Reduced Beam Section Research Articles

An experimental and numerical investigation of reduced beam section connections with horizontal and vertical web stiffeners

Following the 1994 Northridge earthquake, reduced beam section (RBS) was presented as a novel connection detail in the steel moment-resisting frames. Proposed detail aims to debilitate the beam at a limited interval from the connection so that less moment and force are assigned to the connection. Cutting of the beam flange, especially in the narrow sections such as IPE, leads to an increase in the potential for the web local buckling, beam lateral-torsional buckling, and reduction in the beam strength. The hysteresis curve of IPE profiles, particularly in medium and large sections, is highly declining after the buckling. In this paper, a simple method was presented to amend the efficiency of IPE beams with an arched cutting in the flange, using horizontal and vertical stiffeners of the beam web. The experimental results and those of finite element in IPE beams revealed that the simultaneous application of horizontal and vertical stiffeners of the beam web largely prevented the declining of the beam hysteresis curve and left a full effect on its behavior. Therefore, the use of these types of connections in seismic areas with high relative risks is recommended.

Experimental and Numerical Investigation on the Moment Rotation Behavior of Cruciform Moment Connections with Reduced Beam Sections

The unexpected local damage may happen in the beam column connections of steel structures under lateral loads, since the connection is the focal point. Adopting reduced beam section (RBS) is an innovative method to dissipate the energy of such structures since it will enable a plastic hinge to be formed in the beam portion, thus reducing large stress concentration in the connection. Also, the formation of plastic hinge will satisfy the seismic criteria of strong column and weak beam combination. This paper focuses on analyzing the efficiency of cruciform connection fabricated with RBS. The structural components comprise of standard I section beam and column connected through welds. Finite element method is applied and model is created for cruciform connections in which different RBSs are used. The models are validated using experimental analysis applied on the specimen. The monotonic load is gradually applied at the two ends of the beams and moment rotation behavior is established. Similar behavior is studied for the RBS model and double reduced beam section (DRBS) as well. The effective connection stiffness is identified for the different models under consideration. It is observed from the results that RBS and DRBS can sustain large inelastic strains besides limiting stress concentration in the connection.

Comparing the seismic performances of shallow beam connections with accordion webs and reduced sections: An experimental study

As the depth of the beam increases, the probability of the occurrence of lateral-torsional buckling and damage increases as well. Researchers have been able to resolve this problem to some extent by changing the web of the beam into an accordion web. Extensive research has been conducted on deep beams with accordion webs. However, no detailed study has been carried out regarding the performance of the accordion web in shallow beams and its comparison to Reduced Beam Section (RBS) connections. Therefore, seven full-scale experimental samples were constructed and tested under the influence of cyclic quasi-static loading. The samples included conventional end-plate connection, beam with accordion web with angles of 11 and 22°, conventional RBS connection, and RBS connection with accordion web. The results showed that all of the samples satisfy the criteria for special moment resisting frame’s ductility. The RBS sample and the samples with accordion webs were the ones with respectively the highest and lowest ductilities. It therefore seems that using the accordion web in shallow beams, is not properly justified.

Effect of Common U.S. Ground Motion Selection Methods on the Structural Response of Steel Moment Frame Buildings

This study quantifies the impact of two common ground motion (GM) selection methods, included in U.S. standards, on the seismic performance evaluation of steel special moment frames. The methods investigated are a “traditional” approach, herein referred to as the target maximum considered earthquake (TMCE) method, and a newer approach known as the conditional mean spectrum (CMS) method. The TMCE method selects GMs using the risk-based maximum considered earthquake (MCER) spectrum as the target spectrum, while the CMS method uses the CMS that anchors the MCER at multiple conditioning periods. Three special steel moment frames of 4, 8, and 16 stories are designed in accordance with ASCE/SEI 7-10, and their seismic performance is assessed with the nonlinear dynamic procedure prescribed in ASCE/SEI 41-13 using GMs selected and scaled in accordance with the aforementioned methods. A comparison of statistical parameters for the reduced beam sections and column hinges is conducted using the normalized demand-to-capacity ratio ( DCR), as the output parameter. The buildings are evaluated at the collapse prevention performance level for a far-field site located in Los Angeles, CA. In general, the CMS method results in lower DCRs of the frame components and smaller output parameter dispersion. In addition to the spectral shape, the demands are largely influenced by the spectral accelerations prescribed for each evaluated method. The consideration of collapse realizations is also documented as well as the existing and proposed statistical methods to account for these realizations. The study shows that the GM selection process can cause significant differences in structural response that may lead to different retrofitting decisions.

An improved and innovative formulation for calculating amplified elastic story drift induced by RBS connections in steel moment frames

In recent years, the prequalified rigid connections of radius-cut reduced beam section (RBS) have been widely used in the design and construction of steel moment frames. Despite extensive investigations performed on the seismic performance of RBS connections, there is little research into the impact of these connections upon the elastic story drift in steel moment frames. In the current study, a precise formulation (partially based on a previous research) has been developed so as to evaluate the amount of stiffness variation or amplified elastic drift introduced by radius-cut RBS connections in a story of steel moment frames. Herein, by defining a novel and accurate method along with the method of virtual work, the amplified elastic drift of a 2-dimensional one-story single-span moment frame due to utilizing radius-cut RBS connections is calculated. The reliability and accuracy of the presented approach are validated using finite element modeling of the moment frame. Sensitivity analyses are also conducted upon the RBS connection parameters to derive the most effective parameters on the amount of elastic drift in steel moment frames. Then, by employing a statistical approach called response surface method (RSM) and combining findings of this study with some premises considered in a previous research, rigorous amplification factor of elastic drift formulas based on utilizing each of IPE and HEA beams in a story (from the first to the last one) with RBS connections are developed. Ultimately, two simple, linear, and functional amplification factor of elastic story drift formulas for each set of IPE and HEA story beams with RBS connections are proposed for designers. The scope of these formulas will be expanded to plate girders as well.

Parametric Analysis on the Circular CFST Column and RBS Steel Beam Joints.

This research analyzes the results of parametric studies of concrete-filled steel tubular (CFST) columns to the reduced beam section (RBS) beam joint with through diaphragm, using ANSYS. Several indices that are able to characterize the cyclic behavior of the composite joints are investigated, including the stiffness degradation, strength deterioration, stress distribution, and energy dissipation capacity. Four main model parameters, including the distance from the diaphragm edge to the cut start, the cut length, the cut depth, and inner diameter of through diaphragm, are analyzed via comparative studies to investigate their impacts on seismic properties of the joint. Finally, the orthogonal experiment is conducted to study the effects of these parameters on the strength and energy dissipation, the results of which are capable of achieving optimal seismic behavior of the joints.

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.

Cyclic Seismic Performance of Weak-Axis RBS Welded Steel Moment Connections

In this study, cyclic seismic performance of weak-axis reduced beam section (RBS) welded steel moment connections was experimentally investigated. Experimental results of this study again showed that the weak-axis RBS moment connection designed based on the procedure proposed by Gilton and Uang (J Struct Eng 122:1292–1299, 2002) performs well and developed an excellent seismic performance much better than 4% story drift with 80% of beam plastic capacity. However, it was also shown that the sharp corner of C-shaped fillet weld, used for attaching the beam web to the shear plate, can be the origin of crack propagation owing to stress concentration there. Slight trimming of the sharp corner is recommended to reduce notch effect. One specimen in this study eventually failed due to low cycle fatigue fracture as a result of cyclic flange local buckling that occurred at the butt joint between the beam flange and the horizontal continuity plate. The size of the weld access hole beneath the butt joint should be minimized if possible to suppress the occurrence of flange local buckling around the butt joint. The factors affecting the size of C-shaped fillet weld and the beam shear eccentricity for the weld design were also examined through a case study and simple analytical modeling. The sectional factor governing the beam shear eccentricity was shown to be the plastic section modulus ratio of beam section at RBS center to the beam flanges at welded joint. The elastic method neglecting the ductile behavior of the C-shaped fillet weld group was shown to be too conservative and should not be used in design in order to minimize welding heat affection to the thin beam web as well.

Evaluation of reduced beam section (RBS) on castellated beam subjected to a seismic load

The ability of the beam to receive a seismic load can be explained in the experimental study of a castellated beam using RBS (reduced beam section) as a damping factor on earthquake resistant building structure. Evaluation of the structure of the exterior connections was done by subjecting cyclic loads gradually until collapsed to evaluate castellated beam test specimens with and without RBS. The experimental test conducted by cyclic test aimed to determine the ability of deformation and the force that worked due to the influence of RBS placement from the column face on the castellated beam. Investment of RBS positions on the columns in the column face shows that failures occurred due to the bending on the shaft. The results obtained by each load to the drift ratio were as follows. On the test specimens with RBS, the melting point was at 8 kN loading with a 1.05% drift ratio, 2.1% drift load at a 9 kN load, 3.20% drift at a peak load of 12 kN, and necking phase at 10.53 kN load with 4.17% drift.

Seismic Fragility Assessment of SMRFs with Drilled Flange Connections using Ground Motion Variability

Seismic behavior of Steel Moment Resisting Frames (SMRFs) with Drilled Flange (DF) connections as well as Reduced Beam Section (RBS) and Welded Unreinforced Flange-Bolted web (WUF-B) as a Pre-Northridge connection have been compared analytically considering Far-field earthquake-induced. The backbone curves of RBS and WUF connections are extracted from available studies and the backbone curve of DF connection is presented in this study in order to simulating the buildings models. DF, RBS, WUF connections and Panel Zone (PZ) are numerically modeled based on the proposed models provided by the prior researches and these models are applied to analyze low- and high-rise buildings designed in accordance with the relevant standards. Incremental Dynamic Analysis (IDA) process is utilized to evaluate the effects of DF connection on structural seismic response of SMRFs. Afterwards, the structures’ performance in different response levels is probabilistically assessed by means of IDA and fragility curves. The results show that the seismic demand of SMRFs with DF and RBS connections are so similar, specifically for low-rise buildings. Likewise, SMRFs with DF connection provide up to 43% higher seismic demand in high-rise buildings compared to RBS connection. Eventually, DF connection can be used as an authentic option in SMRFs.

Preliminary Finite Element Analyses on the Experimental Mock-Up Frames of FREEDAM Research Project

Recent research ventures focus on structural systems characterized either by low-damage or easily repairable solutions. In this context, moment resisting frames (MRFs) equipped with friction joints represent a viable option. Among the different types of friction beam-to-column connections, the arrangement with symmetric friction devices are very effective to provide stable and ductile friction hysteresis, thus reducing the damage in the structural members (beams and columns) to a minimum or even to zero. Numerical and experimental investigations on the beam-to-column assemblies within the RFCS FREEDAM project testify to the good seismic performance of the friction device when properly designed. However, within this research project full scale pseudo-dynamic tests will be performed for a better understanding of the influence of such joint on the overall structural behaviour of MRFs. In this paper the results of preliminary numerical simulations are presented to give the first insights on the response of the experimental mockup as well as to validate the accuracy of two software with largely different capabilities. Static pushover and nonlinear dynamic analyses were performed and the results for the benchmark structure (frame equipped with reduced beam section joint) and for the structure equipped with the friction devices are presented

Fatigue damage analysis of steel components subjected to earthquake loadings

PurposeExamination of steel moment resisting frames after the 1994 Northridge earthquake showed fatigue cracks presented in the beam–column connections of the frames. These observations indicate that fatigue failure may occur in the steel components of building structures in an earthquake event. To apply the fatigue design approach using the Palmgren–Miner’s rule for steel components of the moment resisting frames requires the knowledge regarding the damage index value at fatigue failure. The purpose of this paper is to perform fatigue tests to give the first damage values of steel components subjected to real earthquake-induced loadings.Design/methodology/approachThe added-damping-and-stiffness steel plates which are used in building structures for earthquake mitigation were fabricated and tested by constant amplitude, SAC block and earthquake-induced loadings to failure. The earthquake loadings were obtained from the dynamic analysis of a steel frame with the mentioned plates. The load cycles of the SAC block and the calculated earthquake loadings were counted using the rainflow-counting method, and the damage index value of each specimen were calculated using the Palmgren–Miner’s rule.FindingsReverse stiffness obtained from cyclic load-displacement loops is a robust and consistent parameter that can be used for determining fatigue failure of tested components. The Palmgren–Miner’s damage values at failure, caused by earthquake loadings, are smaller than 1, and in addition, are also smaller than those obtained from the tests of the SAC block loading. The large-amplitude cycles in the earthquake loading produce large damage on the specimens, and intermediate range cycles also produce damage that should not be neglected in the fatigue analysis.Originality/valueToday’s building design code allows large plastic deformation to occur in steel frames during an earthquake. However, the pre-Northridge earthquake steel frames showed fatigue cracks without the expected substantial plastic deformation at beam flanges. Proposed solutions to this problem were the reduced beam section neglecting the existence of the cracks at beam–column connections. This study considered the fatigue phenomenon in steel frames and provided the first set of tested fatigue damage values for steel components subjected to realistic earthquake loadings, which offered a possible method of dealing with fatigue cracks in the steel components of a building structure.

Design of Optimal Slit Steel Damper Under Cyclic Loading for Special Moment Frame by Cuckoo Search

After the Northridge and Kobe earthquakes, several destruction of the structural beams were occurred. The difficulty and cost-effectiveness of beam replacement after earthquakes is a major problem on steel structures. For this purpose, the idea of using replaceable connection is suggested. A slit steel damper (SSD) is introduced that leads to the further energy damping and the ability to move the plastic joint outside of structural elements. The stiffness and damping characteristics of SSD is related to the thickness, height and number of sheets of the damper. The usage of SSD is more progressive and at the same time, the optimal design of SSDs should be proposed to meet the economical criterion and the reduction of the stress and deflection. In this study, an optimal SSD connection is proposed to enhance the performance of the classical SSD connection to meet the criterion of AISC to utilize in special moment frames. The results demonstrated that increasing the thickness and reduction of the steel sheets in the damper had a greater effect on the performance of the SSD connection, in comparison with the increase in the number of sheets. Therefore, the Cuckoo Search (CS) was utilized to optimize the several SSD parameters. Furthermore, a comparison between the CS-SSD and Reduced Beam Section (RBS) which is known as a common moment steel fixed-connection was accomplished. Results indicate that CS-SSD connection can reach the same function as the RBS connections under cycling loadings, in addition to the easy replacement capability after seismic excitations. The performance of the proposed CS optimization algorithm in designing of the optimal SSD was compared with the traditional Genetic Algorithm and the particle swarm optimization (PSO) with considering several designing constraints. High capabilities of the proposed CS optimization algorithm in terms of weight, energy absorption, stiffness and bearing capacity of SSD connections are simultaneously clarified by the results.

Steel Beam-to-Column RBS Connections: FEM Analysis under Cyclic Loading

In a companion paper [1], an optimization scheme for extended-end-plate Reduced Beam Section (RBS) connections of steel-moment-frames was presented, based on the component method of Eurocode 3, on regression analysis and on principles of Mechanics under monotone loading. European beam and column profiles were utilized, in conjunction with geometric restrictions and constraints of North American and European Standards for prequalified radius-cut RBS. Τhe aforementioned method aimed for an excellent seismic performance, the verification and validation of which is the content of the present study. Using FEM modeling and accounting for the assumptions used in the optimum design, after calibration with existing experimental data, the optimum connections were numerically analyzed under cyclic loading, adopting a well-accepted displacement-based protocol. All optimum solutions exhibited an excellent cyclic response, and met very satisfactorily all the performance criteria for seismic design. Results in terms of hysteretic M-φ curves at three characteristic areas of the connections validate the whole analysis, a fact aiming to assist in incorporation the radius-cut RBS concept in European Steel Design Codes and engineering practice.

Hysteresis modeling for cyclic behavior of concrete-steel composite joints using modified CSO

Concrete filled steel tubular (CFST) column joints with composite beams have been widely used as lateral loading resisting elements in civil infrastructure. To better utilize these innovative joints for the application of structural seismic design and analysis, it is of great importance to investigate the dynamic behavior of the joint under cyclic loading. With this aim in mind, a novel phenomenal model has been put forward in this paper, in which a Bouc-Wen hysteresis component is employed to portray the strength and stiffness deterioration phenomenon caused by increment of loading cycle. Then, a modified chicken swarm optimization algorithm was used to estimate the optimal model parameters via solving a global minimum optimization problem. Finally, the experimental data tested from five specimens subjected to cyclic loadings were used to validate the performance of the proposed model. The results effectively demonstrate that the proposed model is an easy and more realistic tool that can be used for the pre-design of CFST column joints with reduced beam section (RBS) composite beams.

Analytical study of reduced beam sections under monotonic load

This paper presents the result of three different type of reduced beam sections (RBS): radius cut, straight cut, and tapered cut under monotonic load using finite element analysis. Load is applied monotonically and divided into 9 steps, start from 500 N to 4500 N, which increasing 500 N for each step, continuously. The dimension for the beam is WF 500x200x10x16 is used for those three types of RBS. The results show that radius cut gives better performance than the other two types. Stress distribution, load-stress curve, and load-displacement curve of those three types of RBS are compared as the result, then lead to the conclusion that the most effective connection amongst those three types of RBS above is RBS with Radius Cut.

Sloped RBS Moment Connections at Roof Floor Subjected to Cyclic Loading: Analytical Investigation

Reduced beam section (RBS) steel moment connections are the prequalified connections in current design specifications and widely used in the US. Although RBS connection details are for general applications, they are not applicable for sloped beams at roof floors because the prequalified connections were developed based on the orthogonal beam–column conditions in a typical floor. A series of finite element analysis were conducted to investigate the effect of beam slope at roof floor on RBS connections. The results demonstrated that the connections at roof floor were vulnerable to fracture at the heel location when the beam was sloped down from the connections. But the stresses at the heel location were relatively small when the beam was sloping up. The downward beams developed plastic hinges accompanied by buckling, but the upward beams experienced only minor damage. Analysis results also indicated that the beam slope should not be more than 10° to adopt the prequalified connection details. The deformation at the heel location could be limited by introducing a rib plate at the connection and accordingly, the force demand at the connection could be effectively reduced.

EFFECT OF REDUCED BEAM SECTION IN STAGGERED TRUSS FRAME STRUCTURE VIERENDEEL ELEMENT WITH EARTHQUAKE LOAD ANALYSIS

The purpose of this study is to describe the effect of Reduced Beam Section in vierendeel panel element in STF system’s inelastic behavior based on earthquake load analysis. STF system is applied to 4, 6, and 10 storey building that served as office building. The structure’s system used Special Moment Resisting Frame (SMRF) system in longitudinal direction (W-E) and STF system in transversal direction (N-S). The analysis method that used in this study is nonlinear pushover analysis. Application of RBS in vierendeel panel element showed that by the increasing of storey number, the ductility of the structure was increased in both directions, while N-S direction has bigger ductility than W-E direction. Dissipation energy of the structure was also increased, especially in N-S direction. The first yielding process occurred in vierendeel panel element, and then followed by truss and diagonal chord around vierendeel panel element, while the critical condition showed still at life safety level.

Effect of panel zone strength ratio on reduced beam section steel moment frame connections

The effect of different panel zone design ratios (Rv/Vpz) on reduced-beam-section connections as part of steel moment resisting frames under cyclic and monotonic loading is investigated. As design specifications suggest an overestimated value for the (Rv/Vpz) ratio in some cases. To achieve this objective, the balanced design proposed in the specifications is compared to other design concepts in the guidelines. Then a finite element (FE) analysis is conducted representing the beam-to-column connection assemblies under cyclic and monotonic loading to investigate the effect of different panel zone strength ratios. The FE simulation is validated and compared to test results from the literature, then a parametric study is carried out. The parameters included in the study are; the column flange thickness, the panel zone aspect ratio, and the strong column weak beam ratio. Each connection configuration is investigated according to; the amount of energy dissipation achieved, the participation in energy dissipation between the beams and the panel zone, the stable hysteretic behavior and its fracture potential. Results from the analytical study show that for assemblies with thick column flanges the design specifications overestimate the panel zone strength. Therefore, a range for the validity of the panel zone design strength formula is suggested.

Experimental evaluation of rigid connection with reduced section and replaceable fuse

The reduced Beam Section (RBS) connection was proposed after the 1994 Northridge earthquake. By creating a plastic hinge outside the connection area, this connection minimizes the damage inflicted upon the panel zone. But, after the occurrence of a medium-intensity earthquake, the entire beam would have to be replaced due to the concentration of damage in the reduced area, which is practically impossible. This study aims to experimentally investigate the use of the reduced section in a replaceable fuse. Thus, four rigid connections including an Ordinary end-plate Bolted Connection (OBC), an ordinary RBS connection (RBS), a fuse including an RBS connection (RBSF), and a Reduced Depth Section (RDS-F) connection incorporating a fuse were evaluated. The static cyclic load was applied and the moment-drift hysteresis diagram was plotted. The properties of the column and the panel zone were selected so that damage would be sustained by the beam and the performance of the four connections could be compared. Even though all of the samples satisfied the drift criteria of the rigid connection in the special moment resisting frame, the amount of rotation in the panel zone of the OBC sample was 2 to 3 times greater than the other three samples. Therefore, using different types of RBS connections reduces the damage inflicted upon the column and the panel zone. The results show that in addition to having very good ductilities and the ability to be replaced after earthquakes, the RBS-F and RDS-F samples can be suitable replacements for the ordinary RBS connections.