Reduced-scale Structure Research Articles

Experimental validation of vibration similitude laws for stiffened panels excited by a turbulent boundary layer

Stiffened structures excited by a turbulent boundary layer (TBL) occur in many engineering applications, particularly in the aerospace and naval sectors. The structures encountered in these applications are generally large and complex, and carrying out experiments in order to characterize their vibration response can be costly, time-consuming and difficult. To overcome these constraints, the similitude theory can be used to estimate the response of a full scale structure from the response of a reduced scale structure. A simplifying step to determine these similitude laws for stiffened panels consists in modeling the panel as an equivalent orthotropic panel. Then, using the governing equation of the orthotropic panel and considering the TBL excitation, the vibration similitude laws are derived implying similitude conditions to be respected on geometrical and material properties of the structure as well as flow velocity. In this paper, we present the developments of these similitude laws and an experimental validation for panels in air. The similitude conditions are discussed to adequately choose the properties of scaled stiffened panels. Experimental results are presented to show that the structural response of the reference panel can be recovered from the response of the reduced scale panel using the related scaling laws.

Numerical simulation and measurements on reduced scale structures of floating production storage and offloading units

Protecting offshore structures against the effects of lightning strikes, sparks, and short circuits is a considerable challenge to designers and deployment engineers once the numerous exposed structures, pieces of equipment, and devices make the behavior of electromagnetic phenomena difficult to predict. Numerical simulations are important tools to predict such behavior. However, due to the uncertainties involved, they should be compared against laboratory results. This paper presents the results of the measurements performed on three scaled models and compares them to those obtained with two different applications, CST Studio Suite and NEC-4. We elaborate hypotheses regarding the differences between laboratory results and those obtained using simulation software. As an application, after validating modeling techniques, the paper discusses the results of induction on cable loops on “gooseneck” chimneys commonly used in ships and floating production storage and offloading units.

Modern Dimensional Analysis Based on Fire-Protected Steel Members’ Analysis Using Multiple Experiments

Nowadays, the real structures (considered as prototypes) subjected to fire are analysed by means of the behaviours of some reduced scale structures (defined as models). These prototype–model correlations are governed by the so-called dimensional analysis (DA) methods. These methods, starting from the Buckingham theorem, offer several dimensionless variables and based on them is the so-called Model Law (ML), which is able to foresee the predictable prototype’s answer based on the results of the experimental investigations performed exclusively on the model (usually manufactured at a reduced scale). Based on the MDA principles, in a previous paper the authors elaborated the complete ML for the heat transfer in beams with rectangular-hole cross-sections, considering unprotected as well as thermally protected structural elements. The authors, based on meticulous experimental investigations, obtained the validation of this ML for the unprotected steel members. In this contribution, the authors offer in a similar manner the ML validation for intumescent paint-protected steel members and thus the complete validation of their original ML. In their theoretical and experimental investigations, the authors involved both a real column’s element combined with its models manufactured at 1:2 and 1:4, as well as 1:10 scales too. Consequently, the obtained ML can be considered as generally valid, involving a real structural element and its model manufactured at the desired scale.

Boundary effects on dynamic centrifuge testing of onshore wind turbines on liquefiable soils

Centrifuge modelling is an effective tool to assess the response of reduced-scale structures subjected to earthquakes under increased gravity. Space limitations, however, force the model to be contained within relatively small boxes, whose boundaries may affect the seismic performance of the structure under consideration. In this paper, the influence of the proximity of the boundaries of an equivalent shear beam (ESB) container during dynamic centrifuge tests of an onshore wind turbine resting on liquefiable soils is evaluated. To this end, numerical modelling of the ESB box was implemented in the finite-element framework OpenSees, to replicate the results observed in the experiment. The hydraulic and mechanical soil parameters were calibrated against far-field centrifuge results only. From this calibration, the seismic performance of the raft foundation turned out to be in good agreement with the experimental results for a seismic input capable of triggering liquefaction. A larger numerical model, where boundaries do not play any role, was then built, to compare its outcomes with those of the small model, thus allowing the effect of ESB boundaries to be assessed.

Experimental seismic performance of a reduce-scale stone masonry loess cave with traditional buildings

The seismic behavior of the independent type of a stone masonry loess cave (SMLC) is examined. A tri-directional shake table test was employed on 1:4 reduced-scale specimen of SMLC. The specimen represented a typical traditional dwelling of the loess region in China, consisting of unreinforced masonry walls and inner loess, without any seismic treatment. The dynamic parameters directly measured by the test include the acceleration response, displacement response of each key position of the stone masonry loess cave, and the detailed record of the damage form of the structure under each loading. The analysis results indicated that the simplified model design method not only satisfied the main similarity relationship of the reduced-scale structure shaking table test, but also achieved excellent test results. Through the calculation results to determine the torsion angle of the node and the deformation between the layers, corresponding strengthening measures could be proposed for the life extension protection of this traditional building. Finally, according to the analysis of the test results, the damage state and damage level of the prototype of the SMLC is established.

Numerical Analysis of the Effect of Afterburning on Damage to the Concrete Structure Under Interior Explosion

A numerical simulation was performed to analyze how afterburning affects damage to a concrete structure when an explosive detonates inside a room. TNT was used as the explosive for the interior explosion. A reduced scale structure of a Korean apartment room was constructed with reinforced concrete and used as the target room for this research. Interior explosion experiments were performed using TNT 2 kg and TNT 3 kg explosives, to obtain data to verify and calibrate the numerical model. The numerical model was constructed to precisely simulate these interior explosions, especially considering the effect of afterburning, which is a significant phenomenon in enclosed spaces. Numerical simulations were performed with and without the effect of afterburning, and compared with the experiment results. The results showed that the numerical model could well simulate the effects of afterburning in an interior explosion, and that afterburning effects must be considered to more accurately analyze the damage to concrete structures.

Energy capacity and seismic resistance of loess cave structures under field ground motions

A shake table test studied the seismic response of a loess cave structure subjected to ground motions. The test specimen was a reduced-scale structure designed under current building codes and located in a region of moderate seismicity of the Asia area. The specimen was subjected to a sequence of increasing acceleration amplitude tests that represented frequent, design, rare, and very rare earthquakes at the site. The test structure performed well (basically in the elastic domain) under frequent and design earthquakes, approached the boundary between the performance levels of life safety and near collapse under the rare earthquake, and collapsed under the very rare earthquake. Damage concentrated covering at cave face and arch top at inner cave hole. The covering layer energy dissipated about 49.4% of the total energy that contributes to damage, and the rest was dissipated by the arch and cave leg layers. The total energy input on the structure until collapse under the very rare earthquake action was very close to the value obtained in a rare earthquake. The seismic capacity of the loess cave meets the seismic fortification index.

A tuning algorithm for a sliding mode controller of buildings with ATMD

This paper proposes an automatic tuning algorithm for a sliding mode controller (SMC) based on the Ackermann’s formula, that attenuates the structural vibrations of a seismically excited building equipped with an Active Tuned Mass Damper (ATMD) mounted on its top floor. The switching gain and sliding surface of the SMC are designed through the proposed tuning algorithm to suppress the structural vibrations by minimizing either the top floor displacement or the control force applied to the ATMD. Moreover, the tuning algorithm selects the SMC parameters to guarantee the following closed-loop characteristics: (1) the transient responses of the structure and the ATMD are sufficiently fast and damped; and (2) the control force, as well as the displacements and velocities of the building and ATMD are within acceptable limits under the frequency band of the earthquake excitation. The proposed SMC shows robustness against the unmodeled dynamics such as the friction of the damper. Experimental results on a reduced scale structure permits demonstrating the efficiency of the tuning algorithm for the SMC, which is compared with the traditional Linear Quadratic Regulator (LQR) and with the Optimal Sliding Mode Controller (OSMC).

A practical methodology devoted to pool‐type phenomena simulation in safety analysis for sodium‐cooled fast reactor

Accuracy and computational expenses are contradictory when performing simulation to hot plenum where multi-dimensional phenomena voice loudly in sodium-cooled fast reactor (SFR). A 2-dimensional method was preferred in light of the cylindrical shape and drive force behind significant thermal stratification phenomena during transient case. One barrier related to buoyancy effect on turbulent flux transportation has been removed before. Asymmetric arrangement of components in circumferential direction makes another difficulty to reproduce actual flow field within hot plenum of pool-type SFR. A series of special techniques for spatial discretization were implemented to restore geometry reality. Treatments about upper core structure, main heat exchanger and auxiliary heat exchanger were evaluated based on criterion of predicting well-mixed inner plenum and stratification like outer plenum. Finally, optimal model came to an 8-block partition scheme in which whole profiles of upper core structure and auxiliary heat exchanger were described. Validation was carried out through a normal state experiment on a water platform with reduced-scale structure. Agreement of relative temperature distribution demonstrated ability of this model for tackling spatial layout in pool-type SFR with the 2-dimensional library. All real-physics and efficient features indicate that this 2-dimensional methodology is an excellent tool for engineering application when designing or evaluating a new SFR concept.

Scaling Methodology Applied to Buckling of Sandwich Composite Cylindrical Shells

Studying buckling behavior of large shell structures through full-scale test articles can be complex and expensive. Therefore, reduced-scale structures are often preferred for investigating buckling behavior. However, designing reduced-scale structures that are representative of the full-scale structure can be difficult. An analytical scaling methodology for compression-loaded sandwich composite cylindrical shells based on the nondimensionalization of the buckling equations is presented herein. The methodology was used to develop scaled configurations that show similar buckling responses to the full-scale baseline configuration. Finite element analysis results showed that both a baseline and a scaled configuration buckled similarly, when the nondimensional stiffness, defined as the ratio between the nondimensional load and nondimensional displacement, was matched between the different scale models. Limitations of the methodology are discussed and are believed to be a result of neglecting the flexural anisotropy and the transverse shear compliance. A preliminary material failure assessment for the different scales is also considered.

Energy-based collapse assessment of concrete structures subjected to random damage evolutions

The assessment of structural capacity against collapse is conducive to the optimal design of new structures as well as checking the safety of existing structures. However, the evaluation cannot be typically carried out by means of destructive tests on prototype or reduced scale structures. In this regard, the numerical models that adequately represent the prototype structures can be alternatively used. Specifically, both the nonlinearities and randomness as well as their coupling effect of materials need to be represented in a unified manner in structural analysis. The present paper aims at providing an effective approach to incorporate the stochastic nature of damage constitutive relationships in collapse analysis and assessment of concrete structures subjected to earthquake ground motions. Within the framework of stochastic damage mechanics, the spatial variability of concrete is represented by a two-scale stationary random fields. The concept of covariance constraint is introduced to bridge the two-scale random fields such that the scale-of-fluctuation of the random material property is satisfied at both scales. Random damage evolution induced structural collapse analysis is achieved via the nonlinear stochastic finite element method. To address the randomness propagation across scales, the probability density evolution method is employed. By exerting the absorbing boundary condition associated with an energy-based collapse criterion on the generalized probability density evolution equation, the anti-collapse reliability of concrete structures can be evaluated with fair accuracy and efficiency. Numerical investigation regarding an actual high-rise reinforced concrete frame-shear wall structure indicates that the random damage evolution of concrete dramatically affects the structural nonlinear behaviors and even leads to entirely different collapse modes. The proposed method provides a systematic treatment of both uncertainties and nonlinearities in collapse assessment of complex concrete structures.

Structural performance of single-layer reticulated domes under blast loading

Abstract The dynamic performance of single-layer reticulated domes under explosions was analysed experimentally and numerically in this study. The loading was generated by a Gas Blast Shock loading system (GBS) by igniting a gaseous mixture and applying it to the reduced-scale structure to investigate the dynamic response modes. Acetylene/air mixtures with various volumes were designed in different cases. The experimental data, including overpressure–time histories, strain–time histories, and permanent displacement at representative locations, were recorded. The response patterns of the specimens were identified and discussed, the five major failure modes are as follows: no damage mode, insignificant damage mode, moderate damage mode, heavy damage mode, and whole collapse mode. It was shown that the Gas Blast Shock loading system is feasible for studying the response modes of single-layer spherical reticulated shells under external explosion. In addition, a numerical model based on ANYSY/AUTODYN was developed to simulate the experimental tests, and an improved equivalent TNT method was used to simplify the loading system. Good agreement between the experimental and numerical results was achieved and an explosion efficiency factor was proposed.

Investigating the scaling of masonry structures in a blast environment

Full-scale experimental testing of masonry response to blast can be challenging with the requisite need for significant measurement area in conjunction with high construction and material costs. This paper investigates the use of dynamic similitude to produce reduced-scale masonry structures which model the damage state and debris distribution of a full-scale counterpart due to blast loading. An investigation into the fundamental physical components of masonry response to blast loading facilitated the development of a new scaling methodology which maintains the ratio of lateral and vertical force components as prescribed by dynamic similitude. It is shown that this can be accomplished by using a reciprocal scale factor for the reduced-scale structure’s density. Computational models of full and reduced-scale masonry response to blast loading were produced with the Applied Element Method (AEM) to verify the underlying theory of the proposed scaling methodology. These utilised single-storey cuboid structures with non-responding roofs and half-thickness stretcher bond construction. Computational Fluid Dynamics (CFD) models of blast wave interaction with the masonry structures were defined at a range of peak overpressures, enabling a remap procedure into AEM. Importantly, AEM models utilised a constant 1:2 scale factor with masonry material parameters for commercially available units, demonstrating the practical applications of this scaling methodology for blast trials. Analysis of the AEM models demonstrated close qualitative agreement in damage state for full and reduced-scale structures at 55 kPa and 110 kPa peak free-field overpressure. These results also indicated agreement for a range of failure modes with the 110 kPa model showing front-panel collapse versus the 55 kPa model which indicated partial front-panel deflection. Chi-square analysis of the resultant debris distribution at 110 kPa indicated quantitative agreement for the relative quantities of bricks found within the rubble pile as a function of their original panel wall location.

Chemo-mechanical modelling of the external sulfate attack in concrete

This paper is focused on the modelling of the mechanical consequences of external sulfate attack in concrete structures under partially or fully saturated conditions. To this purpose a weakly coupled approach is developed: first the moisture content is computed through a simplified diffusion model, then a reactive-diffusion model allows for the computation of the expansive products of the reaction occurring between the aluminates of the cement paste and the incoming sulfate ions, finally the solution of a nonlinear mechanical problem gives the expansion, the stress state and the degradation induced by the reaction. The mechanical problem makes use of a multiphase elasto-damage model, developed in this work and accounting for both chemical and mechanical damage. The model is validated by simulating various experimental tests on concrete specimens subject to external sulfate attack and then applied to the simulation of a reduced scale structure of a tunnel lining.

Evaluación del diseño de una pequeña mesa vibratoria para ensayos en ingeniería sismo-resistente

<span>Las consecuencias catastróficas de los sismos han incentivado la realización de estudios experimentales para mitigar los efectos de los sismos sobre las estructuras. En el artículo se presenta la evaluación de los diseños mecánico, neumático, estructural, de control y de adquisición de datos, de una pequeña mesa vibratoria uniaxial para ensayos de estructuras a escala reducida. Inicialmente se eligieron los elementos mecánicos que permiten el movimiento de la mesa. Luego se validó el desempeño del sistema a partir de herramientas de simulación. </span><span>Finalmente se estudió la automatización de la mesa por medio de un control con sistema de lazo abierto, implementado en un micro-controlador. La mesa vibratoria propuesta es una herramienta versátil y económica para realizar pruebas experimentales orientadas al análisis y diseño de estructuras sometidas a eventos sísmicos. El dispositivo propuesto promoverá la investigación no sólo en nuevos materiales, sino en diseño y rehabilitación de viviendas, </span><span>edificios y puentes sismo-resistentes.</span>

On the Mechanism of Current Pulse Propagation Along Conical Structures: Application to Tall Towers Struck by Lightning

We discuss in this paper the propagation of lightning current pulses along conical tall structures. Although the dominant mode of an infinitely long conical transmission line is transverse electric and magnetic (TEM), such a structure can also support higher order TE and TM modes which display a gradual cutoff frequency variation with height. Recently, Baba and Rakov's finite-difference time domain numerical analysis revealed that for a perfectly conducting conical structure, while the current pulses suffer no attenuation as they travel from the cone's apex to its base, the attenuation is significant when pulses propagate from the base to the apex. Adopting an analysis method using 1) the COMSOL Multiphysics simulation environment based on the finite element method and 2) the partial equivalent element circuit method, we study the same reduced-scale structure analyzed by Baba and Rakov. The obtained results confirm the conclusions drawn by Baba and Rakov. We also perform simulations for the case of a 100-m tall tower considering different tower-base radii. It is shown that the upward current pulses are affected by a strong attenuation resulting from the field scattering near the discontinuity at the tower base, followed by a weaker attenuation resulting from the propagation along the cone from its base to the apex. A simple way to modify the engineering lightning return stroke models to account for the attenuation of the upward current pulses is suggested. Finally, we report on experiments to study the current pulse propagation along a 1/582 reduced scale model of the Toronto, CN, tower. The obtained experimental data support the numerical simulations.

Laboratory scale tests for the assessment of solid explosive blast effects. Part II: Reflected blast series of tests

An experimental set-up has been developed in order to detonate small explosive charges and to measure their induced blast on reduced scale structures. On the basis of Crank Hopkinson’s Law, the results obtained at reduced scale are used to derive full scale predictive models. A previous paper summarized the development of the reduced scale experimental set-up as well as the first free-field tests performed. The main features of the table, the instrumentation and the pyrotechnics were given.The present paper summarizes a second series of tests investigating the validity of models designed to predict reflected blast waves characteristics. Overpressure maxima, arrival time of the shock wave and impulse have been measured and are presented in this paper as non-dimensional characteristics of the pressure-time history. The results obtained from the experiments are in good agreement in comparison with data from the open literature and may thus be considered as reference results for further numerical or analytical model developments.The series of tests reported in this paper is in addition the final series of tests dedicated to validate the experimental set-up and the experimental procedures associated.

Coupling Control and Analytical Model of the Shaker on a Centrifuge

With the unique function of simulating a combined environment of high static fields and shaking load, centrifugal shakers can provide an effective and advanced testing method to dynamic characteristics and failure mechanism of reduced scale structure models and materials. Their construction has begun both at home and abroad, whereas waveform distortion and instability often happen to the perfect shakers after installed on a centrifuge in the process of their construction. The knowledge about this phenomenon is difficult to find and systems theory is absolutely blank. Practically the construction of centrifugal shakers is still in an exploratory stage. This paper analyses coupling control of shakers on a centrifuge in three aspects of installation method, bucket deformation and arm deformation. Simultaneously corresponding analytical models is also established, aim to guide theory study and equipment design.

Laboratory scale tests for the assessment of solid explosive blast effects. Part I: Free-field test campaign

The definition of blast loads applying on a complex geometry structure is still nowadays a hard task when numerical simulations are used, essentially because of the different scales involved: as a matter of fact, modelling the detonation of a charge and its resulting load on a structure requires to model the charge itself, the structure and air surrounding, which rapidly leads to large size models on which parametrical studies become unaffordable. So, on the basis of the Crank–Hopkinson’s law, an experimental set-up has been developed to support reduced scale structures as well as reduced scale detonating solid charges. As a final objective, the set-up must be used to produce the entry data for numerical assessments of the structure resistance. This set-up is composed of a modular table, sensors and targets and has been designed to conduct non-destructive studies. In the context of security, the general aim is to study the effects of detonation shock waves in the vicinity of facility buildings and to test various shock wave mitigation means that could be implemented for the protection of facilities in sensitive locations. Especially, the set-up offers the possibility to measure the loading in terms of pressure-time curves, even for very complex situations like multiple reflections, combination and diffraction. The present paper summarizes the development of the set-up as well as the first tests performed. The main features of the table, the instrumentation and the pyrotechnics are given. Also the paper summarizes a first qualification tests campaign that has been conducted. In this campaign, free-field blast tests (i.e. blast tests performed without structures) have been conducted. Overpressure maxima, arrival time of the shock wave and impulse are presented as non-dimensional characteristics of the pressure-time history. The results obtained have been found in good agreement with reference curves available from the open literature and numerical model results.

A finite element procedure for prediction of tube hole distortion due to welding of large perforated plates

Large tubesheets are critical components of huge heat exchangers and have to be fabricated by welding together two or more pieces. These parts are previously drilled in order to get reasonable productivity, but are affected by welding-induced residual stress and distortion. The tube holes are important, as several thousand tubes will be fastened to them, so it is crucial to evaluate their welding distortion to properly design the manufacturing process. This paper presents a finite element simulation of multipass welding of large tubesheets in order to compute the induced tube hole distortions. A welding experiment was conducted on a reduced-scale structure, in order to calibrate and validate the numerical model. Good agreement was found between experimental and numerical data.