Bar Connections Research Articles

Evaluation of bond strength of helically ribbed CFRP bar connections as mechanical anchorage schemes

In spite of their outstanding properties, FRP reinforcing bars still face a series of drawbacks that should be addressed to widen their applications. For example, straight FRP bars must be used because those bars suffer significant strength degradation when curved. This implies that FRP bars resist the slip nearly solely through their bond strength with concrete. In addition, helically ribbed FRP bars exhibit bond strength twice higher than sand coated bars but develop weaker bond characteristics with concrete than the steel rebar. Anchorages are thus needed to improve the bond strength but may be difficult to realize in narrow spaces like joints. Therefore, this study examines the combined use of helically ribbed FRP bars with connections like lap-splice, epoxy-filled tube, coupler or expanded ribs as mechanical anchorages. A total of 36 pullout specimens considering the types of connection as parameter are tested to investigate the eventual effectiveness of the connections in enhancing the bond behavior. The use of two expanded ribs appears to be the best option. The predictions of the bond stress-slip models of current design codes are compared to the experimental data to examine whether these models can simulate accurately the connections.

Tensile behaviors and configurations of double-headed bar overlap connections for precast concrete members

The superiorities in construction time, cost, and quality of precast RC structures are accompanied by the challenges in connection, particularly in steel bars connections between RC components. The extensively used steel bar connections for either grouted sleeve splice connection or grouted spiral-confined lap connection requires on-site grouting, which is a concealed construction process with increased the risk of grouting defects, thereby impairing connection reliability. The confined headed bar connection (CHBC) with visible on-site construction has been proposed to address the unseen grouting defects concerns. However, modern structures that use large-diameter or high-strength rebars present higher demands for steel bar connection capacity. For CHBC, it is suggested to use either a long connection length or a large-size head to satisfy the capacity demand. However, the long connection length can increase field wet operation, which is disadvantage in precast concrete construction. The large-size head can help reduce the connection length, but it can cause inconvenience including construction congestion and the increased eccentric bending moment under tension. This paper presents the CHBC with double-head configuration, which can reduce the single head size. The proposed CHBC consists of confinement stirrups and two overlapped headed bars and the stirrup confining the core concrete can also improve the CHBC capacity. Experimental investigation on 21 CHBC specimens has been carried out. The failure modes, bearing capacity, and stirrup strain development have been studied. Effects of the double-head configuration and confinement stirrup ratio have been analyzed through experimental and numerical investigation. Results show that the double-head configuration has a minor impact on the tensile capacity of CHBC while the increase of confined stirrup ratio can improve the tensile capacity of CHBC. Configuration details of size and spacing for stirrup reinforcement in CHBC are proposed. Based on the test results, CHBC tensile capacity prediction method is suggested, which can take the head configuration into account.

Experimental and Analytical Study on the Compressive Arch Action of Precast Concrete Assemblies with Monolithic Connections to Resist Progressive Collapse

Although the first progressive collapse event occurred in precast concrete (PC) buildings due to gas explosions, the behavior of PC buildings resisting progressive collapse is still unclear because related tests are few, and the types of PC connections vary. To study the progressive collapse resistance of monolithic PC beam-column assemblies under a middle column removal scenario, an experimental program including four PC assemblies and one reinforced concrete (RC) assembly is conducted. These PC assemblies contain different types of beam-column connections, including connections of lap-splice, 90° hook anchorage, U-shaped bar, and combined U-shaped bar and 30° hook anchorage. The test results demonstrate that flexural action, compressive arch action (CAA), and catenary action were mobilized in sequence in the beams of RC and PC assemblies. Thus, in general, the development of load resisting mechanisms of PC assemblies is similar to that of RC assemblies, although different types of connections may affect the magnitude of each mechanism. The PC assemblies with 90° hook anchorage and U-shaped bar connections attained a CAA capacity similar to that of the RC assembly, whereas the PC assembly with the combined connections had a greater CAA capacity because of plastic hinge relocation at the beam ends near the middle column. The PC assembly with lap-splice connections achieved the lowest CAA capacity due to the anchorage failure of the beam bottom rebar. However, this PC assembly attained the greatest catenary action capacity, because the beam bottom rebar did not fracture and could still develop tension. Subsequently, an analytical model is proposed to reveal the nature of CAA and to better understand the influence of critical parameters.

Mechanical behavior of semi‐grouting sleeve and steel bar connection

AbstractA series of pull‐out tests were conducted on semi‐grouting sleeve and steel bar connections to investigate the mechanical behavior of the connectors. Taking the steel bar diameter, anchorage length, and grouting material strength as the parameters. The test results showed that when the anchorage length was small and the grouting material strength was low, sufficient anchorage strength cannot be provided, specimens were subjected to bond slip failure, which belongs to the brittle failure. When the anchorage strength between the steel bar and the grouting material was sufficient, the steel bar breaking occurred on the specimens, which belongs to the ductile failure. Then, the semi‐grouting sleeve and steel bar connection was modeled by finite element software, and the bond slip between the steel bar and grouting material was simulated by COMBIN39 element. The results indicated that the failure mode of the specimen obtained by finite element simulation was the same as the test result. The ultimate bearing capacity of the specimen was highly similar to the test result, which verified the feasibility of the modeling method in this paper. Next, the number of specimens and parameter levels were increased through finite element modeling, and the influences of steel bar diameter, anchorage strength, and grouting material strength on the ultimate bearing capacity of semi‐grouting sleeve and steel bar connections were analyzed. The results testified that the ultimate bearing capacity of the specimen increased with the increase of steel bar diameter. In a certain range, the ultimate bearing capacity of the specimen increased with the raising of anchorage length and grouting material strength. While the anchorage length increased, the ultimate bearing capacity of the specimen did not necessarily increase because of the uneven distribution of stress. The steel bar diameter had the most significant influence on the ultimate bearing capacity of the specimen, while the grouting material strength had the most negligible impact, that was, increasing the steel bar diameter is the most effective method to improve the ultimate bearing capacity of the specimen. Finally, according to the results of finite element analysis, the calculation method of ultimate bearing capacity with different failure modes was obtained, and it has been proved to be effective and accurate by the comparison of calculated results and test results. The results of this paper have great significance for improving the calculation theory and design method of relevant engineering.

Behavior of Reinforcing Bar Connection of Hollow-Core Slabs to Steel Beams under In-Plane Forces

Hollow-core slab (HCS) floors supported on steel beams require the use of steel reinforcement as connections to avoid slab displacement caused by lateral loads. However, current North American design codes offer limited provisions on the design and behavior of such connections. In this study, the results of an experimental investigation conducted on such connections to assess their capacity and mode of failure are presented. Eleven full-scale assemblies of HCS reinforcing bar connections to steel beams were tested to failure under monotonic in-plane loading (compression, tension, or shear). Test results revealed that connections tested under compression failed by bar buckling without yielding. Under tension, the connection bar reached close-to-yielding or yielding strains at the unrestrained portion of the bar, followed by grout splitting in the shear key or the grouted core. Finally, the mechanism of failure of specimens subjected to shear was governed by bar yielding.

Behavior of reinforcing bar connections of hollow-core slabs to masonry walls under in-plane forces

Integrity ties are necessary in hollow-core slab floors as connections to prevent floor displacements under lateral loads and maintain the overall structural integrity of buildings. In Eastern Canada, the integrity tie for hollow-core slabs on masonry walls usually consists of a 10M (no. 3), L-shaped steel bar that is hammered into the supporting wall and grouted to the slabs. However, current North American design codes do not offer sufficient provisions to determine the capacity or predict the mode of failure of these ties and connections. This paper introduces the results of testing nine full-scale reinforcing bar connection assemblies under monotonic in-plane forces (compression, tension, and shear) until failure. Test parameters in this study included direction and orientation of the in-plane loading, the bearing type of the hollow-core slabs, and the use of adhesive. Test results showed that connections with dry-fit bars tested under compression failed by bar yielding followed by masonry beam crushing. In addition, the mode of failure under tension forces was governed by the loss of bearing of the slabs due to bar pullout and cover spalling. Under shear forces, the connection failed by bar yielding. Finally, the connections with adhesive had a similar mode of failure compared to their counterparts with dry-fit bars, but these did not show bar pullout from the masonry beam, therefore demonstrating higher stiffness.

The influence of materials on the behaviour of joints with multiple bar connections

Joints made out of tubular steel members are often used in industrial constructions, like offshore, trusses or tree-shaped columns. The joint area is the weakest point in a truss structure. Beside the normal static behaviour many non-linearities due to the geometry or to the welding process have to be taken into account. Steel hollow sections are often used. Much research was made on welded nodes so far. To increase the resistance of a joint it is an option to vary the material choice. Many variations were tested in the past. It is possible to change the overall material of a joint or using composite structures, like a concrete filled column. Or the steel of the welding line can get an additional material component. Another aspect is to reinforce the joint for example by a stiffened ring. In this paper a state-of-the-art review of the influence of different materials to the resistance of welded nodes is given.

ВЛИЯНИЕ НАКЛОННЫХ РЕБЕР ЖЕСТКОСТИ В УЗЛАХ СОЕДИНЕНИЯ ДВУТАВРОВ НА ПЕРЕДАЧУ БИМОМЕНТОВ

Shear-free theory of V.Z. Vlasov remains one of the most reasonable approaches to calculating thin-walled bars taking into account constrained torsion. At the same time, the use of this theory for the analysis of deformations of frame structures still requires research in terms of the conditions for transferring forces in bar connections. As noted in some scientific papers, the balance of bimoments can be significantly broken at the joints of thin-walled bars of an open profile in some designs. This paper deals with this phenomenon for steel I-beam profiles, associated with the presence of inclined stiffeners in joint units. Using shell finite element models, the influence of inclined stiffeners on the appearance of bimoment jumps at the pairwise connection of bars is shown. A dependence is derived that makes it possible to take into account the stiffness of the inclined edge in the bar models from the point of view of the restraint of cross-section warping. On the basis of numerical experiments, it was determined that the introduction of such stiffness into the bar finite element schemes of frame structures allows to reflect the condition of bars interaction in the transferring of bimoments with a sufficiently high accuracy for engineering practice.

Post-installed reinforcing bars – Requirements for their reliable use

Post-installed steel reinforcing bar connections carried out with anchoring mortars sufficiently qualified to fulfill the requirements for safety, serviceability, and durability of concrete structures are capable of developing bond behavior equivalent to cast-in-place steel reinforcing bars. Therefore, their design can be performed according to reinforced concrete design provisions. Post-installed anchorage technology offers fast and convenient solution to introduce new reinforcement in an existing reinforced concrete (RC) structural system, and hence serves as effective method for strengthening and upgrade of existing structures. The behavior of post-installed reinforcement depends on the existing condition of the concrete member. This paper addresses the aspects related to base material, installation, environment, and load conditions in service. This paper also comments on the qualification of anchoring mortars and provides information on the design of post-installed reinforcing bars as reinforcement (RC design perspective) or as anchorage (adhesive anchor design perspective) in the light of corresponding provisions necessary to achieve the same safety level in service compared to connections with cast-in steel reinforcing bars.

Biomechanical study of a low-cost external fixator for diaphyseal fractures of long bones

BackgroundExternal fixation improves open fracture management in emerging countries. However, sophisticated models are often expensive and unavailable. We assessed the biomechanical properties of a low-cost external fixation system in comparison with the Hoffmann® 3 system, as a reference.MethodsTransversal, oblique, and comminuted fractures were created in the diaphysis of tibia sawbones. Six external fixators were tested in three modes of loading—axial compression, medio-lateral (ML) bending, and torsion—in order to determine construction stiffness. The fixator construct implies two uniplanar (UUEF1, UUEF2) depending the pin-rods fixation system and two biplanar (UBEF1, UBEF2) designs based on different bar to bar connections. The designed low-cost fixators were compared to a Hoffmann® 3 fixator single rod (H3-SR) and double rod (H3-DR). Twenty-seven constructs were stabilized with UUEF1, UUEF2, and H3-SR (nine constructs each). Nine constructs were stabilized with UBEF1, UBEF2, and H3-DR (three constructs each).ResultsUUEF2 was significantly stiffer than H3-SR (p < 0.001) in axial compression for oblique fractures and UUEF1 was significantly stiffer than H3-SR (p = 0.009) in ML bending for transversal fractures. Both UUEFs were significantly stiffer than H3-SR in axial compression and torsion (p < 0.05), and inferior to H3-SR in ML bending, for comminuted fractures. In the same fracture pattern, UBEFs were significantly stiffer than H3-DR (p = 0.001) in axial compression and torsion, while only UBEF1 was significantly stiffer than H3-DR in ML bending (p = 0.013).ConclusionsThe results demonstrated that the stiffness of the UUEF and UBEF device compares to the reference fixator and may be helpful in maintaining fracture reduction. Fatigue testing and clinical assessment must be conducted to ensure that the objective of bone healing is achievable with such low-cost devices.

Richard von Mises’ work for ZAMM until his emigration in 1933 and glimpses of the later history of ZAMM

Richard von Mises’ work for ZAMM until his emigration in 1933 and glimpses of the later history of ZAMM

Seismic Retrofitting of Traditional Masonry with Pultruded FRP Profiles

This paper presents a study on the potentiality of seismic retrofitting solutions with pultruded Fiber Reinforced Polymer (FRP) profiles. This material can be used in connected frames providing lightweight, corrosion-free and reversible retrofitting of masonry buildings with the moderate requirements of surface preservation. In a hypothetical case study, an experimental program was designed; monotonic shear tests on a half-size physical model of the sample wall were performed to assess the structural performance before and after retrofitting with a basic frame of pultruded Glass Fiber Reinforced Polymer (GFRP) C-shaped profiles, connected to the masonry by steel threaded bar connections. During the tests, the drift, the diagonal displacements in the masonry and the micro-strain in the profiles were measured. The retrofitted system has proven very effective in delaying crack appearance, increasing the maximum load (+85% to +93%) and ultimate displacement (up to +303%). The failure mode switches from rocking to a combination of diagonal cracking and bed joint sliding. The gauge recordings show a very limited mechanical exploitation of the GFRP material, despite the noticeable effectiveness of the retrofit. The application seems thus promising and worth a deeper research focus. Finally, a finite element modelling approach has been developed and validated, and it will be useful to envisage the effects of the proposed solution in future research.

Grouting Defect Detection of Lapped Bar Connections Based on Impact‐Echo Method

Grouted lap‐splice connections are widely used for connecting precast concrete components. Grouting defects in the connections significantly influence the structural performance of the whole connection, which leads to the need for grouting defect detection. In this study, the impact‐echo (IE) method was used for detecting defects in grouted lap‐splice connections. Grouted connections with different levels of artificial grout defects were prepared in a shear wall, and the IE method was used to measure the frequency response. In addition, finite element (FE) analysis based on ABAQUS was conducted to simulate the tests. Based on the validated FE model, a parametric study was conducted to investigate the effect of the depth of the grout hole on the amplitude spectrum. The results indicated that (1) the IE method offered a good potential for grouting defect detection in grouted lap‐splice connections; (2) the proposed FE model could well predict the frequency response of the grouting hole; and (3) the measured frequency and amplitude of the grouting hole in an impact‐echo test would be considerably influenced by the hole depth.

Evaluation of load–deformation behavior of reinforced concrete shear walls with continuous or lap-spliced bars in plastic hinge zone

This article presents an analysis procedure for evaluation of load–deformation behavior of reinforced concrete shear walls with continuous or lap-spliced bar connections in plastic hinge zones under horizontal loads. For the shear walls with continuous bars, the lateral deformations caused by flexure, shear, and reinforcement slip are evaluated by considering their interaction. The flexural deformation is calculated by conventional fiber model. The shear mechanism is based on modified compression field theory with a softened smeared cracked reinforced concrete membrane element. Both the flexural and shear deformations are estimated separately in the plastic hinge and non-plastic hinge regions. In addition, an approach is proposed for analysis of plastic hinge length based on fracture energies of materials. For the shear walls with lap-spliced bars, due to its complicated behavior and mechanism, a simple way to deal with the lap splice is proposed. The equations regarding bond-slip of the lap splice with minimum spliced length are established and the stress and strain states of lap splices with different spliced lengths are analyzed on the basis of equilibrium of forces with a mean bond stress model. Finally, the validity of the proposed analysis procedure is confirmed by comparing the analytical results with previous experimental data.

Headed Bar Connections Between Precast Concrete Elements: Design Recommendations and Practical Applications

The paper provides an overview of research into the design and behaviour of joints between precast concrete elements in which continuity of reinforcement is achieved through overlapping headed bars, allowing very short lap lengths. A series of tensile and flexural tests were carried out on joints with lapped headed bars of 25 mm diameter with 70 mm square heads and measured yield strength of 530 MPa. The tests studied the influence on joint behaviour of joint concrete strength, transverse reinforcement, geometry, and out-of-plane tolerances. Observations from tests and numerical analysis were used to develop design procedures for headed bar joints based on strut-and-tie modelling and the upper bound theorem of plasticity respectively. A recently completed project using headed bar joints demonstrates the benefits of using this system in precast concrete construction. The potential for further savings in costs and labour when adopting design recommendations stemming from this research is also discussed.

Enhancing the seismic performance of historic timber buildings in Asia by applying super-elastic alloy to a Chinese complex bracket system

ABSTRACTHistoric timber structures widely distributed in East Asia are suffering from the earthquakes. This article aims to develop a technique to enhance the seismic performance of historic timber buildings with the Dou-Gon system. High-strength steel bars and super-elastic alloy bars are used in this study to benchmark conventional Dou-Gon system that uses wood pegs to connect the base Dou and the column. Pushover tests were carried out under different loading conditions. The factors discussed in this study include vertical load imposed on the Dou-Gon system and the pre-strained levels of super-elastic bars. The results from this study has shown that the simple technique enhances the energy dissipation capacity and the ultimate strength of the base Dou-Gon system. Base Dou-Gon system with super-elastic alloy bar connections also show a better aseismic performance due to its constant damping behavior and longer fatigue life. Moreover, pre-strain of the super-elastic alloy bar provides a better damping behavior to the base Dou system.

Flexural behaviour of headed bar connections between precast concrete panels

The use of headed bars in joints between precast concrete elements allows continuity of reinforcement to be achieved over very short splice lengths. The paper describes a series of flexural tests carried out on specimens consisting of pairs of precast elements connected by overlapping headed bars of 25mm diameter. The headed bars overlapped by 100mm within a 200mm wide in situ concrete joint in which transverse bars and vertical shear studs were installed to provide confinement. This type of joint facilitates the construction of continuously reinforced slabs from precast elements thereby enabling significant reductions in overall construction time and improvements in construction quality due to off-site fabrication. The tests investigated the influence on joint strength, ductility and crack width of concrete strength, out-of-plane offset of precast planks and confining shear studs. Ductile failure with yield of 25mm diameter high strength headed bars was achieved with joint concrete having a cylinder compressive strength of 39MPa. A nonlinear finite element model is presented, which gives good predictions of joint strength as well as providing insight into joint behaviour.

DETERMINATION AND RANGING OF ORGANIZATIONAL AND TECHNOLOGICAL FACTORS THAT DEFINE THE RATIONAL DECISIONS OF RE-BARS CONNECTION

Purpose. The paper proposes: 1) determination and formulation of factors that influence the choice of rational method for joining re-bars of vertical support members of reinforced concrete frame; 2) determination of factor parameters; 3) ranging of factors by the expert evaluation (Delphi) method. Methodology. In order to achieve research objectives, it is necessary to carry out analysis of existent rebar connection methods, determination of factors and parameter variation limits for each of the methods. Performing factor ranking by the expert evaluation method. Findings. The results of the questioning materials of 14 experts in the area of monolithic construction allowed setting the following: when choosing the rational re-bars connections, the most significant values are the factors that define the time parameters: possibility of carcassing, time of joining the re-bars, length of rebar cage, prior operation run time, operation time of main lifting equipment. Herewith the factors that define the rebar cage parameters have a direct relation to the work performance time, as they determine the amount of bar connections in the course of building erection over wide range. Economic factors – rebar connection cost and quality control cost – have the less value. It is obvious that in the conditions of considerable construction expenses it is advantageous for an investor to increase the rebar joining cost for the work growth rate. Structural and technological factors have the least value: origin of eccentric load transmission between re-bars, possibility of use of the thermally work-hardened re-bars of А500 and higher grades, work category for implementation of works, necessity to use the scaffold and appurtenances for re-enforcement of constructions. The reason is analogical: a contractor is ready to go to complication of technology with the purpose of reduction of the facility erection terms. As the calculated Pearson’s matching criterion χ2 = 47.24 is higher than the tabular one (22.36203), then the obtained concordance coefficient W=0.26 is not casual value, and that is why the obtained results make sense and can be used in further researches. Originality. The author obtained factors that influence the choice of the rational method for re-bars connection the most. Practical value. Ranging of factors will allow objective approaching to the problem of choice of re-bars connection method, optimizing the labour and material costs, and also reducing the construction time.

일체식교량의 접속슬래브 연결철근 형상에 따른 연결부 구조거동에 대한 실험연구

An experimental study on the structural behavior of connection types between approach slab and integral abutment has been done for three typical bar connections. Typical hinge style reinforcing bar detail for its connection is preferred in order to accommodate rotation of the approach slab among engineers. However, the straight horizontal bars can be used as connection detail accomodate structural capacity. Total six specimens with three types of rebar detail are tested for direct tensile and bending load. The characteristic structural behaviors are carefully monitored and all the strain gauge data obtained are analyzed. It is shown that the structural performance of all the specimens well exceed its design allowance. Several design suggestions are given based on careful reviews on the experiment.

Network and vector forms of tensegrity system dynamics

We write the equations of motion in vector form for any class k tensegrity system dynamics. The network approach yields a connectivity matrix and nodal matrix, providing the dynamics of any network of bars, pipes and cables. The class 1 (bars do not connect) dynamics are described together with a constraint added to allow bar to bar connections. Damping and gravity forces as well as base movement can be handled in a unified framework. The constraint force is eliminated to yield a compact set of vector equations.