Identification of defects in piles with high slenderness ratios using dynamic response to impulse loads

Defects in pile foundations, such as neck defects, bulge imperfections, weak concretes, cracks, and broken piles, can cause a decrease in the bearing capacity and the structural stability of the foundation. Identification of the type of defect is vital in formulating a reasonable repair plan for the pile foundation. In this study, the authors proposed a scheme to identify the types of defects in concrete piles based on a convolution neural network and a low-strain pile integrity test (LSPIT). A batch modeling method of defective pile foundations using Python script was also proffered. The different degrees of signals of five types of defective pile foundations were simulated by this method. The original data were decomposed and reconstructed by wavelet packet decomposition (WPT). To prevent the data from losing too much information after WPT, the data of 400 × 1 after decomposition and reconstruction were processed by dimension-raising to obtain the data of 20 × 20 × 1. Then, the multidimensional feature index of 20 × 20 × 2 was generated by index fusion with the original data. Finally, the data were input onto convolutional neural network (CNN) as a training parameter. Following an improvement of the dataset, the recognition accuracy of the type of defect in the pile foundation by the proposed identification scheme reached 94.4%.

Vibration testing is necessary for functionality and durability evaluations of equipment that may undergo performance degradation due to operational vibrations. Here, we propose a planning method for the vibration testing of equipment with high slenderness ratios, such as missiles, and excitation, support, and measurement locations are important factors for the test configuration. A finite element model was used to calculate the normal modes and suitability indexes, based on the excitation and support locations. The measurement locations were obtained via the effective-independence method. Indexes of the response of dominant modes to excitation were generated for each surface through vibration testing involving various excitation, support, and measurement locations. The suitability indexes of excitation and support locations, exhibited a high correlation with the indexes of response magnitude in the dominant modes of the equipment. Therefore, the vibration test configuration can be planned effectively by exciting dominant modes of equipment under test.

The stress and strain dependent response of THF hydrate

In this study, an Impulse Response Function analysis of pile response to sine-sweep excitation by a low cost, portable shaker was used to identify defects in piles. In straightforward impact-echo methods, echoes from the pile toe and defects are visible in the time domain measurements. However, these echoes are not present in the time domain records of piles subjected to sine-sweep excitations, due to interactions between the input and output signals. For this reason, the impulse response function in the time domain has been calculated and is able to identify the echoes from pile impedance changes. The proposed methodology has been evaluated both numerically and experimentally. A one-dimensional pile-soil interaction system was developed, and a finite difference method used to calculate the pile response to sine-sweep excitation. The numerical simulations indicate that impulse response measurements with a synthesized logarithmic, sine-sweep excitation could be an effective tool for detecting defects in piles. The methodology was further tested with field trials on 6 cast in situ concrete test piles including 1 intact pile and 5 defective piles subjected to sine-sweep excitations by a shaker. In 5 of the 6 cases the echoes from the pile toe could be identified in the deconvoluted waveforms—the impulse response functions. Damage detection is more difficult and dependent on the selection of the optimal regularization parameter. Further research and optimization of the deconvolution process is needed to evaluate the effectiveness compared to standard pile integrity testing methods.

As environmentally friendly pile foundations with small diameters and higher slenderness ratios, micropiles are widely used in fields such as transmission line engineering and building reinforcement. However, the available research has primarily focused on their bearing performance under compressive and horizontal loads, and there is insufficient research on predicting the uplift capacity of micropiles. This study investigated the load transfer mechanism and the behavior of the surrounding soil using model tests and finite element simulations. The ultimate uplift capacities and load distributions of micropiles with different slenderness ratios were analyzed. The results show that as the slenderness ratio increases, the ultimate uplift capacity of a pile gradually increases. However, this rate of increase diminishes gradually. Additionally, the restraining effect and range of the surrounding soil at the lower part of the pile are enhanced. The critical embedment depth of the micropiles shifts further away from the pile tip as the slenderness ratio increases. Finally, this study proposed a novel modification to Shanker’s model of incorporating variations in the critical embedment depth based on the slenderness ratios. Subsequently, a modified model for the ultimate uplift capacity of micropiles was proposed and validated using a model test. The proposed model effectively predicts the uplift bearing capacity of micropiles with high slenderness ratios, which is practical for engineering applications.

Lateral buckling of longitudinal rebars is a crucial phenomenon in the response of reinforced concrete structures since it often anticipates crushing of the core concrete and triggers failure of the member. Owing to this, lateral buckling has been simulated in some uniaxial steel models available in the literature. However, most of these models concentrate their novelties on the compression backbone curve of rebars characterised by low-to-medium slenderness ratios and inaccurately simulate the response of bars characterised by high slenderness ratio or the transition from the compression backbone curve to tension. Moreover, all these models neglect the effects of the concrete cover on the lateral buckling of the rebar. This paper presents a new uniaxial material model of reinforcing steel bars with the aim of achieving a more accurate evaluation of the response of rebars including the effects of lateral buckling. The proposed model is calibrated on the results of numerical analyses conducted on refined models of steel rebars characterised by low-to-high slenderness ratios. The proposed model incorporates the possibility to simulate the behaviour of bare and embedded rebars. The response is path-dependent and described by means of smooth curves so as to guarantee numerical stability in nonlinear finite element simulations. The accuracy of the model has been validated by comparison with the response of laboratory tests performed by other researchers on steel rebars. Finally, applications are shown on six reinforced concrete columns in order to highlight the effects of the simulation of buckling of the steel rebars on the global and local responses.

During a seismic event, first-story end columns in steel Special Moment Frames (SMFs) experience ‎lateral drift demands combined with high levels of axial load due to gravity loading and dynamic ‎overturning moment effects. Deep slender wide-flange sections are typically assigned for the ‎seismic design of perimeter SMFs in North America. The flange and web of these sections have ‎high slenderness ratios, just below the compactness limits specified by AISC 360-10. This makes ‎these sections vulnerable to local and lateral torsional buckling failure modes. ‎ In this paper, the cyclic behavior of deep slender wide-flange steel beam-columns, under combined ‎lateral drift and axial load, is investigated through detailed finite element analysis (FEA). The main ‎design variables that are considered as part of the FEA investigation are (a) the section slenderness; ‎‎(b) the lateral loading protocol; and (c) the axial load level. Emphasis is placed on the effect of ‎these variables on (a) the cyclic deterioration in flexural strength; (b) the column axial shortening; ‎and (c) the lateral torsional buckling of the steel beam-columns. To this end, a range of deep ‎slender wide-flange sections, commonly found in low to mid-rise SMFs, is selected. Two lateral ‎loading protocols are used: 1) a symmetric protocol and 2) a near fault protocol. The axial load is ‎applied in two forms: 1) constant compressive load; and 2) cyclic axial load varying from ‎compression to ‎tension. The latter represents the variation in the axial load experienced by an end ‎column due to the dynamic overturning ‎moments during an earthquake. Using a validated FEA ‎modeling approach, typical first-story end columns of prototype steel SMFs are modeled and ‎analyzed under the aforementioned-coupled lateral and axial loading protocols. The boundary ‎conditions at the base of the columns are fixed. At the top of the columns the flexibility associated ‎with the corresponding exterior beam-to-column connection is considered. Lateral braces are also ‎provided to the column, as per the design code requirements.‎ The FEA simulations demonstrated that higher rates of cyclic ‎deterioration in flexural strength are ‎associated with sections with larger web slenderness, due to severe web local buckling. Sections ‎with large web and flange slenderness ratios also suffer from higher axial shortening levels at lower ‎story drifts compared to more compact sections. It is also concluded that deep slender wide-flange ‎sections with lower resistance to out-of-plane and torsional force demands, experience higher ‎twisting and are likely to be subjected to lateral torsional buckling. The study also shows that axial ‎force demands on the column’s lateral braces exceed their code design capacity at relatively low ‎story drift ratios. Furthermore, the location of the column inflection point and the associated ‎column moment gradient is monitored during the finite element analysis in order to assess the ‎potential of plastic hinge formation at the top of the column once plastification and cyclic ‎deterioration in flexural strength of the column occurs at its base. This is a critical issue since it ‎could potentially lead to weak-story collapse mechanisms.‎

The majority of precast concrete electric pole structures (EPSs) consist of prestressed concrete and reinforced concrete hybrid members with high slenderness ratios. Currently, large prestressing (PS) forces are being applied to EPSs to carry heavier weights of electric transformer machineries and to reduce deflection by increasing structural stiffness. Moreover, when EPSs are exposed to an outdoor environment year-round, their durability decreases. Therefore, the objectives of this study are to transform the failure behavior of EPSs from brittle to ductile by varying the number of PS tendons and steel rebars, and to improve the ductility and durability of the EPS by using short polypropylene fiber-reinforced concrete to control crack formation and propagation. Six different types of EPS specimens were manufactured and tested to evaluate their maximum flexure capacity and to verify their ductility behavior. The results showed that the modified EPSs displayed ductile failure behavior while either maintaining their original flexural strength capacity or improving it compared with the current EPSs sold on the market.

Lightning rod structures are susceptible to wind loads due to their high slenderness ratio, high flexibility, and light weight. The wind-induced dynamic response of a lightning rod is critical for structural safety and reliability. The traditional methods for this response, including observation and simulation, focus on structural health monitoring (SHM), wind tunnel tests (WTTs), or fluid–structure interaction (FSI) simulations. However, all these approaches require considerable financial or computational investment. Additionally, problems such as data loss or data anomalies in the sensor monitoring process often occur during SHM or WTTs. This paper proposes an algorithm based on a long short-term memory (LSTM) network to predict the wind-induced dynamic response and to solve the problem of data link fracture caused by abnormal sensor data transmission or wind-induced damage to lightning rod structures under different wind speeds. The effectiveness and applicability of the proposed framework are demonstrated using actual monitoring data. Root-mean-squared error (RMSE), determination of coefficient (R2), variance accounted for (VAF), and the refined Willmott index (RWI) are employed as performance assessment indices for the proposed network model. At the same time, the random forest algorithm is adopted to analyze the correlation between the data of the different measurement points on the lightning rod structure. The results show that the LSTM method proposed in this paper has a high accuracy for the prediction of “missing” strain data during lightning rod strain monitoring under wind speeds of 15.81~31.62 m/s. Even under the extreme wind speed of 31.62 m/s, the values of RMSE, MAE, R2, RWI and VAF are 0.24053, 0.18213, 0.94539, 0.88172 and 0.94444, respectively, which are within the acceptable range. Using the data feature importance analysis function, it is found that the predicted strain data of the measurement point on the top part of the lightning rod structure are closely related to the test strain data of the two adjacent sections of the structure, and the effect of the test strain data of the measurement points that are far from the predicted measurement point can be ignored.

Stiffness variation is useful in soft robots, especially for continuum manipulators which have a high slenderness ratio. Softness intrinsically gives robots high flexibility of movement, but stiffness gives the robots enough carrying capacity. Many types of continuum manipulators can change their stiffness; however, the both large-range and fast-response stiffness variation has not been realized. In this work, an omnidirectional steering flexible manipulator with large-range and fast-response stiffness variation was designed and fabricated. It was composed of a nested structure with a stiffness variation layer, a tendon-driven layer, and a transport channel. The stiffness variation was achieved by the phase transition of LMPA (Low-Melting Point Alloy) with a melting point of 50 °C. Heating and cooling to the LMPA were performed by a tubular spring-like copper tube embedding in the LMPA. The tube not only acts as a skeleton of the manipulator, but also the material and structure of it bring the fast heat exchange. The motion of the manipulator was driven by tendons going through the holes of a threaded spring-like elastic skeleton which is flexible still can appropriately balance the self-weight of the manipulator. Continuum manipulators with one and two omnidirectional segments were fabricated to show the performance of movement and stiffness variation. Each segment is of 15 cm length and with diameter of 21 mm. The test shows that the stiffness of the manipulator has a factor of 45 variations in 15 seconds, and the stiffness variation is adequate to complete some challenging tasks like screw tightening and loosing.

Bowing of marble slabs: Evolution and correlation with mechanical decay

The Bending-Gradient theory for flexural wave propagation in composite plates

Glass is being increasingly used as a structural material. Its favorable aesthetic qualities have made it popular with modern designers. Recently glass is used for major structural elements such as beams and columns in modern and innovative architectural applications. High slenderness ratios and brittle behavior in tension contribute to the complexity of stability and ultimate strength analysis of glass structures. Moreover the available design curves and methods of ultimate strength analysis of glass structure contain epistemic uncertainty, therefore are not reliable. In this study, based on logical reasoning, concise mathematics and reliable data, a reliable method called the change of state philosophy is developed and expressed in the Persian curve. The Persian curve is used for reliable analysis of glass structures. The validity of the work is verified via its sound basic formulation and comparison of the results with those of the others in the literature.

Application of time domain reflectometer for detecting necking defects in bored piles

Nonlinear amplification is typically done on velocity signals from low-strain pile integrity tests to enhance weak echoes and superimpose any peak reflections. This conventional method may sometimes fail to untangle the hidden information within the signal that is obscured by the presence of noise. In this study, a pile defect identification system based on the conventional nonlinear amplification method and the wavelet packet transform (WPT) was proposed to easily detect the presence of any geometric or material defects by identifying feature parameters. Diagnostic rules, which have been lacking in the literature, were presented to serve as a guide in interpreting decomposed signals and in analyzing various characteristics of peak waveforms that are associated with certain types of defects. In this study, the finite element method was used to simulate the impact echo test of nine cases of defective piles. To verify the proposed scheme, six data sets of the nine cases of defective piles were made, in which a total of 54 piles were analyzed. The results of the study showed that the identification method based on WPT could detect defects 87.04% of the time compared to the conventional method, which only detected defects 64.81% of the time.