Fixity Conditions Research Articles

ANN-Based Bridge Support Fixity Quantification Using Thermal Response Data from Real-Time Wireless Sensing.

Bridges are critical infrastructures that support our economic activities and daily lives. Aging bridges have been a major issue for decades, prompting researchers to improve resilience and performance through structural health monitoring. While most research focuses on superstructure damage, the majority of bridge failures are associated with support or joint damages, indicating the importance of bridge support. Indeed, bridge support affects the performance of both the substructure and superstructure by maintaining the load path and allowing certain movements to mitigate thermal and other stresses. The support deterioration leads to a change in fixity in the superstructure, compromising the bridge's integrity and safety. Hence, a reliable method to determine support fixity level is essential to detecting bearing health and enhancing the accuracy of the bridge health monitoring system. However, such research is lacking because of its complexity. In this study, we developed a support fixity quantification method based on thermal responses using an Artificial Neural Network (ANN) model. A finite element (FE) model of a representative highway bridge is used to derive thermal displacement data under different bearing stiffnesses, superstructure damage, and thermal loading. The thermal displacement behavior of the bridge under different support fixity conditions is presented, and the model is trained on the simulated response. The performance of the developed FE model and ANN was validated with field monitoring data collected from two in-service bridges in Connecticut using a real-time Wireless Sensor Network (WSN). Finally, the support stiffnesses of both bridges were predicted using the ANN model for validation.

Minimizing Superstructure Twist in Irregular Bridges through Optimization of Structural Parameters

Uneven distribution of mass, stiffness, and strength over the length of a bridge can produce rigid body rotation of the superstructure, or “twisting,” during an earthquake. Twisting response modes lead to the concentration or amplification of damage in the bridge substructure and, as such, are undesirable. AASHTO recommends a balanced stiffness approach to limiting bridge irregularities and provides possible modification techniques for bridges that do not satisfy the recommendations. However, there is a lack of research documenting the implementation details and benefits of these modification techniques on the performance of irregular bridges. This study investigated the effectiveness of three techniques to reduce the twisting response modes for geometrically irregular bridges during earthquakes: adjusting the effective heights of the bridge columns, modifying column end fixity conditions, and redistributing the superstructure mass profile. First, analytical equations were derived to estimate the values of parameters needed to achieve a balanced stiffness using each technique for any arbitrary bridge. Next, a finite-element model of an irregular, reinforced concrete, quarter-scale bridge previously tested on a shake table was validated against available experimental data. This model was used in conjunction with a metaheuristic optimization algorithm to determine the optimal values of the parameters such that the superstructure rotation was minimized for the experimental ground motions. Then, a parametric study was performed using a suite of hazard-consistent and spectrally matched ground motions to investigate the sensitivity of the optimization results to the number and spectral characteristics of the ground motions. The parametric study showed that reducing the effective heights of the columns by stiffening the lower portion of the columns was the most effective method to reduce the twisting response of the bridge and that three ground motions with different spectral shapes were the most computationally efficient subset when performing the optimization methodology.

On Effects of Shear Deformation on the Static Pull-in Instability Behaviour of Narrow Rectangular Timoshenko Microbeams

MEMS devices utilize electrostatics as preferred actuation method. The accurate determination of pull-in instability parameters (i.e., pull-in voltage and pull-in displacement) of such devices is critical for their correct design. It should be noted that similar to parallel plate capacitors, the electrostatic force between the surface of the deformable microbeam and stationary ground is non-linear in nature. Hence the analysis associated with MEMS devices is always inherently non-linear. In the literature, these devices have been majorly analyzed as Bernoulli-Euler microbeams with cantilever or clamped-clamped beam end conditions. However, Dileesh et al. (doi: 10.1115/ESDA2012-82536) have studied the static and dynamic pull-in instability behavior of slender cantilever microbeams by developing a six-nodded spectral finite element based on the Timoshenko beam theory (TBT-SFE). They have demonstrated the accuracy of the TBT-SFE by comparing their results with corresponding results of COMSOL-based three-dimensional finite element simulations. In addition, effects of shear deformation also start to play significant role as the beam thickness-to-length ratio increases. In this paper, authors have developed the TBT-SFE based on the work by Dileesh et al. for the case of statics. However, unlike Dileesh et al. where they have developed a six-nodded TBT-SFE, authors have investigated the best combination of number of nodes per element and total number of elements to carry out the study. For this purpose, authors have first calculated results of the maximum beam transverse displacement, for a shear deformable propped-cantilever microbeam under the action of uniformly distributed transverse load, obtained by utilizing the developed TBT-SFE with different combinations of number of nodes per element and total number of elements. These results are then compared with corresponding analytical results available in the literature to arrive at the best combination of number of nodes per element and total number of elements for the electrostatic-elastic analysis. In the second step, the finalized TBT-SFE is utilized to determine static pull-in instability parameters of narrow microbeams with various fixity conditions and beam thickness-to-length ratios. This study highlights the importance of transverse shear effects on pull-in instability parameters of Timoshenko microbeams.

Kinematic seismic response of end-bearing piles to S-waves

In this paper we revisit the problem of modelling analytically the kinematic interaction between a single pile and its surrounding soil under the action of seismic shear waves, by means of a Tajimi-type continuum elastodynamic model in three dimensions. The model provides the steady-state kinematic response of a cylindrical end-bearing pile embedded in a homogeneous viscoelastic soil layer, subjected to vertically propagating harmonic S-waves. Results of the model are first validated against the results of numerical simulations, and the results of an existing, approximate solution. Accordingly, we employ the model in a parametric study, where we investigate the sensitivity of the seismic response of piles to certain key problem parameters, including pile slenderness, soil-pile relative stiffness, excitation frequency and fixity conditions at the pile head. The solution yields closed-form expressions for pile deformations and for the soil resistance developing on the pile, that do not require introducing fitting coefficients.

Behavior, Performance, and Evaluation of Prestressed Concrete/Steel Pipe/Steel H-Pile to Pile Cap Connections

Better understanding of the connection between piles and pile caps/footings is needed so that future designs and design reviews can be adequately performed. This review paper presented discussions on the published experimental and analytical studies on prestressed concrete piles, steel H-piles, or steel pipe piles with cast-in-place (CIP) or precast footings, and/or pile cap connections with a focus on (1) pile embedment to achieve a fully fixed or pinned moment connection; (2) behavior of the pile embedment length when not treated by a special connection detail; and (3) reinforcement details to decrease the embedment length and develop fixity with a plain pile. The length of the pile embedment contributing to achieving a full fixity condition has a critical impact on substructure cost involving pile-to-pile cap (PTPC) connections. The design procedures involving piles subjected to lateral loading require a proper understanding of the fixity and ductility levels provided by the PTPC connections. Discussions on finite-element analyses (FEAs) were presented, including behavior predictions of the connections at both global and local levels. The study points to the need for modifications to the design of the PTPC connections in the existing standard codes/specifications.

Low-Cycle Fatigue Life Prediction of LP Steam Turbine Blade for Various Blade–Rotor Fixity Conditions

Low-Cycle Fatigue Life Prediction of LP Steam Turbine Blade for Various Blade–Rotor Fixity Conditions

Shake table investigation of nonlinear soil–structure–fluid interaction of a thin-walled storage tank under earthquake load

This research experimentally addresses the seismic response of a cylindrical liquid storage tank in terms of six factors, i.e., acceleration, displacement, hoop and axial stresses in the tank wall, sloshing and uplift. Both material and geometrical nonlinear interaction due to sloshing and uplift are considered. The simultaneous effects of the fixity condition, the supporting base flexibility and the aspect ratio on the response are presented in radar plots. The results reveal that, depending on the aspect ratio, the supporting base flexibility governs the overall response by activating uplift. A nonlinear elastic spring–mass model gives a good representation of the results.

Buckling solutions for compression members with end restraints defined along non-principal directions

The classical flexural-torsional buckling solution for compression members with asymmetric cross-sections is intended for members with end fixity conditions defined along the principal directions. In a number of practical situations, member ends can be rotationally restrained about a non-principal direction and rotationally free about the orthogonal non-principal axis. This may occur, for example, when one of the legs of a compression member with an angle or the web of a zed cross-section is connected to a gusset plate. The buckling loads in such cases cannot be determined from the classical solution. Within this context, the present study formulates the variational principle, governing neutral stability conditions, and associated boundary terms based on general non-principal directions, and then develops analytical solutions for the resulting coupled equations. The validity of the solutions is then demonstrated though comparisons with shell and thin-walled beam-based finite element solutions. The solutions are then used to investigate the influence of the orientation of the restraining axes on the buckling load capacity and associated buckling modes for single angles and zed shaped compression members with common end conditions.

Development of Simulation Based p-Multipliers for Laterally Loaded Pile Groups in Granular Soil Using 3D Nonlinear Finite Element Model

The behavior of laterally loaded pile groups is usually accessed by beam-on-nonlinear-Winkler-foundation (BNWF) approach employing various forms of empirically derived p-y curves and p-multipliers. Averaged p-multiplier for a particular pile group is termed as the group effect parameter. In practice, the p-y curve presented by the American Petroleum Institute (API) is most often utilized for piles in granular soils, although its shortcomings are recognized. In this study, we performed 3D finite element analysis to develop p-multipliers and group effect parameters for 3 × 3 to 5 × 5 vertically squared pile groups. The effect of the ratio of spacing to pile diameter (S/D), number of group piles, varying friction angle (φ), and pile fixity conditions on p-multipliers and group effect parameters are evaluated and quantified. Based on the simulation outcomes, a new functional form to calculate p-multipliers is proposed for pile groups. Extensive comparisons with the experimental measurements reveal that the calculated p-multipliers and group effect parameters are within the recorded range. Comparisons with two design guidelines which do not account for the pile fixity condition demonstrate that they overestimate the p-multipliers for fixed-head condition.

Specifying Fixed-Base Columns or Pinned-Base Columns in Computer Analysis and Design of Steel Frames

AbstractThis paper addresses the application of support fixity condition of column bases in computer structural analysis and design of steel frames. All structural analysis software require input d...

Dynamic soil-structure interaction of a shallow founded shear frame and a frame equipped with viscous dampers under seismic loading

It is not uncommon for buildings requiring good seismic performance, with objectives of immediate occupancy and continuous operation, to be equipped with supplemental damping devices. Oil dampers are a popular type of velocity-dependant devices that can be fitted into a structure without incurring considerable changes to the frame stiffness. Seminal experimental work into the seismic performance of structures with viscous dampers is based primarily on large-scale testing of prototype frames fixed to rigid shaking tables overlooking ground flexibility effects. Most literature examining the effects of soil-structure interaction (SSI) on the behaviour of structures with viscous dampers use highly idealised representations of the foundation-soil system stiffness and damping characteristics. This paper utilises high gravity dynamic centrifuge testing to investigate the effects of different base fixity conditions on the seismic behaviour of two model shear structures; one fitted with miniature oil dampers and one left bare for comparison. The base fixity conditions replicated are full base fixity and shallow embedment into dry, dense and loose sand. Experimental results indicate that the damped frame is less responsive to changes in ground compliance relative to its bare frame counterpart. Any apparent drop in damper control with reduced base fixity is driven by improvements in the seismic performance of the bare frame benchmark rather than an increase in the dynamic response of the damped structure. Despite the adverse effects of reduced base fixity on the energy dissipated by the dampers, SSI did not negatively affect the performance of the damped frame.

Impact of Dynamic Soil–Structure Interaction on the Response of Liquid-Storage Tanks

In general, soil-structure interaction phenomena affect considerably the dynamic response of liquid-storage tanks. As it is also observed in the case of ordinary structures, the ground motion transmitted to the superstructure can be amplified (or even de-amplified) due to the presence of the underlying soil layer(s), modifying in parallel the resonant period and the effective damping of structures. Typically, fixed-base liquid storage tanks are characterized by low fundamental periods. However, many such critical structures are located in coastal areas with soft soils, thus, the seismic performance of the superstructure may be notably different compared to stiff soil conditions. Therefore, ignoring soil-structure interaction may lead to unrealistic results. Accordingly, the influence of soil conditions in the dynamic response of liquid-storage tanks is investigated in the present study. More specifically, the dynamic soil-structure interaction of cylindrical steel tanks subjected to different ground motions is numerically examined. The main aim is to investigate the dynamic response and the distress of squat and slender liquid-storage tanks for different foundation conditions. The finite-element models include suitable contact formulations to accurately model the soil-structure interaction for each type of fixity conditions (i.e., anchored and unanchored).

Use of program ANSYS in the analysis of interaction between a rigid foundation and reinforced soil

Work is devoted to model and process the results of interaction between a rigid metal plate and a reinforced sand base using the software “ANSYS”.One of the effective ways to increase the strength is the inclusion of various materials (reinforcement) in the soil below the structure. It increases the resistance of the soil to stress and shear, limits lateral deformation. Organic, synthetic, metal and stone materials are used as reinforcing elements. The studies showed that the effectiveness of the reinforcement and the bearing capacity of the reinforced ground depend on the position, length, roughness and fixity condition of the reinforcement [27].

ЭЛЕКТРОМАГНИТНЫЙ СПОСОБ ЧАСТОТНОГО АНАЛИЗА ПОПЕРЕЧНЫХ КОЛЕБАНИЙ СТЕРЖНЯ

Vibration-based diagnostics of constructions is an obligatory technical procedure in mechanical engineering. In this regard, there is a problem of an adequate theoretical description of vibration processes in various structures with account for end fixity conditions, as well as the creation of experimental facilities for non-contact testing. The paper presents a theoretical basis and experimental verification results for a method of electromagnetic frequency analysis of rod systems. The essence of the method is the experimental determination of natural oscillation frequencies and their comparison with reference values. The main attention is paid to a theoretical description of transverse vibrations of a cantilever conductive rod in an external magnetic field in order to determine reference frequencies of a defect-free sample. The presence of the external magnetic field gives rise to the distributed electromagnetic force exerted on the rod. In the case of natural oscillations, the magnetic force is damping, which leads to a change in damping factors of partial oscillations. The electromagnetic effect is selective towards vibrational modes; hence, the damping factors of various partial oscillations of the rod vary to different degrees. This fact allows one to determine an optimal location of the area with acting magnetic field, as well as its width when measuring a given frequency of sample testing. The proposed method has several advantages: indestructibility of the sample, non-contact measurements, invariability of mechanical properties of the sample during the tests.

The Thermal Response of Concrete Frame Buildings in Arabic Area Considering Time-Dependent Properties of Concrete

The Arabic area is known for its significant changes in climate temperatures within different seasons. Riyadh, Cairo, Abu Dhabi, Ras Al-Khaimah, Kuwait, and Baghdad are subjected to the maximum variance in daily temperatures and presents the mean values of daily temperatures considering the data collected by the Hong Kong observatory in China for the last 30 years. The maximum mean value of daily temperature, which is 44.6 ℃ in Kuwait, and the minimum mean value of the daily temperature in Baghdad is 3.8 ℃. The maximum difference values of daily temperatures are observed in the Arabic Gulf area countries, such as Abu Dhabi, Kuwait, and Riyadh. Hence, these values will be considered in this analysis. This study results fit the thermal requirements of Arabic area and regions with similar temperature variation, accounting for various design aspects considering both methodologies of time-dependent properties of concrete as per CEB FIP 90 code and non-time dependent properties as per ACI 224.3R. For those targets a finite element method is utilized by generating 272 single- storey concrete ETABS models. The models are categorized according to column support conditions and development of concrete properties over time. The study findings are utilized to develop a clear understanding of mentioned variable's effects at thermal deformations and column reactions to aid structural engineers in the thermal design of super-long buildings with similar conditions of this study within time. The thermal displacement grows in proportion to the height of the column and the length of the slab. The findings of the analysis also show that employing thick slabs slightly decreases thermal displacements for models with hinged column supports and a 6 m column height. The thermal reactions increase in proportion to the length of the slab and the fixity conditions. Meanwhile, these reactions are inversely proportional to the height of the column. The ratios of time-dependent deformation and reaction to those of non-time-dependent properties are within the range [159%-163%] and [168%-171%] respectively. Ignoring this difference imposes defects, additional cracks, and damages at the structures and related serviceability conditions for 70 years period.

New Modelling Methodology for Seismic Design of Precast Structures and Performance Evaluation Considering Soil–Foundation System

Precast reinforced concrete (RC) structures are becoming popular in various parts of the globe due to its good quality control and less erection time as compared to monolithic RC structures. However, in Indian construction industry use of such building systems is not gaining impetus due to hesitation of structural designers concerning its seismic safety. In case of precast structure, the joints are not monolithic, and hence, the overall force distribution in the beam–column framing system will be different than that of monolithic RC frame structure. Moreover, the response will also depend on the fixity condition at the ground level. The present work proposes a new iterative process of linear analysis to determine the internal elemental forces, viz. bending moment, shear force and axial load, for precast structure. The performance of the building is assessed using nonlinear static procedure. In addition to this, the effect of soil–foundation system on monolithic and precast structure is also studied. Further, response reduction factor ‘R’ of all the considered buildings is evaluated for collapse prevention performance level. The obtained results show that the performance of precast structure gets enhanced with the reduced value of R considered for design and analysis. Moreover, it is interesting to note that for the precast structure with soil–foundation system the failure pattern follows a more realistic seismic design philosophy as compared to monolithic structure with soil–foundation system.

A field study on pile response to blast-induced ground motion

A series of field tests using controlled blasting was conducted at a location in the North Western part of Singapore to assess the behaviour of pile foundations subjected to ground excitation. The field tests involved three piles with different pile head fixity conditions. The piles were instrumented with strain gauges to evaluate the bending moments and axial force along the piles. For the fixed-head piles, the maximum bending moment occurred at the pile head level. For free-head pile exhibited higher bending moments close to the mid-height of the pile and zero bending moment at the pile head due to the absence of restraints at the top. For all cases, the axial force was maximum at the pile head.

Effective use of geosynthetics to increase bearing capacity of shallow foundations

In this research, a reinforcement mechanism for shallow foundations is determined through laboratory model tests and numerical analyses. The numerical analyses are performed with the finite element program FEMtij-2D using the elastoplastic subloading tij model. The frictional behavior between the reinforcement and the ground is simulated using an elastoplastic joint element. Several tests were performed whereby the installation depth, length, roughness, and fixity conditions at the edges of the reinforcement were varied. Results show that the effectiveness of the reinforcement and the bearing capacity of the reinforced ground depend on the position, length, roughness, and fixity condition of the reinforcement. A significant increase in the bearing capacity can be achieved if the geosynthetics are properly placed at an optimum length with the boundary fixed to the ground. The effect of the loading position is also investigated because in reality the load on a foundation does not always act at the center of the foundation. The numerical results accurately describe the experimental results; the simulations accurately account for the mechanical behaviors of both the soil and reinforcement and the frictional behavior between them. Therefore, the simulation technique can be used to predict the bearing capacity of reinforced ground.

Effects of fixed-piston sampler fixity on clay sample quality

This research studied the application of samplers under three fixity conditions during sampling tube penetration and evaluated the quality of the retrieved samples. Original and portable types of oil-operated fixed-piston samplers with identical sampling tubes were employed at a site in the Nakdong River Delta. Suction, shear wave velocity and consolidation tests were conducted to assess sample quality. The soil fabric, including holes, nodules, roots and shells, rather than soil sensitivity (4 to 28) was found to significantly affect sample quality, particularly suction. The sample quality indicators, except for the suction-based values, near the centre of each sampling tube tended to vary depending on the bulk density of the ground. The original type of sampler fixed at the centre of the steel casing produced samples of slightly better quality than those produced by the same sampler freely located in the borehole and those produced by a portable sampler tightly located in the steel casing. This finding was also supported by different qualities of samples obtained using the oil-operated and mechanical samplers at a different site. It is thus concluded that the fixity of samplers is a minor factor that influences sample quality.

Dynamic Response of Pile Reinforced Soils and Piled Foundations

The rigid-pile soil-improvement technique aims to increase the bearing capacity of the soil and decrease the settlement of the surface structure. The most remarkable difference of this technique from the deep-foundation system is the soil layer between the pile heads and the structure. This soil layer, called the mattress, is made of compacted granular materials and participates in the load transfer through arching and shear mechanisms. In order to understand the dynamic behavior of rigid-pile reinforced soils, tri-dimensional finite-element analyses of a soil-pile-slab system, a soil-pile-mattress-slab system, and a soil-pile-mattress-embankment system are presented in this paper. Different geometric configurations are studied in terms of dynamic impedances. The soil, piles, mattress, and embankment are represented as continuum solids, and the slab is represented by structural plate-type elements. The horizontal and vertical impedances of pile foundations are presented and the results are compared with studies in the literature. This study shows the influence of the mattress stiffness, the geometrical configuration, and head/tip fixity conditions on the dynamic response of the foundation system. A comparison between rigid piles and pile foundations is then presented.