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In practice, reinforced concrete (RC) infilled frames having longer span are provided with tie columns to improve structural integrity but previous work ignored tie columns in studying the effect of masonry infills on progressive collapse resistance of RC frames. Therefore, aim of this study is to explore the effect of tie columns on the progressive collapse resistance of RC infilled frames under column removal scenarios. The high-fidelity-based finite element model of RC infilled frames were first developed and validated with published experimental results. The validated model was then further evolved to an RC infilled frame having tie columns to evaluate the influence of tie columns on the load transfer mechanism and the resistance of the RC infilled frame against progressive collapse. Thereafter, the numerical model was further utilized to study the effects of the pertinent parameters, including the number of stories, the span length of the frame and the longitudinal reinforcement ratio of the tie columns. Finally, equivalent strut model was suggested for macro-modeling of RC infilled frames having tie columns in engineering practice. The results showed that incorporating tie columns makes truss mechanism more fully mobilized in masonry infill walls and the inclined angles of primary struts in each sub-panel of the infill wall are increased, improving the vertical components of diagonal compression in resisting the gravity load. Consequently, introducing tie columns at middle span of infill wall panels helped to increase the initial stiffness (about 19%), peak resistance (about 26%) and post-peak resistance of RC infilled frame under a middle column removal scenario. Such beneficial effect of tie columns becomes more significant for infilled frames with larger spans, and the longitudinal reinforcement ratio of tie columns should be ≤ 0.24%. [ABSTRACT FROM AUTHOR]

A shear tab connection is one of the most common simple beam-to-column connections that is widely used in steel structures. In ASCE41-17 (Seismic evaluation and retrofit of existing buildings, American Society of Civil Engineers, 2017) and DoD (UFC 4-023-03, Design of buildings to resist progressive collapse, Department of Defense, Washington, DC, 2009), the plastic rotation capacity of a shear tab connection is purely a function of the connection depth, which is over- and under-estimated, respectively. To address this shortcoming, the present paper tries to provide a better estimation of plastic rotation capacity under the column removal scenario by employing a validated finite element model and conducting a comprehensive parametric study under various effective parameters such as connection depth, adjacent span length, and connection plate thickness. In addition to improving the plastic rotation capacity relationship, since the connection depth and adjacent span length have a significant effect on the plastic rotation capacity of the connection, a new equation has been proposed in terms of these parameters. Also, the axial-shear force–bending moment interaction of the connection is investigated. [ABSTRACT FROM AUTHOR]

In this paper, the system response, stress redistribution, failure mode, and catenary effect of steel frames with circular tube columns and cast steel stiffener (CSS) joints under a sudden column removal scenario were revealed. Based on this force transfer mechanism analysis, a practical and computationally efficient component-based model considering catenary effects and CSS joint details with a series of springs was established and validated by a detailed solid-element method. By using this component-based model, the proper dynamic response increase factor of the CSS joint frames was investigated. The results show that the great overall stiffness and strength of the CSS limit the deformation of the column front shell. Therefore, the CSS joint frames have superior performance for progressive collapse prevention than the frames using welded joints without stiffeners. In addition, the component-based model is validated to be effective and the dynamic response increase factor of the frames with circular tube columns and CSS joints is smaller than 2.0. [ABSTRACT FROM AUTHOR]

A review of scientific research on survivability and protection of buildings from progressive collapse showed that despite the researchers’ increasing interest in the problem, many tasks in this area are waiting to be solved. The subject of the study in this work was the study of the parameters of the capacity curves of cross-sections of reinforced concrete elements of constructive systems of building frames under their static-dynamic loading conditions. This paper presents a methodology for determining the parameters of "load-deflection" curves and the deformation criterion for accidental limit state of a reinforced concrete element of statically indeterminate frame-rod constructive systems under an accidental action caused by removing one of the load-bearing elements from the constructive system. Two stages of loading such systems are considered: static loading to a specified design level and additional dynamic loading caused by a sudden structural rearrangement of the constructive system from the mentioned action. At the first stage of loading, the relative parametric load of cracking in an arbitrary cross-section of the reinforced concrete element of the constructive system and the sequence of formation of plastic hinges in this element is determined using an extraordinary version of the mixed method of structural mechanics of rod systems. At the second stage, the limit value of the relative parametric load is determined on an energy basis without using the structural dynamics apparatus. An algorithm for calculating parameters of the capacity curve of cross-sections of reinforced concrete elements of constructive systems under the considered actions and calculation results of the "relative parametric load-deflection" curve for the most stressed cross-section of the reinforced concrete frame when a middle column is removed from it are presented. The calculated values of the deforming cross-section parameters are compared with experimental data. It is shown that the use of parametric load in the proposed calculated dependencies for analyzing the sequence of formation of plastic hinges in the constructive system is in satisfactory agreement with the test results of such constructive systems under the considered loading regimes.

The alternative load path method is widely used to assess the progressive collapse performance of reinforced concrete structures. As an alternative to an accurate non–linear dynamic analysis, an energy–based method (EBM) can also be adopted to approximately calculate the dynamic load–bearing capacity curve or the dynamic resistance based on a static capacity curve. However, dynamic effects cannot be explicitly taken into account in the EBM. The model uncertainty associated with the use of the EBM for evaluating the dynamic ultimate capacity of structural frames has not yet been quantified. Knowledge of this model uncertainty is however necessary when applying EBM as part of reliability calculations, for example, in relation to structural robustness quantification. Hence, this article focuses on the evaluation of the performance of the EBM and the quantification of its model uncertainty in the context of reliability–based assessments of progressive or disproportionate collapse. The influences of damping effects and different column removal scenarios are investigated. As a result, it is found that damping effects have a limited influence on the performance of the EBM. In the case of an external column removal scenario, the performance of the EBM is lower as the response is not a single deformation mode according to the results in the frequency domain. However, a good performance is found in the case of an internal column removal scenario in which the assumption of a single deformation mode is found to be sufficiently adequate. Probabilistic models for the model uncertainties related to the use of the EBM compared to direct dynamic analyses are proposed in relation to both the resistances and the associated displacements. Overall, the EBM shows to be an adequate approximation, resulting in a small bias and small standard deviation for its associated model uncertainty. [ABSTRACT FROM AUTHOR]

This paper presents the high-fidelity finite-element-based numerical models for modeling the behavior of prestressed concrete (PC) beam-column substructures to resist progressive collapse under column removal scenario. After careful calibration against data, the validated numerical models are further employed to shed light on the influence of bonded post-tensioned tendons (BPT) with a parabolic profile on the load transfer mechanisms of PC frames against progressive collapse. The effects of parameters, including initial effective prestress, profile of tendon and lateral constraint stiffness at the beam ends, are also investigated. The study shows that, due to the presence of prestressed tendons, the mobilization of compressive arch action in the beam at small deflections demands stronger lateral constraints, and the ultimate load resistance of PC beam-column substructures depends on combined catenary action from non-prestressed reinforcement and BPT at large deflections. For a given constraint stiffness, the initial effective prestress of BPT has less significant effect on the overall structural behavior. For prestressed tendon, a straight profile usually employed in structural strengthening can improve the initial structural stiffness and yield strength, but is less effective in enhancing the ultimate resistance against progressive collapse than the parabolic profile. [ABSTRACT FROM AUTHOR]

In this paper, the system response, stress redistribution, failure mode, and catenary effect of steel frames with circular tube columns and cast steel stiffener (CSS) joints under a sudden column removal scenario were revealed. Based on this force transfer mechanism analysis, a practical and computationally efficient component-based model considering catenary effects and CSS joint details with a series of springs was established and validated by a detailed solid-element method. By using this component-based model, the proper dynamic response increase factor of the CSS joint frames was investigated. The results show that the great overall stiffness and strength of the CSS limit the deformation of the column front shell. Therefore, the CSS joint frames have superior performance for progressive collapse prevention than the frames using welded joints without stiffeners. In addition, the component-based model is validated to be effective and the dynamic response increase factor of the frames with circular tube columns and CSS joints is smaller than 2.0.

Although concrete slabs have an extensive use in structures due to their architectural and executive benefits, the suitability of their behavior against the progressive collapse phenomenon has always been questioned. This study numerically investigates the step-by-step behavior of a support-removed flat slab floor with square panels under the effect of partial overloading. After validation of the modeling method, parts of the designed floor are exposed to increasing downward and uniformly distributed loading during three separate analyses that correspond to the removal of supporting corner, penultimate and interior columns. The pattern of stress in the slab reinforcement and propagation of cracks in the concrete are presented. The findings showed high concentration of slab damage around the corner columns located in the perimeter of overloaded panels and highlighted the role of slab add bars embedded in the vicinity of exterior columns against failure. It was also shown that, unlike the frame-type structural systems, stress redistribution occurs considerably along the diagonals of the slab panels directly connected to the failed support.

The alternative load path method is widely used to assess the progressive collapse performance of reinforced concrete structures. As an alternative to an accurate non–linear dynamic analysis, an energy–based method (EBM) can also be adopted to approximately calculate the dynamic load–bearing capacity curve or the dynamic resistance based on a static capacity curve. However, dynamic effects cannot be explicitly taken into account in the EBM. The model uncertainty associated with the use of the EBM for evaluating the dynamic ultimate capacity of structural frames has not yet been quantified. Knowledge of this model uncertainty is however necessary when applying EBM as part of reliability calculations, for example, in relation to structural robustness quantification. Hence, this article focuses on the evaluation of the performance of the EBM and the quantification of its model uncertainty in the context of reliability–based assessments of progressive or disproportionate collapse. The influences of damping effects and different column removal scenarios are investigated. As a result, it is found that damping effects have a limited influence on the performance of the EBM. In the case of an external column removal scenario, the performance of the EBM is lower as the response is not a single deformation mode according to the results in the frequency domain. However, a good performance is found in the case of an internal column removal scenario in which the assumption of a single deformation mode is found to be sufficiently adequate. Probabilistic models for the model uncertainties related to the use of the EBM compared to direct dynamic analyses are proposed in relation to both the resistances and the associated displacements. Overall, the EBM shows to be an adequate approximation, resulting in a small bias and small standard deviation for its associated model uncertainty.

• High fidelity solid-element based numerical models are built and validated. • Progressive collapse performance of infilled RC frames are studied. • Interaction between infill walls and framed members are illustrated. • Peak resistance of infilled frames is reduced as a function of opening ratio. • Composite action of multi-story infilled frame forms for full-height infill walls. In practice, infilled frame is a common structure but the contribution of infill walls is typically ignored in previous research on progressive collapse. To this end, numerical models based on solid-element are employed to investigate the behavior of reinforced concrete (RC) frames with concrete masonry infill walls under a middle column removal scenario (CRS). The numerical models of bare and infilled frames are initially validated through previous experimental results. Then the numerical models are used to illustrate the effects of infill walls on the load transfer mechanisms of the frames under a CRS and the interaction between infill walls and frame members. Thereafter, the size effect of the frame models is discussed and the numerical models are further extended to study the effects of pertinent geometric parameters on the progressive collapse behavior, including the height of partial-height infill walls, the opening position and area of wall panels as well as the number of stories. The results indicate that the load transfer mechanism of a two-story infilled frame in a middle CRS is the frame action provided by frame members and the truss mechanism provided by the interaction of infill walls and surrounding frame members, in which the latter remarkably enhances the initial structural stiffness and peak resistance. For the multi-story infilled frame with opening in which the geometric and mechanical properties are identical in each story, the load transfer mechanism is basically independent of the number of stories, whereas for the frame with full-height infill walls, the composite effect of multi-story walls is evident, increasing the peak structural resistance. Therefore, if each full-height infill wall is simplified into equivalent strut models in structural analysis, the results are underpredicted but on the safe side. [ABSTRACT FROM AUTHOR]

Highlights • A model is proposed to estimate the responses of composite floors under internal column loss. • The model can accurately predict push-down load–displacement curves. • The model can capture the effects of key parameters. • The procedure of the model can be implemented by a spreadsheet method. Abstract This is the first time that a mechanical model is proposed to estimate the entire load–deflection response of three-dimensional (3D) steel-frame-composite-slab systems (composite floors) subjected to internal column loss. The model incorporates three branches to capture the response at small deformation, transitional and large deformation stages. For ease of hand calculations, the load carried by a 3D composite floor is assumed to be the summation of the respective contributions from the composite slabs and the steel sub-frame. At small deformation stage, the load is resisted through flexural action. But at large deformation stage, structural capacity is mostly contributed by membrane action in the slabs and catenary action in the steel beams. The assumptions and failure criteria in the last stage are based on the results of actual experimental tests. The derivations are discussed in detail, followed by verifications of the model. Compared with actual test results and numerical simulations, the model shows reasonable accuracy in predicting load–displacement curves of 3D composite floor systems under an internal column removal scenario. Besides this novelty and distinct feature from other studies, the model can capture the effects of key parameters, such as slab aspect ratio, joint type, number of joint bolts, slab thickness, reinforcement ratio in the slab and thickness of steel decking. Most importantly, the procedure of the model can be implemented by a spreadsheet method, which provides a simple and numerical robust tool for engineers to calculate progressive collapse resistance of structures for a missing column scenario. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper high fidelity solid-element-based numerical models are used to study the progressive collapse resistance of reinforced concrete (RC) beam-slab substructures. This is a common type of system used in RC framed structures through monolithic construction. After careful calibration of model parameters, the numerical models are validated through comparisons with test data and further employed to investigate the load transfer mechanisms and explore parameters affecting the robustness of RC beam-slab substructures under perimeter column removal scenarios. The parameters studied include lateral restraint stiffness at the substructure boundaries, loading schemes in testing, slab thickness and reinforcement detailing in slabs. The simulation results show that the progressive collapse resistance of the RC beam-slab substructures is developed primarily through compressive arch action of the longitudinal beams and flexural mechanism of the transverse beam at small deflections and catenary action of beams as well as tensile membrane action (TMA) of slabs at large deflections. The slabs enhance structural resistance through working compatibly with beams as L- and T-section “composite beams” at hogging moment regions and developing TMA. If the slabs are just simply converted into equivalent flanges of beams, the resistance at large deflections will be significantly underestimated. [ABSTRACT FROM AUTHOR]

Shear tabs are one of the simplest and the most cost-effective connections in steel frame construction and their behaviour under conventional loading has been well studied. However, their behaviour is highly complex under the scenario when an adjacent column is compromised and this is the subject of ongoing research. In this study, detailed finite element analyses of shear tab connections with three, four or five bolts are performed and the results are compared with available experiments. Characteristic phenomena of the connection, including rotational capacity and failure modes under the column removal scenario, are studied and discussed. The results are compared with available codes and guidelines and new equations are proposed that predict the rotational capacities of shear tab connections for progressive collapse analysis. The shear-axial-moment interaction curves for the connections under study are developed and examined, and conclusions are made based on the observed behaviour. The research uses high-fidelity finite element models of shear tab connections to study both load evolution and failure modes. These models shed more light on the performance and the sources of ductility in shear tab connections. The results can be used to select suitable connection modelling parameters for progressive collapse evaluations of steel frames with similar shear connections. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Five 2D post-tensioned precast concrete sub-assemblages were tested under a middle column loss by multi-point quasi-static loading. • Combined effects of post-tensioned tendons (PT) and precast wet-connection joints. • Investigate the influence of basic parameters such as bonded and unboned PT, T-beam effects and boundary conditions. • PT significantly enhanced compressive arch action and catenary action, and also changed plastic hinge mechanisms. • New design criteria to take advantage of high-strength tendons and avoid failure at critical locations by precast joints. Many previous studies have investigated alternative load path (ALP) mechanisms of 2D reinforced concrete (RC) beam-column sub-assemblages under column removal scenarios. However, how precast and prestressed RC structures respond under a column loss scenario receives far less attention, as evident from a lack of published test data. Herein, five 2D post-tensioned precast RC sub-structures subjected to a middle column loss scenario were tested by multi-point quasi-static loading. In all the specimens, individual precast members such as beams and columns were joined together through wet connection. The study focused on investigating the combined effects of post-tensioned tendon (PT) and wet connection on load-resisting mechanisms and the influence of basic parameters such as unbonded and bonded PT, T-beam effect, and boundary conditions. The test results indicated that both unbonded and bonded parabolic-shaped PT significantly enhanced compressive arch action (CAA) and catenary action (CA) beyond basic flexural capacity, and also changed the plastic hinge mechanism in the precast beams. The specimen with unbonded PT could provide greater residual capacity than bonded PT, but the former was severely damaged by fracture of normal reinforcement and crushing of concrete rather than fracture of the tendon. In addition, the effects of T-beam and boundary conditions are discussed in detail. Finally, new design criteria for precast and post-tensioned concrete structures under column removal scenarios are proposed for structural engineers to take advantage of high-strength tendons and avoid failure at critical locations by precast joints. [ABSTRACT FROM AUTHOR]

This paper describes an experimental investigation on the resistance and failure mode of four precast concrete frames. Each frame comprised a middle beam-column joint, a double-span beam, and two side columns. Two parameters, namely, reinforcement detailing in the beam-column joint and dimensions of the side columns, were investigated in the experimental program. Frame specimens were loaded on the middle joint by a vertical servohydraulic actuator, and corresponding horizontal reaction forces were recorded through load cells connected to the side columns. Experimental results and observations indicate that precast concrete frames were capable of developing compressive arch action (CAA) at the initial loading stage. However, frames with smaller side columns exhibited shear failure in the side joint. This hindered the development of catenary action in the bridging beam. By increasing the dimensions of the side columns, significant catenary action developed in the frames, and eventually flexural failure of the side columns took place resulting from horizontal tension force from the bridging beam. Experimental results suggest that horizontal forces induced by CAA and catenary action in the bridging beam have to be considered in the design of side columns and beam-column joints against progressive collapse. [ABSTRACT FROM AUTHOR]

The transient dynamic response of the substructures above a removed column is the most direct and realistic to demonstrate the structural performance against progressive collapse. To efficiently obtain the transient dynamic response, a nonlinear single degree-of-freedom (SDOF) model is proposed with a tri-linear resistance function that is capable of describing various forms of structural resistance, including catenary action and softening resistance and a loading function consisting of a linear ascending part and an ensuing constant force that considers the effect of column removal time. Moreover, the model accounts for nonzero initial conditions, which are very likely to occur under blast loading. The closed-form analytical solutions of the model are derived with Laplace transform techniques and verified with dynamic experimental results of steel and reinforced concrete assemblies subjected to column removal scenarios. The verification indicates the generality and the accuracy of the SDOF model in predicting displacement responses of substructures. Then, the model is used for parametric studies. The results show that sudden column removal scenarios, which are used for progressive collapse design, provide an upper bound of structural deformation in cases that take a relatively long time to destroy a column but underestimates the structural deformation in cases that have upward displacement and nonzero velocity at the start of the progressive collapse stage. [ABSTRACT FROM AUTHOR]

Under column removal scenarios, initiation of alternate load paths via adjacent bridging beams to redistribute vertical loads requires certain level of ductility and continuity in beam–column joints. Although this approach does not consider the magnitude of the blast event, it is threat-independent and offers a minimum level of robustness against column removal scenarios. This paper studies the behaviour of precast concrete sub-assemblages which comprised two precast beams and a precast column joining together by cast-in-place concrete topping above the two beams and the beam–column joint. The top longitudinal reinforcement in the structural topping of precast beams passed through the beam–column joint continuously. However, the bottom beam longitudinal reinforcement was either lap-spliced or anchored as a 90° bend within the cast-in-place joint. Due to discontinuity of bottom beam longitudinal reinforcement, the ability of such an assemblage to develop compressive arch action (CAA) and subsequent catenary action has to be investigated, in particular, the effect of the top and bottom beam longitudinal reinforcement ratios. Test results show that significant CAA and catenary action developed in the beams under column removal scenarios, with pull-out failure of the bottom beam reinforcement in the joint. The enhancement of CAA and catenary action to structural resistance greatly depends on joint detailing and beam reinforcement ratio. Furthermore, the effectiveness of horizontal shear transfer between concretes cast at different times is examined at large deformation stage. Finally, practical suggestions are given to enhance structural resistance of a similar type of precast concrete sub-assemblages. [ABSTRACT FROM AUTHOR]

• A two-bay steel frame with intermediate moment connection & two strengthened frames were tested under column loss scenario. • First strengthening scheme employed welded double side plates within the connection region. • Second scheme used pretensioned high-strength hot-rolled steel rods within the connection region. • Efficacy of strengthening schemes was evaluated based on the results of experiments and numerical simulation. • The strengthening schemes were able to substantially enhance the progressive collapse resistance. Strengthening of connections in steel framed structures is often required to mitigate the risk of progressive collapse initiated by column removal event. Steel framed buildings with simple shear connections between beams and columns are vulnerable to progressive collapse. Therefore, strengthening such structural systems for existing buildings as well as the designing of robust connections in the new structures is vital. This study was undertaken with the prime objective of investigating experimentally and numerically the efficient strengthening schemes for simple shear beam-column connections in order to minimize the risk of collapse in the incidence of sudden column loss. The experimental and numerical study includes testing a control test frame of a one-third scale single story shear-connection sub-assemblage consisting of two bays. A steel frame with an intermediate moment frame (IMF) connection was used as a target frame. Two schemes of strengthening beam-column connections were adopted for upgrading the control frame. The first scheme employed welded double side plates within the connection region, and the second scheme used pretensioned high-strength hot-rolled steel rods within the connection region. All specimens were subjected to dynamic loading in the vertical direction which essentially simulates extreme load cases like an explosion in which columns are removed. The efficacy of the strengthening schemes was evaluated based on the results of experiments and numerical simulation of the tested specimens. [ABSTRACT FROM AUTHOR]

• The collapse mechanisms of prestressed precast (PC) concrete structure are revealed according to quasi-static pushdown tests. • PC structure presents a different failure mode compared with cast-in-place reinforcement concrete (RC) structure. • PC structure has a higher collapse resistant capacity, and RC structure has a perfect ductility under large deformation. • The equivalent restrained stiffness acted at beam ends in the failed span in 1st story is larger than that in 2nd story. • Compared to RC structure, the effect of compressive arch action is reduced in prestressed PC structure, and the effect of tensile catenary action is strengthened. The prestressed precast concrete frame with dry joints is a novel structural system which has been proved to perform excellently under seismic loads in many researches, but its progressive collapse resistant mechanism need to be investigated further due to the discontinuity of longitudinal bars at joint interface. In this paper, quasi-static pushdown tests were conducted to investigate progressive collapse behavior of three half-scaled plane frame substructures with two stories and two spans under first-story middle column removal scenario. The specimens, including one cast-in-place concrete frame substructure and two prestressed precast concrete frame substructures, were designed according to the same design loads. The test results showed that these two types of frame structures could supply similar vertical resistances in compressive arch action (CAA) stage, while the prestressed precast structure presented a higher bearing capacity in tensile catenary action (TCA) stage, and the load-carrying ability was enhanced with the increase of steel strands area. However, the prestressed precast frame had a smaller failure displacement, which showed a lower ductility. Compared to the cast-in-place concrete frame, the prestressed precast concrete frame showed a distinct failure mode, in which there were almost no obvious cracks in beams, and no plastic hinges formed at beam ends, but a wide through crack appeared at the joint interface. The tension of steel strands decreased the CAA effect in beams, and speeded up the transformation from CAA stage to TCA stage during the progressive collapse process. This phenomenon became prompt with the increase of steel strands area. In terms of the same frame specimen, the peak value of axial compression force of beam in the second story was lower than that in the first story due to distinction of boundary conditions at beam ends in the two stories. However, the strength of steel strands of beams in both stories was fully utilized under large deformation due to a reliable tensile restrained stiffness at beam ends, leading to a similar peak tensile force at TCA stage. [ABSTRACT FROM AUTHOR]

Currently, there are very few numerical and experimental studies of commonly-used fin plate (or shear tab) and welded unreinforced flanges with bolted web joints subjected to impact loads. In this paper, an experimental programme of these two types of joints was presented and five steel specimens were included. Structural behaviour of both types of joints subjected to quasi-static and impact loads was investigated. Besides, numerical simulations were conducted using the commercial software LS-DYNA. Finite element modelling techniques were introduced and validated against the test results. Using validated numerical models, governing parameters including mass, velocity, momentum and energy were investigated. Energy was found to govern structural behaviour of specimens subjected to impact loads. To evaluate the performance of beam-column joints subjected to impact loads, two indices, namely energy ratio and deformation ratio were proposed based on energy of impactor. It was found that deformation ratio was more appropriate and relevant than energy ratio for evaluating residual deformations and energy absorption. [Display omitted] • Five beam-column joint tests. • Load-resisting mechanism. • Comparison of quasi-static and impact loads. • Finite element models. • Two evaluation indices. [ABSTRACT FROM AUTHOR]

• Comparison of evaluation methods for RC slab subject to column removal scenarios. • Tensile membrane action effects taken into account. • Energy-Based Method predicts maximum dynamic responses accurately. • Strain rate effect, damping, and column removal duration investigated. • Model uncertainty when using the energy-based method quantified. The Alternative Load Path (ALP) method is widely used to assess progressive collapse resistance of reinforced concrete (RC) structures by notional removal of one or more load-bearing elements. In general, a nonlinear time history analysis (NTHA) is needed to perform such an analysis if dynamic effects are explicitly taken into account. To avoid cumbersome nonlinear dynamic analyses, the energy-based method (EBM) is a promising technique to predict the maximum dynamic responses of a structural system. In this article, the accuracy and precision of the EBM is evaluated based on a validated finite element model of a tested RC slab subjected to a sudden column removal scenario, in particular in relation to the investigation of tensile membrane action (TMA). Influences of dynamic effects are evaluated, i.e. in relation to strain rate effects, damping, and the time duration of support removals. Strain rate effects are observed to have only slight influences on the dynamic responses. The strain rate dependency of reinforcement is found to have a more significant influence on the responses in TMA stage, although also to a limited extent. The magnitude of the load has a significant influence on the dynamic response, as do increasing damping ratios due to the corresponding significant energy dissipation. Finally, the dynamic response reduces with increasing time duration of the column removal. Based on the results of the stochastic analyses, the EBM is observed to perform well based on a comparison with the results of NTHA in both flexural and TMA stages. Furthermore, in relation to the analyzed case studies on reinforced concrete slabs, the model uncertainty of the responses obtained through the EBM compared with the NTHA is found to be represented well by a lognormal distribution with mean of 0.95 and a standard deviation of 0.20, for evaluating the loads of first rupture of reinforcement. Furthermore, a lognormal distribution with mean 0.96 and standard deviation 0.13 is found appropriate to represent the model uncertainty on ultimate load-bearing capacity predictions. Model uncertainties are also obtained with respect to the model predictions for displacements at the moment of the first rupture of reinforcement, displacements at the ultimate load-bearing capacities, and both loads and displacements at second load peaks. [ABSTRACT FROM AUTHOR]

In this research, the risk of progressive collapse of 2D and 3D single story one-third scale steel frame assemblies having shear beam-column connections was experimentally investigated under middle column loss event. The 2D assembly comprised two beams and three columns; whereas the 3D assembly was the same as the 2D assembly except with the addition of a transverse beam at the inner beam-column connection. A vertical quasi-static loading was placed on the middle column of the assembly in order to represent the event of progressive collapse. The two test frames together with another 2D steel frame assembly having simple joint, tested previously by other researchers, were employed to validate 3D finite element (FE) analysis created with the help of ABAQUS software. Thereafter, the calibrated models were utilized to study the progressive collapse risk for eight steel simple beam-column connections due to removal of the center column. Specimens were divided into two groups (4 specimens in each group), which were 2D and 3D steel frame assemblies. Behavior of different joints was numerically compared with respect to their failure modes and load versus displacement characteristics. Unlabelled Image • Progressive collapse was studied for 2D & 3D steel frame assemblies with shear connections. • Two assemblies (one 2D and one 3D) were tested under middle column loss scenario. • Test results were employed to validate 3D FEA performed using ABAQUS software. • Other researchers' test results were also used in the validation of numerical models. • FEA was used to study the progressive collapse of 4 types of shear beam-column connections. [ABSTRACT FROM AUTHOR]

• Resistance of RC beam-slab assemblies against progressive collapse are studied. • Boundary conditions (BCs) affect assembly behavior only at large deformation stage. • Resistance of assemblies only relies on concerned beams & slabs at strong BCs. • Supporting column stiffness affects resistance of beam-slab assemblies at weak BCs. • Free boundary beams with slab flange increase structural resistance at weak BCs. Perimeter columns of building structures are the most susceptible to accidental loading (say, terrorist attacks or vehicle impact) due to their accessibility to the public, and thus progressive collapse is more likely caused by damage of perimeter columns. Previous studies show contrary or inconsistent conclusions of structural resistance form of RC beam-slab assemblies under an edge column removal scenario (CRS), probably because of inconsistent boundary restraints. Therefore, in this paper two specimens, each of which consisted of two square slab panels and seven beams, were experimentally investigated under a perimeter middle (PM) column removal scenario (CRS) and a penultimate exterior (PE) CRS, respectively, in which the former and the latter represent strong and weak boundary conditions in terms of lateral and rotational restraints to the assemblies in longitudinal direction. During the tests a static-determinate test-up was designed so that all the vertical reaction forces could be recorded. Through systematic instrumentation, progressive collapse resistance and load-transfer mechanisms of the assemblies are demonstrated, and the effects of boundary conditions are illustrated at the global behavior level and micro strain level. Thereafter, high-fidelity numerical models are built to evaluate the validity of the simplified boundary conditions in the tests and to investigate the effects of supporting column stiffness and restraint due to building continuity on progressive collapse resistance of PES. Finally, based on failure modes, analytical models to calculate the progressive resistance are derived using yield-line theory. The results indicate that structural resistance form of PES depends on the stiffness of supporting corner column, and adding slab flanges to the free boundary beam of PES is able to avoid torsional failure of boundary beams, as well as significantly increases structural resistance at large deformation stage. [ABSTRACT FROM AUTHOR]

This paper investigated experimentally the progressive collapse risk of three one-third scale single story, two-bay steel frames under column-loss event. Progressive collapse was simulated by applying a vertical loading on the middle column. In these tests, three different types of steel beam-to-column connections were studied. These included: one shear-connection specimen that signified the prevalent type of steel beam-column joints, and two different steel intermediate moment frame (IMF) connections conforming to ANSI/AISC 358–16. The three tested specimens along with another two 2D steel frames with IMF connection, tested in the literature, were used to calibrate 3D finite element (FE) models prepared using ABAQUS software. The validated FE models were then employed to investigate the risk of progressive collapse for eleven different types of steel IMF beam-column joints under middle column-loss scenario. Out of the eleven specimens, eight connections were designed as per ANSI/AISC 358–16; two joints were in accordance with EN 1993-Eurocode 3 and the last connection was in conformance with the 2007 Turkish-Earthquake Code (TEC-2007). Performance of different IMF connections was compared based on their modes of failure and load-displacement response in both flexural and catenary action stages. • Tested three one-third scale single story, two-bay steel frames under column-loss event. • Three different types of steel beam-to-column connections were used in test specimens. • 3D finite element (FE) models prepared using ABAQUS. • FE models were calibrated using the test results of this study and also the literature. • FE models were used to investigate progressive collapse risk for 11 different types of beam-column joints. [ABSTRACT FROM AUTHOR]

• Collapse behavior of RC beam-slab assemblies studied with static-determinate set-up. • Load transfer mechanisms against collapse illustrated at different deformation stages. • Catenary action of perimeter beams not dominated by pure axial tension. • Seismic design and rebar types affecting progressive collapse behavior of assemblies. • Progressive collapse resistance of assemblies affected by loading positions in testing. The perimeter columns of framed structures are more vulnerable to terrorist attacks due to accessibility. The beams and slabs above the damaged columns are the primary structural members to redistribute gravity load to avoid progressive collapse. Therefore, to investigate the structural behavior of reinforced concrete (RC) beam-slab assemblies against progressive collapse introduced by a perimeter middle column removal scenario, an experimental program and numerical analyses were carried out in this paper. Three 3/10 scaled specimens, each of which consisted of two square slab panels and seven beams, were tested with equivalent uniformly distributed loading (UDL) achieved by a 12-loading apparatus. High-fidelity finite element models were used to conduct parametric studies after the validation, recheck the validity of the loading apparatus and highlight the effect of loading positions. The concerned parameters include the geometric parameters of beams caused by different seismic design intensity, the slab thickness and reinforcement ratio, and the type of reinforcing bars (i.e. deformed and plain). The results indicate that progressive collapse is resisted by compressive arch action and flexural action of the beams and slabs connecting the stub above the removed column at small deformation stage, whereas catenary action of longitudinal beams and tensile membrane action of slabs are more prevailing at large deformation stage. Moreover, higher seismic design intensity results in a larger resistance at small deformation stage, and plain bars cause larger deformation capacity. Finally, loading positions approaching the stub above the removed column tends to show smaller structural resistance. [ABSTRACT FROM AUTHOR]