Low cyclic loading tests and fatigue damage models of concrete bridge piers reinforced with high-strength rebar

Reinforced concrete bridge piers are extremely vulnerable to damage during long-duration ground excitations or main shock-aftershock type earthquakes due to accumulated damage caused by a great number of reversed excursions in elastic-plastic range. However, few studies on fatigue damage of piers can be found in literature. Low-cyclic loading tests of four identical RC bridge piers with high-strength rebar HRB600E (yield strength about 600 MPa) were carried out in this study. One of specimens was taken as the benchmark and was subjected to a conventional load protocol, and the rest was subjected to one, two and three times yield displacement, respectively. The research results showed that the fatigue strength of RC bridge piers tended to drop drastically at about ten cycles and then slowed down gently. It was found that strength degradation rate increased significantly with the displacement amplitude for fatigue tests while the fatigue life decreased dramatically with the displacement amplitude. In particular, when the cyclic loading displacement exceeded 2 times the yield displacement, the fatigue life dropped dramatically. Based on the experimental data, an exponential-type damage model was proposed with the peak lateral force at the first cycle as the coefficient, the cycle count as the base and factor of the loading displacement amplitude as the exponent, which could accurately predict the degraded lateral force of the bridge piers at different constant drifts. An accumulative fatigue damage index was established to evaluate the damage level of bridge piers.

Long-term bisphosphonate treatment coupled with ovariectomy in mice provokes deleterious effects on femoral neck fracture pattern and modifies tibial shape

Aims The processes linking long-term bisphosphonate treatment to atypical fracture remain elusive. To establish a means of exploring this link, we have examined how long-term bisphosphonate treatment with prior ovariectomy modifies femur fracture behaviour and tibia mass and shape in murine bones. Methods Three groups (seven per group) of 12-week-old mice were: 1) ovariectomized and 20 weeks thereafter treated weekly for 24 weeks with 100 μm/kg subcutaneous ibandronate (OVX+IBN); 2) ovariectomized (OVX); or 3) sham-operated (SHAM). Quantitative fracture analysis generated biomechanical properties for the femoral neck. Tibiae were microCT scanned and trabecular (proximal metaphysis) and cortical parameters along almost its whole length measured. Results Fracture analyses revealed that OVX+IBN significantly reduced yield displacement (vs SHAM/OVX) and resilience, and increased stiffness (vs SHAM). OVX+IBN elevated tibial trabecular parameters and also increased cortical cross-sectional area and second moment of area around minor axis, and diminished ellipticity proximally. Conclusion These data indicate that combined ovariectomy and bisphosphonate generates cortical changes linked with greater bone brittleness and modified fracture characteristics, which may provide a basis in mice for interrogating the mechanisms and genetics of atypical fracture aetiology. Cite this article: Bone Joint Open 2020;1-9:512–519.

Stochastic Response of SDOF Self-Centering System

As a new type of seismic resisting device, the self-centering system is attractive due to its excellent re-centering capability, but research on such a system under random seismic loadings is quite limited. In this paper, the stochastic response of a single-degree-of-freedom (SDOF) self-centering system driven by a white noise process is investigated. For this purpose, the original self-centering system is first approximated by an auxiliary nonlinear system, in which the equivalent damping and stiffness coefficients related to the amplitude envelope of the response are determined by a harmonic balance procedure. Subsequently, by the method of stochastic averaging, the amplitude envelope of the response of the equivalent nonlinear stochastic system is approximated by a Markovian process. The associated Fokker–Plank–Kolmogorov (FPK) equation is used to derive the stationary probability density function (PDF) of the amplitude envelope in a closed form. The effects of energy dissipation coefficient and yield displacement on the response of system are examined using the stationary PDF solution. Moreover, Monte Carlo simulations (MCS) are used for ascertaining the accuracy of the analytical solutions.

Theoretical solutions of laterally loaded fixed-head piles in elastoplastic soil considering pile-head flexural yielding

Group piles with a cap under lateral loading behave like fixed-head piles because of the rotational restraint of the pile cap. They are susceptible to flexural yielding at the pile head due to high bending strains. In addition to soil nonlinearity, the pile-head flexural nonlinearity also significantly contributes to nonlinear responses of the piles, which is not covered in most existing analytical solutions. For a fixed-head pile buried in uniform elastoplastic soils, this study derives analytical solutions for the lateral responses to account for soil yielding, and flexural yielding and failure of the pile section at the pile head. The model assumes elastoplastic Winkler-type soil with constant subgrade stiffness and yield strength and a nonlinear moment–curvature curve for the pile section. Examples are provided to apply the solutions to determine the complete capacity curves (moment and horizontal load) of a fixed-head pile and the derived analytical capacity curves are in good agreement with those from numerical analyses that use nonlinear beam elements to reflect the nonlinear flexural behavior of the pile section. The solutions are also applied to evaluate the influences of the yield displacement of the soil and different forms of simplified nonlinear moment–curvature curves of the pile section on the lateral load–displacement curves.

The relationship between Spirulina platensis and selected biomechanical indicators of tibiae in rats

There are several dietary supplements, particularly herbal foods, that have been used in an attempt to improve bone growth. In this study, we aim at determining the effects of low- and high-doses of Spirulina platensis, a “Superfood”, on the bone growth and biomechanical indicators. Thirty Wistar rats, weighing 250 g, at the age of 7–8 weeks were assigned to three groups: The Control group (basal diet), Low-dose group (LDG; 500 mg/kg) and High-dose group (HDG; 1 000 mg/kg) of S. platensis. S. platensis was given daily by oral gavage in a 45-day-trial. At the end of the study, the right tibiae were collected and subjected to bone biomechanical tests (bone weight, bone length, maximum load, stiffness, breaking deflection, fracture toughness, post-yield displacement and yield load). Serum samples were also analysed for the calcium and phosphorus concentrations. There were significant increases in bone weight, bone length, maximum load, breaking deflection, work to fracture, post-yield displacement and yield load (P = 0.025, P = 0.019, P = 0.030, P = 0.015, P = 0.031, P = 0.028, P = 0.049, respectively), whereas stiffness non-significantly increased. However, there were no significant differences (P > 0.05) for any variables between the LDG and the HDG. Although the serum phosphorus concentrations showed no differences among any of the groups, the serum calcium concentration increased significantly in LDG compared to Control group (P = 0.009; 7.14 ± 0.47 and 9.45 ± 0.67, respectively). However, no differences were observed in HDG in terms of serum calcium. In conclusion, S. platensis had positive effects on the bone growth and biomechanical bone features. Therefore, our study supports the use of S. platensis as an alternative food additive for bone growth and health in growing animals.

Anchor Stress and Deformation of the Bolted Joint under Shearing

Bolts are widely used in rock mass engineering, wherein the bolt support improves the safety and stability of the rock mass. To reveal the mechanical behavior of the bolt and failure mechanism of the bolted joint in the shearing process, a direct shear test was conducted by changing the state of grouting, number of bolt, and inclination angle of the bolt. The change in the axial force of the anchor in the shearing process was evaluated by conducting a strain gauge test, and the mechanical behavior of the bolt under the external force was studied. The results showed that under the same normal stress, the yield displacement of the bolt decreased and the stiffness of the joint gradually increased with increased number of bolts. At the same number of bolts, their yield displacement increased with increased normal stress. Analysis further revealed that grouting on the joint improved the force condition of the bolt, increased the yield displacement of the bolt, and coordinated the deformation of the grouting body and bolt, thereby improving the shear strength of the joint. Lastly, when the anchor angles differed, the axial pulling resistance of the anchor changed, and the yield displacement of the anchor with 45° inclination was <90°. The yield displacement of the bolt showed that the supporting effect of the bolt with a 45° inclination was better than that of the bolt with a 90° inclination.

Experimental Investigation on the Mechanical Properties of Curved Metallic Plate Dampers

This study proposes a novel curved steel plate damper to improve the seismic performance of structures. The theoretical analysis of the curved plate damper was carried out deriving formulas of key parameters of the curved plate damper including elastic lateral stiffness, yield strength, and yield displacement. Moreover, a cyclic loading test of four sets of specimens was conducted, and the hysteretic performance, ductility, energy dissipation performance, and strain of the specimens were studied. The results showed that the initial stiffness of the damper was large, no obvious damage was observed, and the hysteresis loop was full. The tested dampers had good deformation and energy dissipation performance. The stress variable rule of the damper was obtained by stress analysis, where the plastic deformation at the end of the semi-circular arc was large. The formula for various parameters of the damper was compared with experimental and numerical results; thus, the value of the adjustment coefficient was determined reasonable. Meanwhile, the rationality of the finite element model was also verified.

Seismic yield displacement of reinforced concrete bridge pier columns in saline soil environment

Corrosion of bridge pier columns in saline soil environment is inevitable, resulting in a gradual decrease in seismic yield displacement. In this study, 10 reinforced concrete bridge pier columns were fabricated, and seismic yield displacement in the saline soil environment was studied. Electrochemical corrosion tests and low-cycle repeated loading tests were carried out. The axial compression ratio and corrosion rate are the main parameters considered in this article. The seismic yield displacement test value of the pier column is determined based on the energy method. Using the static method, the theoretical expression of the earthquake yield displacement is derived. According to our results, when the corrosion rate is constant, the axial compression ratio is within a certain range, and the seismic yield displacement of the pier increases with the increase in the axial compression ratio. Similarly, when the axial compression ratio is constant, the seismic yield displacement decreases as the corrosion rate increases. By comparing experimental results with calculation results, our mathematical expressions have been shown to be effective in predicting seismic yield displacements of pier columns at different times in saline soil environments.