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Beam-to-column connections are crucial for maintaining robustness of a framed structure against progressive collapse, especially in a fire scenario. Existing studies for component-based methods as a balance between accuracy and efficiency primarily focus on ambient-temperature conditions and individual connection types. This study proposes a high-temperature component-based method for top-and-seat-with-web-angle (TSWA) connections, introducing coefficients to enhance accuracy and incorporating temperature-dependent material properties. The influence of plate thicknesses, bolt diameters, edge distances, end distances and temperatures on the load-bearing behaviour of components are examined. A universal theoretical prediction model for the spring stiffness of components is established, applicable to various connection sizes, loading directions and temperature conditions. The accuracy and efficiency of the proposed method are validated against experimental results, with an error margin within 3 %. The advantage of simplification and generalization makes the proposed method suitable for performance-based structural fire design of steel connections. • A component-based method for TSWA connections under fire conditions has been proposed. • The high-temperature component-based model can be applied to connections of various sizes. • The model can be directly used for the collapse analysis of 3-D frame structures under fire. • The prediction error of the high-temperature component-based model is within 3.2 %. • Compared to the solid element model, it reduces computational costs by at least 15 times. [ABSTRACT FROM AUTHOR]

Masonry arch bridges are highly vulnerable to floods and particularly to scour, as demonstrated by the many collapses that regularly occur in Europe. Scour levels that do not directly cause the collapse of a bridge may result in a significant reduction of their capability to withstand traffic loading. Thus, research on the performance of masonry arch bridges under combined scour and traffic loading, and their structural robustness, is of paramount importance. This study evaluates the behaviour of masonry arch bridges subjected to scour and traffic loading by analysing a representative case study with a three-dimensional finite element model developed in Abaqus according to a macro-modelling approach. Traffic load is selected in accordance with different code-based models, including those provided by Italian guidelines for safety assessment of existing bridges. The scouring process is imposed through the progressive removal of elements at the foundation level. Displacements and internal stress distributions for different bridge components are recorded and the capacity of the masonry bridge is estimated under increasing traffic load for different scour levels. Results are obtained in terms of both local and global response parameters to provide useful information on threshold levels for bridge safety and monitoring. The sensitivity of the bridge structural performance to material properties and traffic load position is also assessed. The study results can be useful to inform the decisions to be taken by bridge stakeholders (e.g. close bridge, limit traffic, keep bridge open) based on scour and/or structural response measurements. • Multi-hazard assessment of masonry arch bridges under scour and traffic loading. • Incorporation of traffic load models from Eurocode 1 and recent Italian guidelines. • Multi-scale assessment based on 3D FE macro-modelling and nonlinear static analyses. • Robustness to scour-induced damage based on residual traffic-load capacity. • Derivation of scour-induced damage thresholds for progressive collapse prevention. [ABSTRACT FROM AUTHOR]

The collapse resistance of prestressed precast concrete frames relies on the tie force of the strands, but the influence of the bond conditions and layout of the strands remains unclear. This study investigates these factors through push-down tests on four beam column substructures under penultimate column removal: one reinforced concrete specimen and three precast specimens (unbonded prestressed, bonded prestressed, and bonded with low-prestress parabolic profile). The results revealed that, compared to the reinforced concrete specimen, the precast specimens exhibited similar performance but were uniformly weaker under compressive arch action. Under catenary action, the precast specimens demonstrated significantly greater ultimate deformation and load-bearing capacity. The unbonded prestressed specimen failed due to wire rupture, impairing the flexural resistance of all its joints. In the bonded prestressed specimen, the bonded strand mitigated this issue but fractured prematurely. The bonded low-prestress specimen achieved the highest deformation and load-bearing capacity due to the enhanced deformation capacity of the low-prestress strand. • Push-down tests on bonded/unbonded prestressed precast concrete substructures. • The effects of strand profile and axial compression of side columns were analysed. • The resistance mechanisms of the precast substructures were revealed. • The comparison between the precast and monolithic substructures was involved. [ABSTRACT FROM AUTHOR]

A series of structural collapse failures over the last decades have triggered an increasing research interest to identify ways to prevent progressive collapse when structures are subjected to local damage due to foreseen or unforeseen actions. Strong nonlinearities, dynamic effects, and system behavior should be taken into account in progressive collapse analyses. In order to avoid time-consuming nonlinear dynamic analyses, approaches using a dynamic increase factor or the energy-based method to the static pushdown curve are widely adopted. However, in this article it is shown that reinforce concrete (RC) structures may be evaluated as safe according to a static analysis but essentially unsafe when considering a dynamic analysis due to the dynamic snap-through behavior. This phenomenon results in a dynamic sensitive failure mode (DSFM) that should be identified in relation to progressive collapse analyses. Comparing with the static situation, the dynamic instability may result in a much more brittle failure mode in the dynamic situation due to the dynamic effects. Hence, the structure becomes sensitive to the dynamic effects and this may further lead to danger of brittle failure, which should be prevented in practice. An efficient method is proposed to approximately determine the load-displacement region where the DSFM occurs. This region is designated as the DSFM window. A two-linear-spring system, a RC beam-column substructure, and a RC frame structure are employed to illustrate the DSFM and verify the proposed method. The results demonstrate that the proposed approach can effectively determine the DSFM window for RC structures subjected to column removal scenarios. • The phenomenon and mechanism of the dynamic sensitive failure mode (DSFM) during progressive collapse are investigated. • A method without conducting dynamic analyses for determining the DSFM window is proposed and verified. • The relationship between static instability and dynamic instability due to the dynamic snap-through behavior is studied. • The point of losing dynamic instability during progressive collapse is accurately identified by adopting the phase plan method. [ABSTRACT FROM AUTHOR]

A series of dynamic progressive collapse tests using the uniformly distributed load (UDL) was conducted, to analyze the robustness of three precast concrete (PC) beam–column assemblies using dry connections of top-and-seat angles (TSA-D), strengthened top-and-seat angles (STSA-D), and high-ductility reinforcement (DSTSA-D), under a middle column removal scenario. Finite element (FE) models were also developed to accurately simulate the collapse process. Based on the tests and FE models, comparative studies were conducted on dynamic and static collapse responses of the PC assemblies with identical configuration, loading regime, and boundary conditions. The results showed that: the horizontal reaction force developments were similar under dynamic and static collapse scenarios; under dynamic collapse scenarios, the initial stiffness of the PC assemblies increased due to the strain rate effect, while the peak vertical reaction forces under the compressive arch action (CAA) were smaller than those under static collapse scenarios, attributed to dynamic damage; under both dynamic and static collapse scenarios under UDL, the beam deformed in a downward convex curved shape, with local material deformation becoming more concentrated under dynamic collapse scenarios. An energy-based method for calculating dynamic collapse resistance was evaluated and revised: 1) the traditional method, which converts static responses from static analysis into dynamic responses was found to underestimate the resistance under small deformations due to neglecting the strain rate effect, and to overestimate ultimate resistance by ignoring dynamic damage; 2) by using dynamic vertical reaction forces instead, a more accurate prediction of the dynamic collapse resistance was achieved with a single dynamic collapse analysis. Additionally, dynamic amplification factors for the PC assemblies were calculated based on FE models and static test results providing basic knowledge in whole PC structural level research. • Dynamic progressive collapse tests under uniformly distributed load. • High-fidelity finite element models simulating dynamic collapse of PC assemblies. • Comparison between dynamic and static collapse responses of PC assemblies. • Dynamic collapse resistance considering strain rate effect and dynamic damage. • Dynamic amplification factors (DAF) for simplified collapse resisting design. [ABSTRACT FROM AUTHOR]

This study investigates the plastic seismic design of moment-resisting frames using concrete-filled steel tube columns (MRF-CFST) by utilizing the theory of plastic mechanism control (TPMC). Fundamentally, within the rigid-plastic analysis framework, the TPMC relies on a kinematic approach and an equilibrium curve for mechanisms. Employing this theory in the design of steel frames ensures the formation of a global collapse mechanism while deterring undesirable occurrences like a soft story. Using the TPMC, the kinematically admissible multiplier for seismic horizontal forces linked to the global mechanism is the minimum among all the kinematically admissible multipliers for other generic mechanisms. In this study, three methods were used to address the TPMC conditions: (a) Story-based method in which only the sum of the required plastic moments for columns in each story are unknown—identified as Design Method 1 (DM1); (b) Story-based method in which the sum of required plastic moments for both beams and columns in each story are unknown—denoted as Design Method 2 (DM2); and (c) Member-based method in which the demands for all members are unknown—denoted as Design Method 3 (DM3). A linear programming (LP) problem has been formulated for all design methods that incorporate the TPMC conditions as constraints. The DM1 used in previous studies serves as the conventional method for solving the TPMC conditions. A comparison between the three design methods has been conducted. The results show that DM3 stands out as the superior method, effectively minimizing weight and maximizing the strength of structures. Meanwhile, DM2, despite having fewer unknown parameters compared to DM3, yields the results that are comparable to DM3. To show the practical application of the proposed design methods, seismic design of three MRF-CFST with 5, 10, and 15 stories has been conducted. In order to verify the seismic design goal of achieving a global mechanism, both pushover analysis and incremental dynamic analyses (IDA) were utilized. The results illustrate the successful establishment of a global mechanism and the outstanding seismic performance of the designed structures. • This study proposes seismic plastic design of MRF-CFSTs using TPMC. • TPMC ensures a global collapse mechanism, avoiding undesirable plastic mechanisms. • Three design methods (DM1, DM2, DM3) are suggested to address TPMC conditions through linear programming. • Comparison shows that these methods minimize weight while maximizing structural strength. • Practical design examples are verified through pushover and IDA analyses. [ABSTRACT FROM AUTHOR]

To address the insufficient collapse resistance of the beam-column welded connection (BWC), this study proposes a strengthening beam-column welded connection with truss elements (BWCTE). A static test on a BWCTE specimen with a composite slab was conducted to investigate its failure pattern, final deflection, and the evolution of axial force, bending moment, and collapse resistance. The results indicate that the addition of truss elements increases the peak load and corresponding deformation of BWCTE by 108.6 % and 27.3 %, respectively, compared to BWC. Moreover, the axial force in the composite beam of the BWCTE specimen achieved the yield axial force, facilitating the complete development of catenary action and optimizing beam performance. Subsequently, primary parameters of the truss element were analyzed using refined numerical models. It is recommended that the projected length of the inside leaning limb is within 0.5 to 0.7 times the height of the steel beam, and the thickness of each limb is within 0.6 to 1.2 times the thickness of the beam flange. Then, the working mechanism of BWCTE was explored through theoretical analysis. The resistance process of BWCTE can be divided into elastic, elastic-plastic, plastic, transition, and catenary stages. The loading and deformation synergies between truss elements and composite beams enable the formation of double plastic zones and double strengthening zones at the beam root, effectively delaying the rupture of the stretched beam flange. Finally, a design method for BWCTE is proposed according to theoretical and numerical analysis. • A strengthening beam-column welded connection with truss elements (BWCTE) is proposed. • Failure pattern, final deflection, and the evolution of axial force, bending moment, and collapse resistance of BWCTE are investigated. • Compared to fully-welded connection, the addition of truss elements increases the peak load and corresponding deformation of BWCTE by 108.6 % and 27.3 %, respectively. • The working mechanism of BWCTE is explored through theoretical analysis. • A collapse resistance design method for BWCTE is given. [ABSTRACT FROM AUTHOR]

In the research field investigating the progressive collapse of building structures, event-dependent collapse processes have gained increasing attention in analytical and numerical studies. Different events could cause varying effects on progressive collapse resistance. The alternate load path could be related to events that could also lead to variations in load actions. However, experimental studies on reinforced concrete (RC) frame structures triggering structural column removal by the low-velocity impact have not been reported. Therefore, this paper performed an initial experimental study employing impact loading as the extreme event to investigate subsequent progressive collapse behavior of structures. Tests were conducted on six RC substructures consisting of a two-span beam and a structural column. Gravity loads were applied to the top of substructures, and a pendulum impact setup was utilized to remove RC columns by low-velocity impact. When the column underwent lateral failures, the downward force exerted by longitudinal steel bars of the column, before they fractured, pulled the two-span beam beyond the compressive arch action (CAA) stage, leading to a collapse process entirely different from their event-independent counterparts. The parametric study based on experimental results indicated that low-elevation impact and increase of column longitudinal bars detrimentally affected the performance of two-span beams resisting progressive collapse, while the increase in concrete strength partially improved the residual bearing capacity after impact column removal (ICR). Based on a dynamic model, a simplified calculation method is proposed for quantifying the downward force. • An initial progressive collapse experiment triggering column removal by impact loading. • Specimens under ICR or NCR exhibited significant discrepancies in resistance mechanisms transitions. • Low-elevation impact and higher areas of column longitudinal bars were detrimental when considering ICR. • Residual load-bearing capacity tests under gravity loading were performed and revealed new findings. • A simplified method is proposed to quantify the downward pulling force exerted by impacted columns. [ABSTRACT FROM AUTHOR]

Impact loads often act on columns end during accidents such as vehicle impacts and terrorist attacks. The resulting deformations lead to a redistribution of internal forces in the member as well as a reduction in the load carrying capacity. The integrity of the core area of the joint is threatened, which could trigger a progressive collapse. This study numerically investigated the damage and deformation of reinforced concrete (RC) beam-column joints when they are impacted on the column end. The resistance mechanism of joints and the influence of impact location as well as impact velocity was explored. Besides, the deformation capacity of RC beam-column joints under impact loading was quantified using equivalent plastic hinges. It was found that the damage and plastic deformation is concentrated at the impacted column end. The horizontal reaction force at the beam end provides resistance and plays the role as a bearing. The column bottom reaction force is in the same direction as the impact force. However, it is much smaller than the beam end reaction force. The resistance mechanisms within the joints can be categorized into strut-and-tie near the loading point and arches in the mid-span of the beam and column. The core area rotates and has a significant lateral displacement towards the impact location, whether it is loaded at the beam end or the column end. As the impact location moves away from the core aera, the impacted column end exhibit more bending failure. With increasing impact velocity, the impact force, reaction force and total energy absorption of the joint increase. The deformation increases, with more damage occurring in the core area and spreading to the beam. In addition, the plastic curvature was used to describe the actual plastic deformation of the joints, whose maximum occurring at the interface where the impacted column end meets the core area. A formula for the equivalent plastic hinge length of RC joints was proposed, taking into account the effects of impact location and impact velocity. • The failure mechanism of RC joints under impact load was investigated. • The deformation and internal force of the core area was analyzed. • The influence of impact location and impact velocity was discussed. • A formula for the equivalent plastic hinge length of RC joints was proposed. [ABSTRACT FROM AUTHOR]

Low-cycle fatigue (LCF) failure has been recognized as one of the most common failure modes of structures experiencing a major earthquake; on the other hand, progressive collapse, possibly triggered by impact, blast, or fire, is another dangerous consequence to structures when in the absence of adequate robustness design. Due to varied performance demands, addressing both issues with consistent design solution can be challenging. This paper proposes a new multi-hazard protection concept utilizing iron-based shape memory alloy (Fe-SMA) components to improve both the robustness (against progressive collapse) and seismic resilience (against earthquake-induced LCF failure) of steel frames. Fe-SMA has emerged as a promising material in the field of civil engineering due to its unique thermal-induced shape recovery, superior low cycle fatigue (LCF) resistance, and outstanding ductility. In this paper, the angle form of Fe-SMA components is selected, serving as energy dissipation elements in steel beam-to-column connections which are crucial for ensuring structural safety/integrity against various hazards. Experimental investigations were conducted to understand the basic mechanical properties of Fe-SMA angles under monotonic tensile and LCF loading, followed by comprehensive numerical studies demonstrating the use of Fe-SMA angles for steel beam-to-column connections against cyclic loading as well as 'column loss'. The results confirmed that Fe-SMA is capable of resisting cyclic loading with much improved LCF life, and meanwhile, offers significantly enhanced chord rotation capacity as well as maximum dynamic load resistance of beam-to-column connections against progressive collapse. • A concept of using Fe-SMA components for enhancing both robustness and resilience of steel frames is proposed. • Monotonic tensile and cyclic loading tests on Fe-SMA angles are conducted. • A comprehensive analytical description of considered Fe-SMA angle is provided. • FE model of Fe-SMA angle is established and its feasibility is validated. • The improved robustness and resilience of the Fe-SMA angle-bolted beam-to-column connection are showcased. [ABSTRACT FROM AUTHOR]

As buildings have to face more frequent and intense extreme events due to a changing climate and rising geopolitical tensions, the need for more robust structures that are insensitive to initial failures is now arguably more important than ever before. In this context, this article presents an in-depth analysis of the damage and failures that occurred during the partial progressive collapse test of a purposely built 15 × 12 m precast building with two floors. The building was designed to ensure load redistribution after the failure of single columns and also to segment parts of the structure from a larger initial failure that inevitably triggers a collapse. The ability of the structure to arrest collapse propagation was tested experimentally successfully isolating the collapse to the initially affected area. Based on visual inspections supported by computational simulations, failure mechanisms occurring in the collapsed area and damages in the upright part of the structure are analysed in detail in this article. Mechanisms that enabled the separation of the building in two parts are also studied. The presented results lead to an improved understanding of failure mechanisms causing collapse propagation and provide insights on how such propagation can be arrested. • Post-failure analysis of a full-scale partial collapse test is performed. • Combining inspections and simulations allows deeper investigation. • Different failure modes were observed for various building components. • Mechanisms enabling load redistribution and collapse isolation are identified. • Damages suffered by part of the building remaining upright were assessed. [ABSTRACT FROM AUTHOR]

This paper presents the experimental tests of steel substructures under an internal column loss scenario. Two different models of a column-beam joint were subjected to static loading generated by a hydraulic jack. The first specimen was a classic bolted flush end-plate joint, and the second was the innovative one built based on the latter. The novel joint was reinforced with two horizontal, circular steel plates bolted to upper and bottom beam flanges. This solution aims to mitigate the risk of a progressive collapse phenomenon. Detailed results of the experimental tests are presented: load, strains in steel elements and tension bolts, rotations and deformations. Large deformations were obtained in the flush end-plate specimen, especially in the column. Fracture of tension bolts was the failure mode. The innovative joint's static response presents a significant load capacity increase. Furthermore, a substantial reserve of load-bearing capacity and ductility is observed. The failure of the innovative joint was not achieved in the experimental test. A comparison of both tests presents similar behaviour at the initial phase of loading. The influence of steel rings on the innovative joint behaviour is apparent in subsequent loading phases. Additionally, the analytical model to determine the load-capacity relationship of the innovative joint is presented. It is based on the component method and compared with the classic model. The presented results show good compliance of this model with the experiment. • Experimental tests of steel bolted flush end-plate joint and innovative joint under static loading. • Static response of steel joints under column loss situation. • Measuring strains in tensioned bolts under the experiment. • Comparison result of pure flush end-plate joint and innovative joint. • Significant load capacity of the innovative joint without damage. • Analytical model to assess load capacity and rotational capacity. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study on three-dimensional steel frame structures subjected to progressive collapse, drawing insights from a full-scale steel frame substructure test. The investigation encompasses connection component tests and numerical simulations, focusing on extended end plate and double-angle cleat connections employed in the substructure test. The mechanical properties of the connection components were tested, forming a basis for defining component properties in subsequent connection models. Finite element (FE) models of the test substructure were developed, utilizing hybrid elements for the steel frame part, which include beam, connector, and spring elements based on the component method. To simulate reinforced concrete slabs, a combination of refined solid elements and simplified shell elements was employed. The former accurately captures detailed failure modes, while the latter efficiently simulates the collapse behavior of large-scale steel frame structures. Validation of the established models against test results encompassed load-displacement responses, internal forces in structural members, and failure modes. The validated FE models were then utilized to analyze and discuss the contributions of various structural components in resisting progressive collapse. Specific focus was placed on the development of load-resisting mechanisms in the floor slab and the influence of beam-column connection types on structural behavior. The paper explores the role of bracing systems in a building in resisting progressive collapse. Additionally, it evaluates the effectiveness of the restraint systems used in the test, shedding light on their ability to accurately reflect real restraint effects from the surrounding structure. The findings presented herein contribute valuable insights to the understanding of progressive collapse behavior in steel frame structures, with implications for robustness design of multi-story steel buildings. • A novel component test method for extended end plate connection was proposed. • FE models of steel frame structures for progressive collapse were built and validated. • Contributions of structural members in resisting progressive collapse were clarified. • Evolutions of load-resisting mechanisms in floor slabs were explained. • Effects of beam-column connection types and bracing systems were discussed. [ABSTRACT FROM AUTHOR]

Resisting mechanisms including compressive membrane action (CMA) and tensile membrane action (TMA) are critical for RC beam-slab structures against progressive collapse. However, due to the difficulties in considering the nonlinear behaviors and beam-slab interactions, limited analytical studies are currently available to accurately determine the whole resistance evolution path, let alone a simplified design model. This study presents a unified analytical model and a design-oriented simplified model to predict the whole resistance displacement of RC beam-slab substructures under internal column loss. Both equivalent uniformly distributed load (EUDL) and concentrate load (CL) conditions have been considered. In the analytical model, the beam-slab system is discretized into interior beams and multiple slab strips with consideration of their interaction. The compatibility and equilibrium conditions, curvature equations, and constitutive laws of concrete and reinforcement are incorporated to analyze each beam and slab strip. In the design model, a new strategy is proposed to decompose the structural resistance of membrane effect into summation of two components, CMA and TMA resistances, with explicit expressions. Both developed models are validated through experimental and numerical results and good agreements are achieved. Results of the analytical model show that ignoring the beam-slab interactions can lead to underestimation of structural resistance, especially during the transition from the CMA to TMA. The locations of loading points should be carefully selected to make sure 12-point loading system effective. A rule is also suggested to determine proper locations. The simplified design method gives slightly conservative predictions of structural resistance, and its calculation is easy for engineers to use. • A unified analytical model is developed to evaluate the compressive and tensile membrane actions in RC beam-slab structures. • A design-oriented simplified model is proposed to predict the whole resistance evolution path of RC structures. • Both equivalent uniformly distributed load and concentrate load conditions are considered. • The developed models are validated through experimental and numerical results and good agreements are achieved. [ABSTRACT FROM AUTHOR]

The article presents the results of experimental and theoretical studies of deformation, cracking and fracture of frame structural systems under accidental impacts. The elements of frame structures may exhibit bending with torque as a consequence of the occurrence of an abnormal event. The study provides an energy-based approach to determining internal forces in such structural systems under a scenario in which a column is removed. It also presents a computational model that evaluates the robustness of a reinforced concrete frame of a multi-story building. The model permits the determination of dynamic effects within a structural system, at varying levels of static loads acting within it, and enables the investigation of deformation patterns within the spatial cross-section, including those experiencing bending and torsion. It also allows for the calculation of the robustness parameter of frame systems. In order to validate the proposed computational model, the study employs the outcomes of tests conducted on scale models of reinforced concrete frames with beams subjected to bending and torsion. The experimental frames were loaded in two stages. In the initial stage, the frame structures were subjected to static loading. At the second stage, scaled models were exposed to dynamic loading, which was caused by the sudden removal of columns. A comparison of calculated and experimental results for the investigated structures demonstrates the reliability and efficiency of the proposed calculation model. Consequently, it can be used to calculate the progressive collapse resistance of reinforced concrete frames of multi-storey buildings. • The determination of loadings is carried out using an energy-based approach in combination with force-displacement diagrams. • Parameters of the stress-strain state are determined on the basis of the spatial section model. • Experimental studies were conducted on models of reinforced concrete frames with beams subjected to torsional and bending. [ABSTRACT FROM AUTHOR]

Post-tensioned slab systems serve as an effective method to enhance the progressive collapse resistance of concrete flat plate structures by significantly improving the punching shear and post-punching capacities of their slab-column joints. While the existing research mainly focused on the punching shear behaviour of post-tensioned slab-column joints in small deformation stages, the post-punching behaviour in large deformations has been overlooked. This study aims to fill this research gap thorough experimental and numerical studies. Experimentally, four slab-column joints, including three post-tensioned specimens (with varied prestressing levels) and one specimen without tendons, were designed and tested. Detailed failure process and load-resisting mechanisms of the specimens at different deformation stages were discussed. The test results confirmed the efficiency of post-tensioning in improving the punching shear performance. Varying prestressing levels have influenced the development of punching shear capacity but did not affect the growth rate of load resistance in the initial post-punching stage. Numerically, validated finite element models were used to predict the post-punching capacities of the specimens. The numerical results indicated that post-tensioning helped increase post-punching capacities, but such an enhancement was independent of the prestressing levels. Additionally, the influence of the rupture strains of both tendons and reinforcement on the post-punching capacity was investigated through a parametric study. • Completed failure process and load-bearing mechanisms of post-tensioned flat plate slab-column joint specimens were explored. • The post-tensioning systems in improving the punching shear and post-punching performance was investigated. • High-fidelity 3D FEM models of the post-tensioned flat plate slab-column joints were developed and validated. • The influence of the rupture strains of rebars and tendons on the post-punching capacity was qualified. [ABSTRACT FROM AUTHOR]

To investigate the effect of steel reinforcement corrosion on the progressive collapse resistance of reinforced concrete (RC) structures, this study shows a methodology for establishing a finite element model (FEM) of a beam-slab structure by LS-DYNA software considering steel reinforcement corrosion. The reliability of this methodology was validated by experimental results. The analysis results indicate that considering the effect of the slab causes the beam-slab structure to enter the stages of tensile catenary action (TCA) and tensile membrane action (TMA) at smaller displacement. Moreover, as the corrosion time increases, the influence of the TCA on the structural resistance decreases. After prolonged corrosion, the top reinforcements of the beam at the failed column sides fail to fully utilize their tensile capacity when the structure fails. Furthermore, the corrosion of reinforcement significantly weakens the TCA and TMA to the structure, and neglecting the effect of the slab may lead to an overestimation of the weakening degree of the TCA caused by steel reinforcement corrosion. Additionally, the contribution of the slab's resistance in the beam-slab structure follows an "S"-shaped curve in relation to the displacement of the failed column, and the contribution of the slab to the structural resistance ranges between 40% and 60%. • The reliability of FEM to simulate progressive collapse behavior of substructures considering rebar corrosion was assessed. • Load resistance and load transferring mechanisms of corroded beam-slab structures were investigated. • Effects of rebar corrosion on the membrane action, tensile catenary action of substructures were quantified. [ABSTRACT FROM AUTHOR]

The Alternative Load Path method is widely adopted to perform progressive collapse analyses for reinforced concrete (RC) frame structures subjected to column removal scenarios. Considering that the progressive collapses are usually dynamic phenomena, nonlinear dynamic analyses can yield the most accurate results. However, the associated computational cost is high. Alternatively, the energy-based method (EBM) can be employed to efficiently calculate the maximum dynamic responses. Unfortunately, collapse or dynamic limit states (DLS) cannot be directly determined when the EBM is adopted for the progressive collapse analyses, because it is essentially a quasi-static approach. To tackle this issue, in this paper an EBM-Intersection Point-based (EBM-IP-based) method is proposed to effectively determine the DLS. The method is considering that the dynamic instability will occur once the dynamic capacity curve exceeds the corresponding static capacity curve. Hence, it is possible to determine the DLS by making a correction on the intersection point of the aforementioned two curves without performing nonlinear dynamic time-history analyses. Compared with the other energy components, the kinetic energy at the moment of the peak displacement response is relatively small and the effectiveness of the EBM is verified. Through both static pushdown analyses and incremental dynamic analyses for 48 column removal scenarios of six different multi-story RC frames, the effectiveness of the proposed EBM-IP-based method is verified and the associated coefficients are calibrated. Statistical information and (joint) probabilistic density functions (PDF) of the coefficients are identified. Further, stochastic analyses are carried out in order to quantify the model uncertainties when adopting the EBM-IP-based method. On the basis of all the calculation results, a Gumbel distribution is adopted to capture the PDF of the model uncertainty in relation to displacements, while a lognormal distribution is adopted to fit the PDF of the model uncertainty in terms of resistances. Moreover, a multi-variate Gaussian distribution model with two components is employed to fit the joint PDF. • A method for determining dynamic collapse limit state without dynamic analysis. • Energy evolution features during progressive collapse process investigated. • Static and dynamic instability during progressive collapse compared. • Influence of uncertainties on progressive collapse behavior quantified. [ABSTRACT FROM AUTHOR]

The study is focused on the comparison and discussion of different approaches within the use of the global resistance method (GRM) for safety assessment of reinforced concrete (RC) systems using non-linear numerical analyses (NLNAs). With this purpose, a benchmark dataset, comprising 56 experimental results obtained from tests on 40 RC columns with variable slenderness and 16 non-slender RC elements including walls, deep beams and shear walls, is considered. The NLN models for all the 56 members adopt solution strategies able to optimize the agreement between numerical predictions and experimental outcomes. Then, probabilistic hypotheses have been defined regarding both aleatory (i.e., materials and geometry) and epistemic uncertainties (i.e., model) associated with all the 56 RC members. These assumptions form the basis for developing a comprehensive set of probabilistic analyses of the global structural resistance for each RC member. The results of these probabilistic analyses offer valuable insights into the impact of the different sources of uncertainties on the global structural response. In detail, three distinct approaches for estimating the global safety factors within the GRM are outlined and compared. The purpose is to address the effectiveness of the different approaches for the reliability evaluation of RC members within the GRM together with the relevance of both aleatory and epistemic uncertainties. Ultimately, recommendations are provided regarding the adoption of the GRM in the upcoming generation of design codes. • Three approaches to derive global safety factors according to the GRM are compared. • 40 RC slender and 16 RC non-slender elements are investigated through NLNAs. • Both aleatory and epistemic uncertainties are included in probabilistic analyses. • A sensitivity analysis is developed to identify the dominant role of uncertainties. • Useful recommendations are provided to implement the GRM within design codes. [ABSTRACT FROM AUTHOR]

Previous studies have found that the Vierendeel action (VA) is one of the main mechanisms against progressive collapse of multi-storey reinforcement concrete (RC) frame structures in the event of corner column failure. However, the knowledge remains inadequate in terms of the behavior, mechanism, and quantitative contribution of the VA to the collapse resistance. To fill this gap, quasi-static tests were performed on a 1/3 scale single-storey RC substructure and a three-storey RC substructure with identical geometry, material, reinforcements and boundary conditions under corner column removal scenario. Test results found that, compared with the single-storey substructure experiencing no VA, the three-storey substructure developed additional plastic hinges at the beam ends near the corner column and at the lower end of the corner column as a result of the VA. The ultimate resistance of the three-storey substructure was 10.6% more than the triple ultimate resistance of the single-storey substructure. Then, numerical analysis was conducted to cognize the formation sequence of the plastic hinges and the effect of slabs. Furthermore, two coefficients were defined to reveal the mechanism of the VA in the three-storey substructure, i.e., the displacement-related coefficient e d and the coefficient associated with ultimate resistance e u. Finally, a simplified analytical model was proposed to predict the ultimate resistance of the three-storey structures with consideration of the VA. • A single-storey and a three-storey RC substructure are tested under corner column removal scenario. • The formation sequence of the plastic hinges and the effect of slabs are numerically investigated. • The behavior, mechanism and quantitative contribution of the Vierendeel action are revealed. • A simplified analytical model is proposed to predict the ultimate resistance of the three-storey structures with considering the Vierendeel action. [ABSTRACT FROM AUTHOR]

Evaluating the progressive collapse resistance of truss string structures (TSSs) in the context of key member failure presents a significant challenge, particularly when this indicator is crucial during structural design and performance evaluation processes. Fortunately, machine learning (ML) methods can establish complex and nonlinear relationships between input and output variables during structural performance evaluation. In this study, firstly, 20 practical projects of TSSs are surveyed to determine the critical design parameters and statistical attributes of TSSs, which provide a realistic basis for the subsequent establishment of a sample database of the progressive collapse resistance of TSSs. Secondly, 464 models of TSS are generated by random sampling through MATLAB, and the progressive collapse resistance of TSSs under the failure of cable or lower chord at the support is analyzed based on the validated finite element model, followed by the establishment of the progressive collapse resistance database. Moreover, based on the results of the analysis, the effect of different parameters on the progressive collapse resistance of TSSs is discussed for the two typical failure scenarios. Subsequently, five ML models are introduced, and a framework for predicting the progressive collapse resistance is given. Next, the prediction results of different ML models were compared. The results show that the extreme gradient boosting (XGBoost) model performs best for the failure of cable or bottom chord at the support, with R2 values of 0.988, 0.920, and 0.972, 0.801 for the training and testing sets, respectively. Finally, the physical and quantitative interpretation of the progressive collapse resistance of TSSs obtained from XGBoost model-based predictions under two typical failure scenarios is conducted using the Shapley additive explanations (SHAP) method. • The key design parameters and statistical attributes of TSSs is determined. • A sample database of the progressive collapse resistance of TSSs is established. • ML-based models for predicting progressive collapse resistance of TSSs is given. • XGBoost model performs best for two typical failure scenarios. • An interpretable XGBoost-SHAP ML method is established. [ABSTRACT FROM AUTHOR]

Currently, progressive collapse studies are mostly conducted based on an event-independent assumption. With studies employing an event-dependent premise mainly concerning explosion or fire events, the aftermath of impact loading is seldom reported. Meanwhile, interactions between reinforced concrete (RC) members and superstructures under impact loading need further evaluation. In this paper, finite element models of RC structures subjected to impact loading and progressive collapse are established and validated utilizing LS-DYNA. A valuing methodology of erosion parameters for the continuous surface cap model (CSCM) considering element size is proposed in this process. The influence of impact column removal (ICR) on the progressive collapse performance of RC frame structures is studied at sub-assemblage and structure levels. The parametric study indicates that the ICR process can be described by an impact loading stage and a gravity load stage. It is also found that structures experiencing ICR are exposed to a higher risk of progressive collapse, with the downward force exerted by the impacted columns being a significant contributing factor. Dynamic analyses demonstrate that the acceleration of the column removal point (CRP) can be used to validate and quantify the downward force. The hybrid force-displacement boundary conditions of frame columns give rise to the development of downward force. Recommendations for resisting progressive collapse considering ICR are proposed based on the analytical results of the paper. • A new threat-dependent perspective to progressive collapse research is elucidated, focusing on impact column removal (ICR). • Interactions between impacted columns and superstructures are illuminated, emphasising the influence of downward force. • The boundary conditions of frame columns suggest a new research roadmap for impact response of structural members. • The study on the effects of ICR indicates higher progressive collapse risk due to downward forces and initial damage. • Recommendations of resisting progressive collapse considering ICR are proposed. [ABSTRACT FROM AUTHOR]

This study focused on investigating bond-slip behaviour of headed bars embedded in well-confined concrete. Test results based on eight pull-out specimens under displacement-controlled incremental loading showed that recommended development lengths from ACI318–19 are conservative in developing ultimate strength of headed bars associated with smaller slips than straight bars. Moreover, it was found that different (but sufficiently long) embedment lengths had a significant effect on the free-end slip but negligible effect on the loaded-end slip of headed bars. Additionally, the study highlighted the significance of head bearing in anchorage formation, especially during inelastic stage when bond strength had deteriorated. To address inaccuracy in predicting structural behaviour caused by inadequate anchorage strength of headed bars, a macro bond-slip model was proposed based on correlation in strain distribution obtained from the pull-out tests. Subsequently, the proposed model was incorporated into a component-based model (CBM) and validated against test results of PC joints under seismic loading. A numerical investigation using the validated CBM was conducted to study behaviour of exterior PC joints with headed bars and special detailing. These included Type X joint with X-bent bars and plastic hinge relocation (PHR), and Type A joint with an additional bar layer (ABL) in the beam-joint region, subjected to extreme loads such as seismic action or progressive collapse. The results revealed that seismic behaviour of exterior joints was governed by ductile failure of the beam at the PHR and vertical joint interface in Type X and Type A joints, respectively. Progressive collapse was primarily caused by column failure, with flexural and catenary action effectively preventing collapse. Notably, under both loading conditions, presence of ABL in Type A joint and X-bent bars with PHR in Type X joint improves flexural strength of beams at critical sections and joint shear strength, ultimately enhancing structural performance of joints. Design recommendations include replacing hooked bars with headed bars as well as implementing X-bent bars with PHR and ABL in PC joints. The authors advocate the strong-column-weak-beam design approach as an effective measure to prevent progressive collapse. • Pull-out tests on straight and headed bars embedded in well-confined concrete. • Bond-slip model of headed bars. • Component-based model for PC joints with headed bars. [ABSTRACT FROM AUTHOR]

Improving the integrity of precast concrete (PC) frame structures using dry connections is essential for enhancing their robustness against progressive collapse. Therefore, three PC beam–column assemblies using different dry connections specially designed to ensure the connectivity were experimentally examined under progressive collapse scenarios. And a seven-point loading tree device was adopted to simulate the uniformly distributed loading conditions in practical engineering. The connection details were classified as follows: normal top-and-seat angle connection in TSA, strengthened top-and-seat angle connection in STSA, and strengthened top-and-seat angle with high ductility longitudinal reinforcements in the plastic regions in DSTSA. Under small deformations, STSA had the highest first peak load attributable to its strengthened joint connection. TSA obtained a lower load than that of STSA because of the low joint connection stiffness. Meanwhile, DSTSA achieved the lowest load among the three specimens because the smoother surface of the high-ductility reinforcements (compared with that of normal ones) weakened its bond to the concrete. However, under large deformations, the high ductility of the reinforcements ensured the specimen integrity; consequently, the largest final load was achieved in DSTSA. On the contrary, the high joint stiffness and normal reinforcements in STSA hindered the beam end rotation, and the steel angle failed early in TSA. As a result, the two connections failed to ensure the specimen integrity. To further shed light on the resistance mechanisms of beam–column assemblies under the uniformly distributed loading condition, an iteration-based model was developed to calculate the first peak load under the beam and compressive arch actions. The maximum prediction error achieved was 12%. Based on the model, the load contribution from the friction effect of the loading device was identified as 27%. And it was found that during the collapse, increasing reinforcement deformation region that might be induced by slippage between the concrete and reinforcements could lead to a continuous increase in the horizontal reaction force after the specimen reaching the first peak load. • Conduct progressive collapse tests under uniformly distributed loads. • Propose beam–column connection joints improving structural integrity. • Calculate beam–column assemblies resistance under uniformly distributed loads. • Investigate influence of uniformly distributed loading device on collapse resistance. • Analyze collapse resisting mechanisms under uniformly distributed loads. [ABSTRACT FROM AUTHOR]

This study investigated progressive collapse behaviour of earthquake-damaged interior precast concrete (PC) joints with headed bars. The main objective was to quantify the impact of slight and moderate earthquake damage on their residual collapse resistance. The research began with a numerical investigation to understand the interaction between a 2D frame's global response to seismic loading and its local response during a threat-independent progressive collapse event. Additionally, a separate set of numerical studies examined interior PC joints under cyclic loading to determine maximum drift ratio demands, yielding 2.5% for slightly and 3.5% for moderately damaged joints. Subsequently, an experimental programme under quasi-static loading conditions was conducted on five interior PC joints under cyclic loading, followed by progressive collapse resistance tests. Test results indicated that damaged joints exhibited flexural and catenary action during progressive collapse, but compressive arch action could not be mobilised due to residual cracks from the initial cyclic loading phase. Comparing damaged joints to fully intact ones from a companion study revealed deterioration in stiffness and deformation capacity during a subsequent progressive collapse, with degree of severity proportional to the extent of damage sustained in the cyclic loading phase. Relative to the intact joints without being subjected to cyclic loading, moderately damaged specimens exhibited a degradation of 63% in stiffness and 38% in deformation capacity, compared to slightly damaged specimens, which showed reductions of 38% in stiffness and 28% in deformation capacity. In contrast, strain hardening of steel bars in both slightly and moderately damaged specimens led to slightly greater collapse resistance than the intact joints. Additionally, failure mode shifted from ductile fracture to premature pull-through failure of headed bars in moderately damaged specimens. In this regard, in post-earthquake progressive collapse assessment, it is recommended to use maximum crack width thresholds of 0.2 to 1.0 mm for slight damage and 1.0 to 2.0 mm for moderate damage. • Residual progressive collapse behaviour of earthquake-damaged interior precast concrete (PC) joints. • Recommended respective drift ratio demands of 2.5% and 3.5% in seismic testing to induce slight and moderate damage for PC joints. • Earthquake-damaged interior PC joints exhibited reduced stiffness and deformation capacity but enhanced collapse resistance. • Recommended crack width thresholds (0.2-1.0 mm for slight, 1.0-2.0 mm for moderate damage) in post-earthquake progressive collapse assessment. [ABSTRACT FROM AUTHOR]

Considerable research efforts have been dedicated to understanding the resistance of buildings against progressive collapse. However, these efforts have been relatively limited in the context of bridges, despite the equal, if not more, critical importance of robustness criteria in bridge engineering. In the context of existing bridges, it is crucial not only to assess safety, but also to evaluate robustness using reliable metrics. These metrics can aid managing authorities in prioritizing necessary interventions. Considering this, the paper applies various robustness measures to a specific type of reinforced concrete (RC) girder bridge known as half-joint bridges. As a case study, the Annone viaduct is examined, which collapsed in 2016 due to the passage of a heavy truck. A notional removal approach of critical elements to quantify structural robustness is proposed. This approach considers several load configurations, some envisioned during the design stage, and others representing abnormal loads. The study results reveal specific scenarios that may lead to potential progressive collapse and highlight the preferred metrics for this type of bridges. Ultimately, the assessment of robustness can play a key role in choosing a retrofitting solution over other intervention options for existing bridges. • Assessment of robustness of half-joint RC girder bridges. • Quantifying structural robustness via half-joint removal. • Robustness metrics pivotal for bridge prioritization and retrofits. [ABSTRACT FROM AUTHOR]

Since progressive collapse is essentially a dynamic process, the whole-process resistance evolution path is critical for a structure against progressive collapse. Previous analytical studies focus primarily on one particular phase of a single load transfer mechanism, such as compressive arch action (CAA) or catenary action (CA) capacity. This study develops a unified analytical model to evaluate the whole resistance-displacement curve of reinforced concrete (RC) structures. In the analytical model, compatibility, equilibrium condition, constitutive laws of concrete and reinforcing bars, and fracture of reinforcing bars are considered. To avoid complex iterations, curvature equations and combination schemes of cross-sectional forces are also established. The analytical model is verified against experimental resistances and axial forces of RC beam-column substructures measured in 24 tests, and it is subsequently utilized to illustrate the evolution of cross-sectional forces throughout the loading process. Furthermore, a sensitivity analysis based on the K-L entropy index is conducted to quantify the effects of different parameters on the progressive collapse resistance. The results show that the yield strength and diameter of reinforcing bars are vital to the whole resistance curves. The axial stiffness and beam depth primarily affect CAA capacity, while having a less significant impact on the CA capacity. Fracture displacement of reinforcing bars is the most influential parameter for CA capacity, suggesting the importance of considering the fracture of reinforcing bars in the analytical model. • A whole-process analytical model is developed to evaluate the resistance-displacement curve of RC structures. • The analytical model is verified through experimental resistances and axial forces of RC beam-column substructures. • The evolution of beam cross-sectional forces throughout the loading process is illustrated by using the model. • A sensitivity analysis based on the K-L entropy index is conducted to quantify the effects of different parameters. [ABSTRACT FROM AUTHOR]

• 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]

• The role of sectional strength degradation in progressive collapse resistance is demonstrated. • A theoretical model is developed for the resistance function incorporating strength degradation. • Model is validated and effect discussed with finite element simulation and experimental results. Catenary action is one of the structural mechanisms that could develop in a laterally restrained beam when subjected to large deformation, and it is a primary mechanism in the resistance to progressive collapse of frame structures in the typical scenario of a column removal. Analytical methods for the analysis of the catenary action exists for idealised beams with simple non-degrading yielding conditions. However, in a realistic beam assembly, especially under a column removal scenario, intermediate failure events such as failure of weld or a bolt component at the connection can occur; consequently, the strength of the beam in the plastic region degrades abruptly. This paper presents a simplified theoretical model taking into consideration the degradation of strength in the plastic regions. The basic model is formulated for a generic beam assembly axially restrained with a variable restraining stiffness (flexible axial support), and involves a middle connection. The strength degradation is represented by degraded yield functions. The model subsequently generates a realistic vertical load-deflection relationship, i.e. the resistance function. Comparison of load-deflection relationship so generated with corresponding finite element analysis shows good accuracy. Further comparison of the theoretical model with an existing laboratory experiment also exhibits good agreement. With the generated resistance functions, analysis of the actual dynamic response of beam assemblies in a progressive collapse scenario can be carried out using a standard procedure such as the energy method. [ABSTRACT FROM AUTHOR]

• A method assessing a structure's progressive collapse (PC) vulnerability is proposed. • The proposed energy flow approach (EFA) illustrates an energy – failure correlation. • This EFA predicts failure using Energy Flow vs Energy Flow Rate (E-R) failure thresholds. • This EFA to predicts PC of a damaged mid-rise steel building with repeatable accuracy. • The DOD's Alternate Path (AP) method approaches for PC is shown to be ineffective. This paper explains the correlation of energy flow and the rate energy flow to the susceptibility of progressive collapse (PC) for typical mid-rise steel framed buildings. Energy-time histories of structural members were analysed to determine the energy flow behaviour throughout the structural system. All structural elements, including all connections, were represented numerically to (more accurately) determine the building's susceptibility to PC, including the sequence of failing members. Assessing PC susceptibility with the incorporation of the rate of energy flow for a structure that accounts for the effects of connection resistance functions constitutes a novel approach. Advanced nonlinear transient dynamic finite element simulations were used to reveal the energy flow and its flow rate, and the de facto relationship with building response and failure. In addition, the efficacy of the current DOD approaches for PC assessment are examined in a comparative study with the proposed energy flow approach. [ABSTRACT FROM AUTHOR]

• Proposed a non-iterative design method for progressive collapse design. • The method was based on virtual thermal pushdown analysis. • The method allows to directly determine the amount of reinforcement in beams and slabs. • Redesign of sub-assemblages and an actual building demonstrated the effectiveness. Disproportionate progressive collapse is a structural failure mode with low probability and high consequences. Due to this nature, progressive collapse design is usually treated as a secondary design; namely, a design check after the design for primary loadings. Various design procedures have been recommended, notably the alternate load path method through nonlinear pushdown analyses. However, most of these design procedures are iterative by nature to satisfy the design acceptable criteria. To improve the design efficiency, this study presented an innovative design technique that allows to directly determine the proper amount of reinforcement in beams and slabs of a reinforced concrete (RC) structure in order to withstand the gravitational loads under column removal scenarios. Inspired from fire-induced progressive collapse research, the proposed method employs a virtual thermal pushdown analysis, in which the temperature increase affects only the strength of reinforced steel. With carefully developed strength-temperature relationships of the rebars, the virtual thermal analysis produces a displacement-temperature curve that represents the genuine nonlinear relationship between the amount of reinforcement and structural performance (often represented by vertical displacement). This curve is then used to directly determine the amount of reinforcement at the prescribed performance target. Two beam-column sub-assemblage examples and the Alfred P. Murrah Federal Building were analyzed and redesigned to demonstrate the effectiveness of the proposed method. All three examples showed that the proposal virtual thermal pushdown method can accurately determine the appropriate amount of reinforcements to meet the performance target. [ABSTRACT FROM AUTHOR]

• Fire exposure influences the behaviour of RC connections during collapse. • Variation of reinforcement detailing at beam end influences collapse response of RC connections. • The simplified model accounts for the combined loading states due to axial restraint and fire. • Practical considerations are highlighted to improve the rotational capacity. This paper examines the behaviour of RC beam-column joints with different reinforcement detailing arrangements under pushdown loading before and after fires. The thermal responses of the RC joint specimens are firstly described, including the development of the temperature fields and the horizontal reactions as well as the deflections during the heating and cooling phases. Subsequently, displacement-controlled push-down tests are performed, and the vertical load, horizontal reactions as well as detailed deformations are monitored. The bending moment-rotation relationships are derived from the test results, together with an account of the observed failure modes. The mechanical behaviour is also discussed in detail, including the rotational capacity, with emphasis on the effect of different reinforcement detailing arrangements as well as the post-fire condition. Particular attention is given to comparing the experimental axial-moment strength interaction curves and theoretical predictions under ambient conditions so as to examine the underlying mechanisms. In order to support practical application, a simplified analytical method is proposed for simulating the push-down response, based on the combined loading states and idealised constitutive relations, taking into consideration the restraint effect due to horizontal restraint or fire conditions. The suggested analytical procedure is shown to be a reliable and effective approach for representing the behaviour. Based on the findings of this investigation, practical considerations for enhancing the rotational capacity of RC joints are highlighted. [ABSTRACT FROM AUTHOR]

• The paper proposes indirect approach to design against disproportionate collapse. • The paper presents the approach as well as its implementation at hand of a case-study building. • Tie-force procedure for mid-rise CLT platform-type construction. • Method uses linear-elastic static principles of engineering mechanics. • Estimation of catenary and cantilever actions as collapse-resisting mechanisms. The existing tie-force requirements for disproportionate collapse prevention, as prescribed in the current European codes and American guidelines, are unpractical for the design of Cross-laminated timber (CLT) buildings with platform-type construction. An approach towards meeting structural integrity for such buildings is presented in this paper, specifically an improved procedure to quantify the minimum longitudinal, transverse and vertical tie-force requirements. This indirect approach for disproportionate collapse prevention solely applies linear-elastic static principles of engineering mechanics to satisfy force and moment equilibriums and ensure that CLT platform-type buildings have sufficient strength, stiffness, and ductility to bridge over damaged walls. Cantilever action of the walls and catenary action of the floor panels are identified as the main collapse-resistance mechanisms. Case-study building analyses show that typical seismic design connection detailing can be used to trigger cantilever action, whereas novel connection detailing is required to meet the axial force and deformation demands required for catenary action. The procedure presented herein can aid in designing mid-rise CLT platform-type buildings for disproportionate collapse prevention. [ABSTRACT FROM AUTHOR]

Highlights • Distribution of critical member in different single-layer dome types is investigated. • Method is proposed to identify critical members based on collapse mechanism of domes. • Proposed identification method performs excellent compared to the other two methods. • Methods are proposed to increase progressive collapse resistance of single-layer dome. Abstract This paper presents a method to identify the critical member in a single-layer latticed dome, which in the context of progressive collapse is defined as the member whose removal causes the most severe damage. The distribution of critical members in four typical types of single-layer latticed domes, including the Kiewit dome, the Ribbed dome, the Schwedler dome and the Lamella dome, is investigated through a comprehensive Alternate Path analysis scheme composed of hundreds of individual dynamic nonlinear analyses. The Alternate Path analyses also confirm the progressive collapse mechanism of single-layer latticed domes, i.e., the nodal snap-through buckling at either end of the initially removed member. On this basis, a critical member identification method is established, using an index that implicitly estimates the relative vulnerability to node buckling following the removal of a member to determine the criticality of this member. This method along with two other methods, using either static axial force or free vibration response, are evaluated via comparison against the nonlinear dynamic Alternate Path analysis results, and this proposed method shows a beyond-compare accuracy. Furthermore, based on the established understanding of the progressive collapse mechanism and the factors influencing the node buckling resistance, three methods for increasing the progressive collapse resistance of single-layer latticed domes are presented. [ABSTRACT FROM AUTHOR]

Highlights • IDA was adopted to demonstrate that model uncertainty is not always negligible. • Seismic fragilities is strongly influenced by constitutive model adopted. • ANOVA estimated quantitatively the modelling uncertainty contribution. • The epistemic uncertainty has same order of record-to-record variability. Abstract In the framework of uncertainty propagation in seismic analyses, most of the research efforts were devoted to quantifying and reducing uncertainties related to seismic input. However, also uncertainties associated to the definition of constitutive models must be taken into account, in order to have a reliable estimate of the total uncertainty in structural response. The present paper, by means of incremental dynamic analyses on reinforced concrete frames, evaluates the effect of the epistemic uncertainty for plastic-hinges hysteretic models selection. Eleven different hysteretic models, identified based on literature data, were used and seismic fragility curves were obtained for three different levels of maximum interstorey drift ratio. Finally, by means of analysis of variance techniques, the paper shows that the uncertainty associated to the hysteretic model definition has a magnitude similar to that due to record-to-record variability. [ABSTRACT FROM AUTHOR]

Highlights • A commonly used reinforcing technology was tested on four substructures. • Unpredictable buckling is converted to ductile failure by a double-layer member. • An unbonded member produces two plastic areas to optimise strength failure. • The modified optimisation method is favourable for energy dissipation. Abstract Long-span single-layer spatial grid structures are widely applied in public buildings. Given their long-span and single-layer characteristics, they can easily collapse during accidental events. In this study, a commonly used reinforcing technology was tested on four substructures that were abstracted from single-layer spatial grid structures. Although a fixed end between the member and joint was constructed, the method is not ideal to improve progressive collapse resistance. Therefore, the optimisation method was modified in two aspects to consider different failure mechanisms. First, a double-layer member was used to improve the stability performance. This realised a ductile failure process because unpredictable buckling is converted to ductile failure. Second, an unbonded member was used for optimisation relative to strength failure. The presence of the embedded pipe produced two plastic areas to promote rotation ability. The aforementioned modified optimisation method is favourable for energy dissipation, and thereby improves the performance of single-layer spatial grid structures with respect to stability and strength failure. [ABSTRACT FROM AUTHOR]

Highlights • The reliability of the models for the assessment of the punching capacity. • The modification of the failure criterion for the model based on the CSCT. • Relations between ratio V test / V Rc and parameters influencing the punching capacity. Abstract Reinforced concrete flat slabs are structural members frequently used in building construction. Despite their many advantages, such structural systems suffer from several drawbacks. From a safety point of view, the concentration of shear forces at the vicinity of local supports, e.g., columns, edges and corners of walls, can lead to possible structural failure from punching. Punching is a dangerous phenomenon due to its brittle mode of failure and its ability to spread over a whole structure, which can be followed by a progressive collapse. Several models for the assessment of punching capacity have been developed and calibrated using experimental results from laboratory tests. Some models are fully empirical, e.g., EC2 (2004), while other models reflect the physical nature of the phenomenon; however, their safety level was calibrated using experimental data, as the model presented in Model Code 2010 or the CSCT-based model, which is expressed in a closed form. This paper deals with a statistical evaluation of the safety level of the previously mentioned models for punching resistance without transverse reinforcement. A database which includes the results of more than 600 experimental tests of flat slab specimens has been used. The relations between the safety of the model and the quality of the concrete used, the amount of the bending reinforcement, and the effective depth were found using advanced statistical methods. [ABSTRACT FROM AUTHOR]

Highlights • Various parametric effects on load-resisting mechanisms of 3D composite floor systems under an internal column removal scenario are investigated by FE simulations. • Effects of slab aspect ratio and joint type on robustness of 3D composite floor systems are assessed by an analytical model. • A few joint combinations are recommended for building robust composite floor systems against column removal scenario. • Consistency between the FE simulations and the analytical predictions is confirmed through a comparison of energy stored in different structural members. Abstract This paper presents parametric studies on three-dimensional steel-frame-composite-floor systems (3D composite floor systems) subjected to column loss using macro-based finite element (FE) models and a verified analytical method. The FE modelling method is verified by four actual experimental tests with three important variables, viz. slab aspect ratio, boundary condition and degree of composite action between composite slabs and steel beams. To overcome the shortage of data acquisition in the actual composite floor system tests, the FE models can be used to investigate the effects of these variables on load-resisting mechanisms, such as flexure and catenary action in the double-span girder and the double-span beam over the missing column, and flexure and tensile membrane action in composite slabs. In addition, the parametric studies are extended to include slab thickness. In a similar manner, the analytical model is used to study the effects of slab aspect ratio and joint type on robustness of 3D composite floor systems. After evaluating the robustness of eight sub-structures with different combinations of extended-end-plate, flush-end-plate, web-cleat and fin-plate joints, a few combinations are recommended. Lastly, consistency between FE simulations and the analytical predictions is confirmed through a comparison of energy stored in different structural members. [ABSTRACT FROM AUTHOR]

Highlights • A multi-hazard resistant composite frame structural system is proposed. • Seismic cyclic tests and progressive collapse tests of proposed frame were conducted. • Seismic and progressive collapse failure mechanism of proposed frame is discussed. • The proposed MHRCF has significantly better seismic resilience. Abstract Steel-concrete composite frame is one of the widely used structural systems. Earthquake and progressive collapse due to accidental localized damage are the main hazards that affect the safety of steel-concrete composite frames. Therefore, a seismic and progressive collapse resistant composite frame (SPCRCF) structural system is proposed based on a comprehensive consideration of the seismic and progressive collapse design requirements. The seismic and progressive collapse performances of the proposed SPCRCF were compared with the conventional steel-concrete composite frame using the experimental results of four specimens. The general-purpose finite element software, MSC.Marc, was used to simulate the specimens. The experimental and simulation results show that the proposed SPCRCF has better seismic resilience (low damage, self-centering, and easy repair) and larger progressive collapse resistance compared to conventionally designed steel-concrete composite frames. [ABSTRACT FROM AUTHOR]