composite wall, with steel tubes embedded at the wall boundary elements and fully anchored within the foundation. This arrangement is supposed to enhance the wall’s seismic performance. A series of quasi-static tests were performed to examine the behavior of the ST-RC walls when subjected to high axial forces and lateral cyclic loads. The area ratios of steel tube and of concrete filled steel tube (CFST) (i.e. the ratios of the cross-sectional areas of steel tubes and of CFSTs over that of the wall boundary element, respectively), axial force ratio, and cross-sectional shape of walls were taken as major test variables. All specimens failed in a flexural mode, characterized by tensile yield of the vertical rebars and steel tubes in the boundary elements and compressive crushing of concrete at the wall bottom. The test results showed that the ST-RC composite walls had larger load-carrying and deformation capacities relative to the RC wall counterpart. The deformation capacity of the rectangular-shaped composite walls increased with an increase in the area ratios of steel tube and of CFST and decreased with an increase in the applied axial force ratio. The barbell-shaped composite wall that was configured with edge columns had much larger deformation and energy dissipation capacities than the rectangular-shaped composite walls. In addition, simplified formulation was proposed to evaluate the lateral load-carrying capacity of the ST-RC composite walls. The evaluated results showed good agreement with the test results, with errors no more than 10%.

An Approximate Solution of Scattering From Reinforced Concrete Walls

Scattering from reinforced concrete walls, such as vertical rebar and crossed rebar walls, is analyzed by using an approximate analytical approach. The solution is obtained for a plane-wave incidence with an arbitrary angle of incidence and polarization under the thin-wire approximation. The formulation is based on derivation of the Green's function of a 1-D periodic sources inside a dielectric slab. Since the metal thickness is assumed to be small compared to the wavelength, the induced currents are predominantly axial and the transverse components of the induced currents are ignored. This drastically simplifies the solution for this problem which is obtained using a straightforward point-matching technique. The solution for two-dimensional periodic crossed rebar concrete walls is also cast in terms of the periodic Green's function obtained for the 1-D periodic structure of the vertical rebar walls. Using the finite-difference time-domain (FDTD) simulation results, it is shown that the induced currents on thin crossed re- bars (diameter <lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /5) have primarily a progressive phase equal to that of the incident field in both directions. This observation also points to a simple approximate solution where the problem of scattering from a 2-D periodic crossed rebar concrete wall is decomposed into two coupled 1-D periodic structures of vertical and horizontal rebar walls. The Bragg mode scattered fields of the reinforced walls are computed for different wall parameters and incidence angles for an operation frequency range of 0.5-2.0 GHz and validated by FDTD simulation results.

Designing bar-mesh modules of reinforced concrete structures

This paper introduces a process of developing a system called AUREMSYS, automated rebar module design system. AUREMSYS is a set of computer programs which analyzes reinforcement drawings of a reinforced concrete structure and develops rebar modules for prefabrication. Such rebar modules are also named as bar-meshes because a rebar module is made of a set of horizontal and vertical rebars welded together like a mesh. To develop a bar-mesh design system, a design standard for bar-mesh needs to be established first. The authors determined a bar-mesh design standard by analyzing various factors that are related to design and construction stages. Some of these factors include the Korean standard for reinforced concrete structures, load transportation environment, workmanship of local labor forces, performances of automated welding machines, etc. Considerable effort is made in developing a bar-mesh design algorithm. This paper discusses two specific bar-meshes design algorithms that are devised in the process of AUREMSYS development. The first algorithm is dessigned by focusing on a rebar element itself and the second algorithm is designed by introducing a “Zone Object” concept.