Ductility Of Connections Research Articles

Shear-slip behaviour and ductility of composite dowel connectors with pry-out failure

The flexural strength of steel and concrete composite beams is influenced by both the connectors’ shear capacity and the slip across the steel–concrete interface. However, plastic design strategies commonly used in engineering practice assume a rigid-plastic connector behaviour neglecting the real nonlinear shear-slip characteristic. As to prevent a premature connector failure due to excessive beam end slip, design codes usually allow only ductile shear connectors with a deformation capacity δult of at least 6mm to be used. But, especially for innovative shear connectors like composite dowels, approaches for assessing and describing the nonlinear shear-slip behaviour and deformation capacity are lacking, so far. Therefore, the present paper proposes models for the calculation of the slip capacity and the description of the shear-slip characteristic, considering the composite dowels’ shear capacity and elastic connector stiffness. The proposed models can be used to replace expensive and time-consuming shear tests for the characterization of the composite dowels’ deformation capacity. Furthermore, they allow to define the required material combinations and to carve out the dowels’ geometrical dimensions for a ductile connector behaviour.

Introducing box-plate beam-to-column moment connections

Nowadays, using high-ductility structures in the construction and use of significant buildings is highly appreciated. To use more ductile structures, effort has been made in this research to introduce box-plate beam-to-column connections. They underwent hysteretic loading and it was found from their moment-rotation curves that the bending capacity and ductility of the box-plate connection were more than ordinary rigid connection, and those of the latter were more than those of the normal typical one. It was also shown that stress concentration in box-plate connections disappears over the top and bottom flange plates.

Investigation of Steel Column–Baseplate Connection Details Incorporating Ductile Anchors

It is widely accepted that the behavior of steel column–baseplate connections will profoundly impact the seismic response of steel buildings and other structures. Previous investigations into moment-frame steel column–baseplate connections have focused on the relationship between global metrics such as connection strength and stiffness to basic design parameters such as anchor arrangement and size, baseplate thickness, and column size. However, there is growing recognition that other details, such as the column setting method, baseplate hole size, anchor material selection, and anchor stretch length may significantly affect connection performance. This paper describes nine full-scale connection tests exploring the effects of such details on strength, rotation capacity, and postyield behavior. A comparison of the present with a prior test program on the basis of physical damage evolution and moment-rotation behavior is undertaken. The baseplate setting arrangement and anchor stretch length were found to have the most pronounced impact on the connection cyclic moment-rotation response. Suggestions for connection details that promote connection robustness, ductility, and strength are presented.

Seismic behavior of conxl connections in concrete filled steel Tube Columns (CFT)

Steel box-shaped columns are suitable structural members for structures with moment frames in two directions, but plate connections have several construction problems, including inaccessibility of inside of columns, welding difficulties, etc. ConXL connections are the new proposed details to reduce these problems. This connection consists of collar flange, collar corner, and collar web extension. In this paper, the seismic behavior of these types of connections is investigated using the numerical method. For this purpose, three samples of ConXL connections without concrete filler, with concrete filler and with concrete filler and stiffener plates inside the column were studied using Abaqus software. The results demonstrated that the ductility of the ConXL connection without concrete filler is more than the two other samples, while the strength of this connection is less than the strengths of the two other samples. It was observed that utilizing stiffener plate inside the column has no significant effect on the strength of the connection.

Test and analysis of a new ductile shear connection design for RC shear walls

This paper presents a new and construction-friendly shear connection for the assembly of precast reinforced concrete shear wall elements. In the proposed design, the precast elements have indented interfaces and are connected by a narrow zone grouted with mortar and reinforced with overlapping U-bar loops. Contrary to conventional shear connections, the planes of the U-bar loops are here parallel to the plane of the wall elements. This feature enables a construction-friendly installation of the elements without the risk of rebars clashing. The core of mortar inside each U-bar loop is reinforced with a transverse double T-headed bar to ensure transfer of tension between the overlapping U-bars. Push-off tests show that a significantly ductile load–displacement response can be obtained by the new solution as compared to the performance of the conventional keyed shear connection design. The influence of the interface indentation geometry was investigated experimentally and the failure modes in the push-off tests were identified by use of digital image correlation (DIC). For strength prediction, rigid plastic upper-bound models have been developed with inspiration from the observed failure mechanisms. Satisfactory agreement between tests and calculations has been obtained.

Seismic performance evaluation of Korean column-tree steel moment connections

The purpose of this study was to evaluate the seismic performance of existing Korean column-tree connection type in steel moment frames. The column-tree connection has frequently been used in steel moment frames because it is well known to provide high quality and economy with shop-welding and field-bolting in Korea and Japan. However, there have been no studies on the design details and ductility of column-tree connections. Therefore, in this paper, the seismic performance of non-composite and composite column-tree specimens was evaluated to investigate their problems. Full-scale specimens were fabricated with non-composite/composite strong axis and weak axis column-tree connections, respectively. The testing results showed that, although non-composite column-tree specimens reached a 5% story drift ratio, tiny cracks occurred at beam-to-column connections. In the case of composite specimens, the expected brittle fracture at the bottom flange was not detected. However, severe crushing of concrete slab occurred, and the bottom flange underwent great stress. Therefore, composite action should be considered when composite structures are designed because bottom flanges may fracture.

Probabilistic progressive collapse analysis of steel-concrete composite floor systems

The paper presents a probabilistic analysis of steel-concrete composite floor against progressive collapse considering uncertainties in strength and ductility of steel connections. Using component-based connection model, an analytical framework for developing probabilistic connection models is proposed. The connection models developed are further introduced in probabilistic structure analysis against progressive collapse. Tornado diagram-based sensitivity analysis is performed to determine the influential random variables for structural resistance capacity. Using Latin Hypercube sampling of both random structure variables and external loads, random realizations of structures are generated and progressive collapse analysis is carried out using pseudo-static pushdown method. The proposed framework is applied to study the vulnerability of composite floor. Fragility curves corresponding to three limit states are developed. Discussions on the influences of steel connection and slab continuity on collapse vulnerability are given. Finally, results from the present probabilistic method are compared with those from deterministic approach.

Load-to-grain angle dependence of the embedment behavior of dowel-type fasteners in laminated veneer lumber

Load-to-grain angle dependence of the embedment behavior of steel dowels in laminated veneer lumber, as a consequence of the anisotropic material behavior of wood, is experimentally investigated in this study. As a novel issue, in addition to the stress dependence, the displacement path of the dowel depending on the load-to-grain angle, is discussed. Full-hole embedment tests of screw-reinforced LVL specimens up to dowel displacements of two times the dowel diameter and thus, representative for highly ductile dowel connections were conducted. Tests were performed with unconstrained lateral displacement boundary conditions of steel dowels with a diameter of 12mm and 16mm. Surface deformations were monitored with a full-field deformation measurement system. Increasing the load-to-grain angle caused reduced quasi-elastic limits and loading stiffness. However, for load-to-grain angles of 60° and higher, a pronounced displacement-hardening effect, leading to high embedment stresses at large dowel displacements, was observed. For the investigated dowel diameters, surface strains and plastic deformations around the dowel indicate an almost dowel diameter independent load bearing area, which might explain higher nominal embedment stresses and consequently a more pronounced hardening effect of the smaller dowel diameter. Dowel displacements perpendicular to the initial loading direction, i.e., nonlinear displacement paths of the dowel, were related to the anisotropic stiffness of wood and densification effects close to the dowel. The established experimental dataset was compared to current European timber engineering design equations and could serve as input to analytical and numerical models of dowel connections.

Effects of stiffening sealant thickness on the structural performance of structural silicone glazing (SSG) sealant connections in curtain wall systems

This paper describes the experimental results of tests of the structural behavior of structural silicone glazing (SSG) sealant connections in a curtain wall system in which structural sealant is placed between an aluminum frame and laminated glass panel. Cases often have been found where the structural sealant has been applied at a 45° angle from the inside of the curtain wall. Thus, assessment of the structural behavior of the SSG connections between the aluminum frame and glass panel in a curtain wall system is required. The tests conducted in this research include variables such as the use of Norton tape, the application (or omission) of end sealant, the thickness of the stiffening sealant applied on top of the structural sealant, and the span length of the glass panel. The test results indicate that the use of Norton tape directly affects the initial stiffness and strength values of the SSG connection and that the application of end sealant improves the strength and ductility of the SSG connection. However, the change in the maximum load per unit length caused by the application of stiffening sealant on the existing structural sealant was found to be insignificant.

Hybrid System of Unbonded Post-Tensioned CLT Panels and Light-Frame Wood Shear Walls

Cross-laminated timber (CLT) is a relatively new type of massive timber system that has shown to possess excellent mechanical properties and structural behavior in building construction. When post-tensioned with high-strength tendons, CLT panels perform well under cyclic loadings because of two key characteristics: their rocking behavior and self-centering capacity. Although post-tensioned rocking CLT panels can carry heavy gravity loads, resist lateral loads, and self-center after a seismic event, they are heavy and form a pinched hysteresis, thereby limiting energy dissipation. Conversely, conventional light-frame wood shear walls (LiFS) provide a large amount of energy dissipation from fastener slip and, as their name implies, are lightweight, thereby reducing inertial forces during earthquakes. The combination of these different lateral behaviors can help improve the performance of buildings during strong ground shaking, but issues of deformation compatibility exist. This study presents the results of a numerical study to examine the behavior of post-tensioned CLT walls under cyclic loadings. A well-known 10-parameter model was applied to simulate the performance of a CLT-LiFS hybrid system. The post-tensioned CLT wall model was designed on the basis of a modified monolithic beam analogy that was originally developed for precast concrete-jointed ductile connections. Several tests on post-tensioned CLT panels and hybrid walls were implemented at the Large Scale Structural Lab at the University of Alabama to validate the numerical model, and the results showed very good agreement with the numerical model. Finally, incremental dynamic analysis on system level models was compared with conventional light-frame wood system models.

Introducing the innovative post-tensioned connection with the rigid steel node

Securing strength, stiffness, and ductility in connections is one of the most important goals in structural design. After the Northridge earthquake, most steel moment resistant frames were damaged due to sudden fractures in beam-to-column connections. This resulted in the reconsideration of connection design and performance. As an alternative to weld connections, some researchers proposed the implementation of post-tensioned connections. In the present paper, the innovative connection is a combination of high strength steel frames which, through a rigid steel node above and below the beam flanges, provide the required stiffness, strength and ductility. Advantages of this connection include self-centering capability, lack of residual deformations, and energy dissipation which are analyzed using numerical and experimental methods. The modeling procedure and the cyclic behavior of the connection are studied and the analytical and the experimental results are compared. The results show that the cyclic responses observed in numerical simulations are in agreement with those from the experimental task and that the connection is superior to conventional moment resisting connections. The assumptions employed in simplifying simulations demonstrated high accuracy revealing an improvement in the seismic behavior of the connection.

Interval analysis of nonlinear frames with uncertain connection properties

Semi-rigid connections can often be a more economical solution for a framing system than one with either fully fixed connections or fully pinned ones. In view of the fact that the properties of such ductile and partial-strength connections are not known accurately, this paper presents a method for the obtention of both upper and lower bound responses of semi-rigid frames for possible variations in their moment-rotation properties. The latter are thus assumed to be known within some key upper and lower bound values, namely a constitutive law that is still deterministic but is described in terms of a so-called “interval” model. A mathematical programming approach is used to formulate and solve the problem. In particular, for each load level, a pair of nonstandard optimization problems known as interval mathematical programs with equilibrium constraints (or interval MPECs) are solved to provide the required bounds. A number of examples are provided to highlight the important effects of considering uncertainties in semi-rigid connection properties.

Influence of different types of reinforcements on the embedment behavior of steel dowels in wood

In this study, dowel displacement-embedment stress relationships for different types, numbers and positions of reinforcements were experimentally investigated using a half-hole embedment test setup. Tests were performed parallel to the grain and in compression. Screws with a full or partial thread at different positions below the dowel and oriented strand board, plywood and nail plates on the loaded surfaces of the specimens, served as reinforcements. Test results underline their potential for an increased ductility of dowel-type connections. Comparison of reinforced and unreinforced specimens suggests premature failure of the unreinforced wood and consequently, an underestimation of the embedment strength as it is subsequently used in the design of dowel connections using the European yield model. This was supported by the investigation of cracks on the surface of the specimens visualized by means of a full-field deformation measurement system. It could be demonstrated that the strength in the embedment test even further increases if the reinforcement elements actively contribute to the load transfer. This property however cannot be considered as embedment strength, but represents the strength of a connection system. Test data is compared to the design equation in Eurocode 5.

Studies on structural behaviour of long-span continuous composite beams with flexible shear studs of limited ductility

This paper reports a comprehensive three-dimensional finite element study to examine the structural behaviour of continuous composite beams with the consideration of the flexibility of shear connectors. With the full incorporation of material, geometrical and interfacial non-linearities, the results of the proposed finite element models compare quite well with test results of continuous composite beams with a wide range of geometrical configurations, material properties, arrangements of shear connectors as well as loading and boundary conditions. A comprehensive parametric study is also conducted, which studies the structural behaviour of continuous composite beams with different material properties and geometrical configurations. Composite beams with both ductile and non-ductile shear connectors are included to investigate the effects of flexibility of the shear connectors on the overall structural behaviour of continuous composite beams.

Cyclic flexural tests of hybrid steel–precast concrete beams with simple connection elements

A hybrid H-steel–precast concrete (HSPC) beam system with a simple ductile connection was developed to overcome the limitations of a conventional hybrid beam system with a bolt connection between the H-steel beams supported by columns and H-steel beams embedded into the concrete beams. The developed HSPC beam system has three different sections: H-steel, joint, and reinforced concrete sections. To examine the effectiveness on the moment resistance and ductile performance of the developed HSPC beam system, five fixed-end beam specimens were tested under reversed cyclic one-point concentrated top loads at mid-span. The prepared specimens were designed as follows: H1.0, H0.5, and H0.25 were used to evaluate the effect of the location of the joint on the flexural behavior of the developed HSPC beam; P0.5 was used to introduce the prestressing force; and S0.5 was used to examine the effect of the lap splices of the headed longitudinal reinforcing bars on the stress transfer at a joint. The flexural capacity of the fixed-end HSPC beams was evaluated based on the virtual work principle under the assumption of rigid joints at the connection of the steel and concrete beams. None of the beam specimens showed shear cracks or splitting tensile cracks around the joint region until the beam failed, unlike in the conventional hybrid beam system with a bolt connection. The developed HSPC beam retained a ductility comparable to that of conventional RC beams. From the measured crack propagation, cyclic load–displacement curve, consistency of the recorded strain distribution along the beam length with the calculations, and agreement between the ultimate loads and the calculated collapse loads, the proposed joint element connecting the H-steel and RC beams can be regarded as a rigid connection, resisting the applied moments.

Cyclic behaviour of glulam shear walls with bolted connections

Dowels and bolts are well established types of fasteners for timber connections. They are fast and easy to assemble which made them become a common solution for joining heavy timber elements. This study investigates the feasibility of bolted connections as a possible anchoring solution for heavy shear walls under seismic loads. Fully reversed cyclic tests were performed on glued laminated (glulam) timber shear walls with un-reinforced and reinforced bolted connections. This study builds up on a previous research conducted on single bolted connections tested under quasi-static tensile loading. Testing an entire wall provides additional information on the performance of the connections under the actual load distribution (tensile and shear force) due to horizontal load which varies the shear force distribution in the fasteners compared to the case of a simple connection tested in tension. The influence of the fastener spacing and distance from the loaded and unloaded edge on the failure mechanism was investigated, as well as the effect of the reinforcement on the cyclic response of the connection in comparison to the unreinforced solution. The experimental study proved that conventional bolted connections are a feasible solution for anchoring shear walls to the foundation. Reinforcing the connection area with self-tapping screws resulted in reduced splitting, increased connection ductility and increased equivalent damping. However, bolted connections experience significant irreversible damage, namely plasticization of the bolts and timber crushing at the interface with the bolt, at the end of the seismic event.

Robustness assessment of frame structures using simplified beam and grillage models

Simplified analysis methods derived in previous studies are employed for studying the progressive collapse behaviour of steel and composite buildings. A regular frame building is considered and various scenarios of sudden column removal, each affecting different floor areas in terms of geometry and boundary conditions, are applied. Descriptions of the pseudo-static responses of the various constitutive beams are obtained based on both detailed representations of the nonlinear static responses and by applying a new simplified approach proposed in a separate publication. Comparisons between the results of the two methods confirm that the simplified approach is capable of describing behaviour with reasonable accuracy. By employing a simplified multi-level assessment approach that has been previously derived at Imperial College, grillage-type approximations are obtained and used to examine the floor dynamic behaviour for the various column removal cases. It is found that, although the structural response varies depending on the location of the initial damage, substantial connection strength is required in all cases in order to provide resistance to progressive collapse. In addition, for average levels of connection ductility, failure most likely occurs prior to the development of tensile catenary action in the beams, which indicates that the provision of tying resistance may not be effective in enhancing robustness. Therefore, the combined action of flexure and compressive arching in the beams is likely to form the principal collapse resisting mechanism in common practical applications, which confirms similar conclusions made in previous studies at Imperial. The provision of adequate levels of connection moment capacity – in combination with sufficient ductility supply – is, therefore, the most effective way of securing structural robustness.

An analytical and numerical prediction for ductility demand on steel beam-to-column connections in fire

In this paper a simplified analytical method to assess the ductility demand on connections according to fire resistance requirements is developed on the basis of fundamental structural mechanics principles. An objective is to enable the development of a viable method to allow engineers to take the ductility of connections into account in design practice. Numerical finite element simulations of the single beam model were also performed to validate the simplified analytical model and reveal the important parameters that can influence the ductility demand within the connections. Using both analytical and numerical methods, the principal factors which influence the ductility demand of a connection, such as the span of the connected beam and the required connection strength, are also assessed. It is shown that:1.The compressive ductility of connections is helpful in reducing the push-out of perimeter columns and the possibility of local buckling of beams.2.Provision of high tensile deformation capacity allows large deflection in the beam, substantially reduces catenary forces on the connections, and consequently reduces the risk of structural collapse in fire.3.The ductility demand of the connection is closely related to its stiffness and strength, as well as to the slenderness and load ratio of the connected beam.

Ductility of post-Northridge connections with Angelina beams

Sinusoidal voids of the same size at the beam web can be easily created during fabrication of a castellated beam, when a sinusoidal cut shape is used. Such castellated beams are known as Angelina beams and are available in the market place. This study is designed to investigate the efficiency of using single and multi-sinusoidal voids at the beam web area, to enhance the ductility of the post-Northridge connections. For this purpose, a parametric study is carried out to determine the best configuration of such voids using the finite-element method. The investigation is carried out on more than 440 specimens of different design parameters. The optimum size of sinusoidal voids is proposed to achieve both adequate strength of connections and ductility. Present data indicate that creating a single void at the beam web with optimum size is only effective when the beam overall depth is limited to 750 mm, and the ratio of the beam length to the beam overall depth is equal to or greater than 11·5. For deeper and shorter beams, adequate strength of connection and ductility can be achieved when a modified specimen is used, with either multiple same-size voids with web stiffeners or multiple different-sized voids without web stiffeners.

Numerical modelling of timber/timber–concrete composite frames with ductile jointed connection

Due to the scarcity of experimental data, this article focuses on the application of detailed finite element models for evaluating structural behaviour of timber–concrete composite frames with post-tensioned beam-to-column joints. In the developed finite element models, nonlinear behaviour and failure mode of timber and concrete under biaxial stress state are captured by hypo-elastic constitutive laws based on the equivalent uniaxial strain concept. In addition to material nonlinearities, the effect of geometrical nonlinearities and nonlinearity of contacts at the concrete slab-to-beam, beam-to-column and slab-to-column interfaces are considered in the finite element models. The accuracy of developed finite element models is verified against available experimental data on post-tensioned timber frames, and the validated analytical tool is used to undertake a parametric study. It is shown that elastic modulus of timber and the details of concrete slab-to-column connection can significantly affect the drift response and failure mode, whereas the compressive strength of timber and stiffness of timber–concrete composite connection have only a minor influence on the drift and failure mode of the timber/timber–concrete composite frames with ductile jointed connections.