Beam-column Assemblies Research Articles

A PULL-DOWN DYNAMIC ANALYSIS OF TWO-SPAN STEEL FRAMES SUBJECTED TO PROGRESSIVE COLLAPSE

Progressive collapse, also known as disproportionate collapse, describes a chain reaction of structural element failures in which a primary structural element failure results in the failure of adjoining structural elements. It eventually causes widespread structural damages and a disproportionate collapse. While high level finite-element models incorporating non-linear dynamic analysis will produce more realistic results in progressive collapse scenarios, they are computationally time consuming. Therefore, the development of a non-linear time history pull-down model that is validated with experimental results would be beneficial for producing acceptable and efficient design solutions, particularly for practicing structural engineers. In this paper, a non-linear time history pull-down model of a two-span steel frame is analyzed in ETABS. The ETABS model results are compared with experimental results of two steel frames with two-spans conducted by the National Institute of Standards and Technology (NIST). The NIST experiments include beam-column assemblies from the second-floor framing system of a ten-story building and each span is 20 feet long. The numerical results from ETABS pull-down analysis showed good agreement with the results from the NIST experimental study.

Experimental study of precast dry connections constructed away from beam–column junction under progressive collapse scenario

This paper presents experimental studies conducted on precast dry connections under progressive collapse scenario. Reduced one-third-scale precast beam–column assemblies are constructed as test specimen by providing connection away from beam–column junction. Dimensions of test specimens are extracted from 6-story symmetrical building having overall plan dimensions of 16 m × 12 m. Each specimen consists of two span beams and three columns with removed middle column, which represents progressive collapse scenario. Dry connection between precast elements were constructed by adopting two different types of detailing such as welding and bolting of steel plates at connection region. Monotonic vertical load was applied at the location of removed middle column with the help of hydraulic jack. Behavior of test specimens was measured in terms of ultimate load carrying capacity, deflection profile along the length, failure pattern and crack propagation. The performance of precast specimens having dry connection was also compared with that of monolithic connection. From the results of experimental studies, it was observed that precast dry connection with welded plates was having maximum ultimate load carrying capacity, while the same for precast dry connection with bolted plates was lesser than that of monolithic connection. However, performance of precast dry connection with bolted plates can be enhanced by ensuring appropriate quality of concreting around connection region. Based on the study, it was concluded that building having precast elements connected away from the beam–column junction with adequate connection detailing behaves similar to that of monolithic construction and can be a sustainable alternative of cast-in-place construction.

Strengthening of precast RC beam-column connections for progressive collapse mitigation using bolted steel plates

Being one of the most critical scenarios in extreme events such as blast attacks, the progressive collapse of reinforced concrete (RC) structures has attracted the attention of structural engineering community. As precast concrete buildings are deficient in structural continuity, they are more vulnerable to progressive collapse than cast-in-situ RC buildings. Hence, effective rehabilitation techniques to upgrade beam-column joints in existing precast RC buildings for progressive collapse mitigation are needed. The goal of this study is to investigate the effectiveness of using bolted steel plates on the behavior of precast beam-column connections under sudden column-loss scenario. This study presents experiments involving one half-scale precast RC beam-column assembly, which represented the most prevalent types of existing precast beam-column joints in Saudi Arabia. One monolithic test specimen having continuity of top and bottom beam rebars was used for the sake of comparison. Another precast specimen similar to the control one was strengthened using bolted steel plates within the connection region. The progressive collapse scenario was simulated by removing the central column support and applying a sudden vertical load on this column at a rate of 100 mm/s until failure. The collapse load of both monolithic and strengthened specimens was predicted using a simplified section analysis procedure. The analysis was then used for some useful parametric studies in which the effect of different steel plate parameters on the response of test frames under middle column-loss scenario was investigated.

Flexural, Compressive Arch, and Catenary Mechanisms in Pseudostatic Progressive Collapse Analysis

Progressive collapse has drawn significant attention from researchers because of its potential to inflict severe damage and casualty. Much research has focused on simulation techniques for collapse, but comparatively little has examined the interactions among the three main mechanisms of flexural, compressive arching, and catenary actions by way of simplified methodologies. This paper introduces a theoretical method to calculate the collapse resistance of reinforced concrete frame structures in static progressive collapse analysis. The lumped-damaged plasticity method was used to simulate flexural analysis, and compressive arch and catenary actions were calculated using geometry analogies assuming rigid body rotation after initiation of plastic hinging. The method was validated by comparing the simulation results with experimental data, wherein reinforced concrete beam-column assemblies subjected to column removal were tested pseudostatically. The flexural and arching actions were well predicted by the proposed methodology, though the assumed failure mechanism at the end of the simulated catenary action was conservative compared to the experimental results. Overall, the developed method integrating flexural, compressive arch, and catenary actions during collapse was found to be a reliable tool for quick estimation of collapse resistance in static progressive collapse resistance.

Effects of span-to-depth ratios on moment connection damage evolution under catenary action

This paper proposes an improved method for determining the gravity resistance of a moment resisting beam-column assembly following an interior column loss. The proposed method accounts for the connection's damage evolution and for the catenary mechanism developed by the assembly as it deflects downward. Through a full-scale laboratory test and finite element simulations, the complete responses of moment resisting beam-column assemblies including the connection's damage evolution are investigated under different beam span-to-depth ratios. The welded unreinforced flange-bolted web (WUF-BW) connection method is used for its robustness in developing the catenary action. It is found that, under the same span-to-depth ratio, beam-column assemblies exhibit similar normalized load-rotation relationships, even with different beam depths. The assembly with a larger span-to-depth ratio is able to develop the gravity resistance earlier, and provides a higher ultimate resistance by developing a more effective catenary mechanism. On the other hand, the assembly with a smaller span-to-depth ratio exhibits a more ductile response. A simplified curve model of the gravity resistance development of a moment beam-column assembly with damage evolution has been proposed for a convenient assessment of the progressive collapse resistance following a central column loss.

Investigation of precast RC beam-column assemblies under column-loss scenario

The progressive collapse of reinforced concrete (RC) buildings, being one of the most critical failure scenarios, is a great concern for the structural engineering community. As precast concrete buildings are deficient in structural continuity, these are more vulnerable to progressive collapse than cast-in-situ RC buildings. The goal of this study is to develop a nonlinear finite element (FE) model using LS-DYNA software to predict the performance of precast non-prestressed RC beam-column assemblies, each comprising three columns and two beams, under column-removal scenario. The model takes into account the nonlinear behavior of concrete and steel, strain rate effect on material properties and contact between surfaces at the joints. The FE models were calibrated against three half-scale specimens tested under middle column-removal scenario. Tests included two precast specimens and one monolithic specimen with continuous top and bottom beam reinforcement. The validated FE modeling was further extended to study the progressive collapse potential of seven revised precast connections. As a result of the FE study, new joint efficiency parameters were introduced in this research.

Strength and stiffness of adhesively bonded GFRP beam-column moment resisting connections

For the first time, the feasibility of adhesively bonded connections in FRP frame structures is explored as an alternative to bolted connections. Eight full-scale GFRP beam-column connections are tested and their failure mode, strength and rotational stiffness are investigated. A single pultruded GFRP I-profile is used for the two members. In four of the specimens the beam and the column are connected by epoxy adhesive and GFRP seat angles, similar to the so-called “standard bolted connection”. In the remaining four specimens, the seat angles are supplemented by additional GFRP angles and stiffeners to strengthen the column flange and web. The beam-column assembly forms an inverted L-shape frame, with the column being fixed at the bottom and attached to the beam near the top. The beam, acting as a cantilever, is loaded by a point load near its free end, which subjects the connection to bending and shear. The current standard connection failed by debonding within the column flange while the improved/strengthened connection failed within the adhesive or at the adhesive-column flange interface. The test results reveal that both the standard and improved connection can have at least the same strength as the corresponding bolted connection, irrespective of whether GFRP or steel bolts are used to make the connection. Hence, the current restrictions against the use of adhesive beam-column connections in GFRP frame structures may be unjustified. In making this comparison, the observed failure load of each connection is normalized by the ultimate moment capacity of the GFRP profile in the beam-column assembly.

Robustness of Reinforced Concrete Framed Buildings: A Comparison between Different Numerical Models

According to Eurocode, robustness is the ability of a structure to withstand events like fire, explosions, impact or the consequences of human error, without being damaged to an extent disproportionate to the original cause.Avoiding the progressive collapse of a building in presence of accidental loading conditions is one of the challenges for the designers.The tie-force method is actually one of the most used design techniques for resisting progressive collapse, whereby a statically indeterminate structure is designed with reference to local simplified models determined in accordance to the failure mode considered.In this work a computational study of a reinforced concrete frame is presented. The beam-column assembly represents a portion of the structural framing system of a ten-story reinforced concrete frame building and is subjected to monotonically increasing vertical displacement of the center column to simulate a column removal scenario.Two different finite element models, with distinct levels of modeling, are used in order to compare the numerical results with the experimental ones coming from a full-scale test, and evaluate the ability of the models to simulate the structural behavior of the frame.

Effect of fire insulation delamination on structural performance of steel structures during fire following an earthquake or an explosion

In this article a numerical model is presented for simulating fire performance of steel structures following an earthquake or an explosion event. In particular, effect of fire insulation delamination during seismic and blast loading on fire performance of structural members is studied. The extent of delamination of fire insulation over the structural elements is adopted from previous fracture mechanics-based studies. A sequential thermal-structural analysis is carried out to trace the fire performance of beam-column assembly in a moment-resisting frame and a beam-column member supporting heavy axial loads. Results from the analyses clearly show that consequence of delamination of fire insulation from plastic hinge region developed in the beam near the column can be quite substantial.As such, 25% delamination of fire insulation can reduce failure time from 100min to 64min. The lack of full fire insulation over this region can significantly accelerate the progressive collapse of the moment-resisting frame due to development of flexural and shear failure in beams adjacent to columns. Also, results from post-explosion fire analysis demonstrate that detachment of fire insulation from a steel column under blast overpressure can significantly impair structural stability of columns during fire following blast.The beam-column that undergoes fire insulation delamination up to 25%, experiences earlier loss of capacity around 85min as compared to 120min achieved in case of no delamination of fire insulation.

Reinforced Concrete Frame Structures

According to Comitè Europèen de Normalisation, robustness is the ability of a structure to withstand events like fire, explosions, impact or the consequences of human error, without being damaged to an extent disproportionate to the original cause. Avoiding the progressive collapse of a building in presence of accidental loading conditions is one of the challenges for the designers. The tie-force method is actually one of the most used design techniques for resisting progressive collapse, whereby a statically indeterminate structure is designed with reference to local simplified models determined in accordance to the failure mode considered. In this work a computational study of a reinforced concrete frame is presented. The structure studied is a beam-column assembly which represents a portion of the structural framing system of a ten-story reinforced concrete frame building and is subjected to distributed loads and to monotonically increasing vertical displacement of the centre column to simulate a column removal scenario.

Strengthening of substandard reinforced concrete beam-column joints by external post-tension rods

The efficiency of the proposed strengthening method using externally applied post-tension rods in reinforced concrete external beam-column joints, which do not comply with any code requirements, is investigated. Five full-scale specimens were tested in the laboratory. They have specific deficiencies resulting from the lack of shear reinforcement in the joint and poor material properties including low strength concrete and the presence of plain round reinforcement bars. While all specimens were built with column and in plane beam, one of the tested specimens consists of a transverse beam to demonstrate the applicability of the presented retrofit technique. All specimens were subjected to a cyclic quasi-static loading up to 8% drift ratio to observe different levels of structural damage. Two post-tension rods, which were mounted diagonally at each side of the joint, are utilized as a local retrofit technique. The reference specimen displayed a brittle behavior with the concentration of shear cracks mostly in the joint while the rest of the RC components were almost in their elastic range. The ultimate lateral load capacity was increased considerably for all retrofitted specimens. However, a brittle type of failure mechanism was observed such as a joint shear failure or beam-joint failure in the three retrofitted specimens. A relatively ductile response was observed in the specimen with transverse beam, although the axial force in the post-tension rods was the same with the specimens without transverse beam. After testing all specimens, it is found that the lateral force capacities of the beam-column assemblies can be improved up to code requirements by the proposed retrofitting method.

Structural repairing of damaged reinforced concrete beam-column assemblies with CFRPs

Depending on the damage type as well as the level of damage observed after the earthquake, certain measures should be taken for the damaged buildings. In this study, structural repairing of two different types of damaged RC beam-column assembly by carbon fiber-reinforced polymer sheets is investigated in detail as a member repairing technique. Two types of 1:1 scale test specimens, which represent the exterior RC beam-column connection taken from inflection points of the frame, are utilized. The first specimen is designed according to the current Turkish Earthquake Code, whereas the second one represents a deficient RC beam-column assembly. Both of the specimens were subjected to cyclic quasistatic loading in the laboratory and different levels of structural damage were observed. The first specimen displayed a ductile response with the damage concentrated in the beam. However, in the second specimen, the beam-column joint was severely damaged while the rest of the members did not attain their capacities. Depending on the damage type of the specimens, the damaged members were repaired by CFRP wrapping with different configurations. After testing the repaired specimens, it is found that former capacities of the damaged members were mostly recovered by the application of CFRPs on the damaged members.

Effective properties of spray-applied fire-resistive material for resistance to cracking and delamination from steel structures

This paper presents a study on quantifying the effective properties of spray-applied fire-resistive material (SFRM) to mitigate delamination of fire insulation from steel structures. The crack development in SFRM, its propagation and then delamination is investigated for two types of structural elements; a flimsy steel truss chord, and a beam-column assembly in a moment-resisting frame insulated with three types of SFRM widely utilized in steel construction. In the truss chord, the strain ductility of steel at which delamination initiates and propagates is studied, while in the beam-column assembly, the extent of delamination over the plastic hinge region developed on the beam is monitored. A fracture mechanics-based numerical model is employed to undertake parametric studies to determine effective parameters of SFRM for minimizing delamination. In the sensitivity study, the material properties of SFRM applied on steel members are varied over a wide range. Results from the sensitivity study indicate that the elastic modulus of fire insulation (E), thickness of insulation (t) and interfacial fracture energy (Gc) are the crucial factors that govern the delamination of SFRM from the steel surface. Subsequently, results from an additional set of parametric studies, carried out on a beam-column assembly, also infer that delamination of SFRM occurs over the plastic hinge region and is mainly governed by the critical factors of E, t and Gc. Results from these studies indicate that all three governing factors play a critical role in determining the extent of delamination and such delamination can be overcome if the interface fracture energy in normal mode is enhanced to 350J/m2.

Effect of Earthquake-Induced Damage on the Sidesway Response of Steel Moment-Frame Buildings during Fire Exposure

Spray-applied fire-resistive material (SFRM) is prone to debonding, cracking, and spalling in steel moment-frame plastic hinge regions during inelastic seismic response. To evaluate the effect of experimentally observed earthquake-induced SFRM spall patterns on building sidesway response during an ensuing fire, an analytical case study is developed for a steel special moment-frame building with a seismic hazard representative of coastal California. Response data from numerical earthquake simulations indicate that damage to SFRM insulation in beam hinge regions should be anticipated following ground shaking representative of the maximum considered seismic hazard. Thermomechanical post-earthquake fire simulations demonstrate that earthquake-induced SFRM spalling significantly increases thermal degradation in the affected beam hinge regions during fire exposure, leading to pronounced softening of moment-rotation response for the beam-column assemblies. This temperature-induced moment-frame connection softening increases the flexibility of the structural system for sidesway motion and exacerbates drift demands under the action of residual destabilizing forces.

In-plane behaviour of beam-to-column connections of corrugated web I-sections

This paper presents the results of an experimental study to examine the in-plane behaviour of the moment-resisting beam-to-column connections of thin-walled sinusoidally-corrugated web I-sections. A group of specimens of nearly full-sized beam–column assemblies made of sinusoidal corrugated web I-sections were tested under cyclic loading in the Structural and Earthquake Engineering Laboratory of Istanbul Technical University (ITU). Whereas two of the specimens have corrugated panel zones, the others have flat plate panel zones, according to an existing practice. The results of the experimental works are evaluated in terms of the general behaviour of the specimens and the regional behaviour of the panel zones. Three-dimensional nonlinear finite element analysis (FEA) models are developed in ABAQUS, including nonlinear interaction along with the contacting parts. The analytical results show a satisfactory convergence with the experimental results. The specimens that had relatively thicker flat plate webs in the panel zone were more stable than those which had corrugated web panels.

Development and testing of precast concrete beam-to-column connections

Five half-scale interior beam-column assemblies representing a portion of a frame subjected to simulated seismic loading were tested, including one monolithic specimen and four precast specimens. Variables included the detailing used at the joint to achieve a structural continuity of the beam reinforcement, and the type of special reinforcement in the connection (whether ECC or transverse reinforcement). The specimen design followed the strong-column-weak-beam concept.The beam reinforcement was purposely designed and detailed to develop plastic hinges at the beam and to impose large inelastic shear force demands into the joint. The joint performance was evaluated on the basis of connection strength, stiffness, energy dissipation, and drift capacity. From the test results, the plastic hinges at the beam controlled the specimen failure. In general, the performance of the beam-to-column connections was satisfactory. The joint strength was 1.15 times of that expected for monolithic reinforced concrete construction. The specimen behavior was ductile due to tensile deformability by ECC and the yielding steel plate, while the strength was nearly constant up to a drift of 3.5%.

Experimental investigation of beam-to-tubular column moment connections under column removal scenario

Removal of a column member is a typical scenario of local failure that could trigger the progressive collapse of a frame structure. Under such a situation, the behavior of the beam-to-column connection is expected to greatly influence the structural capacity in resisting progressive collapse. This paper presents two full-scale laboratory tests on steel beam-to-tubular column moment connections with outer-diaphragm, one being welded flange–welded web method and the other being welded flange–bolted web method to connect the beam members to the outer-diaphragm, under a column removal scenario. Test results demonstrate that the beam-column assemblies resisted the load applied atop the center column primarily by flexural action in the early stage of the response, and the resistance mechanism gradually shifted towards relying on the catenary action as the vertical displacement increased. Two types of flexural failure modes, namely a continuous flexural failure throughout the section (in the welded-web connection case) and an interrupted flexural failure without the fracture extending upwards (in the bolted-web connection case), were observed from the tests. The two different flexural failure modes dictated the capacity of the respective specimen in developing an effective catenary action in the subsequent phase of the response, and in this respect the bolted-web connection proved to be more redundant in terms of strength and deformability than the welded-web connection.

Seismic behavior of a type of welded precast concrete beam-column connection

This paper describes testing of a typical practice in Mexico for connecting precast concrete members for moment-resisting frames using welded reinforcement. Cyclic lateral load testing of typical beam-column assemblies were conducted. The connections were also analyzed using a nonlinear analytical model. The results indicate that the welded reinforcement con- nection detail may result in brittle failure under cyclic loading. The authors recommend that emulative con- nections be used instead.

Modeling of Reinforced Concrete Assemblies under Column-Removal Scenario

AbstractThis paper presents a computational investigation of two reinforced concrete beam-column assemblies, each comprising three columns and two beams, subjected to monotonically increasing vertical displacement of the unsupported center column simulating a column removal scenario. One assembly was part of an intermediate moment frame and the other was part of a special moment frame. Two types of models were developed: (1) detailed models with highly refined solid and beam elements to represent the nonlinear material behavior of concrete and reinforcement, and (2) reduced-order models with significantly fewer beam and spring elements to represent the nonlinear behavior of structural components. Reduced models are desirable for analysis of complete three-dimensional (3D) structural systems. Modeling approaches for the detailed and reduced models are described, and the computational results are compared with experimental data from full-scale tests. Good agreement is observed, which demonstrates the capabi...

횡력을 받는 넓은 보-기둥 내부 접합부의 거동 평가

횡력을 받는 넓은 보와 기둥 내부접합부의 거동을 평가하기 위하여 넓은 보-기둥 접합부의 휨강성비와 유효폭 및 슬래브 유무를 변수로 6개의 1/2축소 모델 실험체를 제작하여 구조성능 평가를 수행하여 하중-변형, 연성, 강성 등을 평가하였다. 동 모델을 대상으로 비탄성해석을 수행하여 구조성능평가 결과와 비교하였다. 연구결과 도출된 결론은 다음과 같다. 넓은 보-기둥 내부 접합부는 중진지역인 국내에서 적용하고 있는 부분골조형 구조에 적용이 가능한 것으로 판단된다. 슬래브의 강성이 횡력을 받는 넓은 보-기둥 내부 접합부의 거동에 영향을 미치는 것으로 나타났다. 휨강성비가 2.0이상의 경우 T형보의 플랜지로서 슬래브 유효폭은 넓은 보 유효깊이의 2.0d로 평가되며, 휨강성비가 1.4~2.0일 경우, 슬래브 유효폭의 영향은 고려하지 않아도 되는 것으로 나타났다. An experimental investigation was conducted to study the behavior of RC wide beam-column joints with slab subjected to reversed cyclic loads under constant axial load. Six half scale interior wide beam-column assemblies representing a portion of a frame subjected to simulated seismic loading were tested, including three specimens without slab and three specimens with slab. The primary variables were the ratio of column-to-beam flexural capacity (<TEX>$M_r={\Sigma}M_c/{\Sigma}M_b$</TEX> ; 0.77~2.26), ratio of the column-to-beam width (b/H ; 1.54, 1.67). Test results are shown that (1) the current design code and practice for interior joints(type 2) are apply to the wide beam-high strength concrete column. (2) the presence of a slab have an effect on the performance of the wide beam-high strength concrete column interior joints(type 2). therefore in the design of the wide beam-high strength concrete column interior joints(type 2), the width of slab effective as a T beam flange should be considered. It was show that the case of the ratio of column-to-beam flexural capacity is more than 2.0, the effective width of slab are 2 times of an effective depth of wide beam, however if the ratio of column-to-beam flexural capacity is 1.4~2.0, the effective width of slab are not able to be considered.