Extreme Loading Events Research Articles

Torrent Barrier Under Climatic Extreme Load Events – Forensic Analysis of the Fracture and Failure Processes

Following heavy precipitation in the catchment area of the Pinnisbach torrent in Austria in 2015, a torrent barrier was significantly damaged due to extraordinary water levels and intense bedload transport. The bedload transport triggered by the barrier breach was partly captured in the downstream retention basin and partially diverted into the cone channel. To address this failure, the reverse engineering approach was applied. A failure analysis was performed to better understand the displacement process and quantify the overall load on the barrier structure. A detailed reconstruction of the failure mechanism employing advanced techniques like on-site inspections, numerical methods, and specific monitoring of the region area was conducted. The analysis revealed that the load initially assumed during the design and construction phase of the barrier was underestimated by 40% in certain areas, mainly due to an underestimation of water and debris load. Consequently, a more precise estimation of the applied load and considerations for the most severe scenario combinations are essential for ensuring structural safety. A rigorous analysis and assessment of this type of failure contribute to better implementing the findings into future risk mitigation strategies for similar structures and scenarios.

Detecting abnormal small-scale events by nacelle lidar at the AD8-180 prototype turbine

Identifying anomalous wind events that lead to high aerodynamic loads is essential for mitigating extreme loading on wind turbines. Building upon a recently published method, we herein introduce an extended methodology for early small-scale abnormal wind event detection employing a pulsed nacelle-lidar, potentially triggering preemptive control actions within the wind turbine system. The extended method integrates the consideration of multiple measurement ranges within the induction zone and the evaluation of a horizontally asymmetric wind detection parametrized through a difference in lidar line-of-sight (LoS) measurements. A comparative assessment of various detection algorithms is conducted, leading to the identification of optimal parameter configurations and an evaluation of their reliability. With the proposed methodology, the false positive rate can be reduced with respect to the original methodology, although not all extreme loading events are detected.

Optimization of the Traffic Load Model for Suspenders of a Super-Long-Span Suspension Bridge Considering Influence Line Geometry and Extreme Load Effect Scenarios

Optimization of the Traffic Load Model for Suspenders of a Super-Long-Span Suspension Bridge Considering Influence Line Geometry and Extreme Load Effect Scenarios

Intelligent crack identification method for high‐rise buildings aided by synthetic environments

SummaryCracks can develop in high‐rise buildings because of long‐term environmental changes and extreme loading events such as strong winds or earthquakes. Although deep learning‐based identification methods can efficiently identify cracks, the accuracy of crack identification in high‐rise buildings needs to be improved due to the lack of crack datasets specifically related to high‐rise structures. Moreover, the number of available images of cracks in high‐rise is limited. To this end, this paper establishes an intelligent crack identification method based on a photorealistic synthetic modeling technique. First, a computer graphics (CG) model of a high‐rise building with assumed damage is constructed. Subsequently, the CG model is utilized to generate a dataset that includes photorealistic images of the high‐rise building as well as corresponding labels for various components and types of damage. The generated dataset is then used to train a DeepLabv3 + neural network for structural component and damage identification, followed by validation by employing images of both synthetic and full‐scale high‐rise buildings. The trained network can accurately identify different components in images of the full‐scale, high‐rise building and identify cracks that are intentionally synthesized in those images. The results show that the synthetic dataset generated by the CG model not only allows for fast and efficient labeling for the purpose of neural network training but also outperforms methods that do not consider any application‐specific context in crack identification.

Computational Modelling of Intra-Module Connections and Their Influence on the Robustness of a Steel Corner-Supported Volumetric Module

This paper investigates the robustness of a single 3D volumetric corner-supported module made of square hollow-section (SHS) columns. Typically, the moment–rotation (M-θ) behaviour of connections within the module (intra-module) is assumed to be fully rigid rather than semi-rigid, resulting in inaccurate assessment (i.e., overestimated vertical stiffness) during extreme loading events, such as progressive collapse. The intra-module connections are not capable of rigidly transferring the moment from the beams to the SHS columns. In this paper, a computationally intensive shell element model (SEM) of the module frame is created. The M-θ relationship of the intra-module connections in the SEM is firstly validated against test results by others and then replicated in a new simplified phenomenological beam element model (BEM), using nonlinear spring elements to capture the M-θ relationship. Comparing the structural behaviour of the SEM and BEM, under notional support removal, shows that the proposed BEM with semi-rigid connections (SR-BEM) agrees well with the validated SEM and requires substantially lower modelling time (98.7% lower) and computational effort (97.4% less RAM). When compared to a BEM with the typically modelled fully rigid intra-module connections (FR-BEM), the vertical displacement in the SR-BEM is at least 16% higher. The results demonstrate the importance of an accurate assessment of framing rotational stiffness and the benefits of a computationally efficient model.

Simplified Strength Assessment for Preliminary Structural Design of Floating Offshore Wind Turbine Semi-Submersible Platform

The study aims to develop a simplified strength assessment method for the preliminary structural design of a semi-submersible floating offshore wind turbine platform. The method includes load cases with extreme wave load effects and a load case dominated by wind load. The extreme load effects due to waves are achieved using the design waves. Seven characteristic responses of the semi-submersible platform due to waves are chosen. The design waves for the extreme characteristic responses are all from extreme wave conditions where the significant wave heights are close to the one for a return period of 100 years. The extreme load effects dominated by wind loads are approximated using the modified environmental contour method. The load effects are the tower base shear force and bending moment. The two load effects are correlated, and a linear equation can approximate the relationship between their extreme values. The finite element analysis results show that the frame design of the bottom of the outer column is essential for structural strength. The wave load can also result in significant stress in the area close to the tower base.

Estimating bridge criticality due to extreme traffic loads in highway networks

Around the world, an increasing amount of bridge infrastructure is ageing. The resources involved in the reassessment of existing assets often exceed available resources and many bridges lack a minimum structural assessment. Therefore, there is a need for comprehensive and quantitative approaches to assess all the assets in the bridge network to reduce the risk of collapsing, damage to infrastructure, and economic losses. This paper proposes a methodology to quantify the structural criticality of bridges at a network level. To accomplish this, long-run site-specific simulations are conducted using Bayesian Networks and bivariate copulas, utilizing recorded traffic data obtained from permanent counting stations. To enhance the dataset, information from Weigh-in-Motion systems from different regions was integrated through a matching process. Subsequently, the structural response resulting from the simulated traffic is assessed, and the extreme values of the traffic load effects are obtained for selected return periods. Site-specific bridge criticality as a performance indicator for traffic load effects is derived by comparing the extreme load effects with the design load effects. The outcomes are mapped to facilitate visualization employing an open-source geographic information system application. To illustrate the application of the methodology, a total of 576 bridges within a national highway network are investigated, and a comparison with a popular simplified method is shown. The methodology herein presented can be used to assist in assessing the condition of a bridge network and prioritizing maintenance and repair activities by identifying potential bridges subjected to major load stress.

Analysis of extreme internal load effects in columns in a semi-submersible support structure for large floating wind turbines

Most studies on floating wind turbine floaters focus on rigid-body motion analysis, while analysis of structural load effects of the floaters is lacked. However, load effect analysis of hull structures is essential for the safety design checks throughout the service lifetime. Therefore, this study carries out the analysis of the extreme internal stress of columns in a 10-MW semi-submersible hull. A novel multibody approach is used for the numerical modelling of the hull structure, which can capture the internal forces and moments of the columns in the time domain. The environmental contour approach is employed to conduct the short-term analysis of the column extreme stresses. Important load cases are determined based on the sensitivity study of combined critical wave periods, important mean wind speeds, and the largest significant wave height with respect to column load effects. Long-term extreme values of the column stresses are predicted by the extrapolated method based on the short-term extreme analysis. Relative importance of internal load components on extreme value estimation of columns is studied. Simplified approaches are proposed to more efficiently predict the long-term extreme values of the column load effects, and applied in a case study. The analysis provides a good insight into the important global forces and moments as well as the direct wave load effect on the column stress analysis, which contributes to improve the design efficiency in engineering practice.

Mapping hazardous locations on a road network due to extreme gross vehicle weights

This study investigates the application of hazard maps in identifying critical locations associated with extreme vehicle load events. Examining extreme vehicle loads can offer valuable insights into the reliability of road infrastructure. Weigh-In-Motion (WIM) systems are commonly employed for this purpose; however, their limited availability often necessitates the use of less-sophisticated traffic counters (LSTCs). Unfortunately, LSTCs are unable to measure vehicular axle loads, posing a significant drawback. To overcome this limitation, we propose a methodology that utilizes data from LSTCs to estimate axle loads and map extreme gross vehicle weights. To demonstrate the feasibility of this approach, we present a case study concentrating on the major highway corridors in Mexico. While WIM stations are absent, data from a network of 1,777 counting stations and origin–destination surveys are available. By quantifying Gaussian copula-based Bayesian networks using the existing information, we generate synthetic site-specific axle loads. Subsequently, we calculate gross vehicle weights for selected return periods. These study findings facilitate the identification of hazardous locations. Additionally, we provide an interactive web map and a graphical user interface to generate synthetic axle loads. These tools, along with the proposed methodology, can serve as the foundation for maintenance strategies for existing roads and bridges.

Probabilistic model of traffic scenarios for extreme load effects in long-span bridges

The traffic scenarios that may cause extreme load effects are of great importance to the safety assessment of bridge structures. The traditional simulation method of traffic flow cannot depict the distribution pattern of vehicles on the bridge deck when the maximum effect is induced. In this paper, a probabilistic Gaussian mixture model (GMM) for heavy vehicle scenarios on the bridge deck under free-flow condition is proposed for long-span bridges based on collected Weigh in Motion (WIM) data. The scenarios of extreme response under free-flow occur more frequently than congestion scenarios and are of similar value and relevance in the daily management and safety assessment of long-span bridges.A non-stationary Poisson process is utilized to simulate the uneven occurrence of heavy vehicles in different lanes, and it is assumed that they are located within the artificially defined cells on the bridge deck. Then, Nataf transformation is employed to consider the correlation of gross vehicle weights (GVWs) within close range in the same lane. The numerical study is carried out on a long-span cable-stayed bridge to investigate the effects of correlation in GVWs and stationarity of vehicle distribution location on the structural responses. The load responses calculated by the proposed model and Monte Carlo method for different effects are compared with the values derived from code model. The results show that with the increase of the correlation level of the neighboring GVWs, the simulated responses are more prone to get extreme values, which means an increasing probability of the most unfavorable spatial distribution of on-bridge vehicles. The same results are also found under the non-stationary simulation state for vehicle location. The non-stationary Poisson process provides an efficient, highly feasible method, which is also in the safe side, for simulating the vehicle spatial distribution for specific effects.

The influence of the steel beam‐concrete slab interaction on the robustness capacity of steel frames

AbstractSeismic resistant multistorey steel frames are expected to mitigate progressive collapse even when subjected to extreme loading events. The use of high redundant structural systems and ductile members and connections are key features that provide such structural robustness. The composite action between the steel beam and the concrete slab may also help increasing the robustness, but in practice it is difficult to quantify this contribution if the joint behaviour is not detailed. The paper presents the results of a numerical study that aimed to investigate the contribution of the composite action to the progressive collapse resistance of steel frame structures. The main parameters investigated in the study were the level of the composite action in beams and strength and stiffness properties of beam‐to‐column joints. The numerical model was calibrated using two reference experimental column loss tests, one with steel beams and one with composite beams. The model considers the interaction between the steel beams and concrete slab by means of shear studs and reinforcement detailing. The results showed that the contribution of the slab lessens the demands on the joints when compared with bare steel systems and provides and adequate behaviour at large deformation stage. The progressive collapse resistance may be further controlled by means of joint flexural and axial strengths.

Modeling parameters for predicting the fire-induced progressive collapse in steel framed buildings

Fire is one of the extreme loading events that a building may experience during its service life and can have severe consequences on the safety of its occupants, first responders, and the structure. Steel framed buildings under severe fires can experience high levels of instability at a local or global level, which in turn can lead to the partial or progressive collapse of the structure. However, in current practice, fire resistance of structures is obtained without due consideration to a number of critical factors, and this is mainly due to the high level of complexity in undertaking advanced analysis of structures under fire exposure. This paper presents a parametric study on a ten-story braced steel framed building subjected to fire exposure wherein six different parameters are evaluated: fire severity, fire spread, load paths, temperature-induced creep, local instability, and analysis regime. Results from validated finite element models are utilized to evaluate the influence of the different parameters and recommend critical parameters to be incorporated in the analysis. Results show that the susceptibility of fire-induced progressive collapse significantly depends on the severity of the fire exposure scenario, including fire intensity, fire spread, and extent of burning. Also, accounting for the full effects of transient creep in fire-induced progressive collapse analysis is needed to obtain conservative failure times under severe to very intense fire exposure. Additionally, results from the parametric study infer that the sectional classification of a steel section based on local instability can alter under fire exposure and this effect is more critical in steel columns located in the higher stories of the building; a nonslender column at ambient conditions can transform to a slender section at elevated temperatures. This can induce temperature-induced local instability in the column and lead to an early onset of instability at member and structural levels.

Updated load models for short‐span road bridges in the range of 2‐15 m

AbstractIn recent years, increased effort has focused on developing traffic load models for the safety verification of existing road bridges. In Switzerland for example, this was achieved by updating the alpha reduction factors in the Traffic Load Model 1 (LM1) for new bridges (analogous to the Eurocode LM1 alpha factors), providing a practica approach. The enveloping of the maximum simulated alpha factors, which were calculated by applying measured vehicles to different traffic scenarios, spans and bridge cross‐section types, resulted in the updated LM1 in the SIA 269/1 Standard for Existing Structures. The model development considered spans in the range of 10‐80 m and applied dynamic amplifications as a function of the gross vehicle weight. One limitation of the current updated LM1 is that, as a very high proportion of existing road bridges have spans of less than 15 m, the load effects in this short‐span range tend to be overly conservative. This paper studies the load effect behaviour in the longitudinal and transverse directions, based on actual traffic from Weigh‐in‐Motion (WIM) data. The study utilises all WIM data recorded in Switzerland over the last 10 years. Results from three new short‐span load models are presented which, together, envelope the extreme load effects. A parametric study is presented which highlights some conservatism in the existing code load models. Finally, a case study project is presented, in which these updated models were used to verify the structural safety of a historic road bridge.

Synergistic use of ultra‐high‐performance fiber‐reinforced concrete (UHPFRC) and carbon fiber‐reinforced polymer (CFRP) for improving the impact resistance of concrete‐filled steel tubes

SummaryConcrete‐filled steel tubes (CFSTs) have received growing attention, owing to their rapid construction, reduced labor requirement, and reasonable material cost. While in service, the CFSTs can be subjected to unexpected impact loads, originating from vehicle and vessel collision, as well as water‐ and wind‐borne debris impact. Such extreme loading events often cause a partial or complete failure of conventional CFSTs, risking the safety and performance of the entire structural systems that rely on them. To address this issue, the current study explores how two advanced composite materials, including ultra‐high‐performance fiber‐reinforced concrete (UHPFRC) and carbon fiber‐reinforced polymer (CFRP), can be utilized to provide superior mechanical properties and minimize the vulnerability of CFSTs to impact loads. The composite materials under consideration are appropriate for both new and existing structures, in which normal‐strength concrete can be replaced with UHPFRC, while CFRP sheets can further strengthen the CFSTs. For obtaining in‐depth insights, a computational framework validated with the experimental tests was developed in the current study. Using a set of representative impact scenarios, various response measures, such as internal forces and deflections, as well as the energy absorbed by the CFSTs, were recorded during impact simulations. The investigations were then further extended to capture the influence of the main design parameters related to concrete, CFRP, and steel tube. From the conducted investigations, an energy absorption index was introduced, as a measure to evaluate the performance of CFSTs under impact loads.

A comparison study of power performance and extreme load effects of large 10‐MW offshore wind turbines

AbstractThe utilisation of offshore wind turbines has rapidly increased in the last decade, which has resulted in a steady increase in wind turbine sizes. The global average offshore wind turbine size has increased from 1.5 MW to 6 MW in the last two decades. The research community has started to investigate huge 10 to 15 MW offshore wind turbines in recent years, resulting in the study of very innovative floating wind turbines using various substructure technologies. With this backdrop, this paper will investigate and thoroughly compare the power performance of extreme load effects of a large offshore 10 MW turbine installed on the monopile, spar, and semisubmersible substructures. This is performed by using the average conditional exceedance rate (ACER) and Gumbel methods to predict the extreme responses under the operating conditions of 8, 12, and 16 m/s mean wind speed, representing the below‐rated, rated, and above‐rated regions, respectively. The results show that the power performance and extreme loads experienced depends significantly on the operating regions. The mean power generation between the three different types of offshore wind turbines (OWTs) are closely in the whole operating range, which standard deviations differ significantly. Large standard deviations of power generation appear in the spar turbine under the below‐rated condition. Further, it was observed that the spar wind turbine generally experiences larger extreme loads due to larger platform pitch motion. In addition, the ACER method shows a better prediction for the 1, 2 and 5‐year extreme responses than the Gumbel method, which is due to the relatively poor data fitting of the Gumbel method at the upper tail. The study is believed to consolidate and close the knowledge gap in understanding wind turbine responses across the most common offshore substructure technologies and provide a basis for design and deployment of OWTs.

Estimation of Vehicular Collision Force on Bridge Piers in the Presence of Sub-Standard Intervening Concrete Barriers

Critical infrastructures such as bridges should be resilient to extreme loading events. Among the potential hazardous events, vehicle collisions are becoming increasingly common. Modern American Association for State Highway Transportation Officials (AASHTO) bridge design specifications consider a limit state to account for vehicular Collision Force (VCF) and require addressing it when bridge piers are within the clear zone defined by AASHTO’s Roadside Design Guide. AASHTO specifies two methods to address this limit state: either by providing an intervening structure that is capable of absorbing the VCF and/or redirecting the colliding vehicle, or designing the piers to resist a lateral load equivalent to 2,670 kN (600 kips). Many old bridges that were designed before recognizing the VCF as a limit state fail to meet the modern specifications because either the piers were designed to resist a lower lateral load than the specified limit or the installed intervening concrete barrier is not a standard one that qualifies to solely protect the piers against VCF. This paper introduces a new approach to address the requirements of the modern standards. The approach is based on blended contribution between the available sub-standard barrier and the under-designed bridge pier. The proposed approach is based on a matrix of dynamic simulations using LS-DYNA and considering different crash scenarios. The results showed that in the presence of the sub-standard barrier, the estimated VCF can be at least reduced by about 25%, leaving 75% to be resisted by the existing pier.

Consolidation-induced improvements in plate anchor capacity

Plate anchors are an attractive technology for mooring floating facilities; as relative to piles, suction caissons, and drag anchors, they provide a much higher capacity relative to their mass. Plate anchors may experience an extreme loading event that will cause geotechnical failure, although they will still retain a residual capacity. The displacement associated with bringing the anchor to failure will induce excess pore pressures that initially reduce soil strength but will dissipate over time, leading to regains in soil strength and hence anchor capacity. This paper considers the time scales and magnitude of this anchor capacity regain through a series of model scale experiments conducted in a geotechnical centrifuge. The experiments involved vertical loading of pre-embedded horizontally orientated circular anchors in normally consolidated kaolin clay. The results show that anchor capacity regain is a function of consolidation time and the level of resistance maintained on the anchor, with the longest consolidation time and highest maintained resistance leading to a capacity regain of approximately 60%. These capacity increases are described here using a simple hyperbolic function, which provides a basis for estimating the time needed for the residual anchor capacity to regain sufficient capacity following a movement event.

Performance-Based Retrofits of Long-Span Truss Bridges Based on the Alternate Load Path Redundancy Analysis

Existing long-span truss bridges typically undergo significant retrofits to improve their resistance against extreme load events, such as the seismic loads, for which the bridge may not have been designed or considered originally. However, the conventional seismic retrofit measures recommended in most existing codes and specifications may not be effective in providing sufficient alternative load path (ALP) redundancy to a bridge that is vulnerable to sudden member-loss scenarios because of the significantly different magnitude of demands. This paper proposed a performance-based retrofit (PBR) approach for long-span truss bridges that are vulnerable to sudden member-loss scenarios. The performance of the bridge has been evaluated by using the demand-to-capacity ratio (DCR) or strain ratio (SR) on the member/component level or the displacement factor on the bridge system level as indicators. Based on the analysis results, the argument is made that the proposed PBR approach allows bridge designers to meet the predetermined performance objectives for sudden member-loss loading conditions. Increase in weight of steel because of such the ALP retrofits is less than 10%. The retrofitted bridge would meet the desired performance levels in the event of sudden loss of any members on the primary trusses of the bridge.

Experimental program for characterization of ultra low-cycle fatigue fracture in structural steel

Structural steel components experience cyclic inelastic strains when subjected to extreme loading events such as strong earthquakes. In such cases, the steel may experience ultra-low cycle fatigue (ULCF) fracture, associated with the growth and coalescence of microscopic voids in the material. A number of ULCF criteria have been formulated and used in the literature for the analysis of fracture under cyclic loading. The calibration of such criteria requires experimental test data covering a variety of stress states. To address this need, a testing program, involving both monotonic and cyclic loading protocols, was conducted on 60 circumferentially notched coupons extracted from rectangular steel tube sections made of ASTM A1085 steel. A relatively novel approach was used to achieve a wide range of stress states by subjecting the coupons to combinations of axial and torsional loading. The experimental tests are accompanied by finite element analyses to elucidate the local stress and deformation states at the crack initiation location of each coupon. To demonstrate the importance of the experimental tests for the reliable characterization of ULCF in structural steel, an example calibration is conducted for an ULCF criterion from the literature using the experimental results.

Scour protection effects on the dynamic response of jacket structures under extreme load events

A well-validated Finite Element (FE) model of a jacket structure (OC4 jacket) with pile foundation together with a Computational Fluid Dynamic (CFD) model (OpenFOAM-based) set-up for extreme waves on the structure are applied to perform systematic comparative analyses of the dynamic response of the structure to extreme wave loads with and without scour effects. The pile foundation model is considered by implementing a non-linear soil model to the foundation using load-deflection (p-y) approach to capture the non-linear behaviour of the soil for large pile foundation displacements. The results show that, the scour protection improves the foundation bearing capacity and consequently, decreases the failure probability in terms of both serviceability and ultimate limit states. Moreover, it enhances the dynamic response of the structure under extreme load events caused by breaking waves. Accordingly, an innovative solution for scour protection of offshore-fixed bottom structures has been introduced by “Biodegradable Soft Rock” (sand filled containers produced from bio-degradable geotextile product) which is the newest development in this section.