Load Redistribution Mechanism Research Articles

Arm Back Support Suit (Abs-Suit) for Parcel Delivery With a Passive Load Redistribution Mechanism

Arm Back Support Suit (Abs-Suit) for Parcel Delivery With a Passive Load Redistribution Mechanism

Reconstruct the 3D load redistribution paths of framed structures subjected to local column failure and multi-floor fire

Considering the slabs in the global response of steel framed buildings in fires is important, as deflected floor slabs could redistribute the load to columns leading to column failure and even progressive collapses. Floor slab systems also experience tensile membrane action after deflection, which differs from the behaviour observed using 2D structural fire models. This paper focuses on unfolding the load redistribution mechanisms using 3D steel-concrete composite structure models, while multi-floor spreading fires and local failure of columns are considered. A framed structural model based on the Cardington building is designed and modelled, in which the composite slabs at different floors are simulated using the recently developed high-efficiency slab model to address the slab effect to the global load redistribution. Four hazard scenarios representing different states of the central column (i.e., fully protected, partially protected or failed) are given as the initiating local failure. The analyses with the 3D structural models show that the slabs can effectively pull up the failed column and the vertical loads are transferred to the adjacent columns. The worst scenario has been identified as the case experiencing column failure both at the upper floor and the lower floor, which leads to the middle floor without pulling action and causes significantly large deflection. This may be a scenario that local column failure triggers progressive collapse of a real building and should be prevented.

An integrated section model to enable simulating composite slabs in fire simply as modelling a flat slab

Modelling slabs is vitally necessary when simulating building structures in fire. The composite action between slabs and beams and tensile membrane action after fire induced deflections significantly reshape the load redistribution mechanisms and failure patterns. Modelling slabs particularly composite slabs of ribbed sections as a common choice of modern steel structures is a challenging task. Currently, those existing modelling approaches, such as beam & shell model and strip models, require elements for ribbed and flat parts separately, which produces much complexity and ideally it can be solved using a suitable section model to appropriately represent the behaviour. This paper begins with formulating the theoretical base of state determination in the integrated composite section, which enables correct deformation distribution to ribbed and flat subsections. While being compatible to typical modelling infrastructure for slabs, the integrated section model has demonstrated excellent capability in comparison to the strip models and fire tests. Even using a coarse mesh scheme (e.g. 8 × 6 elements), the prediction error of mid-span deflection remains low (around 10%). The appealing benefit is further shown in modelling Cardington Corner fire test, which significantly reduces the efforts of determining geometric details, the number of nodes & elements, and the computation time cost.

The response of diagrid structures to fire

The diagrid structural system has seen significant uptake in medium to high rise buildings because of the architectural and resource advantages that it provides. These arise mainly as a result of flexibility in the topology that this particular structural solution provides. However, as a result of the way in which diagrids carry both horizontal and vertical loading, the diagrid structure itself may be susceptible to fire in ways which are not immediately obvious on the basis of our understanding of more traditional rectilinear construction forms. This study addresses this to improve our understanding of diagrid structures' response to fire. A comprehensive structural analysis on 45 fire load cases is conducted, considering different fire locations and sizes, using parametric design tools and finite element analysis software. The results provide valuable insights into the load redistribution and collapse mechanisms of diagrid structures in fire conditions.

Percolation phase transition in the heterogeneous multi-coupled interdependent network

In previous studies, complex system models have often been set up in an oversimplified manner and the failure mechanism for nodes affected by each other is not correspond to reality. Accordingly, we develop a new model of heterogeneous multi-coupled interdependent networks and analyze the change in the giant component of the network after the attack in conjunction with the load redistribution mechanism. We first analyze the cascading failure process of the network analytically by recursive methods, estimate the critical phase transition threshold and the size of the giant component of the remaining network after network percolation, and find that the threshold of the WS sub-network is positively related to the average degree, and the BA sub-network is related to the power exponent λ of the distribution. When λ is less than or equal to 2, there is always a giant component in the network. After λ is greater than 3.4788⋯, there is no giant component. When λ is between 2 to 3.4788⋯, we found that the threshold of the sub-network is more influenced by the proportion of attacks (p1 and p2), and when p1 is larger, the sub-networks interact with each other, resulting in a smaller threshold for the BA sub-network, while the threshold of the WS sub-network is smaller in other cases. Finally, the cross-sectional comparison illustrates that as λ increases to 3.4788⋯, the robustness of the BA sub-network increases, contrary to the findings for single-layer networks. Overall, the heterogeneous multi-coupled interdependent network is more robust, and the giant component is greatly affected by the low robust sub-network, but its existence is related to the stock of nodes.

Cascading Robustness Analysis of Wireless Sensor Networks with Varying Multisink Placement.

In practical wireless sensor networks (WSNs), cascading failures are closely related to network load distribution, which in turn strongly relies on the locations of multiple sink nodes. For such a network, understanding how the multisink placement affects its cascading robustness is essential but still largely missing in the field of complex networks. To this end, this paper puts forward an actual cascading model for WSNs based on the multisink-oriented load distribution characteristics, in which two load redistribution mechanisms (i.e., global routing and local routing) are designed to imitate the most commonly used routing schemes. On this basis, a number of topological parameters are considered to quantify the sinks' locations, and then, the relationship between these quantities with network robustness is investigated on two typical WSN topologies. Moreover, by employing the simulated annealing approach, we find the optimal multisink placement for maximizing network robustness and compare the topological quantities before and after the optimization to validate our findings. The results indicate that for the sake of enhancing the cascading robustness of a WSN, it is better to place its sinks as hubs and decentralize these sinks, which is independent of network structure and routing scheme.

Progressive collapse mechanisms of multi-story steel-framed modular structures under module removal scenarios

Due to different assembly requirements and novel integration strategies, the modular structures may have undesirable loading mechanisms and distinct structural behaviors. However, there are limited data on the progressive collapse performances of modular structures, which impedes their industrial applications. This study numerically investigates the structural robustness of multi-story steel-framed modular structures against progressive collapse by using pushdown analysis. A six-story structure with five modules per floor is designed and established. The applied load, failure process and load redistribution mechanisms are studied under module removal scenarios. Parametric analyses are also conducted to investigate influences of cross-sectional dimensions of frame members, number of stories and bays, arrangements of bracings, and module removal scenarios on the robustness of the modular structures. The results show that the prototype modular structure designed with Hong Kong design code can withstand collapse under module removal scenarios. The load redistribution mechanisms among columns are significantly influenced by the overturning action and bracing effects. The ceiling beam in the lower module and the floor beam in the upper module form a double beam system. The column section, beam section, arrangement of bracings and number of stories significantly influence the robustness of the modular structure. Finally, a design method based on buckling load equation for columns is proposed to estimate the progressive collapse resisting capacity of modular structures with good accuracy and efficiency.

Finite element modelling of the progressive collapse of post-and-beam mass timber building substructures under edge and corner column removal scenarios

The structural behaviour of mass timber buildings under a load bearing element removal scenario is complex. To well understand this phenomenon and ultimately develop scientifically based design guidelines against progressive collapse for this type of buildings, there is a need to develop a Finite Element model which accurately captures the non-linear response, load redistribution mechanisms (alternative load paths) and failure modes of such buildings. As such, this paper presents for the first time how mass timber post-and-beam systems can be accurately modelled using Finite Element under edge and corner column removal scenarios. The model was validated against published 2D and 3D experimental tests performed on scaled-down substructures. The non-linear structural responses of the main structural connections were experimentally quantified and inputted into the model using the component method. Results show that the model accurately replicated load redistribution mechanisms, ultimate loads, failure modes, and strain developments. The use of the model was then illustrated by running parametric studies to quantify (i) the influence of the Cross Laminated Timber (CLT) floor panels layout and (ii) one alternative load path, typically ignored in design, on the progressive collapse resistance capacity.

Gravity-Induced Progressive Collapse Response of Precast Corner-Supported Modular Buildings

Compared with conventional buildings, the design of interconnections in multistory modular buildings requires special attention, due to different load distribution mechanisms, level of redundancy, integration strategies, and the stability requirements, in particular under accidental load conditions. In this paper, the robustness of corner-supported modular steel buildings, subject to various sudden loss scenarios, is studied through investigating the collapse-resisting capacities and the probable gravity-induced progressive collapse mechanisms. The aim is to evaluate the minimum requirements and special considerations for the design of interconnections for robustness. To that end, archetypal precast room-sized volumetric modules with different heights are analyzed using the finite-element macromodelling method and the alternate load path approach. The load redistribution mechanisms are inspected, nonlinear dynamic responses are determined, and probable collapse mechanisms are identified for buildings with different heights and configurations. The dynamic increase factors are computed and compared with the values suggested by the design codes. Given that the individual modules are mainly made of inherently robust structures, the focus of this study is on the performance of interconnections and interactions between an assemblage of units. The results indicate that due to high redundancy in these systems, there is great reserve strength against gravity-induced progressive collapse scenarios, triggered by instantaneous removal of components.

The resilience of logistics network against node failures

This paper studies the resilience of logistics network against node failures in the context of express industry owing to disruption in the network. By considering the flow capacity between the nodes and the impact of each node's failure, we propose a load redistribution mechanism in the presence of cascading failures which is akin to a criticality-based resilience assessment or stress testing the supply chain. To further investigate the impact of the node/nodes failure, we simulate and propose algorithms for two cascading failure scenarios, illustrating the different adjustment schemes for resilience improving strategies. A sensitivity analysis with managerial insights is also performed to investigate the effect of the adjustment schemes on the criticality of the nodes and the resilience of the express logistics network.

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.

Progressive collapse mechanisms of post-tensioned reinforced concrete frames considering effect of infill walls

This study numerically investigates progressive collapse mechanisms of post-tensioned RC frames under column removal scenarios by using the finite element method. The primary objective of this study is to investigate the influences of infill walls on mechanical behavior of post-tensioned (PT) frames against progressive collapse. Three experimental conventional reinforced concrete (RC) frames are modified into PT reinforced concrete frames: PT bare frame, PT full infilled frame and PT partial infilled frame. The numerical study was performed and the applied vertical load, horizontal displacement, failure process, load redistribution mechanisms, and re-centering capacity of PT frames are investigated under middle column pushdown scenario. Parametric analyses are conducted to study influences of material properties, arrangement of PT tendons, and magnitude of PT forces. The results show that masonry infill walls enhance the resisting capacity; however, change the failure modes and reduce the re-centering capacity of the PT frames against progressive collapse. The presence of the opening reduces the contribution of the infill walls. The PT bare frame have better resilience performance compared with the conventional bare frame. The bed joints, strength of masonry bricks, arrangement of PT tendons and magnitude of PT force greatly influence the progressive collapse resisting capacity of PT frames. Finally, a preliminary design method is proposed to predict the progressive collapse resisting capacity of PT frames with good accuracy and efficiency.

Effect of inter-module connections on progressive collapse behaviour of MiC structures

Modular integrated Construction (MiC) is a game changing construction approach which could significantly increase construction efficiency, quality, and sustainability. For steel MiC structures, modules are manufactured in the factories and assembled at the construction sites through inter-module connections. The concern of the vulnerability of these connections under abnormal hazards leading to disproportionate progressive collapse will impede industrial applications of MiC structures in the construction community. This study investigates the structural robustness of corner-supported modular steel buildings with the focus on different inter-module connections. A sub-structure extracted from a five-storey modular building is analysed by high fidelity finite element analysis under a corner column removal scenario. The load redistribution mechanism and failure modes are investigated thoroughly, based on which the simplified macro model is proposed where connections are modelled with rotational springs. Good consistence is found for the simplified model and the detailed FE analysis in terms of the pushdown curve. It is also found that the inter-module connection types will affect the beam-column joint properties which dominate the progressive collapse resistance of MiC structures. Therefore, attention should be paid when selecting different inter-module connections. The current rigid beam-column joint connection may lead to nonconservative evaluation.

Experimental study on the quasi-static progressive collapse response of post-and-beam mass timber buildings under corner column removal scenarios

This study presents the results from two quasi-static experimental tests performed on a ¼-scale, 2 × 2-bay, post-and-beam mass timber substructure, with Cross Laminated Timber (CLT) floors and Laminated Veneer Lumber (LVL) frames, subjected to corner column removal scenarios. One substructure was constructed and tested twice under different CLT panel configurations. In the first test (Test CM-1), the bay adjacent to the removed corner column consisted of three CLT floor panels, with two panels spanning one bay and one panel extending past the bay and spanning two bays. A constant Uniformly Distributed Pressure (UDP) was first applied to the bays not adjacent to the removed column and the UDP on the bay adjacent to the removed column was increased till failure through a six-point loading tree. As failure did not extend past the vicinity of the removed column in CM-1, the damaged elements were replaced and the specimen was re-tested by removing the diagonally opposite corner column. In this second test (Test CM-2), the bay adjacent to the removed corner column now consisted of one CLT floor panel spanning one bay and two panels spanning two bays. A similar loading protocol to that of Test CM-1 was used in CM-2. The substructure was assembled from a commonly used commercial beam-to-column connection. The failure modes, load redistribution mechanisms (alternative load paths) and overall behaviour are presented in this paper. The two test results showed that the applied load was mainly transferred to the three columns, the closest to the removed column. The ultimate load in Test CM-2 was 1.62 times higher than Test CM-1, demonstrating that the layout of the CLT panels plays a critical role against progressive collapse. A simplified analytical model, consistent with the current industry practice and pre-defined alternative load paths, was developed for the tested specimen to predict the ultimate resistance capacity and was also compared to the experimental tests. On average, the experimental failure load was 2.7 times higher than the predicted one, indicating that simplified current design practice is conservative.

Collapse performances of steel frames against fire considering effect of infill walls

This study numerically investigates effect of infill walls on collapse performances of three-dimensional (3D) steel frames against fire using explicit nonlinear dynamic analysis. Two eight-story, five-span with three-bay composite steel frames were designed, one of which has no infill walls (bare frame) while the other one has infill walls (infilled frame). The fire resistance, load redistribution mechanisms of the structures subjected to single compartment fire are investigated. Frame analysis against traveling fires is also conducted. Moreover, the effect of parameters including load ratio, opening percentage and number of infill walls are also studied. Results indicate that the damaged compartment moves down in fire and drags the other part of the frame, causing overturning deformation in the frame in the short span and the twist of the entire structure. The infill walls decrease the buckling temperature of the heated columns, whereas significantly increase the collapse temperature of the steel frames. The strut action of infill walls drastically changes internal forces within adjacent columns, significantly altering load redistribution path of steel frames against fire. The collapse modes of the steel frames can be greatly altered by the presences of infill walls. The steel frames collapse at a lower temperature with increasing load ratio and opening percentage of infill walls, whereas collapse at a higher temperature with more infill walls. Finally, a preliminary analytical approach is proposed to predict collapse modes of steel frames against traveling fire.

Cascading failures of overload behaviors on interdependent networks

The increasing dependence between infrastructure networks and the threat of cascading failures on network security motivated the analysis of the overload behaviors on interdependent networks. Here, considering the node load redistribution mechanism, we study the robustness of interdependent networks with different attack strategies, connection strategies and load distribution mechanisms considering attack strength (r), coupling strength (q) and different network topologies (Erdős–Rényi network model and Albert–László Barabási network model). Compared to the different connection strategies, with q and qc, we conclude that the response of the cascade failure is more evident in the assorted connections which keep more concentrated load nodes. Meanwhile, by the proportional allocation of load in interdependent networks, we observe that the BA–BA networks with DC strategy (connect nodes with high load in network A and nodes with low load in network B) have the worst robust level against cascading failures in the case of the intentional attack. In addition, by considering the charge distribution mechanisms, we demonstrate that interdependent networks have the greatest capacity to withstand risk when the charge distribution preference (β) is closed at the initial charge of the nodes (τ). Theoretical solutions are useful for revealing cascading failures of overload behaviors in large-scale networks composed of multiple networks in the real world.

Cascading Failures Analysis of Wireless Sensor Networks With Varying Routing Schemes

In practical wireless sensor networks (WSNs), routing schemes determine the message-forwarding direction of sensor nodes, which have a significant impact on the cascading process of WSNs. However, the existing cascading failure models fail to consider the impact of routing schemes. Therefore, in this work, we propose an actual cascading failure model for WSNs considering routing factors. In this model, a sink-oriented load metric is proposed to characterize the actual load distribution of WSNs, and four load redistribution mechanisms (i.e., latency-sensitive global routing, congestion-sensitive global routing, capacity-sensitive local routing and latency-sensitive local routing) are proposed to mimic the most commonly used routing schemes. Through extensive simulations, we have obtained some meaningful results: 1) there is a critical threshold for node capacity. When the node capacity is greater than the threshold, the lifting effects of capacity expansion are saturated; 2) there is a cascade interval for the attack range. Cascading failures occur only if the attack range is within this interval; 3) among the four routing schemes, congestion-sensitive global routing has the strongest cascading robustness; 4) flat-structured topology is the most robust, followed by clustering topology, and finally scale-free topology. The obtained results can provide theoretical support for improving the cascading robustness of WSNs.

Experimental study on the quasi-static progressive collapse response of post-and-beam mass timber buildings under an edge column removal scenario

Mid-rise to tall mass timber buildings are becoming internationally popular and are required, for instance in the Eurocode or the Australian building code, to be designed against progressive collapse. Designing against progressive collapse is especially important for mass timber buildings as, when compared to reinforced concrete and steel, timber is a more brittle construction material and mass timber buildings are deemed to be more elastic and have limited rotational capacities at the beam-to-column connections. However, whilst the ability of reinforced concrete and steel buildings to resist such an extreme event has been widely researched, limited studies were carried out on mass timber buildings. Their load transfer mechanisms and structural response after the loss of a load-bearing element are currently unclear. Consequently, this paper presents the outcomes of three experimental tests performed on three scaled-down, 2 × 2-bay, post-and-beam mass timber substructures under an edge column removal scenario. The capacity of the 3D substructures to resist progressive collapse was investigated for two types of beam-to-column connectors, namely two tests performed with a connector type commonly used in Australia in mass timber buildings and one test with a proposed novel connector. In the tests, Uniformly Distributed Pressures (UDP) were applied to the floors in two stages: (i) a constant UDP of 4.8 kPa was first applied to the bays not adjacent to the removed column and (ii) an idealised UDP was then increasingly applied to the remaining two bays through a hydraulic jack connected to a six-point loading tree. The load redistribution mechanisms or alternative load paths, structural response and failure modes were recorded and are presented in this paper. Results showed that the applied load was principally transferred to the three columns closest to the removed column and that the Cross Laminated Timber (CLT) panels spanning over two bays were efficient in resisting and transferring the load. The substructure with the proposed novel connector showed an 8.5% increase in capacity and higher ductility than the substructures assembled with the commonly used connector. A simplified theoretical model consistent with the methodology currently used by industry to predict the collapse resistance capacity of post-and-beam mass timber buildings was compared to the test results. The model underpredicted the test capacity by 53%.

A case study on geogrid-reinforced and pile-supported widened highway embankment

The objective of this case study is to evaluate the development of soil arching in an embankment being widened. In two test sections, earth pressures at different locations were monitored to reveal the load redistribution mechanism due to soil arching. Tensile forces in uniaxial plastic geogrids were measured to evaluate the performance of geosynthetic reinforcement in the widened embankment. The soil arching effect was quantified in terms of stress concentration ratio, soil arching ratio and geomembrane effect. Several existing methods were selected to compare with the measured results. The results of the field tests indicate that a two-dimensional plane soil arch with some eccentricity affects the fill load distribution on pile caps and subsoil between piles, and a realistic critical arch height of less than 2.0 m with a ratio of 1.4 times the pile clear spacing was identified. Although few existing methods could generate close results comparable to the measured values, most of the methods overestimated the load carried by the geosynthetic layer. The results also suggest that the foundation soil reaction underneath the geosynthetic layer should be taken into account in the load transfer mechanism of geosynthetic-reinforced and pile-supported widened highway embankments.

Experimental investigation of the breaking load of a dowel-pinned connection in granite cladding

A dowel-pinned anchorage in granite cladding has been widely used in modern granite cladding systems. However, current studies on the breaking loads of dowel-pinned connections are limited, and the damage behavior is not yet clear as a result of insufficient experimental studies. In accordance with current code provisions, monotonic static loading tests were carried out to investigate the breaking load and failure modes of a dowel-pinned cladding connection. The specimens were 200 × 200 × 30 mm granite panel with holes drilled in the middle of each side for steel pins with three different diameters, that is, 5 mm, 6 mm, and 7 mm. The results show that when using the 5 mm steel dowel pin, the steel pin yielded with a certain plastic deformation prior to the V-shape fracture of the granite near the dowel hole. When using the 7 mm and 8 mm steel pins, the U-shape fracture of the granite panel occurred first. Then only slight bending deformations of the steel pins were observed. The experimental breaking load was compared to the computational results following the formula recommended in the literature. A conceptual loading skeleton curve was developed to illustrate the damage progress of the connection. It revealed the load redistribution mechanism in the connection in a representative manner with which the calculated stress in the steel pin is corresponded to the experimental breaking load. This study is helpful for understanding the damage behavior of the novel connection in stone claddings.