RBS Connections Research Articles

Prevention of brittle failure for steel connections utilizing special devices

The present paper proposes the use of a special design procedure devoted to the prevention of brittle failure for welded steel connections. In particular, reference is made to steel frame structures made up of beam elements with I-shaped cross-sections where the connections between columns and beams are usually welded and/or bolted. The proposed new design procedure consists of four subsequent analysis steps; it is based on the identification and analytical definition of new appropriate brittle safe domains defined in the N,V,M space and on the use of suitably designed devices (LRPD), recently proposed by the authors, belonging to the class of the RBS connections. The latter are able to impose prefixed values of internal forces on selected beam element cross-sections, avoiding any modification of the elastic stiffness features of the involved beam. The main novelties of the present study consist in the introduction of new brittle safe domains for I-shaped cross-sections, in the specialization of the LRPD optimal design to the present context and in the definition of a special design strategy which allows to contextually obtain structures safe from the risk of brittle failure as well as being able to dissipate an appropriate amount of plastic strain energy. The numerical application, devoted to plane steel frames, confirms the sound reliability of the procedure and the great flexibility of the utilized LRPDs.

Experimental investigation using demountable steel-concrete composite reduced web section (RWS) connections under cyclic loads

This paper presents an experimental study of demountable steel-concrete composite reduced web section (RWS) connections for use in seismic areas. Four composite connection specimens were subjected to sagging and hogging moments to investigate the performance of RWS connections under reversible actions. For such purpose, a single web opening was created near the beam-column joint. Focus was made on the effects of perforation location and composite beam-slab action induced by bolted shear studs within the protected zone. Results indicate that employing RWS connections for seismic retrofit is a viable solution capable of achieving a performance similar to that observed for RBS connections in new buildings. Notably, a Vierendeel Mechanism is formed, allowing for plasticity development in the beam. All RWS connections were capable of achieving an interstory drift larger than 4%, thereby complying with the performance targets set by ANSI/AISC 358-16, ANSI/AISC 341-16 and EC8. Moreover, the deformation of beam-slab shear-transfer bolts was small enough to allow for disassembly after the tests, indicating that the proposed solution could enable reuse and post-earthquake retrofitting.

A new energy-dissipating RBS connection with double-nut-bolts: Seismic performance assessment and design methodology

This article investigates analytically the seismic performance of a new RBS beam-column connection system with Double Nut Bolts (RBS-DNB) and double shear tabs. The bolts in the proposed RBS-DNB connection are used to: i) increase the strength of the beam’s reduced section, ii) increase the energy dissipating capacity of the connection, and iii) delay damage of the beam in the plastic hinge zone. A reference RBS connection from the literature is first modeled and calibrated in Abaqus® software. A set of RBS-DNB connections are then designed (according to a new proposed method) and subsequently modeled in Abaqus®. The analytical results from the reference RBS connection and RBS-DNB connections are then compared in terms of cyclic response, beam strength, dissipated energy, beam hinge deformation and damage. The results show that, compared to the reference RBS connection, the proposed RBS-DNB connection system can increase the strength of the beam’s reduced section and dissipate more energy by up to 16 % and 12 %, respectively. Moreover, a damage assessment shows that RBS-DNB connections can efficiently delay the initiation of beam hinging, and reduce buckling deformation and damage to the beam hinge compared to the reference RBS connection. The new proposed design method ensured the safe load-carrying mechanism of RBS-DNB connections, and it also predicted well the potential damage at the components of the RBS-DNB connections. Finally, the results of pushover analyses on typical 4-storey buildings with RBS-DNB connections demonstrated their higher lateral load bearing capacity compared to those with conventional RBS connections. This study contributes towards developing more robust energy-dissipating connections for steel buildings located in seismic areas.

Numerical Investigation to Assess the Cyclic Performance of RBS Connection using Cover Plates

This paper presents the result of four welded connections under cyclic loading using finite element analysis, those are: Conventional Connection (C-Con), Cover Plates Connection (CP-Con), Reduced Beam Section Connection (RBS-Con), and RBS Connection using Cover Plates (RBSCP-Con) using the same material properties, including dimension of the profile and its span. Previous research indicates that, even though the fact that the initial seismic yield must be located far from the column’s face, it still occurs on the majority of RBS connections on occasion. By adding a cover plate to the top and bottom flanges of the beam, the research can provide a seismic design improvement for RBS connections. The analysis results observed that cyclic performance of RBSCP-Con was much superior to the other connections in terms of plastic hinges location, panel zone’s performance, and failure modes. However, CP-con has the maximum value of energy dissipation, which correlates with its hysteresis curves. Furthermore, all of the four types of connections already satisfy SMF category requirements in accordance with AISC 341-16.

Building retrofit with reduced web (RWS) and beam (RBS) section limited-ductility connections

Softening of beams near the moment connection can be regarded as one of the most convenient methods for retrofitting non-seismically designed buildings. This can be done by drilling the beam web near the connection (aka RWS–Reduced Web Section) or cutting the beam flanges (aka RBS–Reduced Beam Section). Thus, becoming a cost-effective option for retrofitting and erection of new buildings, while optimising use of steel; making the most of this high environmental impactful material. This study presents a comprehensive investigation of the effects of using extended end-plate bolted RWS and RBS connections in building behaviour when subjected to strong ground motion. For that purpose, incremental dynamic analyses (IDA) were performed in a low height building fitted with RWS and RBS connections capable of limited ductility. This was done with the aim of characterising the seismic behaviour of buildings with deficient earthquake design after being subjected to seismic upgrade. It is observed that empirical fragility curves show that probabilities of exceeding 1%, 2% and 4% inter-story drifts are significantly lower (more than 25%) for the building that is designed with RWS connections instead of RBS connections; while the probability of collapse is reduced by at least one third. This indicates that deployment or RWS connections can reduce the risk of both; severe structural damage and, large economical loss due to damage of non-structural elements when seismic demands are moderate.

Numerical Study on Seismic Behavior of Flexural Frames with Semi-Rigid Welded Steel Connections Considering Static and Reciprocating Loads: A Performance-Based Earthquake Approach

This paper aims to apply a performance-based earthquake engineering approach to assess the assurance of flexural frames whose members are jointed together by using new modified RBS connections, namely, semi-rigid welded steel connections, which obey a progressive failure mechanism. First, the structural members and connections are modeled and predesigned in ETABS software, and then, using OpenSees software, a series of nonlinear progressive failure analyses are performed on the built models extracted from ETABS. To this end, three types of multi-story structures with 3, 10, and 15 are modeled. The models are subjected to 15 earthquakes, such as Northridge (1994), Kobe (1995), Chichi (1999), Bam (2003), Tabas (1978), and so on. The connections are modeled by a series of rotational springs whose nonlinear behavior is estimated by a three-line curve that is established based on the modified Ibarra–Krawinkler deterioration model. Finally, obtaining the maximum ground acceleration versus the maximum relative drift of the floors, the fragility curves of the structures for a collapse level (CP) are determined, through which the seismic performances of the models are evaluated. The results show that by reducing the number of structural floors, the ductility of structures was reduced, and by increasing the ductility of structures, higher drifts in structures were achieved at the same seismic level. The average amount of ductility reduction coefficient in structures with RBS was 1.06 times those without RBS, which indicates that the energy dissipation capacity in structures without RBS connection is higher than in those with RBS. Local analysis of connections shows a 9% increase in the plastic rotation capacity if RBS connections are used. The ductility of all frames with RBS connection increased slightly compared to frames without RBS.

The Behavior of Welded Moment Connections in Box-Columns and Investigating Applied Demands for Different Cyclic Loading Protocols

The strength and deformation capacity of structural elements are influenced by their cumulative damages received through load cycles. Loading protocols would provide the estimation of the structural components’ capacities and a proper simulation of the seismic reality, which are used to assess the performance of connections. Given the widespread application of I-beam to box-column moment connections, the seismic performance of moment connections in the box-columns should be thoroughly investigated. However, most regulation details for the qualification of moment connections are presented for the H-shaped columns. Accordingly, in this study, 12 prequalified types of RBS and WUF-W moment connections in the box-columns are selected and analyzed under different cyclic loading scenarios. Therefore, the seismic behavior of such moment connections is studied for the drift qualifying criteria and collapse threshold when they are subjected to the SAC and ATC-24 loading protocols. Besides, energy dissipation, demand, loading protocols’ impact, and their influences are examined.

Pseudo-dynamic testing of a full-scale two-storey steel building with RBS connections

The paramount role of beam-to-column joints on the seismic response of MRFs is universally recognised and many studies have been devoted to the improvement of their ductility and energy dissipation capacity, mostly after the seismic events of Northridge and Kobe. Nevertheless, while much experimental work has been already carried out on the seismic behaviour of beam-to-column sub-assemblages, the available experimental results on full-scale structures are still quite limited.Aiming to provide a contribution to fill this knowledge gap, an experimental program has been planned at the STRENGTH Laboratory of the University of Salerno concerning the pseudo-dynamic testing of a full-scale two-storey steel building equipped with five different connection typologies.In this paper, the results of the first series of tests, dealing with MRFs with RBS connections, are presented. The mock-up has been subjected to a sequence of seven accelerograms and the results have shown that, if properly designed, RBS connections provide large energy dissipation capacity and plastic rotation supply. Nevertheless, the failure of welds due to damage accumulation can be still possible, even adopting full penetration butt joint welding details conforming to Eurocode 3 and Eurocode 8 provisions.The test results are complemented with a numerical model developed with the software SeismoStruct. The comparisons between the numerical simulations and the experimental results show that the applied mixed distributed and concentrated plasticity approach, based on the adoption of phenomenological spring models, can capture accurately the global response of the structure, but it predicts with lower accuracy the local response parameters.

The effect of RBS connection on energy absorption in tall buildings with braced tube frame system

The braced tube frame system, a combination of perimeter frame and bracing frame, is one of the systems used in tall buildings. Due to the implementation of this system in tall buildings and the high rigidity resulting from the use of general bracing, providing proper ductility while maintaining the strength of the structure when exposing to lateral forces is essential. Also, the high stress at the connection of the beam to the column may cause a sudden failure in the region before reaching the required ductility. The use of Reduced Beam Section connection (RBS connection) by focusing stress in a region away from beam to column connection is a suitable solution to the problem. Because of the fact that RBS connections are usually used in moment frames and not tested in tall buildings with braced tube frames, they should be investigated. Therefore, in this research, three tall buildings in height ranges of 20, 25 and 30 floors were modeled and designed by SAP2000 software, and then a frame in each building was modeled in PERFORM-3D software under two RBS-free system and RBS-based system. Nonlinear time history dynamic analysis is used for each frame under Manjil, Tabas and Northridge excitations. The results of the Comparison between RBS-free and RBS-based systems show that the RBS connections increased the absorbed energy level by reducing the stiffness and increasing the ductility in the beams and structural system. Also, by increasing the involvement of the beams in absorbing energy, the columns and braces absorb less energy.

Experimental and numerical evaluation of rigid connection with reduced depth section

After medium or strong earthquakes, damage in the reduced portion of RBS connections occurs due to plastic deformations. The purpose of this paper is to numerically and experimentally investigate the reduced depth section connection as a replaceable fuse. In this regard, three commonly used rigid connections with RBS, a replaceable fuse with RBS, and a replaceable fuse with Reduced Depth Section (RDS-F) were evaluated. All specimens were subjected to quasi-static cyclic load until failure. Although the final strength of the RDS-F is lower than that of the other two, laboratory results showed that it had the maximum ductility among the three samples. The numerical models of all three laboratory samples were constructed in ABAQUS, and the results were verified with great accuracy. The results of more than 28 numerical analyses showed that the RDS-F sample is more ductile than the other specimens. Moreover, the thickness of the web and the plastic section modulus increasing, the final strength would be equal to the other specimens. Therefore, the modified RDS-F with replaceability after an earthquake can be a better alternative for RBS connections.

Seismic behaviour of RWS moment connections to deep columns with European sections

The design of reduced web section (RWS) connections is becoming more often, while the research is focusing on both areas: (a) the performance of beams with large web opening areas near the supports, and (b) the seismic behaviours of steel beam-to-column connections of different types when subjected to cyclic loads. The beam web opening is used as a mechanism to counterbalance the low inelastic capacity of the welds and to shift the stresses from the welding and shear panel zones to the weak beam section in line with the concept of “weak beam-strong column” often employed for the seismic-resistant design of Moment Resisting Frames (MRFs). However, limited data yet exists on the design and seismic behaviour of such connections and those are only for UKB and UKC section profiles. This paper presents an extensive FE study of the seismic performance of fully welded RWS moment connections to deep columns. The effect of using different beam spans is also investigated together with the effect of web opening size and position (web opening at different shear-moment interactions). European HE and IPE profiles are employed for this study in an effort to introduce RWS connections in Eurocode 8, and understand potential variations in design when using European sections as previously reported in RBS connections. Finally, a step-by-step design procedure is presented based on AISC 358 (AISC 358[3]).

A comparison of seismic low cycle fatigue and extremely low cycle fatigue on steel moment frames with reduced beam section connection (RBS)

Damage to the connections of buildings with steel moment frames during past earthquakes indicates that the damage to some buildings cannot be justified by the mechanisms of common failure. This is due to the effects of low cycle fatigue (LCF) and extremely low cycle fatigue (ELCF) on the moment frame connections. On the other hand, studies on moment connections have mostly focused on the permitted connection capacity, and the issue of fatigue (LCF and ELCF) has not been considered in these types of connections. The present study focused on the effects of LCF and ELCF on a prequalified reduced beam section connection (RBS) in the steel moment frames of a 6-floor commercial complex and compared these two effects. The results showed that although the RBS connections of the studied structure were not damaged by LCF and ELCF, low cycle fatigue was effective when the behavior of the structure’s material was linear. The extremely low cycle fatigue index was significant when the connection’s material stresses, caused by an earthquake, exceeded the yield stress. In addition, the comparison of the damage ratios induced in the LCF method and the failure indices in the ELCF method on different floors indicated that the ratios of these two indices were consistent across all the floors.

Multi-resolution analysis of the SAC steel frames with RBS connections under fire

Vulnerability of steel-framed buildings in many recent fire events has motivated structural engineers to understand their performance under such hazard. Numerical finite element models have been relied upon heavily to evaluate fire behavior of steel structures. Previous studies utilized either isolated models of connections or line-element based models of frames to assess structural response under fire. This paper aims to combine both modeling approaches to evaluate the performance of three SAC steel moment-resisting frames, designed for LA region. These frames are of particular importance since they have been the focus of many previous research studies under earthquake excitations. All moment-resisting connections in the frames are redesigned to reflect post-Northridge design where reduced beams section connections are typically employed. A multi-resolution modeling technique is devised where detailed 3-D representation of the connection is seamlessly integrated with line elements to create full frame models. This technique allows for investigating the global response of the frames under various fire scenarios while accurately evaluating the local behavior of the RBS connections. The results indicate that the beams and columns do not satisfy the fire design limit states in accordance with Appendix 4 of the AISC 360-16 Specification. The RBS connection exhibits significant local buckling developed in the flange and web of the reduced section. The axial force and bending moment assessment at the location of the reduced section and beam-to-column interface reveal very large demand on the RBS connections that needs to be addressed during the structural fire design process. The results support better understanding of the response of steel moment-resisting frames with RBS connections under fire loads.

Steel beam-to-column RBS connections with European profiles: I. Static optimization

In this work we present an optimization procedure for achieving best beam-to-column reduced beam section connections in steel moment frames under static loading. The whole scheme is based on selecting European I-profiles for beams and columns, using the component method of Eurocode 3 for evaluating the design moment resistance of the joint, regression analysis to correlate the geometrical and inertial characteristics of the various components and elementary principles of Mechanics. In doing this, were are led to a constraint optimization problem, which is formed according to the design restrictions of European and North American Standards for RBS connections, regarding expected seismic behavior. Adopting advanced numerical procedures, optimum results are obtained, regarding either combinations of connection components or existing steel frames, a fact validating the flexibility and applicability of the proposed method. The calibration, verification and validation of the excellent seismic performance of the results obtained will be demonstrated in a companion paper, via FE analyses under cyclic loading. Hopefully, this work will contribute in adopting RBS connections in the design of steel moment frames in European engineering practice, as well as in the relevant codes.

Experimental behaviour of friction T‐stub beam‐to‐column joints under cyclic loads

AbstractEurocode 8 has introduced the possibility of adopting partial‐strength joints for seismic‐resistant MR frames, provided it is demonstrated that connections perform adequately under cyclic loads. A programme of experiments devoted to investigating the cyclic behaviour of traditional joint details has recently been carried out by the authors. Within this programme, the analysis of the results obtained has revealed that even though connections designed to dissipate the seismic energy in bolted components can provide significant advantages because they are easy to repair after a destructive seismic event, they possess reduced dissipation capacity when compared with RBS connections and traditional full‐strength joints. An advanced approach aimed at enhancing the hysteretic behaviour of double split tee (DST) joints and the ambitious goal of preventing joint damage is presented here. The system proposed is based on the idea of using friction dampers within the components of beam‐to‐column joints. A preliminary set of prototypes has been tested experimentally and the performances of joints under cyclic loading conditions have been compared with those of traditional joint details. The experimental work was carried out at the Materials

Seismic Behavior Assessment of RBS and AW-RBS Moment Resistant Connections with Double I-Beam

Today Steel moment resistance connections are the most common type of connections that are widely used in steel structures and certainly play a significant role in retrofitting, reliability and economic benefits. It is used also for improvement of the quality and useful life cycle of buildings. Considering the motions importance and after the Northridge earthquake, the invention of RBS connection was a turning point in resistant steel moment connections. This research observes the seismic behavior of the RBS connection with reduced section of flange and a new AW- RBS with reduced section of web in a double section beams that are applicable in Iran. These observations are performed using ABQUS finite elements software. Many models are implemented under cycle loads and ductility. Energy absorption and plastic behavior in the reduced section were observed. Among the studied connections, AW- RBS with the least resistance and stiffness decrement had bearded 8% off the rotation of the story. These models have acceptable behavior in cyclic loads.

Fragility analysis of steel moment frames with various seismic connections subjected to sudden loss of a column

In this study the progressive collapse potential of steel structures having WUFB, RBS, and WCPF connections was investigated considering the uncertainty in material properties such as yield strength, live load, and elastic modulus. The beam-end rotation was used as a member-level limit state for progressive collapse. Pushdown analyses of the model structure with three different connection types were carried out after removing one of the first-story columns. Fragility curves were obtained based on the probability of exceeding a given limit state for vertical displacement using the First-Order Second Moment (FOSM) method. The analysis results showed that the RBS connections showed the highest load resisting capacity against collapse due to their highly ductile behavior and that the loss of an exterior column turned out to be more vulnerable for progressive collapse than the loss of an interior column.

A plastic collapse method for evaluating rotation capacity of full-restrained steel moment connections

An analytical method to model failure of steel beam plastic hinges due to local buckling and low-cycle fatigue is proposed herein. This method is based on the plastic collapse mechanism approach and a yield-line plastic hinge (YLPH) model whose geometry is based on buckled shapes of beam plastic hinges observed in experiments. Two limit states, strength degradation failure induced by local buckling and low-cycle fatigue fracture, are considered. The proposed YLPH model was developed for FEMA-350 WUF-W, RBS and Free Flange connections and validated in comparisons to experimental data. This model can be used to estimate the seismic rotation capacity of fully restrained beam-column connections in special steel moment-resisting frames under both monotonic and cyclic loading conditions.

接合部パネルの塑性化を伴うH形鋼柱梁より成る2層鉄骨造骨組の地震応答性状に関する振動台実験

This paper presents the results of shaking table test on earthquake response behavior of 2-story space frames composed of H-shaped steel beam and column with shear yielding of joint panel zone. The main objective of this test is to investigate an influence of shear yielding of joint panel on maximum inter-story drift and energy absorption capacity of the frames. The results of the shaking table test are summarized as follow. 1) Maximum inter-story drift in each story of the specimens becomes smaller, as the thickness of doubler plates becomes greater. 2) Energy absorption capacity of the H-2 specimen yielding both beam and panel becomes largest in the test. 3) Fracture of the RBS connections in the H-4 specimen is not observed at the end of the test.