Lowest Natural Frequency Research Articles

Nonlinear liquid sloshing in an upright circular container: Modal responses and higher-order harmonics

Nonlinear sloshing in an upright circular container near the lowest natural frequency is analyzed by using a fully nonlinear overset-mesh-based harmonic polynomial cell method, two weakly nonlinear Narimanov–Moiseev-type multimodal models and a linear multimodal method. Modal responses are extracted from the fully nonlinear results based on a simple but accurate least-square procedure using the time series of free-surface wave elevations, which provides new ways to delve into the underlying modal responses and energy transfer between modes, as well as to verify the validity of ordering assumptions in the weakly nonlinear models. Wavelet analyses are also performed for the wave elevations and generalized coordinates of the modes to better understand the time-frequency information of the higher harmonics of the sloshing responses and energy transfer in a nonlinear process. Planar harmonic sloshing state, swirling harmonic sloshing state, and periodically modulated sloshing state are analyzed. It is found that the energy is more dispersed among different modes in the periodically modulated sloshing state, which means higher natural modes are consequential. In general, energies are found to transfer from lower to higher natural modes and between symmetric and antisymmetric natural modes. The results also show that the O(ε1/3) and O(ε2/3) responses are dominated by only first and second harmonics, respectively, while the O(ε) response contains non-negligible first and third harmonic contribution. At last, the influence of initial disturbance is examined, demonstrating that different initial disturbances may lead to the different rotation direction of the swirling waves and the sloshing-wave responses in the transient stage, while the main characteristics of the sloshing waves are robust and independent of initial conditions.

ДИНАМІЧНИЙ СИНТЕЗ МОБІЛЬНОЇ ЗЕМЛЕРИЙНОЇ МАШИНИ БЕЗПЕРЕРВНОЇ ДІЇ З ЛАНЦЮГОВО-БАЛКОВИМ РОБОЧИМ ОРГАНОМ

This article depics the mobile continuous earth-moving machine dynamic model with a chain-type trenching equipment in order to determine the effect of changes in the elastic-inertial system parameters on its own resonant frequencies. The object of the research: natural resonant transmission frequencies of the soil-working working equipment drive in the mobile earth-moving continuous action machine with a chain-beam working body. The purpose of the work: to determine the system elements, in which the elastic-inertial parameters affect the lower natural resonant frequencies. To give practical recommendations on the parameter choice of such elements in order to deviate from the resonant frequency. Creation of strong, reliable and durable machines depends on the dynamic load accuracy and ways to reduce it. Modern earthmoving machine structure formation can be realized using the modular principle, but always with taking into account the full picture of possible machine loads and their drives in the process, these load magnitude and their dynamic characteristics. In accordance with the dynamic synthesis conditions, the dynamic system machine parameters should be selected so that the external load machine change frequency in its maximum productivity mode doesn’t coincide with the dynamic system natural frequency. According to the theoretical and strain gauge research results, it is established that the frequency of the first form of earth-moving machine drive torsional oscillations is close to 9 Hz. The first frequency oscillation shape has a node between the gearbox masses, which can lead to fatigue shaft destruction, joints or gears of the base tractor transmission part destruction. The frequency of the natural oscillation system is close to the traction chain link engagement frequency with the sprocket teeth in the earth-moving machine maximum productivity mode.

A Highly Stable and Sensitive MEMS-Based Gravimeter for Long-Term Earth Tides Observations

Precision measurements of local gravitational acceleration variations are of great importance in geophysical surveys. With advantages such as cost-effectiveness and portability, Micro-Electro-Mechanical system (MEMS)-based gravimeters have shown the potential for long-term gravity measurements. In this paper, aiming to further improve the stability of the instrument, the design considerations and system evaluations of a MEMS gravimeter are presented. With a linear spring design for the silicon proof-mass, a low natural frequency of ~14 Hz and a large linear range of ~10300 mGal are achieved with an ultra-low self-noise floor of 1.2 μGal/√Hz@1 Hz. By implementing a vacuum chamber system, the pressure variation is reduced from hundreds of Pa/day in atmosphere to a linear variation of ~6 Pa/day. In addition, an active temperature control system can suppress temperature fluctuations by 2 to 3 orders of magnitude within the band from 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Hz to 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> Hz. The stability of the proposed MEMS gravimeter is demonstrated via long-term Earth tides observations within a 30-day time span, giving a correlation coefficient of 0.957 with the reference. An excellent bias instability of ≤4 μGal is demonstrated within the 8-3000 s averaging time range, representing one of the best performances to date in terms of stability for MEMS gravimeters. This shows the potential of high-performance MEMS gravimeters for petroleum and mineral prospecting, seismology and other geophysical applications.

An Optical Interferometry Based MEMS Accelerometer Applicable to Seismic-Grade Measurement

Traditional seismometers and gravimeters normally have large volumes and entail high manufacturing costs. Aiming at the miniaturization of such geophysical instruments, a portable Micro-electro-mechanical Systems (MEMS) accelerometer applicable to seismic-grade measurements is fabricated and characterized in this work. This accelerometer is operating on the deformable grating-based MEMS interferometer, where a new MEMS out-of-plane sensing chip employing double layers of 10 &#x03BC;m-thickness spring is first proposed. The simulation results show that this geometry has a low natural frequency of 52.23 Hz and undergoes a low stress of 26.94 Mpa under the gravitational acceleration. The test of the fabricated prototype is implemented in a quiet environment for tracking the inherent micro seismic peaks of Earth. The evaluation results demonstrate that the fabricated prototype has achieved a voltage sensitivity of 510.6 V/g. Furthermore, the noise estimation indicates that the self-noise of this sensor has reached 20 ng/&#x221A;<i>Hz</i> from 0.3 Hz to 20 Hz, and a bias stability of 235.4 ng is obtainable at room temperature. Combined with the compact package size of 40.5 cm³, the proposed device exhibits a promising prospect in the seismic-grade applications.

Dynamic Analysis on RCC and Composite Structure for Uniform and Optimized Section

Abstract: This study examines the composite structure that is increasing commonly in developing countries. For medium-rise to high-rise building construction, RCC structures is no longer economical due to heavy dead weight, limited span, low natural frequency and hazardous formwork. The majority of commercial buildings are designed and constructed with reinforced concrete, which largely depends on the existence of the constituent materials as well as the quality of the necessary construction skills, and including the usefulness of design standards. Conventional RCC structure is not preferred nowadays for high rise structure. However, composite construction, is a recent development in the construction industry. Concrete-steel composite structures are now very popular due to some outstanding advantages over conventional concrete and steel structures. In the present work, RCC and steel-concrete composite structure are being considered for a Dynamic analysis of a G+25-storey commercial building of uniform and optimized section, located at in seismic zone IV. Response Spectrum analysis method is used to analyze RCC and composite structure, CSI ETABS v19 software is used and various results are compared such as time period, maximum storey displacement, maximum storey stiffness. Maximum storey shear and maximum stoey overturning moment. Keywords: RCC Structure, Composite Structure, Uniform Section, Optimized Section, Shear Connector, Time Period, Storey Displacement, Storey Shear, Storey Stiffness, Response Spectrum method, ETABS

Lattice structure design parameters optimization for the structural integrity of passive vibration isolator

Passive vibration isolator with lower natural frequency has always been a challenge due to structural integrity issues. This study presents the use of RSM statistical tool to analyze and optimize the mechan-ical responses of BCC lattice structure for structural integrity in a passive vibration isolator application. The optimization was done to obtain low stiffness for low natural frequency but high yield stress for optimum load-bearing capability with unit cell size and strut diameter design parameters tweak. From the results, the significance and contribution of each design parameter on each mechanical response through compression test can be understood. Results indicated changes in strut diameter produced lin-ear growth while changes in the unit cell size produced inverse exponential responses. From optimiza-tion, a combination of 3.9 mm strut diameter with 10 mm unit cell size produced the optimum result. Therefore, it was demonstrated that RSM can provide statistical importance and contribution between input factors and their influence on each mechanical response with minimal test and cost.

Research on topological configuration of lightweight high natural frequency plate structure

Because the resonance of the structure is related to the fundamental frequency in most cases, in order to avoid the resonance of the structure, this paper takes the square plate structure as the research object, and uses the variable density method and draft technology to optimize the square plate, so that the optimized stiffened plate has a higher first-order natural frequency. The optimized stiffened plate is compared with the unreinforced plate and the conventional stiffened plate. The results show that under different mass constraints, the four optimized stiffened forms tend to be consistent in the distribution of main materials. Under the working condition of this paper, when the mass of the stiffened plate is 18.737 Kg, the first natural frequency of the optimized stiffened plate can be increased by 107.68%, 21.54% and 10.03% respectively compared with the first natural frequency of the unreinforced plate and the first natural frequencies of two conventional stiffened plates. When the first natural frequency of the stiffened plate is 537.39 Hz, the mass of the optimized stiffened plate can be reduced by 53.95%, 62.70% and 21.44% respectively compared with that of the unstiffened plate and the two conventional stiffened plates. The optimized stiffened plate is redesigned, and it is found that the designed stiffened plate still has advantages over the unstiffened plate and the conventional stiffened plate. This means that this paper provides a reference for the stiffener of square plates with low mass and high natural frequency.

Dynamic behavior of T-beam resonator with repulsive actuation

Electrostatic MEMS transducer driven by repulsive force is an attractive possibility and has advantages of avoiding the pull-in instability, tuning the natural frequency, and achieving high sensitivity by applying high enough voltages. In this work, a “T”-shaped beam, which is formed by attaching a secondary beam perpendicular to a primary cantilever at the tip, is introduced and its nonlinear dynamics is analyzed. A reduced-order model is derived from mode shapes formed from electromechanical coupling effects respectively. Generalized forms of forced Mathieu equation of motion are derived, and then, dynamic behaviors are investigated through the theory of multiple scales. The resonant responses, including both primary and principal parametric resonances, reveal softening behavior originating from quadratic and cubic nonlinearities in the governing equation. The behavior of the T-beam is compared with traditional cantilever structure. The resonance under repulsive force demonstrates that the T-beam has several advantages over a traditional cantilever: Lower natural frequency but higher resonant responses can improve the signal-to-noise ratio; with an attached micropaddle, the T-beam has a larger surface for absorption of targeted analytes for mass sensing. We conclude that an electrostatic MEMS resonator with a “T”-shaped beam is potentially appropriate for the new generations of sensors and actuators.

Vibration analysis of a strain gradient plate model via a mesh-free moving Kriging Interpolation Method

The strain gradient elasticity theory is adopted to capture the small-scale effect of micro/nanostructures to derive the governing equation by using Hamilton's principle. A mesh-free method on the basis of moving Kriging interpolation, of which shape function possesses C2 continuum, is developed to investigate the vibration of the strain gradient plate. Owing to the inherent Kronecker delta function property of the moving Kriging interpolation, essential boundary conditions can be directly implemented. One superiority of the moving Kriging interpolation over the traditional moving least square approximation is that it can ensure the continuity and stability at the third order derivatives, which is confirmed by comparison with analytic solutions. Parametric studies in terms of small-scale parameters, boundary conditions and side length on the frequencies and mode shapes are carried out. Natural frequencies evaluated by strain gradient elasticity theory are found to be lower than those of classical plate model. Some lower natural frequencies corresponding to ultrahigh-order mode shapes appear. This demonstrates that the mesh-free moving Kriging interpolation method has high precision and good stability in the construction of the C2 continuum shape functions.

Microzonation of Probolinggo Fault Using Microtremor as an Effort for Earthquake Disaster Mitigation

Earthquake can be caused by several things, one of which is due to an active fault. To mitigate earthquake disasters that can be caused by the Probolinggo Fault, measurement of the microtremor geophysical method is carried out to analyze seismic vulnerability. In this study, the microtremor measurements were carried out on 30 measurement points. The data obtained from measurements are then processed using EasyHVSR software to obtain natural frequency (f0) and natural amplification (A0) values. This value is then used to create a microtremor microzonation map, which is seismic vulnerability index, based on natural frequency and amplification . it founded that low natural frequency mostly founded on eastern of measurement area, caused by thick sedimentary from Lamongan volcanic. High amplification mostly founded from middle to western area, and high seismic vulnerability index founded on western of measurement area, include Maron and Krucil Sub-district. It means seismic wave can very destructive on those area.

Model-based approach to investigate equipment-induced error in pressure-waveform derived hemodynamic measurements

Objective. Advanced hemodynamic monitoring systems have provided less invasive methods for estimating pressure-derived measurements such as pressure-derived cardiac output (CO) measurements. These devices apply algorithms to arterial pressure waveforms recorded via pressure recording components that transmit the pressure signal to a pressure monitor. While standards have been developed for pressure monitoring equipment, it is unclear how the equipment-induced error can affect secondary measurements from pressure waveforms. We propose an approach for modelling different components of a pressure monitoring system and use this model-based approach to investigate the effect of different pressure recording configurations on pressure-derived hemodynamic measurements. Approach. The proposed model-based approach is a three step process. (1) Modelling the response of pressure recording components using bench tests; (2) verifying the identified models through nonparametric equivalence tests; and (3) assessing the effects of pressure recording components on pressure-derived measurements. To delineate the application of this approach, we performed a series of model-based analyses to quantify the combined effect of a wide range of tubing configurations with various damping ratios and natural frequencies and monitors with different bandwidths on pressure waveforms and CO measurements by six pulse contour algorithms. Results. Model-based results show the error in pressure-derived CO measurements because of tubing configurations with different natural frequencies and damping ratios. Tubing configurations with low natural frequencies (<23 Hz) altered characteristics of pressure waveforms in a way that affected the CO measurement, some by as much as 20%. Significance. Our method can serve as a tool to quantify the performance of pressure recording systems with different dynamic properties. This approach can be applied to investigate the effects of physiologic signal recording configurations on various pressure-derived hemodynamic measurements.

Glass Suspension Footbridge: Human-Induced Vibration, Serviceability Evaluation, and Vibration Mitigation

Glass suspension footbridges or glass-bottomed suspension footbridges, such as the Zhangjiajie Grand Canyon Glass Bridge in China, are increasingly constructed in scenic regions as tourist attractions due to the glass-bottomed and transparent characteristics. Being installed in the second phase as a constant load, the glass deck is not designed to participate in the load bearing of the whole structure and thus the structure requires larger load-bearing capacity from other parts and a slightly different load-bearing system. More importantly, the glass deck weakens structural integrity and results in slightly smaller structural stiffness, which may result in globally lower natural frequencies. These structures may inevitably be more sensitive to the pedestrian-induced excitations. This paper investigates the human-induced vibrations of a typical glass suspension footbridge in China. The investigations are first performed based on intensive parametric studies. Except for the boundary conditions, the interlayers of glass, the vertical rise–span ratio and the axial stiffness of main cables, other governing parameters of the modal parameters for this type of structures are also identified, including the pavement load of the bridge deck system, the central buckle, and the prestress of wind-resistant cables. It may guide designers to take effective measures to improve the dynamic behavior of this type of structure. Next, the vibration serviceability assessments were performed based on different standards and guidelines such as the UK National Annex (NA) to Eurocode 1 (2008), the Bro2004 specification of Sweden, the HiVoSS guideline, and the ISO standard. Comparisons show that the UK NA to Eurocode 1, the HiVoSS guideline, and the ISO standard are more suitable for the vibration serviceability evaluations. These three codes consider the pedestrian-induced vibrations in both lateral and vertical directions and also consider both a small number of pedestrians and crowd-induced loads with different densities. Furthermore, comfort limits are not defined by only one limiting value but sorted into different levels. This may provide reference for the vibration serviceability evaluations of the other glass suspension footbridges. The assessments also show that changing the structural parameters can only partially reduce vibration levels and may result in different modal parameters to dominate the vibration responses. Thus, they recommend taking vibration mitigation measures to improve the vibration serviceability and suggest performing careful response calculations to ensure the effectiveness of vibration serviceability design if via changing the structural parameters. Finally, tuned mass dampers (TMDs) are installed to the structure. The results show that the popular TMD may effectively reduce the vibration levels.

Comprehensive Design Method of a High-Frequency-Response Fast Tool Servo System Based on a Full-Frequency Error Control Algorithm.

With the development of optoelectronic information technology, high-performance optical systems require an increasingly higher surface accuracy of optical mirrors. The fast tool servo (FTS) based on the piezoelectric actuator is widely used in the compensation machining of high-precision optical mirrors. However, with the low natural frequency of mechanical structures, hysteresis of the piezoelectric actuators, and phase delay of the control systems, conventional FTS systems face problems such as a low working frequency and a large tracking error. This study presents a method for the design of a high-performance FTS system. First, a flexure hinge servo turret with a high natural frequency was designed through multi-objective optimization and finite element simulations. Subsequently, a composite control algorithm was proposed, targeting the problems of hysteresis and phase delay. The modified Prandtl–Ishlinskii inverse hysteresis model was used to overcome the hysteresis effect and a zero-phase error tracker was designed to reduce the phase error. The experimental results reveal that the tracking error of the designed FTS system was <10% in the full frequency range (0–1000 Hz).

Neurovascular Coupling Is Impaired in Hypertensive and Diabetic Subjects Without Symptomatic Cerebrovascular Disease.

The mechanistic link between hypertension, diabetes and cerebral small vessel disease (CSVD) is still poorly understood. We hypothesized that hypertension and diabetes could impair cerebrovascular regulation prior to irreversibly established cerebrovascular disease. In this study, 52 hypertensive patients [54% males; age 64 ± 11 years; 58% with comorbid diabetes mellitus (DM)] without symptomatic cerebrovascular disease underwent transcranial Doppler (TCD) monitoring in the middle (MCA) and posterior (PCA) cerebral arteries, to assess vasoreactivity to carbon dioxide (VRCO2) and neurovascular coupling (NVC). 1.5T magnetic resonance imaging was also performed and white matter hyperintensity volume was automatically segmented from FLAIR sequences. TCD data from 17 healthy controls were obtained for comparison (47% males; age 60 ± 16 years). Hypertensive patients showed significant impairment of NVC in the PCA, with reduced increment in cerebral blood flow velocity during visual stimulation (22.4 ± 9.2 vs. 31.6 ± 5.7, p < 0.001), as well as disturbed NVC time-varying properties, with slower response (lower rate time: 0.00 ± 0.02 vs. 0.03 ± 6.81, p = 0.001), and reduced system oscillation (reduced natural frequency: 0.18 ± 0.08 vs. 0.22 ± 0.06, p < 0.001), when compared to controls. VRCO2 remained relatively preserved in MCA and PCA. These results were worse in hypertensive diabetic patients, with lower natural frequency (p = 0.043) than non-diabetic patients. White matter disease burden did not predict worse NVC. These findings suggest that hypertensive diabetic patients may have a precocious impairment of NVC, already occurring without symptomatic CSVD. Future research is warranted to evaluate whether NVC assessment could be useful as an early, non-invasive, surrogate marker for CSVD.

Fundamental frequency analysis of endohedrally functionalized carbon nanotubes with metallic nanowires: a molecular dynamics study.

The endohedral functionalization of carbon nanotubes (CNTs) with nanowires (NWs), i.e., NWs@CNTs, has been the center of attention in a lot of research due to the applications of NWs@CNTs in nanoelectronic devices, heterogeneous catalysis, and electromagnetic wave absorption. To this end, based on the classical molecular dynamics (MD) simulations, the effect of four pentagonal structures of encapsulated metallic nanowires (mNWs), namely the eclipsed pentagon (E), the deformed staggered pentagon (Ds), staggered pentagon (S), and staggered pentagonal structure without the monatomic chain passing through the centers of the parallel pentagons (R) configurations on the vibrational behavior of CNTs, is investigated. Also, the effects of geometrical parameters such as length and radius of CNTs on the natural frequencies of simulated models are explored. The results illustrate that by increasing the length, the natural frequency of pure CNTs and mNWs@CNTs decreases. In a similar length, mNWs@CNTs possess lower natural frequencies compared to the pure CNTs. According to the results, the highest and lowest natural frequencies are calculated by inserting the S structure of sodium NW and Ds structure of aluminum NW inside their proper armchair CNT, i.e., Na-S NW@ (9,9) CNT and Al-Ds NW@ (7,7) CNT, respectively.

A technique for inversely identifying joint stiffnesses of robot arms via two-way TubeNets

Joint stiffnesses of robot arms play a critical role in the control of the posture and movement of the arm tip. This work develops a systematic approach for inverse real-time quantitative identification of the stiffnesses of joints for robotic arms using the TubeNet proposed by Liu. To start with, a finite element (FE) model for a six-axis tandem robot arm is established. Experiments are then conducted to measure the first few lowest natural frequencies of the robot arm to be compared with numerical results for the validation of the FE model. Using the validated FEM model, sensitivity analyses of the joint stiffnesses to the natural frequencies are carried out to ensure sufficient sensitivity for inverse analyses and a neural network data set is established. The selection of appropriate TubeNet layers and activation functions is exposited. Subsequently, the direct-weights-inversion (DWI) formulae for the TubeNet is adopted to inversely compute the joint stiffnesses explicitly in real time. The predicated joint stiffness using the currently proposed DWI formulae of the TubeNet is accurate with the maximum root-mean-square of test errors less than 0.0020 N·m/rad.

Comparison of Analysis and Design of Regular and Irregular Configuration of Multi Story Building in Seismic Zones

Abstract: This paper is concerned with the effects of various vertical irregularities on the seismic response of a structure. The objective of the project is to carry out Response spectrum analysis (RSA) and Time history Analysis (THA) of vertically irregular RC building frames and to carry out the ductility based design using IS 13920 corresponding to Equivalent static analysis and Time history analysis. Comparison of the results of analysis and design of irregular structures with regular structure was done. The scope of the project also includes the evaluation of response of structures subjected to high, low and intermediate frequency content earthquakes using Time history analysis. Three types of irregularities namely mass irregularity, stiffness irregularity and vertical geometry irregularity were considered. According to our observation, the storey shear force was found to be maximum for the first storey and it decreases to minimum in the top storey in all cases. The mass irregular structures were observed to experience larger base shear than similar regular structures. The stiffness irregular structure experienced lesser base shear and has larger inter-storey drifts. The absolute displacements obtained from time history analysis of geometry irregular structure at respective nodes were found to be greater than that in case of regular structure for upper stories but gradually as we moved to lower stories displacements in both structures tended to converge. . Lower stiffness results in higher displacements of upper stories. In case of a mass irregular structure, time history analysis gives slightly higher displacement for upper stories than that in regular structures whereas as we move down lower stories show higher displacements as compared to that in regular structures. When time history analysis was done for regular as well as stiffness irregular structure, it was found that displacements of upper stories did not vary much from each other but as we moved down to lower stories the absolute displacement in case of soft storey were higher compared to respective stories in regular structure. Tall structures were found to have low natural frequency hence their response was found to be maximum in a low frequency earthquake. It is because low natural frequency of tall structures subjected to low frequency earthquake leads to resonance resulting in larger displacements. If a high rise structure (low natural frequency)is subjected to high frequency ground motion then it results in small displacements. Similarly, if a low rise structure (high natural frequency) is subjected to high frequency ground motion it results in larger displacements whereas small displacements occur when the high rise structure is subjected to low frequency ground motion.

Comparative Study Seismic Analysis of RCC, Steel &amp; Steel-Concrete Composite Frame: A Review

Abstract: This study examines how composite structures are gaining popularity in developing countries. For medium and high levels in RCC buildings are no longer economical due to increased weight, range limitations and low natural frequencies and dangerous formwork. Steel and concrete composite structures are becoming increasingly popular today and are safe throughout their useful lives. Round Steel and concrete structures are the best solution for modern buildings. In this article we will discuss the various results of building construction for RCC, metal and composite construction taking into account different studies. Keyword: RCC Structure, Steel Structure, Composite Structure, Joint Displacement, Base Shear

Considerations for increasing the survivability of Gannet-inspired drones during diving

Many external sources of excitation can interfere with the performance of drones, such as propeller or flapping motion. This work investigates the effects external excitation plays on the nonlinear system's dynamics of Gannet-inspired drones. Using Euler-Bernoulli beam theory, the partial differential equations of motion are solved following the Hamilton's principle. Galerkin discretization is then applied to convert the equations of motion to ordinary differential equations in which to obtain an approximation for the nonlinear dynamic response before buckling occurs. The model is inspired by the Gannet bird, where the head is simplified to a cone shape, the neck is resembled by a soft beam, and the rest of the body is represented by a stiff, thicker beam to suggest a buckling behavior physical for the diving bird if buckling is to occur. The results observe the direct effects the external forcing, the damping, the impact velocity, and the boundary conditions have on the resonant frequency range and deflection. The results estimate that increasing the forcing amplitude will in turn amplify the hardening behavior and the maximum displacement. To maintain a lower forcing amplitude consistent with a reasonable external acceleration, a smaller drone mass should be employed. It is observed that higher damping should be applied to minimize the drone's nonlinear effects. The speed at which the drone impacts the water should be chosen to be lower than the estimated buckling speed to avoid lower natural frequencies and to reduce the hysteresis region in order to minimize the nonlinear behavior. From the boundary conditions explored, stiffer boundary conditions should be employed to increase the buckling speed and reduce the deflection when entering water.

Free Vibration Analysis of Horizontally Curved Composite Concrete-Steel I-Girder Bridges

The presence of a crack in civil structures has a significant effect on their vibrational characteristics. The objective of the current research is to investigate the effect of crack on vibrational characteristics of bridges using FE simulation. The CAD modeling and modal analysis are conducted using ANSYS software. The natural frequencies are computed and mode shape is generated for each frequency. Then I shaped a girder with the crack that has lower natural frequencies as compared to bridge design without crack. The deformation obtained for bridge deck with cracked I-shaped girder beam is higher than bridge deck design without cracks.