A revisit of common normal method for discrete modelling of non-spherical particles

© 2017 Elsevier B.V. Discrete element method (DEM) is prominent for studying granular materials at particle scale. However, how to model non-spherical particles in DEM is still challenging. In light of the present contact detection algorithms in the literature, common normal (CN) and geometric potential (GP) are two methods used for particles with smooth surfaces. Yet it has been long believed that CN gives erroneous results while GP is more preferable since they were firstly proposed for ellipsoidal particles decades ago. A revisit of CN in this work identifies two problems in the original CN, and then a new CN is proposed which can overcome these problems. Based on the comparison to sub-particle scale finite element analyses, the new CN has been further shown to be able to predict the contact plane more accurately than the original CN and GP. Such an advantage is found for the modelling of ellipsoidal and superquadric particles. The study not only proposes an improved CN algorithm but also demonstrates that CN should receive more attentions in DEM, though GP is now much more widely used

Seismic vulnerability assessment of a case study anchored liquid storage tank by considering fixed and flexible base restraints

© 2017 Elsevier Ltd Liquid storage tanks are among vital infrastructures, and their seismic vulnerability assessment plays a pivotal role in uninterrupted operation of an industrial plant. Technically, vulnerability notion relates capacity of each tank component's resistance to failure subjected to different seismic hazard levels. The predominant source of damage to liquid-containing tanks is steel shell buckling which can be intensified by base flexibility of fully anchored tanks. This flexibility is mainly resulted from anchor bolt failure and base concrete damage. In other words, the presence of anchors does not necessarily preclude anchorage failure or loss of concrete support, especially during large earthquake events. A case study on a liquid storage tank is introduced in this paper to highlight the influence of base flexibility on the seismic performance of fully anchored tanks. The tank is initially designed in accordance with the requirements of American Petroleum Institute, API-650. Fluid-structure interaction (FSI) is simulated by means of so-called added-mass approach. Two sets of finite element models are constructed, namely; fixed base (FB) and flexible base (FLB) tank. Nonlinear time history analysis based on a suite of twenty-two multi-directional spectrally matched acceleration time histories are conducted. Simultaneous input motions of two horizontal components are of particular importance as they shift the position of failure mechanism to directions being more affected by combined accelerations. Critical intensity measures (IMs) according to incremental dynamic analysis are then determined. Fragility curves are obtained by introducing conditional probability of failure as a function of critical IM. Results reveal that modelling base flexibility may contribute to lower value of critical IMs compared to that obtained from restrained support model. More specifically, FLB model demonstrates lower value of endurable peak ground acceleration (PGA) compared to the initial value selected for the tank design as per API requirements

Damage diagnosis in bridge structures using rotation influence line: Validation on a cable-stayed bridge

© 2019 Elsevier Ltd This study proposes a new damage identification technique for condition assessment of bridge structures. The method is based on the concept of rotation influence line (RIL) at the bridge bearing locations, and solely relies on measurements obtained from two points at either end of the bridge e.g., RIL R and RIL L . The sensitivity of the rotation measurement to damage is first investigated using a 1-D simply supported beam model and it is demonstrated that unlike conventional strain measurements, the rotation measurement is capable of providing useful information about damage even though it is far from the measurement point. Further, an existing cable-stayed bridge is considered to validate the capability of RIL in damage identification. A comprehensive three-dimensional finite element model (FEM) of the bridge is established and calibrated using the measured static and dynamic responses. Numerous field tests are conducted on this large-scale structure to extract static and dynamic characteristics, including natural frequencies, mode shapes and influence lines (ILs). Sixteen hypothetical damage scenarios are induced in the numerical model, including symmetric and asymmetric cases of cable loss. A damage index based on the normalised discrepancy of RIL between the benchmark state and an unknown state is introduced and through extensive investigations it is demonstrated that regardless of damage location, either RIL R or RIL L (or both) can successfully identify the induced damage in all of the sixteen damage scenarios. In contrast, the success of strain-based measurement is highly dependent on the closeness of damage to the sensor location, thus a much higher number of strain gauge sensors is required not to misidentify damage. The contribution of this work is four-fold. First, a novel and robust damage identification technique based on RIL is proposed which has not been reported in the literature. Second, the method solely relies on two measurement points e.g., two tilt meters at either end of the bridge and it is capable of identifying damage, even far from the sensor location. Third, the validation of the technique is demonstrated through extensive numerical and field test investigations on a statically indeterminate cable-stayed bridge structure. Finally, it is demonstrated that the conventional strain-based measurement is very likely to misidentify cable damage even with extreme case of cable loss

Evaluation of contact force models for discrete modelling of ellipsoidal particles

© 2017 Elsevier Ltd Discrete element method (DEM) has been widely used to study granular materials. However, how to model non-spherical particles is still challenging. Ellipsoidal particles are a typical kind of non-spherical particles in DEM simulations. There are three common methods to calculate the overlap and contact force between two ellipsoidal particles, namely, geometric potential (GP), common normal (CN) and overlap region (OR) methods. These methods are based on different physical concepts and hence will give different results. However, the comprehensive evaluation of these methods is still lacking, leaving DEM users no solid reference for selecting algorithms. In this paper, we conduct detailed comparisons on the penetration depth, contact plane and contact point predicted by the three methods. Particularly, using the orientation discretization method, the results are compared in all orientations quantitatively. It is shown that the difference between GP and CN is the largest whereas OR is always in between. The difference in contact point is relatively small when overlap ratio is small, whereas the difference in contact plane can always be large. Further, the results are directly compared to those obtained from sub-particle scale finite element analyses, which reveals that CN can always accurately predict contact plane and most times contact point, while GP are relatively better in predicting force magnitude. This study not only gives a more clear and comprehensive evaluation of different contact force models for ellipsoidal particles but also establishes an effective framework for comparing and verifying contact force models for general non-spherical particles

Prefabricated hybrid steel wall panels for mid-rise construction in seismic regions

Light steel framing systems are increasingly used in a variety of low-rise buildings, while are gradually penetrating to the mid-rise residential construction. The increased demand for mid-rise light weight steel frames in recent years has resulted in various research activities being undertaken in order to enhance the performance of these systems in compliance with the increased demands of mid-rise construction. This paper investigates the performance of a hybrid wall panel system (HWPS) made of a hot-rolled steel panel and a cold-formed steel panel in seismic regions. The hot-rolled steel panel resists the shear force, while the gravity load is distributed proportionally between the hot-rolled and cold-formed panels. The research employs the results of an experimental program to investigate the performance of the proposed hybrid panel, and a numerical study on the seismic performance of a 4-storey building made by HWPS. A practical design procedure is suggested for the system according to relevant specifications, and the results are compared with those obtained from employing a fully hot-rolled steel frame (moment resisting frame) and then a fully CFS frame system. The structural performance and the construction cost of two systems are then compared

Seismic collapse assessment of a hybrid cold-formed hot-rolled steel building

This paper investigates seismic collapse potential of a hybrid cold-formed hot-rolled system in order to quantify the response modification factor (R-factor) through a procedural method proposed in FEMA-P695. A series of hot-rolled steel (HRS) knee-braced frames in conjunction with cold-formed steel (CFS) stud walls are proposed to resist lateral and gravity loads. ASCE7-16 does not provide seismic performance factors for this hybrid HRS/CFS structural topology in lightweight steel construction and as a result, more sophisticated assessment is needed to measure reasonable seismic performance. A nonlinear numerical model that simulates post-peak response of HRS knee-braced frames is calibrated with experimental data. Post-buckling behaviour of CFS studs are measured according to various techniques in terms of finite strip method (FSM), finite element method (FEM) and AISI-S136-16 analytical formulations. The modelling approach is implemented into nonlinear analytical models of a six-storey steel building which is designed in accordance with ASCE7-16, ANSI/AISC360-16, and AISI-S316-16. A suite of twenty-two bidirectional far-field ground motions are chosen from PEER/NGA database subset and scaled to conditional mean spectrum (CMS) relevant to Urban California region. A set of nonlinear static analysis as well as incremental dynamic analysis (IDA) is conducted to measure collapse fragility and seismic performance of the building. It is concluded that initially assumed R-factor for the proposed structural system maintains the collapse prevention criterion as recommended by FEMA-P695 and is appropriate to be considered for design purposes

Transmissibility performance assessment for drive-by bridge inspection

This paper presents results of numerical and experimental investigations for the feasibility assessment of drive-by bridge inspection, where the health status of a bridge is solely identified from the dynamic response of an inspection vehicle, passing over a target bridge. To this aim, a novel metric based on the transmission performance between the vehicle and the bridge is proposed, and it is demonstrated that the vehicle having a higher sensitivity index has superior performance, in moving state, to identify bridge frequencies, and consequently separate different states of a bridge from one another. In addition to numerical validation, experimental validation from a laboratory-based experiment is presented, where a two-axle vehicle model is adopted as an inspection vehicle and a single-span steel beam is employed as a target bridge. Vibration data are collected using three highly sensitive accelerometer sensors installed on the vehicle, one at the middle of each axle, i.e., the front axle, and the rear axle, and the third one at the center of gravity of the vehicle, to measure the pitching and bouncing modes of the vehicle. The contributions and the key findings of the work are as follows: First, this paper presents one of the early attempts of experimentally validating the feasibility of drive-by bridge inspection to identify an induced change in the structure of a bridge which has caused less than 2% variation in the fundamental frequency. Second, a novel index for quantifying the performance of transmission between the bridge and the vehicle is proposed. It is verified that a vehicle with a higher sensitivity index captures more bridge-related information, hence making the bridge modal frequencies more identifiable. Third, through experiments, successful identification of the first three vibration modes of the bridge, obtained from the moving vehicle tests, is verified under the condition of constant and slow speed, when there is ongoing excitation on the bridge. Fourth, the indices and criteria presented herein would be helpful in designing a real drive-by inspection system which can be potentially used for drive-by bridge inspection of a network of bridges with similar structural design in practice

Lateral behaviour of hybrid cold-formed and hot-rolled steel wall systems: Experimental investigation

The seismic design of light steel frames (LSF) can not only rely on the application of cold-formed steel (CFS). Some mixed systems and integrated solutions such as hybrid systems can offer new possibilities, in particular with regard to applications in mid-rise construction. A hybrid solution is to replace some CFS chord studs with hot-rolled square hollow section SHS, in order to achieve higher capacity. This paper provides the results of experimental studies on the lateral behaviour of a hybrid light-weight steel panel and investigates the implication of any further system improvements for mid-rise construction. Each hybrid wall panel (HWP) consists of a hot-rolled SHS frame, laterally incorporated in a cold-formed panel. The study includes investigating the lateral performance of HWP, while a CFS top chord acting as a load collector, and a hot-rolled steel frame acting as a lateral load resisting system. The behaviour of specimens is investigated under monotonic and cyclic loads, and the step-by-step enhancement is implemented according to the results. The outcomes revealed that although the hysteretic behaviour of the HWP represents pinching effect, mainly due to poor performance of the cold-formed steel collector, by strengthening the top chord design the behaviour is improved. Relying on the cold-formed part to resist the major portion of gravity loads, while the hot-rolled collector transfers the entire lateral load to the hot-rolled frame, results in significantly improved hysteretic behaviour

Non-contact structural health monitoring of a cable-stayed bridge: case study

© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. In this article, the condition assessment of a cable-stayed bridge using remote sensing is presented. The displacement influence line (DIL) of the bridge under the live load tests is measured for a discrete number of target points. Three different remote sensing techniques including, laser scanning, terrestrial robotic total station and digital levelling are adopted for this purpose. It is demonstrated that DIL obtained by non-contact system is capable of identifying an emulated damage in an actual operating system. The contribution of the work is fourfold. First, a damage index based on the displacement profile of the bridge under the weigh-in-motion is extracted from the non-contact sensing system. Second, our study compares three different remote sensing techniques, namely, digital levelling, robotic total station and laser scanning and uses the measurements to validate the finite element model. Third, the effectiveness of the proposed method for structural damage identification is validated in a real-world large-scale operating structure. Finally, it is validated that strain-based influence line is highly likely to misidentify damage especially when the location of damage is not in the close proximity of the sensor; however, DIL is a better damage indicator even if damage occurs far from the measurement point

Lateral force resisting systems in lightweight steel frames: Recent research advances

Lightweight Steel Frames (LSF) made by framing thin gauge cold-formed steel (CFS) into different structural elements such as walls, trusses and joists are commonplace in Australia and many parts of the world. The great progress in the knowledge of CFS structures achieved in the past two decades, together with the modern design and fabrication methods supported by progressively improved specifications, have equipped the industry of the lightweight steel construction with tools and confidence to play an important part in the future of building construction. Despite the ever-increasing demand on the use of cold formed steel (CFS) framing into more complex and taller structures, the lateral load resistance capacity of lightweight steel frames has proven to be a major hindrance and a major concern. This paper reviews and summarises the research developments made in the area of lateral load resistance capacity of lightweight steel frames (LSF) as published in leading journals and codes’ provisions in the area. Research advances in conventional systems such as shear walls clad with face sheathings and LSF strap-braced wall systems in addition to other less conventional systems such as special bolted moment frames are reviewed here, and the solutions for improving the lateral performance of these systems are classified