Ductility Of Connections Research Articles

Structural behaviour of post-installed reinforcement bars in moment connections of wall-slabs

Post-installed reinforcement (PIR) bars helps to facilitate retrofitting works, mitigate misplaced reinforcement problems, as well as support newly casted additions. However, the use of PIR has not been addressed in the major reinforced concrete (RC) design codes worldwide. Recently, the European standards have introduced a beneficial coefficient of moments in EN 1992-4 2018 for concrete fastenings which allows compliant PIR systems to be designed by using the bonded anchor (BA) design method. However, when applying this method to wall-slab connection design, the moment resisting capacity is often limited by the lack of bar spacing and small concrete covers. This means that the method neglects long embedment depths and the connections designed based on this method are prone to brittle failure. In this paper, the strut and tie model (STM), which can better describe PIR with long embedment depths, together with the fundamental reinforced concrete (RC) theory is used to improve the ductility of moment connections with PIR bars. An experimental study is conducted to explore the structural behaviour of applying PIR bars that connect the wall and slab. Validations on the proposed STM and supplement to the BA design methods are made. From the experimental findings, measures are then proposed to enhance the ductility of the moment connections.

Resistance of steel and composite connections under combined axial force and bending including group effects: Analytical procedures and comparison with laboratory tests

The paper presents some different developments performed in the last years at the University of Liège. In particular, complete component-based analytical procedures to predict the resistance of steel bolted connections subjected to combined bending moments and axial forces (M-N interaction) are introduced and commented. These analytical procedures consider so-called “group effects” between successive bolt-rows and cover possible ductile or brittle connection responses. Then they are extended to composite steel-concrete connections. In a second part, the analytical predictions are compared to available test data. The correlation between the experimentally measured resistances and those provided by the analytical procedures may be considered as close. Finally, the recommendations in terms of M-N interactions provided by Eurocode 3 are critically analysed.

Uncertainty assessment of field weld connections and the related effects on service life of steel buildings

Delay in construction schedules has raised concerns regarding the quality of field weld connections. To address this issue, the effects of field weld quality are evaluated using test data and literature review. The evaluation provides evidence supporting the assumption that for the marginal workmanship quality, the strength of steel connections can reduce by approximately 5%. The effects are then used to predict the service life of 9-story and 20-story steel buildings with pre-Northridge connections. The prediction adopts a seismic risk-based framework, and considers the uncertainties of steel material and workmanship quality. The example buildings are highly vulnerable to earthquake damage owing to their lack of connection ductility. That also amplifies the effects of poor field weld quality. The result indicates that in the case of marginal quality workmanship, the service life can be reduced by 20 years, with taller buildings more significantly affected.

The concrete frame strength test is filled with brick against horizontal loads with several types of column beam joints and several types of brick wall connection frames

Damages occurred in brick walls in a simple structure because of an earthquake show that walls also withstand the load. The damages may occur due to a lack of quality in the materials and the joint between each element of the wall. Therefore, this research aimed to examine the connections between the wall elements in order to investigate the strength of the walls with ductile and non-ductile connections and its crack behavior. The connection investigated in this study was the connection between the brick wall and beam, the brick wall and column, also the beam and column. The ductile wall connections were built according to the SNI brick walls, and anchors were put on the wall-to-beam and wall-to-column connection. The brick walls were then given load in a horizontal direction. Based on the results, the ductile walls with anchor had the greatest strength with 2.204 kg whereas the ductile walls without anchor had lower strength with 1.929 kg. On the other hand, the strength of the non-ductile walls was 1.653 kg. In conclusion, the absence of anchors that bind the wall-to-column and wall-to-beam reduced the strength by 12.5% whereas the use of non-ductile connections reduced the strength by up to 25%.

Seismic design of the double-cell accordion-web reduced beam section connection

The Accordion-Web Reduced Beam Section (AW-RBS) connection consists of a corrugated web, replacing the flat web, within a limited area near the column face, which reduces the beam flexural strength and forces the plastic hinge to form in the corrugated zone. This paper presents a seismic design procedure in the framework of the U.S. design standards for a double-cell AW-RBS connection, which consists of two box sections oriented such that an accordion web is formed within a limited area in the beam. The analytical evaluation of the flexural strength of the beam with a corrugated web was first presented. Then, a step-by-step design method was proposed to size the connection. Quasi-static cyclic tests were conducted on two relatively identical specimens including double-cell AW-RBS connections. A detailed numerical model of a connection with deep beams and columns designed in accordance with the proposed design procedure was developed and a nonlinear analysis was performed to investigate the seismic response of the connection and verify the adequacy of the proposed design method for the beam sizes frequently used in steel moment-resisting frames. The results confirmed the satisfactory response of the double-cell AW-RBS connection under cyclic loading. The stable plastic hinge formed in the reduced area of beams as predicted in design and no yielding was observed in the beam-to-column groove weld. The findings of the numerical simulations confirmed that the AW-RBS connection with deep beams and columns could potentially exceed the plastic rotation limit for ductile moment connections without significant instability and strength degradation.

Review of experimental tests and numerical study on masonry vaults reinforced through fiber-reinforced mortar coating

The paper deals with the seismic vulnerability of masonry vaults: a review of the results of an experimental campaign recently performed by the authors and a numerical study are presented. In particular, the effectiveness of a strengthening technique based on the application, at the vault extrados or intrados, of a 30 mm thick mortar coating with glass fiber-reinforced polymer meshes embedded is discussed, referring to vaults carrying their self-weight and subjected to a lateral transversal load. The role of the connection of the vault with its abutments is particularly emphasised in this review of the experimental results, evidencing how slides and/or uplifts at the spring sections can reduce the strengthening effectiveness. The numerical study, based on non-linear static analysis, allowed to investigate in depth into the effects of an inadequate skewback configuration: it was found that a ductile connection, which allows plastic rotation at the spring section while maintaining the bending resistance and avoiding shear slip and uplift, is the optimal solution. Moreover, the results of a sensitivity study are also presented, as this permits to evaluate the incidence of the different mechanical and geometrical parameters on the behavior of the vaults and provides useful indications towards a proper design and optimization of the strengthening intervention.

Experimental investigation of strengthening slab-column connections with CFRP fan

Abstract This paper investigates experimentally the punching shear behavior of flat slabs-column connections strengthened with CFRP (Carbon fiber reinforced polymer) string having a fan shape as a new strengthening technique. Six square slabs with middle column heads were casted monolithically at the center of the slabs. All the specimens had the same dimensions and the same reinforcement detailing. Four slabs were strengthened with CFRP fan, one strengthened with steel bolts while the reference specimen was tested without strengthening. The results showed that the proposed strengthening technique improved the punching shear capacity and enhanced the ductility of the flat slabs column connections compared with un-strengthened connections. Punching shear failure load achieved by experimental testing showed a reasonable agreement with the calculated punching failure loads based on formulas adopted by different codes: ECP (Egyptian code of practice), ACI-318 (American concrete institute) and JSCE (Japan society of civil engineers).

Investigation of a steel connection to accommodate ductility demand of beams in fire

In fire conditions the provision of connection ductility is key to the prevention of brittle failure and progressive collapse of steel and steel-concrete composite framed structures. This paper describes the development and testing of a novel connection concept intended to provide appropriate ductility enhancement compared to that of conventional connection types. The connection consists of two connection pieces, each of which takes the form of a web cleat which includes a semi-cylindrical section. This section allows the beam-end to move towards or away from the column-face by deforming plastically. A simplified analytical model has been developed to simulate the mechanical behaviour of the proposed connection, and this model will eventually enable the incorporation of this new connection type into global frame analysis to be used in performance-based structural fire engineering design. The model has been tested against FEA simulations and against model-scale experiment results, indicating that it can predict the behaviour of the connection with satisfactory accuracy. Preliminary sub-frame analysis results indicate that the proposed connection behaves similarly to an idealized pinned connection under ambient-temperature conditions, but provide significantly larger ductility, and resistance to disproportionate collapse, compared to conventional connection types.

End Plate–Stiffener Connection for SHS Column and RHS Beam in Steel-Framed Building Modules

This paper investigates the mechanical performance of beam-to-column connections for steel-framed building modules with RHS and SHS sections of relatively small member sizes. In the experimental program, three forms of connections are tested under static bending, including a completely welded connection (W), a bolted end plate connection (EP) and the proposed end plate–stiffener connection (ST), and the failure modes, moment–rotation behaviours and strain responses were recorded during experiments. Results show that ST connection exhibited superior connection strength, stiffness and ductility compared with the EP connection. In addition, a detailed finite element analysis was conducted to model the connection behaviours, considering bolt pretension, contact behaviours and detailed bolt geometry, showing good comparison with experiments. Furthermore, a parametric study was performed on the end plate thickness and stiffener thickness for the proposed end plate–stiffener connection. Such results finally enable a comparative study to investigate the performance of module during road transportation, highlighting the effects of connection stiffness.

Rotational demand and capacity of conventional single-plate shear connections subjected to gravity loading

This study aims to investigate the adequacy of the ductility requirement presented in the 15th edition of AISC Steel Construction Manual for conventional single-plate (CSP) connections when the threads of the used bolts are excluded from the shear plane (i.e., using X-bolts). For this purpose, by changing different parameters, 72 specimens were designed in accordance with the AISC Manual. All specimens were analyzed based on finite element (FE) method using ABAQUS software. FE results indicated that the most effective parameters which affect the connection rotational demand are loading type and beam length-to-beam depth ratio while connection rotational capacity is mostly affected by the number of bolts, beam depth and bearing strength of beam web/shear tab. To make a more accurate judgment about connection ductility, first, the existing equation for estimating rotational demand, usually leading to a non-conservative prediction, was modified. Then, using the FE results, three predictive models for connection rotational capacity was proposed. Comparing these models showed that the one which was developed on the basis of the artificial neural network is of the highest level of accuracy. Finally, using Monte Carlo method, a reliability analysis was conducted which showed that the ductility requirement of AISC Manual can be too conservative in most cases when the number of bolts exceeds five.

Strength and stiffness variability study of viscoelastic adhesive butt joints

ABSTRACTViscoelastic structural adhesives show great promise in industrial and civil applications where a highly ductile connection between dissimilar components is required, at both low and high load frequencies. Suitable chemistries for these applications are silicones, polyurethanes and MS polymers. These adhesives show the required low modulus and high elongation at joint failure, while retaining satisfactory strength for long-term applications.However, the reliability of these adhesives has not been studied extensively. Partly because of the high initial variability on the mechanical performance of these joints. The aim of this research is to identify and correlate potential factors that have an effect on the performance of an MS polymer joint. Both experimental and numerical studies are used to attain these results.Both studies show that bond geometry has a marked effect on its mechanical performance. The experiments show that the assumption of a solid, uniform adhesive joint is invalid. However, if the size and location of defects in the bond line can be estimated, and an appropriate finite element model can be constructed, the joint performance can be estimated.

Numerical study on seismic force modification factors of hybrid light wood frame structures connected to a stiff core

Seismic force modification factors of structures are specified in building codes or design standards based on structural materials and types of lateral load resisting systems (LLRSs). For a hybrid building system consisting of multiple structural materials and/or more than one type of lateral load resisting systems, the National Building Code of Canada requires that the lowest value of seismic force modification factors of these LLRSs be assigned for the seismic design of the whole building. This requirement is conservative and in need of a review. In this project, the seismic force modification factors of hybrid building systems were investigated through numerical modeling analysis. These hybrid buildings consist of a mid-rise light wood frame sub-system connected to a reinforced masonry core sub-system by a ductile connection. Incremental dynamic analysis was conducted on these hybrid buildings using commercial software, ABAQUS, together with a subroutine that describes the hysteretic performance of the sub-systems and their inter-connections with the Bouc-Wen-Baber-Noori model. Results show that a value larger than the lower value of the two sub-systems can be assigned for seismic design of these hybrid buildings. In this study it was demonstrated that meaningful relationships exist between the seismic modification factors of a hybrid structure and the mechanical characteristics, including the relative strength and ductility, of the sub-systems and their inter-connections.

Novel shearhead reinforcement for slab-column connections subject to eccentric load and fire

Traditional means of reinforcing concrete flat slab-column connections against punching shear, such as increasing slab thickness, or provision of shear links, all have drawbacks. This paper proposes a novel type of punching shear reinforcement in the form of a shearhead to enhance connection strength and ductility. The structural behaviour of the connection is explored experimentally by testing nine specimens under various loading conditions including eccentric loads that produce combined axial and bending effects. Specimens with and without shearheads are compared. This is followed by a detailed numerical investigation using finite element analysis to obtain more in-depth insights into the connection behaviour when shearheads are present. The performance of the proposed system is also investigated under fire conditions for the first time. It is found that the proposed shearheads improve the performance of slab-column connections in all conditions and particularly under concentric loading and in fire conditions.

Efficient design of cold-formed steel bolted-moment connections for earthquake resistant frames

Cold-formed steel (CFS) sections can be designed in many configurations and, compared to hot-rolled steel elements, can lead to more efficient and economic design solutions. While CFS moment resisting frames can be used as an alternative to conventional CFS shear-wall systems to create more flexible space plans, their performance under strong earthquakes is questionable due to the inherited low local/distortional buckling of thin-walled CFS elements and limited ductility and energy dissipation capacity of typical CFS bolted-moment connections. To address the latter issue, this paper presents a comprehensive parametric study on the structural behaviour of CFS bolted beam-to-column connections with gusset plates under cyclic loading aiming to develop efficient design solutions for earthquake resistant frames. To simulate the hysteretic moment–rotation behaviour and failure modes of selected CFS connections, an experimentally validated finite element model using ABAQUS is developed, which accounts for both nonlinear material properties and geometrical imperfections. Connection behaviour is modelled using a connector element, simulating the mechanical characteristics of a bolt bearing against a steel plate. The model is used to investigate the effects of bolt arrangement, cross-sectional shape, gusset plate thickness and cross-sectional slenderness on the seismic performance of CFS connections under cyclic loading. The results indicate that, for the same amount of material, folded flange beam sections with diamond or circle bolt arrangements can provide up to 100% and 250% higher ductility and energy dissipation capacity, respectively, compared to conventional flat-flange sections with square bolt arrangement. Using gusset plates with the same or lower thickness as the CFS beam may result in a premature failure mode in the gusset plate, which can considerably reduce the moment capacity of the connection. The proposed numerical model and design configurations can underpin the further development and implementation of CFS bolted-moment connections in seismic regions.

The effect of beam section property on the behavior of modular prefabricated steel moment connection

The specially prefabricated steel moment connections with pyramid head is one of the significant innovations in the steel structures forms to improve the installation time and simplify the construction procedure. The beams in this structure form are supported by two top and bottom angles and web double angles. Such a configuration despite its advantages increases the welding operation and filed installation time and costs. In this paper, the effect of using beams with channel and I section in three classes of seismically compact, seismically non-compact, and slender section according to width-to-thickness ratio on the behavior of the connection was investigated under monotonic and cyclic loading. Modeling was performed by ABAQUS and verified by the results of an experimental specimen. The findings indicated that using I and channel section instead of angle section reduces the amount of welding materials as well as easing the installation procedure. However, it has no significant effect on the ultimate strength and ductility of the connection. Furthermore, if the beam section is seismically compact, this form is considered as a special moment frame that has a rotation capacity up to 0.04 radians without any reduction in connection moment resistance.

Fragility Assessment of Pre-Northridge Steel Moment Frames Using Finite-Length Plastic Hinge Elements and Concentrated Plasticity Fracture Elements

Although pre-Northridge earthquake steel moment resisting frame buildings have been shown to be susceptible to brittle connection failures, they still represent a large fraction of the existing steel buildings in the United States of America. In this study, the performance of the 3- and 9-story Los Angeles pre-Northridge SAC buildings are analyzed considering ductile and brittle beam-column connection failures, and their uncertainty. This paper contributes to understanding the influence of uncertainty associated with connections brittle fracture on building interstory deformation capacity and its impact on bias and variability of fragility functions and loss assessment. The results show that considering brittle connections leads to significantly larger drift demands and to higher repair costs, particularly under intense ground shaking. New fragility curve parameters are derived that account for the effect of the uncertainty of the strength and deformation capacity of brittle connections.

An experimental study on the effect of CFRP on behavior of reinforce concrete beam column connections

The aim of this research is reinforcing of concrete with variety of fiber reinforced polymer (FRP) configurations and investigates the load capacity and ductility of these connections using an experimental investigation. Six scaled-down RC exterior joints were tested under moderately monotonic loads. The results show that, the shape of the FRP had a different effect on the joint capacity and the connection ductility coefficient. The greatest effect on increasing the ductility factor was seen in the sample where two reinforcement plates were used on both sides of the beam web (RCS5 sample). For the sample with the presence of FRP plates at the top and bottom of the beam (RCS3 sample), the ductility factor was reduced even the load capacity of this sample increased. Except for the RCS3 sample, the rest of the samples exhibited an increase in the ductility factor due to the FRP reinforcement.

Analysis of different types of bolted joints for i-beams and their comparative laboratory research

The article considers the results of laboratory tests for different types of bolted joints for the I-beams. The tests were subjected to a steel I-beam cross section with splice joint by friction, flange and combination of the splice joint by friction and flange joints under a bending. A brief review and analysis of existing solutions for bolted joints for girders are given, the main difficulties with the use of welded joints and bolted joints are indicated. The results of laboratory tests of different types of bolted joints for I-beams, using high-strength bolt connections, are presented. The pictures of stress distribution in the elements of joints are investigated, and the deflections of beams using different types of joints are determined. It is shown that the deflections of beams with splice joints are significantly influenced by the ductility of the entire connection. According to the test results, the conclusions about the obtained pliability values are made and the graphs of joint deformation at various stages of loading are given. The obtained pliability values may be used in further virtual experiments.

Capacity design of traditional and innovative ductile connections for earthquake-resistant CLT structures

Traditional connections in earthquake-resistant cross-laminated timber buildings are susceptible of brittle failures, even when buildings are designed and supposed to be ductile. This is mainly due to the large underestimation of the actual strength of the ductile components, with consequent increased strength demand for the brittle parts, which may fail if designed with insufficient overstrength. Recent studies demonstrate that the use of steel connections characterized by a well-defined mechanical behaviour can improve significantly ductility and dissipative capacity of cross-laminated timber structures and the reliability of the capacity design. In this paper, the conceptual model of capacity design is discussed, proposing some modifications to improve its reliability for traditional and high-ductility connections for CLT structures. Results from quasi-static cyclic-loading tests of an innovative ductile bracket are presented and the corresponding overstrength factors are computed using the proposed conceptual method and compared with values available in the literature for traditional connections. Finally, a comparative application of the capacity criteria to the design of the innovative bracket and of a traditional nailed connection is presented and discussed.

On the required ductility in beams and connections to allow a redistribution of moments in steel frame structures

The structural behaviour of beam-column joints plays a critical role in the seismic response of frame structures. In the case of steel rack systems, uprights (columns) are usually 3rd or 4th class open thin-walled cold-formed steel members according to Eurocode 3, as such, local buckling occurs before the plastic moment resistance (in the case of the 3rd class) or yield stress (4th class) is achieved. Beams are usually 1st or 2nd class, so they can form a plastic hinge with the rotation capacity required from plastic analysis without reduction of the resistance (1st class) or with a limitation on the rotation capacity due to local buckling (2nd class). For this reason, only beams and connections, which can be easily equipped with additional bolts to increase their structural performance, can provide a post-elastic behaviour with energy dissipation. This paper presents and develops a method capable of evaluating the rotational capacity, or ductility, required by beams and connections to perform a plastic design. The proposed approach allows for the failure mode of the structure to be predicted starting from mechanical properties of structural members, and it is valid for both service and seismic loads. The analytical method could be applied to different structural systems and materials; in the present work it is presented with reference to industrial pallet racks. Theoretical assessment herein of the required plastic rotation for beams and connections provides useful information for bolted connection design to increase capacity of the racks with minimal additional cost.