Beam Bottom Flange Research Articles

Investigation of Composite Slab Effect on Seismic Performance of Steel Moment Connections

In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange was prevalent. Increased strain demand in the beam bottom flange due to the presence of concrete slab and resulting composite action was speculated to be one of the critical causes of the prevalent bottom flange fracture. Past experimental studies available in the literature after the 1994 Northridge earthquake were reviewed first in this paper. The review showed that different observations and recommendations were made for the slab effect on seismic performance among researchers depending upon the details of beam-to-column connections and concrete slab. In order to augment the test database and emphasize possible negative effect of a composite slab on seismic performance, full-scale cyclic seismic testing was also conducted for three welded steel moment connections. The pre-Northridge (PN) type connection specimen designed with a low degree of composite action (15% fully composite) exhibited a significant upward shift of the neutral axis under a positive (sagging) moment, thus experiencing larger strains in the bottom flange, and showed a poor seismic performance because of the bottom flange fracture at 3% story drift. The specimen designed by using reduced beam section (RBS) connection and with the details of minimized composite action exhibited a superior seismic performance without experiencing any fracture or concrete crushing, close to that of bare steel counterpart. Based on the review of previous experimental studies and test results of this study, it is clearly shown that welded seismic steel moment connections and concrete floor should be designed and constructed to minimize the composite action, if possible, especially when shallow beams and conventional connections are involved.

Study on an Innovative Flange Bolted-Welded Connection

This paper proposes a new type of spliced joint, named flange bolted-welded hybrid joint, which is designed to splice I-shape beam to facilitate the construction of industrialized buildings. The flange, welded with the bottom beam flange as well as the web close to bottom flange, are jointed by high strength bolts. Stiffening plate is welded at top of the flange while downhand welding and high strength friction grip bolts are used for the top beam flange and the beam web, respectively. The connection reduces the construction period and costs. In this paper, monotonic and reversed cyclic loading tests were conducted on three full-scale specimens of this innovative joint to investigate its load-bearing capacity, energy-dissipating capacity and failure modes. The results indicate that the joint has high load-bearing capacity and great ductility. The failure mode is due to the slippage of flange bolts as well as the gap development between the two flange plates. The experimental studies enabled improvement of the design of the connection to be used in moment-resisting steel frame structures.

Panel Zone Deformation Capacity as Affected by Weld Fracture at Column Kinking Location

Three full-scale specimens were tested to evaluate the cyclic performance of rehabilitated pre-Northridge steel beam-to-column moment connections. A Kaiser bolted bracket (KBB) was used on the beam bottom flange for all specimens, but different rehabilitation schemes (another KBB, a notch-tough beam flange replacement weld, or a double-tee welded bracket) were used to strengthen the top flange. All specimens were able to sustain an interstory drift angle of 0.04 radian, with large inelastic deformations in the panel zone. Two specimens experienced fracture at the replacement complete-joint-penetration (CJP) welds, mainly due to the large shear deformation in the panel zone. Because it may not be economically feasible to mitigate weak panel zones in seismic rehabilitation, an analytical model was developed to predict the panel zone deformation capacity and the associated strength. In this model, it was postulated that the ultimate panel zone deformation capacity corresponded to that when each column flange was fully yielded and excessive kinking would cause fracture of the beam flange CJP welds. This postulation was verified by the test data of two specimens that experienced weld fracture due to excessive panel zone deformation. It was shown that the deformation capacity is a function of db/tcf (beam depth-to-column flange thickness ratio). The effect of column axial load was also studied.

Normal, high and ultra-high modulus carbon fiber-reinforced polymer laminates for bonded and un-bonded strengthening of steel beams

Abstract This paper studies the elastic behavior of steel beams strengthened with normal, high and ultra-high modulus CFRP laminates using bonded and un-bonded systems. The elastic behavior of retrofitted beams provides useful information for design of fatigue strengthening systems. A total of seven steel beams including one control unstrengthened beam and six strengthened beams were tested statically until failure in a simply supported four-point bending set-up. The steel beams were retrofitted by normal modulus (NM), high modulus (HM) and ultra-high modulus (UHM) carbon fiber-reinforced polymer (CFRP) laminates with nominal Young’s moduli, ranging from 165 to 440 GPa. Each type of laminate was attached to the steel beams using bonded reinforcement (BR) and un-bonded reinforcement (UR) systems. There is no direct comparison between the BR and the UR systems in the literature. The main goal of the paper is to provide a better understating about the stress distribution along the beam bottom flange when the BR and the UR systems are used for strengthening. All specimens failed due to lateral-torsional buckling (LTB). The effect of different strengthening methods on buckling capacity of the retrofitted specimens was also studied. Experimental results have shown that strengthening using bonded UHM laminates could increase the stiffness of the composite section so that the steel profile has yielded prior to buckling and a larger reinforcement efficacy was then achieved.

Failure Behavior of a Top and Seat Angle Bolted Steel Connection with Double Web Angles

AbstractThe article presents a three-dimensional nonlinear finite-element model that was developed for the study of a top and seat angle bolted steel connection with double web angles. Unilateral contact and friction laws were used to simulate the interaction among the connected parts. Large displacements as well as the von Mises failure criterion were also considered for the steel parts and bolts. Numerical results were compared with the experimental research conducted on the same steel joint. This study finds yielding of the top angle as well as shear failure of the bolts connecting the beam bottom flange with the seat angle to be very important for the overall response of the structure. For this reason, a parametric investigation of the influence of the top angle on the behavior of the structure and the study of the prying forces developed at the top angle were conducted.

Modelling partial end-plate connections under fire conditions

This paper presents a simplified robust 2-noded connection element for modelling the behaviour of partial end-plate connections under fire conditions. In this new model the partial end-plate connection is modelled as a 2-noded nonlinear spring element. The characteristics of the spring – such as stiffness, tension, compression, shear strengths and bending moment resistance – are determined based on each component of the connection. It is well known that the rotational response of a partial end-plate connection comprises of two stages, due to the shift of the compression centre of the connection from the end of the endplate to the centre of the beam bottom flange at large rotation. This two stage behaviour is considered in the model proposed. Compared to normal component-based models the most significant of the current model is that this simplified model has very good numerical stability under static solver condition. The model also retains the advantages of both simple and component-based models. Fourteen tests of partial end-plate connection previously conducted by other researchers were used to validate the proposed model. It is evident that the model is capable to predict the behaviour of flexible end-plate connections under fire conditions. In order to investigate the influences of the connections on the behaviour of steel structures, a series of numerical studies has been conducted on a 2D steel frame, subjected to ISO834 Fire and Natural Fire. It is clear that the model can be used to represent the partial end-plate connections in performance-based fire resistance design of steel-framed composite buildings.

Effects of Composite Floor Slab on Seismic Performance of Welded Steel Moment Connections

Traditionally, domestic steel design and construction practice has provided extra shear studs to moment frame beams even when they are designed as non-composite beams. In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange side was prevalent. The upward moving of the neutral axis due to the composite action between steel beam and floor deck was speculated to be one of the critical causes. In this study, full-scale seismic testing was conducted to investigate the side effects of the composite action in steel seismic moment frames. The specimen PN700-C, designed following the domestic connection and floor deck details, exhibited significant upward shift of the neutral axis under sagging (or positive) moment, thus producing high strain demand on the bottom flange, and showed a poor seismic performance because of brittle fracture of the beam bottom flange at 3% story drift. The specimen DB700-C, designed by using RBS connection and with the details of minimized floor composite action, exhibited superior seismic performance, without experiencing any fracture or concrete crushing, almost identical to the bare steel counterpart (specimen DB700-NC). The results of this study clearly indicate that the beams and connections in seismic steel moment frames should be constructed to minimize the composite action of a floor deck if possible.

The Study on Mechanical Property for Welded Haunch-Reinforced Seismic Connections of Steel Frame

To study on the mechanical property for haunch-reinforced seismic connection, the simplified calculation model of haunch connection is introduced. The force analysis of beam with haunch connection is carried through to research value influence on mechanical property of haunch connection; Combining American design specification with Chinese design specification, the design procedure for welded haunch connection is shown, and the differences in design between two countries are summarized. At last, it is studied through beam-column joints case analysis that the influence of the different node parameter on the mechanical property of haunch connection. The results can be got that with the increase of thevalue, the beam moment at column face becomes smaller, which will be better to protect the beam form brittle fracture. But if is larger than one,beam web shear in the haunch region will increase. In the design the reasonable value should be close to one but less than two. When haunch height is invariant, with the increase of haunch length,became smaller; the value and the top and bottom beam flange groove weld stress reduce gradually; The required minimum section area of haunch flange became larger; The axial force of haunch flange and haunch web shear increase gradually. The reasonable haunch length should be as close to the middle of the (0.5 to0.6) times of height beam as possible. The reasonable haunch flange section area should make close to, such materials are made full use. Key words: haunch connection; force analysis; parameter of reinforced-plate; mechanical property

Behavior and design modification of RBS moment connections with composite beams

Reduced beam section (RBS) connections are innovative connections for steel beam composite frames. The current design procedure for RBS connections is based on bare steel beams. For RBS connections with composite beams, the potential of fracture at the beam bottom flange increases because the strains at the corresponding location increase. This paper focuses on the behavior of RBS connections with composite beams. A mathematical model of the RBS connections with composite beams is derived first. Then an amplification factor is proposed for the design of the RBS connections with composite beams. Parametric analyses based on the mathematical model are conducted to examine the influence of the slab dimension, reinforcement and the beam dimension on the amplification factor. The simplified design formula is proposed for the amplification factor based on a large number of theoretical data from the parametric analysis. Finally, the mathematical models are verified by the finite element (FE) models. It is found that the proposed design formula can give satisfactory results for considering the slab effects in the design of the RBS connections with composite beams.

Effects of Different Bolted Connection Types on Seismic Performance of Column-Tree Steel Moment Connections

This paper experimentally presents the effects of different bolted connection types on the seismic performance of column-tree steel moment connections used in moment resisting frames. Two full scale test specimens were fabricated and tested: one specimen with slip critical bolted beam splices and the other one with bearing type bolted beam splices. The bearing type splice specimen was expected an improved deformation capacity by means of bolt slippage. The experimental results showed that the slip critical splice specimen successfully developed ductile behavior without brittle fracture until 5% story drift ratio, however for the bearing type splice specimen, the beam bottom flange fractured at 4.0% story drift ratio due to stress concentration around the weld access hole area. However the energy dissipation capacity of the bearing type splice specimen was better than that of the slip critical splice specimen until 4.0% story drift ratio.

Finite element simulation of new RHS column-to-I beam connections for avoiding tensile fracture

Many beam-to-column connections, consisting of rectangular hollow section (RHS) columns and wide flange I-beam connections, sustain brittle fracture of welded connections at the beam ends during a large earthquake. These fractures most frequently occur in regions around the beam bottom flange groove welds. A series of tests was conducted on an improved RHS column-to-I beam connection. The aim of the tests was to find possible solutions for avoiding premature occurrences of brittle fracture in RHS column-to-I beam connections. Research results show that the improved connection does not fail by fracture as observed in the conventional connections and has a larger energy dissipation capacity than the conventional types. This paper describes the finite element modeling method employed to analyze the new RHS column-to-I beam connection. The ABAQUS finite element package is used to simulate the experimental behavior, and three highly detailed 3D finite element models are created. These are complex models accounting for material nonlinearity, large deformation and contact behavior. The connection models have been analyzed through the elastic and plastic ranges up to failure. Comparisons with experimental data show that the models have high levels of accuracy.

Finite Element Analysis of Large Capacity Endplate Steel Connections

Problem statement: Extended endplate connection is one of the most widely used beam-to column steel connections because of its fabrication simplicity, good overall performance and cost effectiveness compared with other connection types. The objective of this research is to develop three-dimensional finite element models to study the behavior of large capacity eight-bolt extended un-stiffened wide endplate steel connections, using current-technology elements instead of legacy elements which were previously used by other researchers. Approach: A finite element software package (ANSYS, version 11.0) was used to create and analyze three finite element models. Two of the finite element models were compared with previously reported experimental results to validate the accuracy of the finite element models. The third model was based on a modification of the second finite element model to improve bolt force distribution. Eight-node brick solid elements were used to model the connection members. The bolt shank was modeled using one three-dimensional spar element that connected the bolt head and nut together. Pretension in bolts, contact algorithm and material nonlinearity were considered in the finite element models. Results: Results of the first and the second finite element models were compared with experimental data. The comparison was based on moment-beam rotation and moment-endplate separation of the finite element models and the corresponding tested specimens. The results of the finite element models were used to compare the behavior of the bolts in the tension region adjacent to the beam bottom flange. Conclusion: The comparison showed good correlations between the finite element models and the corresponding tested specimens which confirmed the validity of the proposed models. Thus, a modified connection was proposed to improve the connection response. A finite element model of the modified connection was modeled, analyzed and compared to the original finite element model prior to modification to show their correlation.

Development of the self-centering Sliding Hinge Joint with friction ring springs

The Sliding Hinge Joint (SHJ) is a low-damage beam–column connection used in steel moment-resisting frames. It allows large beam–column rotation with minimal damage through sliding in asymmetric friction connections. It is however, subject to elastic strength and stiffness degradation and potential residual deformations under earthquake shaking and inelastic action. This paper describes the development of the self-centering SHJ incorporating ring springs installed to the beam bottom flange to improve the dynamic re-centering properties and reduce strength degradation. Five 10-storey frames with ring springs generating up to 50% of joint moment capacity were studied analytically using a suite of 10 earthquake records. Ring spring contribution of 40% improved the self-centering properties, resulting in residual drifts less than 0.1% under design level shaking. They also reduced strength degradation.

Cyclic Seismic Testing of Composite Concrete-Filled U-Shaped Steel Beam to H-Shaped Column Connections

In this study, the cyclic seismic performance of a concrete-filled U-shaped steel beam to H-shaped steel column connections was experimentally evaluated. The concrete-filled U-shaped steel beams were compositely attached to the concrete floor slab. The test was conducted in two stages. The first testing program was carried out on one-sided moment connections to find the most promising connecting scheme. The strengthening scheme, or welding steel plates to the beam bottom flange with minimized stress concentration, was shown to be the most satisfactory, and it was used in the second-stage test on two full-scale cruciform specimens. Considering the unique constructional nature of the proposed composite connections, the critical limit states such as weld fracture, local buckling, concrete crushing, and rebar buckling were carefully addressed in designing specimens. Test results showed that the connection details and design procedures proposed in this study can successfully control the critical limit states mentioned previously. The proposed connection detail successfully pushed the plastic hinging to the tip of the strengthened zone as intended in design, thus effectively protecting the more vulnerable beam-to-column welded joint. The specimens typically exhibited a maximum story drift capacity of more than 5.5% rad, exceeding the minimum limit of 4% rad required of special moment frames. Four of the five specimens tested in this study eventually failed because of a low-cycle fatigue fracture across the beam bottom flange at a high story drift greater than 5.0% rad.

Influence of steel shim hardness on the Sliding Hinge Joint performance

The Sliding Hinge Joint is a flexural connection designed for use at the ends of beams in steel moment resisting frames. It is an asymmetric friction connection, where energy is dissipated through sliding in slotted bolted connections in the beam bottom flange. The friction resistance and the repeatability of the hysteretic force displacement curves are dependent on the type of shim used. Brass or mild steel shims have been researched and used in construction to date. This paper describes tests on the sliding components conducted to investigate the use of steel shims of different hardness. The materials tested were mild steel, high strength quenched and tempered steel, and abrasion resistant steel. The beam and cleats which these shims slid against were Grade 300 mild steel. The specimens were tested dynamically at earthquake rates of loading. The abrasion resistant steel, the hardest material, produced the most stable and consistent sliding characteristics for all displacement cycles on different specimens. It also had the least material wear and the highest friction capacity per bolt. Compared to brass, high strength steels are more readily available, less costly, more easily erected, and less susceptible to corrosion, making them a preferred solution.

Study of connection detailing on SMRF seismic behavior for unequal beam depths

This study investigates the differences in the seismic behavior of Special Moment Resisting Frames (SMRF) with unequal beam depths, which can be affected by connection detailing arrangements. The studied connection detailing arrangements consist of continuity plate arrangements, such as straight or inclined continuity plates, coverplate, flange plate connections, and the haunch connection system on the shallow beam side, which can create some alternatives for the connection of the shallow beam and the deep beam to the column. In spite of the occurrence of this special case in everyday engineering practice, the codes do not take these special cases into consideration. Six full-scale experiments were performed in order to improve the understanding of the seismic behavior associated with this special case. Companion analyses were then conducted in order to consider the effects of the above-mentioned connection detailing arrangements on the analytical rupture indices. The results of the experiments and analyses have shown that the described connection detailing arrangements can achieve performance corresponding to story drift ratios of at least 4% to 6% before experiencing 20% strength degradation. By the use of a specific combination of a flange plate connection with a haunch connection system, crack propagation on the deep beam's bottom flange, which is observed in most connection detailing arrangements for this special case, can be eliminated.

An experimental study of relative structural fire behaviour and robustness of different types of steel joint in restrained steel frames

This paper describes the experimental results of ten fire tests on medium-scale restrained steel sub-frames to investigate the relative behaviour and robustness of different types of steel joint in steel framed structures in fire. The ten fire tests were designed to investigate the effects of two column sizes (simulating two different levels of axial restraint to the connected beam) and five different types of joint, including fin plate, web cleat, flush endplate, flexible endplate and extended endplate connections. Each test frame, in the form of “rugby goalpost” consisting of one beam and two columns, was connected through two identical beam to column joints. All the steelwork was unprotected except for the top flange of the beam which was protected to simulate the effect of a concrete slab in reducing the beam top flange temperature. The column ends were restrained to examine the effects of axial restraint on the beam and the joints. This paper presents the observations of structural fire behaviour, including joint failure modes and beam limiting temperatures, the development of deflections at beam middle span and axial forces in the joints at elevated temperatures. The main conclusions are: (1) failure (fracture) was observed only in joints when the beam was in catenary action and a variety of joint failure modes were observed which provides valuable data in understanding joint behaviour; (2) the medium-scale steel beams were able to undergo very large deflections ( span / 8 ∼ span / 6 ) without failure; (3) the specimens with stronger connections such as extended endplate reached higher than their limiting temperatures, defined as the beam bottom flange temperature at middle span at which the axial load in the beam returned to zero. But the difference in beam limiting temperatures using different types of joint is small, less than 50 °C; also the column size had little effect (less than 30 °C) on the beam limiting temperature; (4) the beams connected to the larger column experienced less deflections, but higher axial force due to the higher axial restraint to the beam, which led to fracture of the joint components in these tests; in contrast, the lighter columns visibly deformed and formed plastic hinges at the joints, but there was little evidence of connection fracture in the test frames using the light columns; (5) the web cleat connection appears to have the best performance.

Seismic design and shake table tests of a steel post-tensioned self-centering moment frame with a slab accommodating frame expansion

Post-tensioned (PT) self-centering moment frames were developed as an alternative to welded moment-resisting frames (MRFs). Lateral deformation of a PT frame opens gaps between beams and columns. The use of a composite slab in welded MRFs limits the opening of gaps at the beam-to-column interfaces but cannot be adopted in PT self-centering frames. In this study, a sliding slab is used to minimize restraints to the expansion of the PT frame. A composite slab is rigidly connected to the beams in a single bay of the PT frame. A sliding device is installed between the floor beams and the beams in other bays, wherever the slab is allowed to slide. Many shaking table tests were conducted on a reduced-scale, two-by-two bay one-story specimen, which comprised one PT frame and two gravitational frames (GFs). The PT frame and GFs were self-centering throughout the tests, responding in phase with only minor differences in peak drifts that were caused by the expansion of the PT frame. When the specimen was excited by the 1999 Chi-Chi earthquake with a peak ground acceleration of 1.87g, the maximum interstory drift was 7.2% and the maximum lateral force was 270 kN, equal to 2.2 times the yield force of the specimen. Buckling of the beam bottom flange was observed near the column face, and the initial post-tensioning force in the columns and beams decreased by 50 and 22%, respectively. However, the specimen remained self-centering and its residual drift was 0.01%. Copyright © 2011 John Wiley & Sons, Ltd.

Hysteresis behavior of concrete filled square steel tube column-to-beam partially restrained composite connections

This study presents the development of an improved detail for a Concrete Filled Steel Tube (CFT) square column-to-beam partially restrained composite connection (PR-CC) and the evaluation of its hysteresis behavior under cyclic loading. The detail of the connection was designed to prevent brittle failure at the bottom of the connection due to the composite effect and to simplify its fabrication process. The suggested connection is a welded type of bottom beam flange connection and the existing PR-CC is a bolted type with a seat-angle. To evaluate their hysteresis behavior, specimens were fabricated at full scale and tested under cyclic loading. Results revealed that the suggested type fractured at the welding zone without a drop in capacity due to the anchors inside the steel tube, and reached over 0.04 radian of the plastic rotational angle. The stiffness of the suggested type was about 10% greater than the existing type because the bolted connection allowed more deformation than the welded connection. Both connections were classified as Semi-rigid connections by stiffness analysis according to Eurocode3. The suggested type can be classified as a partially restrained connection overall under cyclic loading because it reached more than 0.03 rad of an inelastic rotation angle 80% capacity of the maximum moment capacity. This type was evaluated to exhibit equal or more ductility than the existing type. As a result, it is concluded that the welded bottom beam flange connection type can be used in practice for the CFT column–beam connection instead of the existing bolt connection using a seat-angle.

Post-tensioned self-centering moment connections with beam bottom flange energy dissipators

This work presents results of experimental and analytical studies of self-centering moment connections. The connection subassembly consists of post-tensioned steel beams, a reinforced concrete column, and energy dissipators placed only below the beam bottom flange for simplicity of construction, ease of replacement, and no interference with the composite slab. Two types of steel energy dissipators are proposed: one includes a reduced section plate restrained by two flat plates, and the other uses cross-shaped steel plates. Cyclic tests are conducted on three full-scale post-tensioned connection subassemblies and six energy dissipators. Finite element analysis is performed to investigate the cyclic performance and likelihood of fracture at critical regions in the energy dissipators. Cyclic test results show that (1) energy dissipation, moment, and flexural stiffness of the beam in positive bending are larger than those of the beam in negative bending, (2) the location of the compression toe at the end of the beam stabilizes at the junction between the beam flange and web after an interstory drift of 1.5%, in which the gap opening angles of the beams are similar in both bending directions, and (3) the shoulder radius equal to 2.5 times plate thickness results in a premature fracture along the shape transition of the reduced section plate. This study also develops an iterative analytical procedure for predicting un-symmetrical cyclic responses of post-tensioned connection subassemblies.