Alternative Load Research Articles

Evaluation on the progressive collapse resistance of infilled reinforced concrete frames based on numerical and semi-analytical methods

The effect of infill walls on the progressive collapse behaviour of reinforced concrete (RC) frames should be fully considered due to the formation of alternative load paths. To explore the configuration that could affect the progressive collapse resistance of infilled RC frames, parametric analyses are performed based on the validated simulation method. In the study, the impact introduced by the extra tie and integrity rebars in walls is emphasised. It is indicated that the variation in the reinforcement ratio of tie rebars leads to a limited change in structural progressive collapse resistance. In contrast, with the arrangement of the integrity reinforcement, the progressive collapse resistance of the infilled frames is enhanced and the propagation of the diagonal cracks in partial infill walls is suppressed. The notable re-ascending trend in the resistance curve for models with integrity rebars reveals the significant development of the catenary action in the integrity reinforcement. Then, with the help of linear regression and gene expression programming (GEP) techniques, a semi-analytical model is proposed for the prediction of the load-bearing capacity of infilled frames after column loss. In the prediction model, the linear effect of geometrical parameters and the non-linear effect of openings on the progressive collapse resistance are included by introducing three modification coefficients. Based on the statistical assessment, the capability of this explainable model is verified, and the appropriate amount of integrity reinforcement to mitigate the progressive collapse is discussed.

FE modelling progressive collapse assessment of steel moment frames-parametric study

ABSTRACT Progressive collapse is the failure of primary structural components produced by natural or abnormal events that may result in a total or partial collapse of the structure. In this paper, several structure models, under different column removal scenarios, were modeled and analyzed using the FE program SAP2000 to evaluate the effect of span length, the strength of structural members, and cross-section on the steel building’s resistance against progressive collapse. The Alternative Load Path method was carried out using the linear static and nonlinear dynamic analysis following the GSA 2003 guidelines for this investigation. The material and the geometric nonlinearities must be considered in the nonlinear dynamic analysis. The contribution of span length, steel grade, and cross-section to the response of the structural system was studied through the Demand Capacity Ratio for the linear static analysis. The variation of several parameters, such as bending moments, plastic hinges status and their rotations, displacements, and ductility, was discussed based on the response of the nonlinear dynamic analysis. The main objective of this study is to demonstrate the impact of the latter parameters on the structural enhancement and the reduction of the damage level triggered by the failure of a primary structural component.

NODES OF HOLLOW CORE PROFILE TRUSS BRIDGES WITH INCOMPLETE WELDS

Among the various types of steel bridge girders, trusses seem one of the most powerful and reliable. The individual members may be simple, but the nodes are the complex parts, especially if more than 4 members are connected. In view of the complexity, internal welds become inevitable and are inaccessible after finishing, which is overlooked easily during construction. The proposal is to eliminate those of the inside welds that may be dispensable. If some of them would be eliminated, the consequences may be acceptable, or require minor modifications only. The assumption is tried out on the example of a structure replacing a historic bridge. This structure is composed of rectangular hollow profiles, connected at nodes. Member forces and bending moments, have been determined. If the internal welds are about to be eliminated, an alternative load path is found. The calculation of stresses in the various fillet welds is based on the assumption of evenly distribution of forces. The weld stresses can be calculated by the derived formulas. The results show some welds require strengthening, due to the elimination of the more complicated internal welds. This concerns ultimate limit state. Thus, the possibility is confirmed that some of the internal welds may be eliminated, although there still is no proof the method may be applied to more complex nodes. Research must be continued to clarify this. For the type of hollow core members being considered, omitting of some internal welds may well be an acceptable alternative.

Disproportionate collapse prevention analysis for post and beam mass timber building

Disproportionate collapse prevention of mass timber buildings is becoming a relevant design consideration due to the increasing height of these buildings. Linear static alternative load path analysis (ALPA) is well-established with extensive details and worked examples for concrete, steel, and light-frame wood buildings. The research presented herein illustrates its application to a mass timber building with floors supported by beams and columns, referred to as ‘post and beam’ system. The performance of a six-storey post and beam mass timber building, using predominantly wood-to-wood bearing-type connections, following three notional element removal scenarios (removals of ground floor internal, edge, and corner columns) is investigated. It is evaluated whether the case study building’s structural system can trigger floor collapse-resistance mechanisms by checking if its primary elements and connections satisfy disproportionate collapse acceptance criteria, defined by the demand-to-capacity ratio after element removals. The results demonstrated that: i) element removal analyses should be conducted in 3D and should include all primary structural elements, with floor carrying its own weight at every level; ii) compressive arching is dominant for members with small span over depth ratios, whereas catenary action governs for larger spans; iii) continuous beams and slabs help the floor system to cantilever over the removed elements; and iv) the considered connection detail may be improved by the addition of axial connectors to trigger catenary action.

NODES OF HOLLOW CORE PROFILE TRUSS BRIDGES WITH INCOMPLETE WELDS

Among the various types of steel bridge girders, trusses seem one of the most powerful and reliable. The individual members may be simple, but the nodes are the complex parts, especially if more than 4 members are connected. In view of the complexity, internal welds become inevitable and are inaccessible after finishing, which is overlooked easily during construction. The proposal is to eliminate those of the inside welds that may be dispensable. If some of them would be eliminated, the consequences may be acceptable, or require minor modifications only. The assumption is tried out on the example of a structure replacing a historic bridge. This structure is composed of rectangular hollow profiles, connected at nodes. Member forces and bending moments, have been determined. If the internal welds are about to be eliminated, an alternative load path is found. The calculation of stresses in the various fillet welds is based on the assumption of evenly distribution of forces. The weld stresses can be calculated by the derived formulas. The results show some welds require strengthening, due to the elimination of the more complicated internal welds. This concerns ultimate limit state. Thus, the possibility is confirmed that some of the internal welds may be eliminated, although there still is no proof the method may be applied to more complex nodes. Research must be continued to clarify this. For the type of hollow core members being considered, omitting of some internal welds may well be an acceptable alternative.

Shear-stud restrained composite slab strength prediction model

The use of shear-stud in composite slab (CS) construction can significantly enhances its longitudinal shear capacity. However, the strength capacity determination of such complex construction is still marred with the capital-intensive challenge resulting from the complex laboratory procedures for establishing its longitudinal shear capacity. To address this challenge, this article develops a simplified numerical function for determining longitudinal shear capacity for CS with shear stud. This study considers slope-intercept method for the longitudinal shear estimate and the adaptation of rational approach in developing the performance function. Furthermore, limit state function was from the use of shear capacity violation that considers experimental failure load (FL) value and design load estimated form the shear contributions. The resulting outputs from the rational analysis facilitated the formulation of numerical function for the determination of FL in a more simplified form. The developed FL algorithm exhibits strong performance in determining the alternate CS failure load though there is stillroom for further improvement especially if span length would be taken into consideration. Further development that incorporates span length might give much favorable result in determining CS strength especially when considering slope-intercept method

Analysing local failure scenarios to assess the robustness of steel truss-type bridges

Many of the steel bridge collapses occur in truss-type bridges. This is, in fact, the focus of this study involving an assessment of the robustness of this type of structures based on an actual bridge that the authors had extensively monitored and controlled. Robustness was assessed by means of computer simulations of various Damage Scenarios (DSs) to analyse the structural bridge capacity to efficiently activate Alternative Load Paths (ALPs). The computational models have been previously validated with the results of load tests on the bridge and a laboratory test on a full-scale bridge span. The DSs have considered a series of non-simultaneous failures in different elements. The results indicate that the structure is capable of not triggering a disproportionate collapse after each of the DSs with the help of the efficient activation of ALPs that required the contribution of other elements with extra-strength capacity as well as from the superstructure and the joints working under bending moments. The results were used as the basis for practical recommendations for: i) the design of new steel bridges and the retrofit of existing ones and ii) monitoring the structure for the optimal position of sensors to predict local failures that could spread to the rest of the bridge.

A multi-model framework for assessing long- and short-term climate influences on the electric grid

Climate change influences many aspects of the electric grid, but prior work and industry practices often ignore the potential effects of changing climate, or they only consider a single effect or individual effects in isolation. Challenges vary with each grid and include adapting to long-term trends such as changing temperature and precipitation or shorter-term events such as drought or storms that could increase in frequency or intensity. Here we present a multi-model framework designed to analyze the effects of long and short-term climate impacts in combination. This framework couples capacity expansion and production cost models with hydrologic models and future climate scenario data to analyze alternative climate and energy futures at high spatial, temporal, and process resolutions. We constructed and evaluated the results of a suite of simulated scenarios exploring climate impacts on capacity investment and stress-tested the resulting future infrastructures using hourly dispatch modeling under alternative drought and load conditions. We demonstrate the approach through a case study of the U.S. Western Interconnection, where climate impacts depend on interactions between temperature-induced load, water availability for hydropower, technology competitiveness, and demand flexibility. Changes in 2038 generating capacity range from −8.5–16.6 GW, and changes in 2038 transmission capacity range from −1–2 GW. Capacity increases are driven by higher load from higher temperatures, while capacity reductions can be achieved in scenarios with higher future hydropower availability and increased demand flexibility. Scenarios requiring additional capacity cost an additional $5–$17 billion (discounted) from 2018 to 2038; however, scenarios with capacity reductions cost $1–$18 billion less. Stress tests on four 2038 infrastructures demonstrated that the identified systems were able to serve at least 99.999% of load and 99.96% of reserves. However, drought and unexpected high-load conditions can result in reduced capacity to respond to contingency events we did not model. Although these results are system and scenario specific, they highlight the importance of considering multiple climate change impacts simultaneously in long-term planning efforts and demonstrate a multi-model, multiscale approach that can be flexibly applied to any system and set of climate change concerns.

Enhanced thermal fatigue resistance of 20Mn2SiCrMo bainitic steel by microstructure stabilization design

Thermal fatigue properties of 20Mn2SiCrMo bainitic steel used for alternative thermal load environment were analyzed. The results showed that thermal fatigue resistance is closely related to microstructure stability. The higher microstructure stability, the higher thermal fatigue resistance. There are smaller microstructure morphology variations during thermal cycles for the structure of ferrite and carbides in 600 °C tempered condition, demonstrating relatively higher thermal fatigue resistance. While for the mixed structure of ferrite, RA and ε- M2C in 300 tempered conditions, the samples show stronger thermal fatigue sensitivity because of stress accumulation by dynamic carbide precipitation and the transformation from retained austenite to martensite. Moreover, a new evaluation rule for thermal fatigue is proposed to quantitatively analyze the damage degree of steel suffering alternative thermal loads effectively.

Reliability of fully assembled precast concrete frame structures against progressive collapse

Although fully assembled precast concrete (PC) frame structures using dry connections have been widely promoted, limited studies have been carried out on anti-progressive collapse performance. This study presented a methodology to analyze and compare the reliabilities of entire structural systems between RC and fully assembled PC frame structures, which evaluates the risk and vulnerability of a structure when column failure occurs under accidental loads. First, based on the existing test results of three half-scale moment sub-structures, nonlinear finite element (FE) macro-models were established in OpenSEES software. The verified FE models were applied to three seven-story frame structures (one RC frame and two PC frames). To evaluate the progressive collapse resistance, nonlinear pushdown analysis was conducted on the frame structures using the alternate load path (ALP) approach. Combining the Nataf transformation-based point estimate method (i.e. the improved point estimate method (IPEM)) with the nonlinear analysis results, various load amplification factors (αmax) were estimated. Finally, the reliability indices of the intact and damaged frame structures were calculated using the Zhao-Ono high-order moment method (HOMM). The results revealed that the fully assembled PC frames had higher failure probability (2.1–6.6%) than the RC frame (0.2%) in a catastrophic event.

Compressive membrane action in axially restrained hollow core slabs: Experimental investigation

AbstractWhen a concrete slab is excessively loaded due to an accidental event, compressive membrane action can be activated in order to generate an alternative load transfer to the remaining supports, which can considerably enhance the load‐carrying capacity. This can increase the structural reliability of the components, delay or even prevent a progressive collapse and consequently increase the robustness of concrete structures. Although the beneficial effects of compressive membrane action in reinforced concrete slabs have been recognized for decades, only limited research has been focusing on this effect in prestressed concrete members, like hollow core slabs. Therefore, a novel real‐scale test set‐up has been developed in order to assess this phenomenon in real‐scale hollow core slabs. The results indicate that, despite the presence of the large hollow cores, compressive membrane action can significantly improve the load‐carrying capacity of hollow core slabs under excessive loading. In parallel, a numerical finite element model has been established in order to model the structural behavior (i.e., compressive membrane action) as per the investigated test program. The model was able to correlate the results from the experiments very well, and emphasized the large influence of the concrete tensile strength on the ultimate failure load of concrete hollow core slabs in compressive membrane action.

Validation and application of rational tying method for robustness design of post-and-beam timber buildings

ABSTRACT The lack of reliable tying methods for the design of low and medium-rise mass timber buildings with regard to progressive collapse constitutes a strong limitation for the timber structural engineering community. In this context, the current work aims to validate and apply a recently developed rational tying force method for robustness design of post-and-beam timber buildings falling in low and medium consequence classes. This rational tying force method, which considers the requisite resistance via tying to the loss of a load-bearing member, preserves the simplicity of the prescriptive rules present in some current standards, while utilising a mechanical basis similar to that considered in alternative load path analysis. An extended example of the application of the new tying force method to a post-and-beam timber structure is reported in this work. The example highlights several critical aspects for achieving a robust design of post-and-beam timber structures. The results presented in this work show that the new tying force method represents a promising and practical strategy for robustness design of post-and-beam timber buildings.

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.

Numerical simulation of heat recovery potential of hot dry rock under alternate temperature loading

Alternate-temperature loading can cause cyclic thermal stress; therefore complex artificial fracture networks are formed in hot dry rock owing to thermal expansion and contraction under alternating thermal loading. In this study, the heat recovery potential of hot dry rock with a complex fracture network under alternating temperature loading was simulated numerically based on the hydrothermal coupling algorithm and compared with that of hot dry rock with a single fracture. The results show that the complex fractures formed under alternate thermal loading have better heat recovery performance than a single fracture, and the heat recovery performance of a complex fracture network is closely related to the intersection angle of fractures in the network. The temperature at the fracture outlet increases with an increase in the intersection angle between the branch fractures and the main fracture. At the angle of 30° between the branch fractures and main fracture, the average temperatures at the fracture outlet were 0.031% and 0.025% higher than those at the angles of 90° and 60°, respectively. Simultaneously with the water-injection, the temperature in the branch fractures gradually increased with an increase in the angle of the crossing fractures. When the angle between the branch and main fractures was 90°, the temperature fields on the two sides of the branch fracture were symmetrical. When the angle was 60° or 30°, the temperature fields on the two sides of the branch fracture were asymmetrical. The results of this study provide theoretical and technical support for the extraction of deep geothermal energy.

Calculation of fatigue fracture in structural elements using a multimode damage model

The problem of nucleation and development for the fatigue fracture process in element of aircraft structure is solved using previously developed cyclic damage model. The model contains the evolution equation for the damage function depending on stress state and amplitudes of the cyclic loads. On this basis, calculations of the initiation and development of crack-like fracture zones are carried out for an aircraft structure element of a disk with a blade in a flight loading cycle (HCF mode). The damage function was calculated based on the elastic solution for the element of structure loaded by centrifugal forces in disk and blade and aerodynamical pressure on blade. An alternative cyclic loading regime associated with high-frequency vibrations of the blades was investigated. Fatigue fracture in this process corresponds to the VHCF mode. Simulation of the combined cyclic loading mode was also carried out. In all these modes, the real time of the nucleation and development of quasi-cracks was estimated up to reaching the observed disk surface.

Collapse Resistance Model and Deformation Mechanism of Shear Wall Replacement

Structural elements can be damaged or defective due to man-made and natural disasters, which will result in structural resistance below the design value. Component replacement can effectively improve the safety of the structure and does not affect its later use. However, this construction process must dismantle defective components. If the process is not properly operated, it will cause progressive collapse of the structure. There are few studies on progressive collapse resistance based on field tests. In this paper, the alternate load path method is used to study the deformation mechanism of reinforced concrete shear wall structures when partial members fail. The maximum vertical displacement and strain of the beam end near the failure walls are obtained, and load transfer laws are analyzed. Based on the component replacement construction of a high-rise shear wall structure, the progressive collapse resistance mechanism of the shear wall structure is verified, which provides theoretical guidance for component replacement construction.

IMPACT OF LOAD LAYOUT ON INTERNAL VENTILATION DURING THE TRANSPORT OF BROILERS

The characterisation of ventilation during the transport of broilers is essential to identify and solve problems related to the microclimate of the load. This study aimed to evaluate the ventilation patterns in alternative load layouts, i.e., with a central span or an air corridor, used in Brazil. Computational fluid dynamics simulations and wind tunnel tests were performed on virtual models and small-scale prototypes, respectively. The results showed that the load layout with a "central span" promoted positive ventilation patterns and increased wind circulation between the crates. However, it reduces the load transport capacity by approximately 20% (70 fewer crates), increasing the number of journeys. Therefore, the modification of the layout can optimise ventilation throughout the load; however, it becomes necessary to reduce the number of crates stacked on the truck, which is not feasible for the commercial transport of broilers because it makes it more onerous.

The Anatomic Reconstruction for Chronic Lateral Ankle Instability Utilizing Absorbable Synthetic Graft (ATLAS Procedure): Description and Short Term Follow Up

Category:AnkleIntroduction/Purpose:The traditional modified Brostrom-Gould has long been the procedure of choice for addressing chronic lateral ankle instability. However, more robust techniques including anatomic reconstruction are favored for larger deformity, poor tissue quality and revision surgery. Given the inherent disadvantages of allograft and non-elastic synthetic materials, material science has sought to impact the graft choice providing alternative load sharing, permeable materials with high strength. Our institution has performed anatomic lateral ligament stabilization, termed the 'ATLAS' procedure, using a novel synthetic graft (Artelon, Marietta, GA) with anecdotal success in recent years. In this study, we present our surgical technique and early experience.Methods:A retrospective cohort review was performed, evaluating the failure rate, complications and radiographic changes in anatomic alignment. A review of all patients undergoing the ATLAS procedure was performed, a total of 27 patients were included. The study included only those patients with at least 1 year follow up from the procedure.Results:Only one patient had symptoms of early failure of the procedure, while none of the patients required a revision procedure. The most common complication was minor wound dehiscence (11.1%). Overall, our short-term results with this procedure were excellent.Conclusion:We conclude that the ATLAS procedure is a predictable and effective lateral ligament reconstruction technique with no donor site morbidity, no donor tissue transmission risk, low risk of over-constraint and accomplished with a predictable technique.

Effect of hanger system on resistance against progressive collapse of RC buildings under column removal in alternate storeys

Progressive failure refers to the continuous disintegration of a structure until partial or complete collapse, resulting in severe economic losses and mass casualties. The present study proposes a feasible solution against progressive collapse by the introduction of a hanger system and highlighting its effect on R.C. building under the progressive collapse scenario. The hanger system mainly comprises of a structural wall enacted at the terrace level in place of the parapet wall, which acts as a highly stiff member connecting the columns in various bays and allowing load transfer between these bays, thus creating an alternate path for loads. This study follows the removal of various critical columns on the alternate storeys, in contrast to previously conducted researches which were focused on only removing selected columns on the ground storey. CSI's ETABS 19 software has been used to simulate the progressive collapse of a reinforced concrete building, assessing the behaviour of a 10 storey reinforced concrete building under progressive collapse, with and without designing structural wall at the parapet, using static linear analysis of progressive collapse compliant with the general service administration guidelines (2016). In all the cases of removal of critical columns, the results are compiled in respect of DCR values (Demand Capacity Ratio) at critical locations and joint displacement at the top of removed columns. The hanger system used in this study provides an innovative and feasible solution to prevent progressive collapse by providing alternate load paths and increasing the robustness of the building.

Progressive collapse analysis of corner-supported composite modular buildings

In this study, the alternate load path analysis is carried out to investigate the progressive collapse resistance of multi-storey modular buildings constructed using corner-supported composite modules. Three sudden column loss scenarios are considered: a corner column, an edge column at the long side, and an edge column at the short side. The non-linear dynamic analysis is first conducted to investigate the peak dynamic response of the locally damaged structures. The dynamic analysis results show that the 4-storey building prototypes remain elastic whilst partial yield in the joint regions is developed in the 12-storey counterparts; however, no collapsing tendency has been observed in cases under design loads. In the non-linear static analysis, the capacity curve and the overload factor for each case are then identified by increasing the gravity load until failure occurs. Four possible failure mechanisms are identified, varying among the different column loss scenarios and the building height. The suitable dynamic increase factors (DIFs) for the proposed composite modular buildings are identified on the basis of achieving the same maximum vertical displacement at the point of column removal when the non-linear static method is carried out to predict the peak dynamic response to sudden column removal. The DIFs of 1.90 and 1.60 are recommended for the 4- and 12-storey modular buildings, respectively.