Ductility Of Connections Research Articles

Fire performance of axially ductile connections in composite construction

To enhance the robustness of steel-framed structures in fire, a novel, axially ductile connection has previously been proposed. In this paper its performance is investigated when it is used to connect composite beams to steel columns in composite steel-concrete construction. The ductile connection is designed to satisfy the ductility demands of the composite beam at elevated temperatures. A reinforcement component has been added to the bare-steel ductile connection model to establish a component-based model of the composite ductile connection. The connection model has been incorporated into the software Vulcan, and is validated against detailed Abaqus FE models using solid elements. Results show that the proposed component-based model can efficiently represent the behaviour of the connection given by the detailed Abaqus simulations. Parametric studies using Vulcan have been carried out, varying three parameters; the connection thickness, the semi-cylindrical section radius, and the density of longitudinal reinforcing bars. Finally, a 2-D Abaqus composite frame model has been created to investigate the influence of shear studs on the behaviour of the composite ductile connections under different stud spacings.

Cyclic Experiments on Sidelap and Structural Connectors in Steel Deck Diaphragms

This work studied the cyclic shear performance of connectors used for sidelap and structural connectors in bare steel deck roof diaphragms. Typical bare steel deck roof diaphragms include thousands of such connections, and knowledge of the connection shear performance is important for understanding and predicting diaphragm performance under seismic loads. The experimental program included four types of sidelap connectors: screw nestable, top arc seam weld, button punch, and the proprietary PunchLok II crimp. The testing included three types of structural connectors: power-actuated fasteners, arc spot welds, and arc seam welds. A total of 64 sidelap and 60 structural connector tests were performed under a cyclic loading protocol, to evaluate behavior at small deformations as well as under large excursions, providing the shear response up to and through the peak response, including postpeak response until material separation and failure. The connection test results were used to establish experimental multilinear backbone curves. From these backbone curves, average connection shear stiffness, strength, and ductility can be estimated and compared for all studied configurations. The connection response for the mechanical connections and welded connections exhibited significantly different behavior, particularly in terms of postpeak ductility and residual force. The connector backbone response is appropriate for use as nonlinear shear springs to model the response of the connections in high-fidelity models of bare steel deck roof diaphragms.

Analysis of a more ductile connection between steel beams and concrete-filled tube columns

To resolve the problem of ductile fracturing of beam flanges in traditional connections with concrete-filled steel tube columns using site-welded side plates, an alternative connection design requiring minimal site welding was designed and studied. An explicit analysis method, considering the fracture strain criterion developed by the stress triaxiality path, was adopted to study the seismic performance and fracture resistance of the proposed connection. The results of a verified finite-element model indicated that the steel beam flange in the connection never fractured. The connection exhibited a notably higher maximum bending moment and ductility than the traditional connection. The side plates in the proposed connection were tension-bending or compression-bending elements, unlike in the traditional connection. The failure modes, maximum bending moment, ductility and stress distribution of the side plates were compared and analysed parametrically. Based on the stress distribution of the side plates according to the finite-element results, a fitting formula related to the width and thickness of the side plates was obtained. A formula to calculate the flexural capacity was also developed according to the force model of the proposed connection. The calculations and finite-element results were in good agreement.

Shaking table test on insulated sandwich concrete wall building structure

To solve the problems of concrete sandwich panels such as cold bridge, weak connection, and poor ductility, a new configuration, namely the insulated sandwich concrete wall (ISCW) is proposed in this paper. Shaking table test is performed on a 1:3 scaled model of 3-storey ISCW building structure to study the seismic behavior of an ISCW structure. Results indicate the ISCW building structure has large stiffness and can work in elastic state under small earthquakes. The stiffness degrades continuously and the structure arrives in a plastic state as the PGA increases, but the degradation process is relatively slow. The cracks or damages mainly occur on the wall with big opening of floor one, but they do not spread through the walls and no collapse happens. With a rational arrangement of the cold-formed thin-walled steel tube columns, the structural integrity and ductility are significantly improved. Three types of seismic waves were input in the test, and significantly different structural responses were caused. Hence the ground motion spectrum characteristics cannot be ignored when the seismic response of an ISCW building structure is analyzed.

Tensile behavior of bolted through-diaphragm connections to SHS columns

Steel hollow section (SHS) columns have been increasingly popular in structural systems due to high compressive capacity, excellent torsional strength, and symmetric bending stiffness. In moment-resisting frames, the structural performance of connections to SHS columns is important to ensure the ability to carry bending moment. Conventional connections utilize welds to connect the adjacent components to the columns. Brittle fracture often initiates at the welds and reduces the ductility of connections. In this research, bolted connections were proposed to connect the beam to the SHS column through through-diaphragm. Tensile tests were conducted on four specimens to evaluate the structural behavior of connections. Failure mode, load-displacement response, strength, stiffness, and ductility were analyzed. The test results showed that the specimens were failed by the necking down of the flange plate. All specimens had high load-carrying capacity and good ductility. Strain distributions at key locations were investigated to study the force flow path. The application of through-diaphragm was found to deliver the tensile load to larger area and thus reduced the stress concentration at the connecting zone. Moreover, large amount of stress was transferred directly through the diaphragm, and the required stress demand for the SHS column was decreased. Design method was then proposed to predict the yield and the ultimate strength of the bolted connections. Good agreement was found between the experimental results and the theoretical predictions.

Nonlinear Static Seismic Response of a Building Equipped with Hybrid Cross-Laminated Timber Floor Diaphragms and Concentric X-Braced Steel Frames

Simplified seismic design procedures mostly recommend the adoption of rigid floor diaphragms when forming a building’s lateral force-resisting structural system. While rigid behavior is compatible with many reinforced concrete or composite steel-concrete floor systems, the intrinsic stiffness properties of wood and ductile timber connections of timber floor slabs typically make reaching a such comparable in-plane response difficult. Codes or standards in North America widely cover wood-frame construction, with provisions given for both rigid and flexible floor diaphragms designs. Instead, research is ongoing for emerging cross-laminated-timber (CLT) and hybrid CLT-based technologies, with seismic design codification still currently limited. This paper deals with a steel-CLT-based hybrid structure built by assembling braced steel frames with CLT-steel composite floors. Preliminary investigation on the performance of a 3-story building under seismic loads is presented, with particular attention to the influence of in-plane timber diaphragms flexibility on the force distribution and lateral deformation at each story. The building complies with the Italian Building Code damage limit state and ultimate limit state design requirements by considering a moderate seismic hazard scenario. Nonlinear static analyses are performed adopting a finite-element model calibrated based on experimental data. The CLT-steel composite floor in-plane deformability shows mitigated effects on the load distribution into the bracing systems compared to the ideal rigid behavior. On the other hand, the lateral deformation always rises at least 17% and 21% on average, independently of the story and load distribution along the building’s height.

Development and seismic retrofit of an innovative modular steel structure connection using symmetrical self-centering haunch braces

As an efficient and environment-friendly technique that combines off-site manufacturing of modules and on-site assembly, modular buildings have been adopted in many areas of the construction industry. This paper proposes an innovative modular steel structure (MSS) connection with bolted cover plates, which inserts a cross-shaped plug-in connector inside the adjacent columns to achieve accurate positioning in the module installation and provide partial shear capacity. A seismic retrofit method using hinged diagonal self-centering (SC) haunch braces is proposed for the MSS connection to improve the seismic performance and achieve functional recoverability after earthquakes. Hysteretic behaviors of the MSS connection and the self-centering MSS (SC-MSS) connection are investigated due to cyclic loadings. Results demonstrate that compared with the MSS connection, the SC-MSS connection is expected to improves strength and stiffness and exhibits stable flag-shaped hysteretic responses with a satisfactory self-centering capability. The hinged diagonal SC haunch braces are capable of reducing the average stress level around the connection core region, controlling the development of plastic damage, avoiding the local failure, and improving the ductility of connection. The present study provides practical connections and important guidance for the applications of modular steel structures in the medium- and high-intensity seismic regions.

Performance improvement of rigid connection by reduced web section with cellular beam and column subjected to cyclic loading

The behaviour under cyclic loading of a cellular beam and cellular column on the beam-to-column rigid connection was studied. The main focus was at the connection when openings were implemented to the beam and column members. The study aims to determine the increase of the yield and ultimate strength of the whole rigid connection, the ductility of connection and the energy dissipation in each loading cycle. Finite element analysis with both nonlinear material and geometry was conducted. The results indicated that the implementation of a cellular member could increase the yield strength of the overall rigid connection by 28.81%, the ultimate load by 30.93% and the stiffness by 36.21% compared to the original.

Flexural response of Ultra-High-Performance Concrete (UHPC) hybrid bridge deck connections made with local materials

Although using of UHPC improves the mechanical properties of concrete, it is not widespread used, due to several factors; for instance, its high material cost, special curing requirements, and lack of widely accepted design guidelines. This study aims to find an appropriate Ultra-High-Performance Concrete (UHPC) mixture, using locally available materials. In addition, the flexural response of this locally made UHPC mixture in hybrid bridge deck connections has been studied. The effect of different parameters such as percentage of silica fume, fine aggregate type, glass powder content and steel fiber volume fraction were investigated. The compressive strength results showed that a 28 days-strength of 140.3 MPa (20.35 ksi) has been achieved by using basalt as fine aggregate and 20% silica fume by weight. To investigate the applicability of this locally produced UHPC in bridge joints closure, two full scale, 8 ft. clear span, hybrid bridge deck slabs with UHPC closure were constructed and tested under monotonic loading. The load–deflection behavior of the hybrid connection confirmed that the deflection increased linearly until the initiation of cracks, after that it was increased non-linearly up to the failure of the connection. UHPC experienced less strain than high strength concrete (HSC) under compression. The hybrid connection produced with local UHPC showed moment capacity increase by more than 80% compared to NSC. The Moreover, a moment–curvature analytical model of hybrid bridge deck connection was developed to predict ductility, curvature, and stress distributions in closure connections. The model was able to predict initial stiffness and ultimate flexural moments of UHPC composite connections very well.

Experiments on laminated bamboo lumber nailed connections

Laminated bamboo lumber (LBL) nailed connection made with LBL main member and sandwiched by LBL side members is introduced, which plays an essential role in the safe and reliability of bamboo buildings. However, little research attention has been paid on to the mechanical behavior of connections. This paper reports an experimental study on the effect of parameters on reference loads (yield load and maximum load), stiffness and load-slip behavior of the connections, including nail diameter, number of lines and rows of nails. For single nail connection, nail pull-through and plastic hinges forms in LBL members. Mode cross-over from ductile response to brittle failure is observed in multi-nail connections with one or more splitting cracks develop from the nails hole edges toward the ends of the side or middle members. A decreasing ductility of connections is investigated with increasing diameter and number of nails. The capacity of effective number of nails in a row is evaluated and compared with that obtained from existing analytical formulas. Predictions from existed various theoretical modes are compared with experimentally determined capacities of nailed connection per shear plane per nail and conclude that the load carrying capacity of connections with laterally load nails can be applied to accurately estimate using European yield models (EYMs). Furthermore, the model developed by Folz is capable of predicting the load-slip relationship of LBL nailed connections. The findings of this study will provide the scientific basis and useful information for the design and finite element numerical simulation of engineered bamboo structures containing LBL nailed connections and promote the rational use of bamboo resources in the field of civil engineering.

Influence of steel properties on the ductility of doweled timber connections

In the seismic design of structures according to the dissipative structural behaviour, the connection ductility is crucial in order to ensure the desired level of energy dissipation of the overall structure. Therefore, in case of ductile zones composed of dowel-type fasteners arranged in series, it is important to ensure that all the fasteners can fully develop their energy dissipation capacity by plastic deformations. However, when different types of connections made of two symmetrical and serially arranged assemblies of dowel-type fasteners are tested, it often appears that only few fasteners fully work in the plastic region while most of the remaining ones exhibit very low yielding.Looking at the causes of this dysfunction, a possible explanation is due to the fact that the rules for the seismic design of dissipative zones in timber structures given in international codes and used in common practice often make reference only to the steel quality of the dowel-type fasteners specifying a minimum tensile strength or sometime, like is the case of the current version of Eurocode 8, only to maximum values of the dowel-type fastener diameter and of the thickness of the connected timber or wood-based members. Also, the research conducted so far about the ductile behaviour of serially arranged connections was not focused on the post-elastic properties of steel. However, for the seismic design of ductile zones of other materials, such as for example is the case of reinforced concrete walls, post-elastic characteristics of steel are required for the reinforcing bars, in order to achieve the desired dissipative behaviour.Inspired by this fact, timber connections composed of serially arranged dowels made of steel grades with different hardening ratio and elongation at maximum tensile stress were fabricated and tested. The purpose of this work is to understand if the use of steel with significant post-elastic properties may help to solve the problem of limited yielding in serially arranged dowel-type connections.The tested specimens were composed of two symmetrical timber members made of Glulam and LVL connected to two 6 mm thick slotted-in steel plates by means of 9 steel dowels with a diameter of 6.0 mm, which were subjected to monotonic and cyclic tests carried out by implementing dowels made of steel with favourable post-elastic properties.The results showed that the simultaneous yielding of two serially arranged dowelled assemblies is possible, although not fully. Moreover, assuming as reference the steel grade with the lowest post-elastic properties, the connection ductility and strength measured through monotonic and cyclic tests increased by about 30% for the steel grades with the highest hardening ratio and elongation at maximum tensile stress, whereas the displacement at maximum strength was about five times higher.In addition, it was found that confinement of the timber members and shaping of holes were crucial in order to avoid undesired and premature brittle failures and to increase the connection strength and ductility.The results obtained may be useful in order to bring a reassessment of the code requirements regarding the steel properties of ductile connections as well as of certain principles of dimensioning and detailing.

Experimental verification of seismic performance of built-up beam-to-column connections with ductile detail

The sections of hot-rolled H-beams produced by steel manufacturers in South Korea do not fully meet the requirements of recent buildings. Although custom-made built-up H-beams meet field requirements, they require a highly reliable manufacturing process through batch production and quality control. To this end, a steel manufacturer in South Korea recently constructed a production line for manufacturing built-up H-beams with uniform quality, and has attempted to address the problems of inefficient sections of hot-rolled members and the lack of large sections with a beam depth of 900 mm or greater. Furthermore, the steel manufacturer has attempted to produce highly ductile connection details that can be applied to large H-beam sections with a maximum depth exceeding that of existing hot-rolled H-beams to ensure seismic performance. In this study, real-scale structural performance tests were conducted on the beam-to-column connections of large built-up H-beams with ductile connection details suitable for site conditions. Built-up H-beams with a beam depth of 1000 mm were selected as specimens. The authors used a beam-end seismic welding detail and horizontal stiffener reinforcement detail, which were verified in a previous experimental study on beams with conventional depths. The test results were described in accordance with the connection performance verification process presented in the AISC standard and the seismic design code adopted in South Korea. The structural stability of the applied connection details was analyzed.

Experimental study on beam-to-column connections with reduced beam section against progressive collapse

Steel frame structures with ordinary weld connections often suffer from brittle fractures; to mitigate this issue, measures have been taken to integrate plastic hinges and improve the ductility of the steel connections. A new reduced beam section (RBS) connection, consisting of radius cuts in the beam flanges and circular openings in the beam webs, is proposed to allow for more ductile connections. RBS connections may increase structural deflection and crack width in the steel frame structure under large deformations. Therefore, it is extremely important to consider the bearing capacity and progressive collapse resistance of steel frame structures with RBS connections. In this study, nine different specimens with double-span frames were constructed and tested under a central column removal scenario: one without an RBS (RBS0), one with flange reductions (RBS1), and seven with web and flange reductions (RBS2). The mechanical behaviours of the different specimens were determined, including their failure modes, vertical force-deformation responses, and contribution of catenary action to resist progressive collapse. The results indicated that the RBS0 specimen with brittle failure did not effectively resist progressive collapse. RBS1 exhibited superior ductility and resistance compared to RBS0; however, its resistance to progressive collapse after cracking required further improvement. RBS2 with web openings formed an arch structure to dissipate energy through continuous deformations. Further, two important factors in RBS2 that influenced its mechanical properties were the diameter and the distance of the web opening, which were related to the development of arching and catenary action.

Connection and System Ductility Relationship for Braced Timber Frames

AbstractBraced timber frames (BTFs) are an efficient lateral load-resisting system for wind and seismic loads. This paper derived a relationship between the connection ductility and the system duct...

Numerical and experimental study on the behavior of drilled flange steel beam to CFT column connections

Numerous studies conducted on different types of rigid connections such as reduced beam section (RBS) and drilled flange (DF) showed their ductile behavior. This paper evaluates the seismic performance of moment resisting connections with drilled flange (DF) beams as a simple and efficient alternative for RBS connected to concrete-filled steel tube CFT columns. Different patterns are proposed and the shear capacity of panel zone and ductility of connection are assessed to obtain the optimum case. For this purpose, the finite element Abaqus version 6.14 is used for the numerical models with the thickness ratio of beam flange to web, the type of hole and the number of holes in beam flange as the main parameters. The best arrangement of zigzag hole pattern is first determined by cyclic loading and then experimentally validated. Based on the obtained results, the type and number of holes significantly affect the ductility of connection and decrease stress concentration in the area of beam to column connection. In addition, the idea results in changing plastic hinge from the area of beam-to-column connection and panel zone to the beam in its drilled flange region and reducing the possibility of brittle failure at connection. Further, the arrangement can decrease flexural stress in the groove weld of direct beam-to-CFT column connection by 41% and increase the flexural capacity of panel zone by 31%. Furthermore, a 35% enhancement was observed in the shear capacity of panel zone by increasing the flange to web thickness ratio.

Experimental and numerical study on the T-Stub behaviour with preloaded bolts under large deformations

The bolts and their arrangement strongly influence stiffness, strength and ductility of T-Stub connections. Preloaded bolts are typically adopted to improve the stiffness and limit the opening of the connection under serviceability conditions. EN1993-1-8 allows using two types of high resistance preloaded bolts, namely the German HV (acronym of Hochfeste Bolzen mit Vorspannung, which is the German for High Resistance bolts for pretension and the British/French HR (acronym of High Resistance) without making any distinction. However, the tensile failure modes of these bolts are different (i.e. nut stripping for HV and shank necking for HR) and may affect the ultimate tensile response of the T-Stub connections with weak bolts (e.g. failure mode type 2 and 3). Furthermore, despite the effects of geometrical non-linearities at large deformations are not specifically addressed in current codes, the membrane action developing in the flange and the shear force and bending moment in the bolt may influence the reserve of ductility of the T-Stub that plays an important role in case of seismic and robustness scenarios. In addition, the presence of initial imperfections (e.g. the misalignment of the web as respect to the bolts and the flange bowing) can influence the non-linear behaviour of the connection. The influence of all these aspects on resistance and ductility of T-Stub connections are investigated by means of both experimental monotonic tests and parametric finite element analyses. On the basis of the obtained results, design rules are discussed with the aim to guarantee extra ductility of T-Stub connections.

Experimental study on local compressive behaviour of ECC ring beam connections

To avoid problems associated with cracking and ensure that a connection has sufficient strength in compression for a through-beam ring beam connection, a novel Engineered Cementitious Composite (ECC) ring beam connection is proposed and experimentally studied. Local compressive tests on 15 ring beam connections with various matrix type, width ratio, height-to-width ratio and reinforcement ratio are reported. According to the test results, ECC connections showed a higher load carrying capacity and more ductile behaviour than regular concrete connections. Compared with concrete ring beam connections, the displacement ductility of the ECC ring beam connections was improved by 25.7%-62.2%. The energy dissipation of the ECC connection specimens was 1.5 and 1.7 times larger than that of the concrete connections, respectively. The test results also indicated that the width ratio had the greatest impact on delaying cracking in the ECC connection. Increasing the reinforcement ratio can efficiently increase the peak load. Increasing the width ratio resulted in an obvious increase in the energy dissipation of the ECC ring beam connections.

An Analytical Method to Predict the Structural Behavior of Timber-Concrete Structures With Brittle-to-Ductile Shear Connector Laws

Timber-Concrete Composite (TCC) structures allow taking synergistic advantage of the properties of both materials to optimize the overall performances in terms of lightness, slenderness, acoustic insulation, vibrational behaviour and environmental footprint. In the last years, ductile shear connectors have been developed to allow the structural ductility of TCC structures. Considering the limitations of current design methods, this work aims at developing a closed-form solution for accurately predicting the nonlinear structural response of a TCC structure directly from the materials' property and the shear law of ductile connectors. In particular, we have assumed a generalized shear law based on 3 parameters which allow considering shear law from a pure elasto-plastic to pure brittle behaviour.After a short introduction, Section 2 briefly presents the basics of the well-estabilished elastic theory for a 2-layer composite beam with a linear shear law in terms of horizontal shear vs. slip (Vh-s) law; Section 2.3 extends the semi-analytical method proposed by Bažant and Vitek for composite structures with a generalized shear law; Sections 3 extends the previous method by developing a new closed-form analytical solution for predicting the TCC structural response for a generalized elasto-plastic Vh-s shear law described by an initial linear response up to Vmax followed by a plastic plateau at a constant load, which can range from 0 to Vmax; Section 4 compares the model results with the ones of existing methods and FEM analysis. Furthermore, a parametric analysis is carried out to investigate the model sensitivity to the connector parameters; Finally, Section 5 presents a simple point-by-point design procedure of which prediction accuracy of the ultimate moment, deflection and slip was statistically assessed for a large range of possible TCC structures against FEM analysis. Eventually, the effect of concrete cracking is also considered and a correction factor is proposed for engineering purpose.

Behavior of novel CFST circular column-to-foundation connections under cyclic loading

Despite several advantages of Concrete Filled Steel Tube (CFST) columns, these are still not very popular in construction due to the non-availability of well-established, efficient, and ductile connections of these columns with the reinforced concrete (RC) foundations. The present study proposes two novel CFST column-to-foundation connections that are efficient and have adequate ductility to withstand inelastic deformation cycles under moderate to extreme seismic loading. The first proposed connection was developed by shape modification of the cavity used in the first-stage concrete of the RC foundation, whereas the second proposed connection was developed with a concept of mechanical interlocking. The connections were tested under cyclic lateral loading for simulating the seismic excitations. Two more connections representing controls were also tested. The first was a conventional CFST column-to-foundation connection, and another was a regular RC column-to-foundation connection. The test results establish the better performance of the proposed connections. Finite element (FE) models were developed to predict the behavior of experimentally investigated RC and CFST column-to-foundation connections. The prediction was in good agreement with the experimental results.

LVL–structures made as combined section with small-compliance connections, consistent with stress-strain state of structural elements

The use of composite structures of LVL elements of I-beam and box-shaped cross-section is the most expedient from an economic point of view. Applications for connecting the individual parts of the cross-section of ductile connections - self-tapping screws, allows you to assemble such structures at a construction site. The use of self-tapping screws installed in accordance with the direction of the tensile internal forces of the structure makes it possible to force self-tapping screws to work mainly in tension. At the same time, the most rational work of the joints is achieved — minimum work on bending and maximum work on stretching. According to the results of tests, manufactured and designed beams of composite section, it was concluded that box-shaped beams have greater load-bearing capacity and less deformability. The ratio of actual and calculated bearing capacities of composite box-section beams is higher, the required value of the safety factor, and the destruction of the test beams occurred wood, which indicates the safety margin of the connection of the elements of the composite section on the stretched self-tapping screws.