Column Loss Research Articles

Simplified analytical method for the robustness assessment of precast reinforced concrete structural systems

The article presents the simplified implementation of alternative load path method based on the energy balance approach. This method should be used to check the global resistance of a damaged structural system after the occurrence of an accidental event. Basic assumptions of simplified analytical models for modelling resistance of horizontal ties in a damaged structural system, taking into account the membrane (chain) effects, were presented. An approach to modelling the dynamic resistance of a damaged structural system based on the energy balance method is described. Calculated dependencies for checking the robustness of a prefabricated multi-storey building with hollow-core slabs after the loss of the central column are proposed and considered using an example. On the considered example, a comparison of the required tie sections area with the dynamic resistance designed using the energy balance method (EBM) and according to the current standards, and a statistical assessment of the reliability of the load-bearing capacity models are carried out. In the end, a brief algorithm for the simplified calculation of the dynamic resistance of a damaged structural system is proposed.

Analytical prediction on static collapse resistance of steel frames in multi-interior-column-removal scenarios

Previous studies on the collapse performance of steel frames mainly focused on a single column loss. Even though the multi-column loss has been increasingly studied recently, it is generally based on the numerical analysis and relevant analytical study is rare. Moreover, the behavior of the frame after the multi-column loss should be more sensitive to its boundary condition due to a greater affected zone, compared with the single-column loss. However, this boundary condition effect has not been given enough attention in existing studies. To fill in these research gaps, this paper investigates the collapse resistance of the steel frame in two-interior-column-removal scenarios. To quantify the boundary condition effects, four column-removal scenarios, namely, two general-interior columns (GG), one general-interior column and one edge-interior column (GE), two edge-interior columns (EE), and one edge-interior column and one corner-interior column (EC) are studied. It is found that in the scenarios of GG and GE, the resistance-displacement relationship of the frame shows a tri-linear characteristic. In the scenarios of EE and EC, however, a bi-linear characteristic is exhibited. Compared with the GG scenario, the ultimate bearing capacities of the frame in the GE, EE and EC scenarios decrease by about 12%–20%, 33%–43% and 39%–52%, respectively.

Retrofit of non-code conforming moment resisting beam-to-column joints: A case study

The non-linear monotonic and hysteretic behaviour of as-built and retrofitted beam-to-column joints of an existing non-code conforming steel multi-storey building was investigated by means of finite element simulations. The building case study was originally designed to resist only gravity and wind loads without any capacity design principle. The as-built joints have continuous beam and interrupted columns that are bolted with end-plate splice to the tapered flange of the beams in the nodal zone. These joints were analysed to assess their non-linear response. The finite element analyses showed adequate resistance against gravity loads, but poor performance under seismic and column loss scenarios. Two retrofit strategies were designed in order to enhance the local and global performance in order to guarantee the weak beam-strong column condition, namely: (i) strengthening the column and its connection; (ii) weakening of the beam. Different types of interventions were designed per examined strategy and numerically analysed to assess the global and local response of both external and internal joints. A TOPSIS multicriteria analysis was also performed to identify the most suitable type of intervention.

Robustness of inter-module connections and steel modular buildings under column loss scenarios

This paper investigates the progressive collapse behaviour of laterally braced steel modular buildings under various column removal scenarios. Unlike the conventional building with continuous floor slab at each storey, steel modular building is constructed by connecting individual modules at the corners. The floor slab in each individual module is often discrete and not connected to each other. Therefore, it is critical to check the robustness of the inter-module connections to ensure adequate redundancy for load redistribution under a column loss scenario. Nonlinear static analysis was performed to study the response behaviour and force transfer mechanism of low-to high-rise modular buildings under various column removal scenarios. The results were compared with those obtained from nonlinear dynamic analysis. The dynamic amplification factor used in the nonlinear static analysis was shown to overestimate the displacement response of the modular building structure under column loss events. The effect of column removal on the inter-module connections was studied for modular steel buildings of 5- to 30-storey height. Design implications related to minimum tie force requirement and its effect on inter-module connection were discussed.

Residual axial capacity of square recycled aggregate concrete-filled steel tube columns after blast loads

To study the residual axial capacity of square recycled aggregate concrete-filled steel tube (RACFST) columns after close-range explosion loads, the explosion tests and the axial compression tests were performed. Firstly, 18 square RACFST columns were fabricated, and 15 of them were subjected to explosion tests. Secondly, the axial loads were applied to 3 intact columns and 9 blast-damaged columns. The curves of the load-displacement, the load-strain, and the load-mid-span deflection of all columns are analyzed. After the tests, the failure modes of the columns and core concrete were analyzed. Furthermore, the theoretical axial capacity of the RACFST columns was calculated according to the Chinese standard GB50936-2014. Based on the residual load-bearing capacity, the damage index is adopted to assess the damage degree of 9 blast-damaged columns. It indicates that both the blast-damaged and intact columns show diagonal shear failure under axial compression; the range of bearing capacity loss of the blast-damaged columns is approximately 1.9–32.1%, of which 5 columns are slightly damaged and 4 columns are moderately damaged, indicating that the columns still maintain a strong axial capacity reserve after explosion loads.

Process design and multi-objective optimization for separation of ternary mixtures with double azeotropes via integrated quasi-continuous pressure-swing batch distillation

An multivessel double-column pressure-swing batch distillation process and a triple-column integrated quasi-continuous distillation process were designed to separate ternary mixtures with double azeotropes of ethyl acetate-methanol-water. The process feasibility was analyzed based on ternary phase diagram. Based on a production capacity of 100 kmol/batch, the corresponding control strategy was proposed to achieve the effective separation of the mixture. Taking the minimum total annual cost as the objective function, the process parameters of the double-column process were optimized to determine the optimal operating pressure. Exergy loss of distillation column was calculated to evaluate the thermodynamic performance of the process. To further improve the separation efficiency of the process, a third column was integrated in the double-column batch distillation process to separate the methanol-water mixture directly. Combined with sequential iterative optimization sequence and multi-objective optimization algorithm, the triple-column process parameters were optimized, and the trade-off between equipment cost and carbon dioxide was realized. Thermal integration technology was adopted to further reduce the energy consumption of the triple-column process. The results show that the carbon dioxide emissions and total annual cost of the double-column process are 1.323 × 106 kg/y and 2.107 × 105 $/y. The purity of ethyl acetate and water was 99.9 mol%. While the purity of methanol was 99.5 mol%. The total annual cost and emission of the triple-column process were 1.993% and 1.890% higher than that of the double-column process, respectively, but the methanol purity was further improved to 99.9 mol%. Compared with the double-column process, the total annual cost and carbon dioxide emissions of the triple-column process with thermal integration technology were reduced by 1.281% and 17.337%.

Retrofit of existing steel structures against progressive collapse through roof-truss

The paper presents the results of a comprehensive study on the evaluation of the effectiveness of a retrofit strategy of existing steel buildings against progressive collapse. In this respect, it investigates the performance and the design of a retrofit solution to increase the robustness of steel Moment Resisting Frame buildings. A truss steel system added at the building's rooftop level (i.e., ‘roof-truss’), and intended to define an alternative load path, was investigated as a retrofit solution. The numerical model key components, including the plastic hinges and the beam-column connections, were validated against available experimental results. The validated models were then used to study the robustness of the structure under column loss scenarios by means of non-linear static and dynamic analyses performed in OpenSees. The simulations allowed for the identification of possible failure modes and alternative load paths together with the definition of the Dynamic Increase Factor (DIF). In this regard, it is shown that column buckling is critical for the selected case study. Moreover, the outcomes showed how the proposed retrofit solution allows the definition of effective alternative load paths when subjected to column loss scenarios and informs on the critical details that should be checked by employing this retrofit system.

Dynamic response of RC beam-slab substructures following instantaneous removal of columns

High fidelity solid-element-based numerical modeling is employed in this paper to study the dynamic response of reinforced concrete (RC) beam-slab substructures following instantaneous removal of columns. After validation of the numerical model against experimental results, incremental dynamic analysis is conducted to obtain the dynamic structural resistance to progressive collapse. The numerical results for development of internal forces and distribution of stress and strain are used to illustrate the load redistribution and load transfer mechanisms of RC beam-slab substructures subjected to sudden loss of columns. The numerical model is further employed to explore parameters affecting the dynamic response and structural resistance of beam-slab substructures under sudden column removal scenario. The parameters studied include damping ratio, slab thickness, location of removed columns, multi-column removal sequence and interval. The numerical result shows that the dynamic progressive collapse resistance of the beam-slab substructure is much smaller than the quasi-static test result due to a dynamic increase factor (DIF) of 1.14. Moreover, a minimum slab thickness of 1/45 of span length is found vital for the improvement of progressive collapse resistance of RC beam-slab substructures through developing compressive membrane action (CMA) and tensile membrane action (TMA) as well as enhancing the development of compressive arch action (CAA) and tensile centenary action (TCA) of beams. When the damping ratio is smaller than 5%, the increase of damping ratio can significantly benefit the structural resistance against progressive collapse by producing smaller deflection and milder vibration. The additional loss of the corner column besides the penultimate perimeter column is found to form the most hazardous situation among the double-column removal scenarios. In contrast to the location of removed columns, the column removal sequence and interval tend to little affect the maximum deflection developed and consequently the progressive collapse resistance of substructures.

Robustness of multistory corner‐supported modular steel frames against progressive collapse

SummaryIn current practice, a relatively wide range of different interconnections and assembly systems is being used for the design of modular buildings made of volumetric prefabricated frames. The integration strategies and design criteria for providing structural robustness or prevention of disproportionate collapse for such buildings are completely different with those with conventional systems. This is mainly because of their fundamentally different configuration of joints and the redundancy they provide for the structure. This paper studies the robustness of multistory modular structures, made of prefabricated volumetric steel frames, against progressive collapse, which can be triggered by various column or module loss scenarios. In this study, the flexibility of modules and propagation of the damage inside the individual modules are taken into account, and the buckling of columns, as well as the effects of material nonlinearity on the responses and capacity of the entire system, are also considered. Load redistribution patterns and possible gravity‐induced collapse mechanisms are inspected in various collapse scenarios for a typical mid‐rise modular steel frame. In the numerical model, each individual prefabricated volumetric module is composed of discrete beam and column frame elements, connected through conventional rigid beam–column connections. The volumetric modular steel frames are connected via corner vertical and horizontal intermodule joints; each one is modeled utilizing one axial and two shear springs with predefined nonlinear force–displacement behavior in a 3D finite element analysis. The local damage scenarios are assumed to be due to instantaneous loss of columns or entire modules, for which the interactions between the modular units, overall structural robustness, load redistribution, possible collapse mechanisms, and progressive collapse response of the frames are investigated. The results indicate that the additional redundancy may help the modular steel frames survive from some corner column or single module removals but will not be able to maintain their robustness under combined module removal scenarios.

Comparison of Different Procedures for Progressive Collapse Analysis of RC Flat Slab Structures under Corner Column Loss Scenario

Progressive collapse is the failure of the whole structure caused by local damage, which leads to significant economic and human losses. Therefore, structures should be designed to sustain local failures and resist subsequent nonproportional damage. This paper compared four procedures for a progressive collapse analysis of two RC structures subjected to a corner column loss scenario. The study is mainly based on the methods outlined in the current Russian standard (linear static (LS) pulldown, nonlinear static (ND) pulldown, and nonlinear dynamic), but also includes LS and NS pushdown procedures suggested by the American guidelines and linear dynamic procedure. We developed detailed finite element models for ANSYS Mechanical and ANSYS/LS-DYNA simulations, explicitly including concrete and reinforcement elements. We applied the Continuous Surface Cap Model (MAT_CSCM) to account for the physical nonlinearity of concrete. We also validated results obtained following these procedures against known experimental data. Simulations using linear static pulldown and linear dynamic procedures lead to 50–70% lower results than the experimental because they do not account for the nonlinear behavior of concrete and reinforcement. Displacements obtained from the NS pulldown method exceed the test data by 10–400%. It is found that correct results for both RC structures can only be found using a nonlinear dynamic procedure, and the mismatch with the test data do not exceed 7%. Compared to static pulldown methods, LS and NS pushdown methods are more accurate and differ from the experiment by 28% and 14%, respectively. This relative accuracy is provided by more correct load multipliers depending on the structure type.

The influence of loading rate on the ultimate capacity of bolted T-stubs at ambient and high temperature

In case of accidental actions, the robustness of multi-storey steel frame buildings depends to a large extent to the performance of the beam-to-column joints. For an extreme scenario that involves a column loss under fire, the dynamic process can induce high strain rates and therefore needs to be considered when evaluating the resistance and ductility of the beam-to-column connections. End plate bolted connections are widely used in the steel framed constructions. The simple macro-component through which the response of such a connection is assessed, consists in the T-stub model included in EN1993-1-8.The paper presents the results of an experimental program on T-stubs subjected to normal and high temperatures, in both quasi-static and high loading rate conditions. While under normal temperature the response of the T-stubs are not significantly influenced by the strain rate, at high temperatures the strain rate determines a change in the failure mode and in the ultimate resistance and deformation. The experimental study highlights the necessity of assessing the strength reduction factors of bolts function of both temperature and strain rate, a critical information in the progressive collapse assessment of structures under fire, in order to obtain realistic results.

A Retrofit Method to Mitigate Progressive Collapse in Steel Structures

AbstractAccidental events, such as impact loading or explosions, are rare events characterized by a very low probability of occurrence. However, their effects often lead to very high human losses and economic consequences as are likely to trigger the progressive collapse of the buildings. The progressive collapse of structures attracted the attention of many researchers and the topic has been widely investigated in recent years. In addition, increasing interest has been shown also on the definition of retrofit strategies able to increase the robustness of existing structures. The present work investigates the performance and the design of a retrofit solution to increase the robustness of steel moment resisting frames. A case study structure is selected and modelled in OpenSees, including both mechanical and geometrical non‐linearities. Non‐linear static analyses have been carried out on the frame, simulating a column loss scenario to investigate the subsequent load redistribution. The simulations showed that the case study was unable to redistribute the load and hence retrofitting was required. Among others, a truss system was added at the rooftop level of the building allowing the definition of an alternative load path. The analyses outcomes showed how the proposed retrofit method allows to increase the robustness of the case study structure and allowed for critical remarks on the checks required when this retrofit system is employed.

Anti-collapse performance of composite frame with special-shaped MCFST columns

A spatial steel frame with concrete slab and special-shaped multi-partition concrete filled steel tubular columns was tested in a simulated scenario of progressive collapse. The load-deformation relationship of the frame with damaged middle column under vertical load is discussed. Parametric analysis is also conducted by using a validated FE model. The results show that the failure of the specimen resulted from the fracture of the bottom beam flange eventually. The configuration of side plate connection and multi-partition concrete filled steel tubular column both could provide a reliable load-redistribution path for additional loads due to single column loss. The designed frame exerts good robustness to resist the collapse resulting from middle column loss. The simulation shows that single-column removal would cause additional lateral force and moment on the adjacent columns, besides axial force. The length of side plate exhibits more influence on the performance of the frame than the thickness of side plate.

Performance of Fully Restrained Welded Beam-Column Connections Subjected to Column Loss

Performance of Fully Restrained Welded Beam-Column Connections Subjected to Column Loss

Blast-induced progressive collapse of steel moment-resisting frames: Numerical studies and a framework for updating the alternate load path method

The Alternative Load Path (ALP) method is widely used to assess progressive collapse resistance of steel framed structures. Code-based ALP is a threat-independent methodology, implicitly focuses on a very special triggering event, i.e., a small near-field blast, that can lead to complete and sudden column loss. However, in a real blast-induced progressive collapse scenario, characteristics of the triggering event and subsequent initial damage control the structural response. To study these effects, a wide numerical investigation is carried out. First, the code-based ALP method is applied to assess the threat-independent dynamic column removal responses. The results emphasize the importance of initial damage location and building’s size. Then, the model structures were analyzed in different blast scenarios. A meaningful difference in the obtained results compared with code-based ALP is observed in both quantity and quality. Finally, a novel methodology (modified ALP) is suggested to update the code-based ALP method to capture the threat-dependent parameters, i.e., column removal time (CRT) and damage level. To serve this purpose, a substructure techniques, i.e., equivalent column model, is developed and validated. The results of three methods (threat-independent code-based ALP, threat-dependent blast analysis (BA) and the proposed modified ALP) are compared, and it is observed that the modified ALP method can effectively adjust the dynamic column removal response to reflect the blast effects.

Coastal eutrophication drives acidification, oxygen loss, and ecosystem change in a major oceanic upwelling system.

Global change is leading to warming, acidification, and oxygen loss in the ocean. In the Southern California Bight, an eastern boundary upwelling system, these stressors are exacerbated by the localized discharge of anthropogenically enhanced nutrients from a coastal population of 23 million people. Here, we use simulations with a high-resolution, physical-biogeochemical model to quantify the link between terrestrial and atmospheric nutrients, organic matter, and carbon inputs and biogeochemical change in the coastal waters of the Southern California Bight. The model is forced by large-scale climatic drivers and a reconstruction of local inputs via rivers, wastewater outfalls, and atmospheric deposition; it captures the fine scales of ocean circulation along the shelf; and it is validated against a large collection of physical and biogeochemical observations. Local land-based and atmospheric inputs, enhanced by anthropogenic sources, drive a 79% increase in phytoplankton biomass, a 23% increase in primary production, and a nearly 44% increase in subsurface respiration rates along the coast in summer, reshaping the biogeochemistry of the Southern California Bight. Seasonal reductions in subsurface oxygen, pH, and aragonite saturation state, by up to 50 mmol m-3, 0.09, and 0.47, respectively, rival or exceed the global open-ocean oxygen loss and acidification since the preindustrial period. The biological effects of these changes on local fisheries, proliferation of harmful algal blooms, water clarity, and submerged aquatic vegetation have yet to be fully explored.

Progressive Collapse Analyses of Buildings Subjected to Earthquake Loads

Progressive collapse is a partial or total failure of a building that mostly occurs when the build-ing loses primary structural elements (typically columns) due to accidental or natural hazards. The failure of structures due to an earthquake is one of the most important and frequent types of progressive collapse. In this study, the finite element method is used to assess the response of multistory reinforced concrete buildings subjected to column loss during an earthquake. Three-dimensional nonlinear dynamic analyses are carried out using SAP2000 V.20 program. The ef-fects of different parameters on the progressive collapse behavior are investigated, namely: the location of the removed column within the ground floor; the method of column removal (sudden, in two-steps, and in four-steps) and the removal timing during the earthquake. It is demonstrated that the collapse occurs when all or most of the hinges at the bases of the ground floor columns reach their collapse level. The chosen column removal timing and policy affect the structural behavior considerably. It is realized that, the risk of building collapse increases when the removal timing harmonizes with the peak ground acceleration timing. Based on the adopted earthquake characteristics and building configurations, it is found that, the two steps removal scenario is the most dangerous one.Keywords:Progressive collapse, Concrete buildings, Seismic load, Nonlinear dynamic analysis, Plastic hinge.© 2014

Effect of dynamic load redistribution in reinforced concrete flat-plate structures under collapse

An initial punching shear failure or column loss in a reinforced concrete flat-plate structure may lead to the dynamic load redistribution to the surrounding connections. The dynamic demand and capacity at these connections will determine if subsequent failures occur leading to collapse of the structure. Therefore, the effect of dynamic load needs investigation in order to evaluate the collapse potential of the structure. This paper analyzes previous experimental work as well as new computational models to determine the effect of the dynamic load in terms of the dynamic load amplification as well as the dynamic capacity. The dynamic load amplification in flat-plate structures may be much less than what is used in current design guidelines. In addition, while the dynamic strength is relatively unchanged from the static, the ductility may be greater for connections with low reinforcement ratios. The results show that including the dynamic capacity can increase the load of the flat-plate system considered in this study to resist collapse progression by only approximately 0.5 kN/m2 (10 psf).

Influence of the Bolt Material Properties on the Ultimate Capacity of End Plate Bolted Joint Subjected to Column Loss

This paper investigates the performance of extended stiffened end-plate bolted beam-to-column joints subjected a column loss scenario by means of finite element simulations. An advanced numerical model was developed, and its effectiveness was validated against the experimental results. The influence of the bolt strengthening on the column loss action was investigated changing the grade of bolts. The results showed that the joint performance under column loss scenario are deeply related to the development of the catenary action that depends from the connection ductility; therefore increasing the bolt material strength will provide beneficial effects on the joint capacity under the column loss.

Bilateral Weighted Regression Ranking Model With Spatial-Temporal Correlation Filter for Visual Tracking

Many discriminative correlation filter (DCF)-based methods have successfully leveraged the guidance for solving two problems (i.e., the boundary effect and temporal filtering degradation) as a model prior to visual tracking. The intuitive motivation of these methods is to control the degeneration of the updating loss of the objective function with a structural framework. While these methods rely mostly on various regularization items, they always ignore the loss from data fidelity term. Therefore, we propose a bilateral weighted regression ranking model termed as BWRR. Here, we resort to two procedures for solving the above problems. First, BWRR introduces a bilateral constraint into the data fidelity term to control the loss of rows and columns of the filter learning data term. The weighted matrices could impose an adaptive penalty for large data loss during the learning process to avoid the model degradation problem. Second, the data of the updated weighted matrices is not directly applied to the calculation of the filter during each iteration. Instead, a new weighted product matrix is obtained by ranking and numerical transformation for updating the filter. We show that the proposed model converts the original correlation filter regression problem into a regression-with-ranking problem, thus avoiding the problem of positive and negative sample imbalance. Overall, the BWRR model is iteratively solved by the alternating direction method of multipliers(ADMM). Qualitative and quantitative evaluations demonstrate the effectiveness and superiority of our proposed method by extensive and quantitative experiments on the OTB, VOT, and UAV datasets.