Uplift Force Research Articles

Characterization of Flow under Impervious Dams: Dimensionless Groups and Universal Solutions

As far as we know, no dimensionless solutions for infiltrated flow under dams in anisotropic media exist since those that can be found in manuals refer to isotropic soils. The novelty of this work is the presentation of universal solutions in the form of abaci for water flow, average exit gradient, uplift force, and its application point for this type of soil. These solutions are obtained by the application of the discriminated nondimensionalization technique to the governing equations in order to find accurate dimensionless groups that control the results of the problem. In particular, the ratio of permeabilities corrected by a geometrical aspect relationship appears as a governing group, so anisotropy can be considered as input information. In this way, the sought solutions are a function of the emerging groups. Numerical solutions are used to successfully verify the results obtained, which in turn are compared to those of other authors for isotropic scenarios.

Experimental and numerical investigation of combined countermeasure for mitigating tsunami forces on typical coastal T-girder bridge deck

Tsunamis can cause destructive damage to coastal bridges resulting in huge economic losses and human casualties. Different types of countermeasures can reduce the horizontal and vertical uplift forces of tsunami on coastal bridges, thus protecting them from tsunami damage. However, previous studies have mainly focused on a single measure for the bridge deck. This study concentrates on the integrated countermeasure of the fairing and opening holes on typical coastal T-girder bridge deck. The effect of the integrated measure on reducing tsunami forces under different tsunami bore conditions is investigated through experiments and numerical simulations. The optimal fairing shape is also examined using an adaptive machine learning based technique. Results show that: (a) The openings on the bridge deck can effectively reduce the vertical uplift tsunami forces on the T-girder, but will slightly increase the horizontal tsunami forces on it; (b) With installation of the fairing only, the horizontal force on the T-girder can be effectively reduced due to the diversion effect of the fairing; however, in some cases, this measure will adversely increase the vertical uplift tsunami force on the T-girder; and (c) The combination of the fairing and opening-hole countermeasures for tsunami has a favorable mitigation effect on both horizontal and vertical uplift forces applied to the T-girder bridge deck. The mitigation effect increases beneficially with the increase of the tsunami bore wave height for the studied cases.

Performance of grouped stud connectors in precast steel-UHPC composite bridges under combined shear and tension loads

Ultra-high performance concrete (UHPC) becomes an innovative resolution for facilitating the construction and structural performance of prefabricated bridges. Head stud shear connectors are commonly applied in prefabricated steel–concrete composite bridges and can withstand uplift forces as well as shear forces. In this paper, six standard push-out tests (SP-Test) and ten modified push-out tests (MP-Test) are prepared and tested. In the MP-Test, stud connectors are subjected to shear and tensile forces simultaneously. The stud diameter, stud aspect ratio and the ratio of tension to shear are considered for their effects on shear resistance, shear stiffness and ductility. Besides, the finite element (FE) analysis and parametric analysis are conducted to further investigate the behavior of stud connectors under combined loads. The formulas for predicting the shear-tension interaction strength and shear resistance of stud connectors in UHPC under combined loads are proposed. The results show that the stud connectors exhibit both shear and tension failure characteristics under combined loads. The lower ratio of tension to shear has less effect on the shear resistance and ductility. Meanwhile, the stud aspect ratio is positively related to shear resistance and ductility but has no effect on shear stiffness under combined loads. As the ratio of tension to shear increases, a considerable reduction is observed in the shear resistance and relative slip as well as shear stiffness of stud connectors. However, the attenuation effect of tensile stress on the shear strength of studs can be neglected when the tensile shear ratio is less than 0.17. The unevenness of the force on single stud in stud connectors can be ignored and excessive strength of UHPC has limited effect on improving the strength of studs under combined shear and tension loads. The proposed shear strength reduction factor takes into account the ratio of tension to shear and gives more reasonable predictions of shear resistance under combined loads. Besides, compared with other available equations, the proposed shear-tension interaction model predicts the interaction strength more accurately.

Simulation of Heavy Precipitation and the Production of Graupel Related to the Passage of a Cold Front over the Australian Snowy Mountains

Abstract The case study of a heavy precipitation event associated with the passage of cold front over the Australian Snowy Mountains (ASM) on 3 August 2018 has been examined using the observational data from an intensive field campaign and high-resolution (1 km) Weather Research and Forecasting (WRF) simulation. We divided this event into prefrontal, cold front, and postfrontal periods. The cold front and postfrontal periods were characterized by higher production of graupel, while relatively low graupel was produced in the prefrontal period. Overall, aggregation along with deposition are likely the main growth mechanisms of snow in the prefrontal clouds, while heavy rain was produced below the melting level over windward slopes of the ASM. The simulated melting level is lower compared to the observations, which is consistent with model cold bias. Stronger orographic uplift and frontal forcing were mainly responsible for the enhanced supercooled liquid water (SLW) production over the ASM in the cold front period. A drop in elevation of the freezing level and increase in low-level relative humidity further enhanced the SLW production. The production of graupel through riming processes was highly efficient in the cold front period given the high concentration of ice-phase hydrometeors in the frontal clouds and the development of clouds comprising supercooled liquid water. The orographic updrafts and embedded convection were the main dynamical processes generating postfrontal SLW clouds and graupel. Ice initiation processes were activated once SLW cloud tops reached −15°C level followed by graupel production through riming processes.

Large-eddy simulation of wind loads on a roof-mounted cube: application for interpolation of experimental aerodynamic data

Failure of roof-mounted equipment in extreme wind events such as hurricanes is a significant contributor to wind-related property loss. Proper understanding of the nature and magnitude of wind-induced loads on these structures is essential to ensure their safety during severe storm conditions. Previous studies showed that wind loads on roof-mounted equipment vary significantly depending on the shape, size, location, and elevation relative to the roof surface. In this study, using large-eddy simulation (LES), wind loads on cube-shaped equipment mounted on the roof of a low-rise building are investigated to study the effect of equipment elevation on the wind loads aiming to complement existing experimental aerodynamic data. The accuracy and reliability of the LES results are assessed stage by stage. First, the characteristics of the incident wind profiles reported in the experimental study are reproduced in LES. Then, surface pressure distribution and force coefficients found from the LES are validated against the experimental aerodynamic data for selected representative cases. Finally, the variations of peak drag and uplift force coefficients with different equipment elevations are investigated in detail with the help of the flow field acquired from the LES near the rooftop equipment. The results from the current work show that the peak uplift force coefficients are significantly reduced for elevated equipment. However, considering all the cases studied, the peak drag force coefficients remain relatively unchanged with equipment elevation.

Experimental study of wind-induced dynamic response and collapse of a mechanically-attached membrane roofing system installed on metal substrate of flat-roofed low-rise steel building

This paper investigates the wind-induced dynamic response at design wind speed and the collapse of a mechanically-attached membrane roofing system installed on metal substrate of flat-roofed low-rise steel building, based on a wind pressure loading test using full-scale assembly specimens. Three kinds of waterproofing membranes with different mechanical properties are tested. The wind loads employed in the test are generated by using the time histories of wind pressure coefficients on the roof of a flat-roofed low-rise building with low parapets, which were obtained from a wind tunnel experiment. Both the temporal and spatial variations of wind pressures are reproduced by using three Pressure Loading Actuators (PLAs). A step-wise pressure loading test commonly employed in the conventional wind-resistant performance evaluation in Japan is also carried out for a comparative purpose. The results indicate that the fasteners connecting the disks with the metal substrate are subjected to not only vertical uplift forces due to negative wind pressures (suctions) but also horizontal forces related to the membrane deformation. The maximum value of horizontal force is nearly equal to that of the vertical force. Although the general correlation between vertical and horizontal forces is not so high, the maximum values of these forces may occur almost simultaneously. Larger the membrane deformation, larger the horizontal forces are. The horizontal forces reduce the failure loads and affect the failure modes significantly. Therefore, the effect of horizontal forces should be considered in the evaluation of wind-resistant performance of mechanically-attached membrane roofing systems. The failure load under step-wise pressure is generally smaller than that under fluctuating wind pressures. Therefore, the conventional step-wise pressure loading test can be used as a simple method for evaluating the failure load of the roofing system on the safer side.

Horizontal penetration of a finite-length intruder in granular materials

In recent years, bio-inspired burrowing robots and other intruder problems in granular media have attracted significant attention. Many of these, especially related to traditional penetration problems in geotechnical engineering, cover vertical penetration. Modelling these types of problems numerically using the discrete element method (DEM) is typically done ignoring gravity by controlling the stresses in the selected representative volume. Additionally, most problems involve infinitely long intruders from a modelling point of view. However, in horizontal penetration there is enough evidence to show that intruders are affected by an uplift force that affects the penetration and needs to be considered. In this paper we use the DEM to demonstrate the impact of considering vertical uplift and gravity for a finite-length intruder penetrating in a dense granular media through particle level and macro-behaviour. Additionally, through the numerical study, important mechanisms emerge during horizontal penetration, including four different distinct stages on the surrounding soil, or the extent of disruption, that are fundamentally distorted when gravity is ignored.Graphical abstract

Evaluation of the influence of pantograph cracks on contact forces in the interaction between pantograph and catenary

Pantograph-catenary (P-C) systems operating under cracks would induce potential risks to railways. Previous relevant research efforts lack consideration of the dynamic characteristics of P-C systems under crack damages. A rigid-flexible P-C coupling model with pantograph cracks for a metro line is developed in this study. The evolution law of P-C contact forces under the condition of cracked propagation is described, and the influence of speed, uplift force, span length, and pullout value arrangement on P-C contact force and the sensitivity of crack influence are explored. Results show that the dispersion of contact force values decreases continuously upon the existence and expansion of cracks on the pantograph. Proper selection of operating conditions is advised because of the sensitivity differences of the five influencing factors indicates. Metro trains should not be operated at 100 km/h for the rail section with an 8 m-span catenary, and further inspections on the pantograph should be scheduled for trains operating over 100 km/h. The uplift force should be adjusted to the 90-100 N range. The 8 m span length is suggested in a metro catenary, and the splayed arrangement of pullout value is recommended as a priority when the situation allows.

Dynamic performance of simply supported girder bridges subjected to successive earthquake-tsunami events

Coastal bridges are susceptible to severe damage when subjected to successive earthquake-tsunami events. Previous studies mainly consider the tsunami loadings as hydrodynamic forces, whereas other hydrodynamic forces such as uplift and slamming forces, are not fully investigated. Moreover, there are limited studies on the dynamic performance of simply supported girder bridges under the earthquake-tsunami sequences. To this end, this paper aims to conduct an in-depth investigation on dynamic performance of simply supported bridges subjected to sequential earthquake and tsunami hazards by means of a high-fidelity wave force simulation approach. More specifically, a typical, already constructed simply supported girder bridge is taken as the example bridge, and the numerical model of this bridge is built using the analytical platform OpenSees. The applied time series of tsunami wave force with five wave heights are generated based on a refined computational fluid dynamics (CFD) model, and are separately combined with the time histories of 21 pairs of far-field earthquake records to generate the sequential earthquake and tsunami loadings. Subsequently, nonlinear time history analyses (NTHAs) are carried out to obtain the structural dynamic responses, and the effects of preceding earthquakes and wave heights on the performance are investigated. Results indicate that the bearings are susceptible to the damage in the longitudinal direction of the bridge under the sequential earthquake and tsunami loadings, while the piers sustain more damage in the transverse direction. The preceding earthquakes have a significant effect on the bridge performance, and the effect becomes more pronounced with the increase of the ground motion intensity. The contribution of the tsunami loadings to the bridge response increases as the wave height rises. In particular, the transverse pier drift is dominated by the tsunami loadings when the wave height is higher than 7 m. The outcome of this study could aid the design and management of coastal bridge subjected to successive earthquake-tsunami events.

Hydrodynamic Uplift Forces on Submerged Bridge Decks during Bedform Migration

Hydrodynamic Uplift Forces on Submerged Bridge Decks during Bedform Migration

Estimation of the Uplift Resistance for an Under-Reamed Pile in Dry Sand Using Machine Learning

Abstract Under-reamed piles are extensively used to resist uplift forces and settlements. The objective of the present study is to develop various machine learning models (linear and non-linear) and determine the best model to estimate the ultimate uplift resistance of under-reamed piles embedded in cohesionless soil. The machine learning models were developed considering the following input parameters: the density index, dry density, base diameter, angle of an enlarged base with a vertical axis, shaft diameter, and embedment ratio. A linear equation is proposed to estimate the ultimate uplift resistance based on Multivariate Linear Regression analysis with a mean absolute error equaling 0.25kN and 0.50kN for loose and dense sands respectively. The Decision Tree Regression model provides an excellent degree of accuracy with a mean absolute error of 0.02kN and 0.02kN in cases of loose and dense sands respectively.

Reline Jacket: Efficient Reduction of Frost-Heave Uplift of Piles in Warming Permafrost

Air temperature in the Northern Hemisphere has been progressively warming in the recent decades, and the ground temperatures have increased correspondingly. The air temperature increasing due to the climate change induces degradation of permafrost and frost heaving activation. The frost heaving forces cause unevenly distributed damaging displacement of foundations and thus poses problems to the development of Arctic regions. Frost-heave uplift forces can be reduced by protecting piles with an OSPTReline (or Reline) polymer heat-shrinkable jacket. The interaction of heaving soil with a pile covered with the Reline jacket is modeled in laboratory to estimate the uplift force and the related shear strength of frozen soil along the soil-pile adfreeze surface at temperatures from −6 to −1 °C. The data are obtained for silty sand and silty clay soils and mortar (1:5 cement-sand mixture). The experiments show that frost-heave uplift forces on Reline-protected piles are 52% to 85% lower than on uncovered steel piles (steel grade 09G2S—analog to European steel grade S355JR), depending on soil type and temperature.

Numerical study on aeroelastic behavior of spoiler on low-rise building based on fluid-structure interaction method

Technical mitigation devices have been demonstrated to be efficient for decreasing the peak and mean wind pressures on the roof systems of low-rise buildings through disrupting vortex formation. Previous investigations have regarded mitigation devices as rigid bodies and neglected aeroelastic behavior. However, the relatively large uplift force induces small-stiffness mitigation devices vibration and introduces a considerable aeroelastic effect. These effects might result in an increase in negative wind pressure on the building roof and the aeroelastic instability problems of mitigation devices. Therefore, the current study conducted two-way fluid-structure interaction (FSI) numerical simulation for a low-rise flat-roof building with mitigation devices (metal and polymer spoilers) to investigate the influence of the aeroelastic effect of spoiler on the wind pressure on the building roof. The large eddy simulation (LES) method and the finite element analysis (FEA) model are used to solve the flow field and the structural responses of the spoiler, respectively. The computational model and method are verified by comparing the calculation results with previous experimental results and benchmark computational tests. The dynamic response of spoiler, the flow instabilities and the wind pressure on the building roof are analyzed. The influence mechanism of the spoiler vibration on the flow field over the building, the wind pressure on the building roof and the uplift force on the spoiler were revealed considering the influencing factors, including the vibrational amplitude of spoiler and the wind direction. The results show that the aeroelastic effect of spoiler increases the negative wind pressure on the roof, and the larger the vibrational amplitude of spoiler is, the more the negative wind pressure increases. The negative wind pressure on the eave also increases with increasing wind direction angle. The main reason for this phenomenon is that the spoiler vibration increases the degree of flow separation over the building roof.

Numerical limit analyses on the uplift capacity of strip plate anchor in anisotropic spatially random cohesive soil

Plate anchors are used to resist uplift forces of both offshore and on-land structures. Numerous studies have been carried out to date to evaluate the ultimate uplift capacity of anchor plates. Most of them assumed the soil as homogeneous and isotropic. Naturally, the soil is heterogeneous and anisotropic. The undrained shear strength of soil may vary significantly within a seemingly homogeneous layer. This paper analyzes the impact of spatial shear strength variability of cohesive soil on the uplift capacity of strip anchors through numerical limit analyses. Repeated Monte Carlo simulations within a selected range of both isotropic and anisotropic spatial correlation length and coefficient of variation of undrained shear strength are conducted using the random field theory to evaluate the effect of probabilistic uplift capacity of shallow and deep strip anchors. The outcomes are then compared to those of a similar study in which the probabilistic uplift capacity was investigated using a displacement-based analysis employing the local averaging subdivision of the random finite element method (LAS-RFEM). The comparison depicts the requirement of a higher safety factor than the published results of RFEM. The findings suggest considering the spatial variation of soil strength for the safe design of plate anchors.

Flow-structure interference effects with the surrounding structure in the choked quasi-steady condition of tsunami: Comparison with traditional obstruction approach

To understand tsunami flow interaction with the structure occupying a certain percentage of flow channel width in the field, several authors modelled experiments or numerical simulations with the single structure occupying the same obstruction ratio, denoted by b/W (b = single structure width; W = flow channel width, considered as flume width). This setup resulted from laboratory width constraints or computational time reduction in numerical modelling, leading to the inherent assumption of symmetrical flow conditions at the flume walls. In this study, we compare the flow and force characteristics of the b/W setup with the results from the unsteady flow-structure interaction experiments conducted along with the adjacent structures in the flow choked condition, thereby replicating a realistic flow behaviour (termed as b/W* setup, here W* represents the actual flow width replicating the real field). The results from the experiments indicate that the change in the bore height at the structure front (h*f) is marginal or almost negligible between the two setups, independent of Fr. However, the bore height at the structure backside (h*r) is significantly altered. The existence of the hydrostatic pressure at the structure front and backside still ensured F*≈ρbg(hf*2−hr*2)/2 in flow-choked condition for the b/W* setup. Further, the 'h*f' prediction using the conservation of mass and momentum equations is improved. In regards to the vertical uplift force (F*u), the ratio of F*u/F*ub (F*ub = buoyancy force calculated using the average of h*f and h*r in the b/W* setup) is found to be approximately 1.15 times higher than Fu/Fub in the b/W setup (Harish et al., 2022a).

Measurement of pile uplift forces due to soil heave in expansive clays

Piled foundations are widely used to limit the movement of foundations in expansive clays. These piles are subjected to large uplift forces as the clay swells (and downdrag forces when it shrinks). The appropriate method to estimate these forces is not well understood, and estimates from various methods presented in literature result in large variations. The uplift force generated in piles by soil heave was derived from strain measurements in full-scale field tests where a pile was installed in a highly expansive soil profile and flooded for several months. The results were compared to available theoretical estimates to comment on the most appropriate procedure to estimate the generated tension in the pile. The results showed that the use of an elastic solution related to the expected soil heave combined with a limiting shaft friction estimated from total stress capacity methods provided the most appropriate match to the measured results both in terms of the magnitude of the developed tension in the pile and shape of the tension profile along the depth of the pile.

Wind Loading of Photovoltaic Panels Installed on Hip Roofs of Rectangular and L-Shaped Low-Rise Buildings

Many residential houses in Japan have hip roofs with pitches ranging from 20° to 30°. Recently, roof-mounted photovoltaic (PV) panels have become popular all over the world for environmental conservation. The design of PV systems in Japan is usually based on the Japanese Industrial Standard (JIS) C 8955 (2017). However, the standard does not provide wind force coefficients for PV panels installed near roof edges (up to 0.3 m from the edge) because flow separation at the roof edges causes large up-lift forces on such panels. In this paper, we investigated the wind force coefficients for designing PV panels installed on hip roofs of rectangular and L-shaped low-rise buildings. The roof pitch was set to 25° as a typical value. Rectangular panels were installed almost over the whole roof, including the edge zones. Because the thickness of PV panels and the distance between PV panels and the roof are both as small as several centimeters, it is difficult to make wind tunnel models of PV systems with the same geometric scale as that for buildings. We focused on a numerical simulation using the unsteady Bernoulli equation to estimate the pressures in the space between PV panels and the roof. In the simulation, we used the time histories of wind pressure coefficients on the bare roof, which were measured in a turbulent boundary layer. We propose installing PV panels with small gaps between them along their short sides. The gaps may reduce the wind loads not only on the PV panels but also on the roofing due to pressure equalization. We discuss the optimum gap width from the viewpoint of wind load reduction.

Measurement of tangential frost heaving stress for steel pile in clay silt

Tangential frost heaving stress is the main force causing the frost jacking of pile foundation in frozen ground. It is of great engineering and scientific significance to measure the exact value of tangential frost heaving stress economically and quickly. In this study, a set of test equipment was designed and manufactured to measure the tangential frost heaving stress of steel pile in clay silt during freezing. At the same time, the soil temperature, vertical displacement and moisture content of the soil were measured. The results indicate that the measuring equipment is feasible, and a complete variation process of uplift force during the freezing-thawing cycle was obtained. In addition, some interesting phenomena were found in the tests, such as the slow reduction and rebound of the uplift force. Also, it was found that the water migrated horizontally to the pile during the freezing process, and the vertical ice films were formed in the soil around the pile.

Experimental Investigation of Wave Force on the Pavement behind Crown Wall of Rubble Mound Seawall

Physical experiments were conducted to establish an empirical formula that predicts the wave force on the upside of the pavement behind crown wall of rubble mound seawall due to wave overtopping as well as the uplift force on the downside of the pavement. The experiments were performed by different conditions of the parapet, water depth, relative freeboard, and thickness of the armour layer. Then, the wave force on the upside and downside of the pavement behind the crown wall was analyzed. The parameters that affect the wave overtopping force and the uplift force were identified and empirical formulae were suggested for evaluating the forces on the pavement.

Experimental Study of Forces Influencing Vertical Breakwater under Extreme Waves

In order to understand the extreme wave acting on the vertical breakwater, a series of experiments were constructed in the wave tank to measure the variations of pressure on the front, rear faces, and below the caisson due to overtopping waves. The front and backward horizontal forces and the uplift forces were estimated by integrating the dynamic wave pressure distributions. The COBRAS numerical model was also used to calculate the wave loads under various overtopping waves. The measured wave pressures and wave forces were compared with the predictions of numerical results and showed good agreement. It was found that the forces acting on the backward side of the vertical structure induced by the wave overtopping should be considered. From the experimental data, new semi-empirical equations for calculating the maximum wave forces are proposed using a least squares approximation.