Moment Frame Connections Research Articles

Fatigue Reduction in Tubular Web Connections in Semi-deep Beams

In this paper, the experimental and numerical results of a new type of Reduced Beam Section (RBS) connection called (abbreviated of Holed Tubular Web), are investigated. The HTW-RBS connection is a kind of accordion-web RBS connection by which contribution of the web to the flexural capacity of the beam would be reduced. In HTW-RBS connection, a steel tube is used instead of part of the beam web to place the plastic hinge in the expected location and also holes are created on the beam web. Similar to the corrugated webs, HTW-RBS connection has also adequate shear capacity but has little contribution to the flexural stiffness and capacity. Specimens of semi-deep beams with HTW-RBS connections are prepared and tested under cyclic loading. Results of proposed connection (HTW-RBS), indicated that the fatigue caused by stress concentration at the connection of the tube and beam is decreased and no fracture and cracking occurred at this area. Holes as second fuse actuated After the tube (primary fuse) and the length of plastic hinge is increased. Specimens provide at least 7% story drift, without any significant strength loss, which is more than current requirements for qualifying connections in special moment frames. Finally, the HTW-RBS specimen is simulated by finite element software and the results are compatible with the experimental results.

Effect of weld specifications on seismic performance of built-up box columns

Application of built-up box sections as columns of steel moment frames is very favorable due to their bi-directional flexural resistance. According to seismic provisions, for built up box columns, within a zone adjacent to the beam flange, flange and web plates of box columns shall be joined by complete joint penetration (CJP) groove welds. In spite of the fact that fillet welds are much easier to execute and are economically more efficient, they are prohibited within the panel zone of columns. In this paper the effect of weld specifications on seismic performance of panel zone in built-up box columns with corner welded joints is experimentally investigated, mainly focusing on the comparison of performance of fillet and groove welds used for fabrication of this type of columns. Based on the results of the experimental program on 1:2 scale specimens, representing beam to column connections of steel special and intermediate moment frames built using fillet and groove welds in panel zones, it is concluded that the performance of fillet welds could be as good as CJP groove welds in panel zones of built-up box columns for the qualifying drift angle of steel moment frames.

Assessment of SMC Frames under Different Column Removal Scenarios

Throughout the past decades, failure of structures threatening the lives of humans had been popular whether through structure failure due to human error such as Hyatt Regency walkway collapse, 1981, terrorist attacks on the American embassy attack in Nairobi, Kenya 1998 and the famous 9/11 attacks in 2001 and many more. As a result of these incidents, The Unified Facilities Criteria (UFC) was developed concerning the progressive collapse issues by analyzing different types of structures under column loss and studying the overall structural behavior. However, the (UFC) didn’t scope on the local behavior of the structural components and its connection under column loss. In this research, the main objective is to study the local behavior of the special moment frame connection (SMC) under column loss. A detailed study is conducted on a 3D model fully designed by adopting the strong-column weak-beam approach following the ACI318-14 regulations. Two frames are selected from the designed structure, interior and exterior frames, to apply the column loss scenario in different locations and different floor levels. The Applied Element Method is adopted in the study. Non-linear time-dependent dynamic analysis is implemented to apply the different column removal scenarios. Twelve case studies are modeled in detail using the Extreme Loading for Structures (ELS) software at which all elements are modeled and analyzed in a 3D model technique. After analyzing the different case studies, structure behavior is observed. Some cases encountered total collapse, other cases encountered partial /local collapse and finally, some survived the column loss scenario. Many parameters are involved and studied in the research. Failure pattern is observed for collapsed cases, the cause of failure is monitored and studied. Special moment connection behavior is studied concerning the shear connection capacity. The location of the column removal with the type of frame selected played an important role in changing the structural behavior from one case to another. As a result, it is not applicable to assume that due to the special moment connection ductility, the structure will be able to resist the column loss in all cases.

Effects of design factors on the cyclic response of sloped RBS moment connections

Abstract Non-orthogonal structural framing is essential in modern architectural design; and sloping beams are often required in structures, particularly at the roof level. In this paper, a sensitivity study is conducted to statistically evaluate the effect of beam slope angle along with different design factors on the cyclic response of Reduced Beam Section (RBS) moment connections. To this end, 3D finite element models are developed and verified based on two sets of experimental studies. The verified models are then used to carry out a sensitivity analysis employing the design of experiment (DOE) method. Thirteen design factors, related to the connection material or geometry, are considered. The analyzed RBS connection models, which are generated based on DOE, conform to the AISC seismic design requirements for RBS connections in special moment frames. The cyclic response of RBS connection models is assessed by evaluating different response variables, including initial stiffness, rupture index, plasticity index, moment capacity, yield moment, hysteretic energy dissipation, and strength degradation rate. The results show the relative significance of each factor and interaction effects on the cyclic response of RBS connections. Based on the results, slope angle and beam depth are the most significant factors influencing all the response variables. The beam slope angle and beam depth both have increasing effects on the cyclic response characteristics. With up to 80% contribution, the effect of beam slope angle is found to be significant and greater than the beam depth effect. RBS connections with smaller beam slope angle and beam depth experience lower strain demands and less potential for fracture.

A two-step method for damage identification in moment frame connections using support vector machine and differential evolution algorithm

The main aim of this study is to introduce a two-step method for damage identification in moment frame connections using a support vector machine (SVM) and differential evolution algorithm (DEA). In the first step, the potential location of damage in connections is determined through SVM leading to reducing the dimension of the search space. Then, the accurate location and precise amount of damage in connections are determined in the second step via DEA with a high speed. In order to simulate damage in connections, a moment frame is modeled through semi-rigid beam to column connections and the analytical model is used to randomly generate structures with damaged connections as data. Then, SVM is trained and tested using this data, to facilitate natural frequencies are considered as input data and the characteristics of damage in beam to column connections are considered as output data of the network. Now, the possible location of the damage in connections can be determined using the SVM trained. The accurate location and severity of damage are determined by DEA based on the prediction of SVM in the first step. In order to assess the efficiency of the proposed method, two numerical examples are considered with different damage cases and considering noise. A comparative study is also made to judge the performance of the method with that of a work available in the literature. The outcome shows the high efficiency of the proposed method to identify the location and severity of the damage in moment frame connections.

Cyclic Behavior of Replaceable Shear Fuse Connections for Steel Moment Frames

Most steel moment frames rely on beam yielding to safely handle the inelastic drifts that will be imposed by severe earthquakes. More resilient steel moment frame concepts such as post-tensioned self-centering frames and friction connections have not seen widespread use, perhaps because they deviate dramatically from current practice. Experimental work was performed to investigate the performance of a new moment frame connection called the replaceable shear fuse connection. The replaceable shear fuse connection uses beam flange plates with special cutouts to prevent beam damage. Seven reduced-scale specimens were tested to investigate the effect of fuse geometry on drift capacity, connection strength, and connection stiffness. The specimens achieved maximum drifts of 0.06–0.10 rad without experiencing strength degradation prior to fuse tearing. The shear fuses experienced strain hardening of 1.39–1.75 times yield, similar to high strain hardening that has been observed in shear link testing. The connections had more-than-adequate stiffness to be classified as fully rigid. Full-scale testing should be conducted to evaluate the effects of beam depth, column depth, and fuse parameters.

A Repairable Connection for Earthquake‐Resisting Moment Frames

ABSTRACTMost steel moment frames are designed so that the beams act as structural fuses during earthquake loading. This approach, while effective for life safety, has poor resilience because it is difficult to replace beams in a building. A repairable steel moment frame connection has been developed that may be attractive to engineers and constructors. The connection has a fuse plate bolted to the beam bottom flange which experiences shear yielding during severe earthquakes, protecting both the column and the beam. After an earthquake, the fuse plates can be removed and replaced to unlock residual frame deformations and prepare the building for another event.Full‐scale experiments were conducted to validate the performance of this repairable connection. Two tests were performed with a W690×125 beam and a W530×196 column. The tests demonstrated rotation capacities of at least 0.05 rad, which exceeded AISC requirements for special moment frames. The fuse plate was effective in preventing beam damage in each test, and the second test demonstrated that the connection could be repaired by replacing the fuse plate.

An Experimental and Numerical Investigation on Seismic Retrofit of Steel Moment Frame Connections

ABSTRACT A series of five large-scale steel moment frame connections containing one reference and four retrofitted connections were tested under cyclic loads. The retrofit techniques focused on the beam bottom flange in which a “ductile fuse” was defined through “drilling” and “heat induction”. It was concluded that “drilling” of the beam would increase the connection sensitivity to out of plane buckling. A considerable advantage of “heat induction” technique was low stress demands in near weld region. Meanwhile, as the beam was heated with no material removal, the out of plane buckling resistance was similar to that of the reference specimen.

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 Study on Behavior of Steel Beam to Reinforced Concrete Column Connections in Moment Frames

The steel beam to concrete column connections in moment frames has been studied during recent years. This system has more advantages compared with steel and concrete systems. There are two kinds of joints, including “through-beam-type” and “through-column-type”. One of the most important parts of the composite structures is their connections. Recently, the various experiments have been performed on the steel beam-concrete column composite structures. In this research, one kind of the joints was modeled in the ABAQUS finite element program and its seismic performance was verified by the experimental results. A parametric study was conducted on the eccentric load, axial load in the center of column, the compressive strength of the concrete, the resistance of the steel and the steel cover plate height effects. The obtained results showed the effectiveness of these parameters on the behavior of connections.

Mechanical and seismic behaviors of bottom-flange-bolted upper-flange-welded through-diaphragm connections

The through-diaphragm connection to a square steel tubular column is a regular connection type that is used widely in Asian countries. However, connections with traditional welded flange designs often fail through bottom flange to diaphragm weld fractures owing to the unstable quality of on-site intermittent welding. Therefore, a new type of bottom-flange-bolted upper-flange-welded through-diaphragm connection is presented to avoid premature fracture failure at the bottom flange. The construction process of the new connection is more convenient than the traditional construction. In this study, three specimens were designed and tested to investigate the mechanical properties and seismic behaviors of this new connection, and different web connecting methods were considered. The failure modes, hysteretic curves, backbone curves, and energy dissipation abilities were evaluated. Together with finite element simulations, the deforming pattern and load bearing performances were discussed in comparison with the traditional welded flange. The results indicate that the bolt and plate slip would present a slight reduction in the energy dissipation ability of the new connection. However, the strength development is similar to that of the traditional welded flange connection, and the ductility satisfied the seismic provisions in moment frame connections. A two-row regular-bolted web connection can ensure a sound load transfer at the beam web.

Balanced Damage Concept for Beam-to-Column Connections of Special Moment Frames Using HPFRC

Balanced Damage Concept for Beam-to-Column Connections of Special Moment Frames Using HPFRC

Performance of Low-yield Strength Plates in Beam-column Connections against Progressive Collapse

In many studies about progressive collapse in steel moment frames, the effect of connections in all of structural frames has not been investigated. This is while the connections can have a significant impact on structure behavior against abnormal loads. This study highlights the effect of mechanical and geometric properties of beam-column connections in steel moment frames against progressive collapse. For this purpose, variable parameters include beam-column connection type (Welded Flange Plate connection or WFP and Welded Unreinforced Flange-Welded web connection or WUF-W), mechanical properties of beam-column connection (St37 building steel and low-yield strength plates) and the column removal location in different stories (without removing column and removing column on the ground, first and second floors). Three-storey steel frames which are designed to withstand earthquakes are examined. Finite element ABAQUS software is used for simulation. Moreover, the alternative load path method in which the structure response is examined against the column removal is used to evaluate steel frames response against progressive collapse. The results show that the use of low-yield strength steel plates depending on connection type can be efficient in improving the behavior of steel moment frames against progressive collapse.

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.

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.

Column axial load effects on the performance of skewed SMF RBS connections

The selection of an appropriate seismic steel system (braced frame, moment frame, shear wall, etc.) is often influenced by architectural considerations. Moment frame configurations offer the most architectural flexibility, but are somewhat limited by a lack of guidance in the AISC seismic provisions regarding the use of non-orthogonal (skewed) beam-column connections. A recent study investigating laterally skewed moment frame connections indicates adequate seismic performance; however, it was observed that skewed connections increase the potential for column twist and column flange yielding. It is unclear how larger column axial loads present in medium-to-high-rise structures will affect the performance of skewed special moment frame connections.This study investigates the effects of column axial loads on skewed special moment frame connections containing reduced beam sections (RBSs). Detailed finite element analyses are used for all investigations, and several beam-column connection configurations are considered, representing: 3 beam-column geometries (shallow, medium and deep columns); 4 levels of skew at the beam-to-column connection; and 4 levels of applied column axial load. Results indicate that for axial loads less than 50% of the column axial capacity, there is little effect on connection rotation capacity. Beam-skew angle is the main contributor to resulting column twist. With the exception of the deep column geometries that experienced local flange buckling at 50% ΦPn axial load, increased column axial loads have little effect on resulting column flange yielding in skewed RBS moment connections.

Seismic Demands in Column Base Connections of Steel Moment Frames

Methods for the seismic design of base connections in steel moment frames are well-developed and routinely utilized by practicing engineers. However, design loads for these connections are not verified by rigorous analysis. This knowledge gap is addressed through nonlinear time history simulations using design-level seismic excitation that interrogate demands in column base connections in 2-, 4-, 8-, and 12-story steel moment frames, featuring base connections that reflect current U.S. practice. The results indicate that: (1) for exposed base plate connections, lower bound (rather than peak) estimates of axial compression are suitable for design because higher axial forces increase connection strength by delaying base plate uplift; (2) even when designed as pinned (as in low-rise frames), base connections carry significant moment, which can be estimated only through accurate representation of base flexibility; and (3) the failure of embedded base connections is controlled by moment, which may be estimated either through overstrength or capacity-based calculations.

Impact of Loading Protocol on the Performance of the Steel Moment Frame Connections

Today, for the moment frame structures, seismic provisions of the structural engineering design codes depend on the inelastic deformation as well as inelastic capacity of the connections. A cyclic loading protocol is normally exercised for measuring such capability. This paper investigates the deformation capacity of steel moment resisting frame’s connections subjected to different loading protocols. To evaluate the performance of the connections subjected to various cyclic loads, behavior of three types of connections is studied. Behavior and capacity of each connection are assessed subjected to different loading protocols; namely ATC, FEMA and SAC. The results from this research indicate that the ATC and FEMA loading make greater demands on the connections; while SAC basic loading shows a better agreement with the target values of the loading protocol. A loading protocol has been developed taking some criteria into account in order to match the target values presented in SAC study for steel moment connection’s bam to column sub-assemblies. Then the connections were subjected once again to the proposed loading protocol and results compared to those of other loading protocols. The results reveal that the connections subjected to the proposed loading protocol provide greater deformation and strength capacity. Also, lower equivalent plastic strain and lower dissipated energy were observed when the connection is subjected to the proposed loading protocol.

Parametric Finite-Element Modeling for Exposed Steel Moment Frame Column Baseplate Connections Subjected to Lateral Loads

AbstractThe behavior of steel column baseplate connections in moment frames subjected to lateral loads has been the subject of numerous experimental investigations, giving rise to a variety of meth...

Seismic lessons from the full-scale testing of SidePlate: An innovative steel moment frame connection

Seismic lessons from the full-scale testing of SidePlate: An innovative steel moment frame connection