Post-Fire Behavior of Non-Prismatic Beams with Multiple Rectangular Openings Monotonically Loaded

The main objective of this paper is to study the behavior of Non-Prismatic Reinforced Concrete (NPRC) beams with and without rectangular openings either when exposed to fire or not. The experimental program involves casting and testing 9 NPRC beams divided into 3 main groups. These groups were categorized according to heating temperature (ambient temperature, 400°C, and 700°C), with each group containing 3 NPRC beams (solid beams and beams with 6 and 8 trapezoidal openings). For beams with similar geometry, increasing the burning temperature results in their deterioration as reflected in their increasing mid-span deflection throughout the fire exposure period and their residual deflection after cooling. Meanwhile, the existing openings situation was compounded. The burned NPRC beams were left to gradually cool down under ambient laboratory conditions, and afterward, they were loaded until failure. The influence of temperature on the residual ultimate load-carrying capacity of each beam was studied by comparing these beams with unburned reference beams. Increasing exposure temperature reduces the ultimate strength of solid NPRC beams exposed to temperatures of 400°C and 700°C by about 5.7% and 10.84% respectively. Meanwhile, NPRC beams with trapezoidal openings showed ultimate strength reductions of 21.13% and 32.8% (for beams with 8 openings) and 28% and 34.4% (for beams with 6 openings) under the same burning conditions. The excessive mid-span deflections for these three types of beams were 2%–30.8%, 1.33%–21.8%, and 1.5%–17.4% under the same burning conditions.

Computational and experimental determination of the parameters of the Cowper — Symonds hardening materials model for metal beams

The study introduces a method for the computational and experimental determination of the parameters of the Cowper — Symonds material model for steel beam structures under shock loads, the method being based on the finite element method. A full-scale experiment was carried out on a developed and manufactured installation that implements dynamic shock loading of metal beams according to the three-point bending scheme. The results of the practical approbation of the proposed method are presented on the example of determining the parameters of the Cowper — Symonds model for beams of steel 20. The difference between the calculated and experimental values of the residual deflection of the beam did not exceed 5%. Computer simulation of the experiment was carried out in the ANSYS LS-DYNA software package. The above methodological approaches are proposed to be used in the calculated assessment of the strength of the power structure of passenger vehicles for compliance with the requirements of UN Regulation No. 66.

Study on a straightening process by reciprocating bending for metal profiles

The straightness is a critical quality parameter of metal profiles, and straightening is a necessary process in metal profile production. Due to the limitations of the existing straightening methods, the straightening process by reciprocating bending for metal profiles is proposed. The curvature is unified by multiple reciprocating bending, and then the straightening is completed by reverse bending. The process has the advantages of high straightening efficiency, flexibility, and wide straightening range. In order to verify the feasibility of the process, numerical simulation and physical experiment are carried out with the rectangular section profile with “C” shape and “S” shape. The results show the profiles of different shapes are unified into arcs of the same size after multiple reciprocating bending. In addition, the smaller the elastic area ratio (ratio of elastic deformation to overall deformation) is, the better the effect of unification curvature is. The residual deflection is basically the same after straightening, and straightness is within 0.1%.

Research on Bearing Capacity of Simply Supported Skew Bridge Based on Load Test

The purpose of this paper is to use static and dynamic load tests to evaluate the mechanical performance of a simply supported skew slab bridge and to evaluate its actual bearing capacity. Firstly, the Midas Civil software is used for theoretical simulation, and secondly, the deflection, strain, and dynamic response of the key section of the bridge are studied through static and dynamic load tests. Finally, the measured values and theoretical values are compared and analyzed. The results show that: under static load, the relative residual deflection and relative residual strain of the measuring point of the structure are between -13.8%～-0.4% and -16.7%～1.8% respectively; Under dynamic load, the first-order vertical natural frequency of the test section is 7.813, and the damping ratio is 0.0316, indicating that the bridge is in an elastic working state under the test load, and the stiffness and bearing capacity can meet the requirements of the current code.

Impact Resistance Behavior of Reinforced Concrete Beams Deteriorated due to Repeated Freezing and Thawing

Many existing reinforced concrete (RC) structures constructed more than 50 years ago now require maintenance. This is especially true in cold, snowy regions where significant frost damage deterioration of RC structures becomes a severe problem. In this study, falling-weight impact tests were performed to investigate the impact resistance behavior of RC beams degraded by frost damage. An RC beam was subjected to approximately 900 freeze-thaw cycles to emulate the frost damage before the execution of the impact test. The surface of the beam was remarkably scaled, and its coarse aggregate was exposed. The degree of deterioration was evaluated by the distribution of ultrasonic propagation velocity. The following conclusions were drawn. (1) The ultrasonic propagation velocity of RC beams was significantly reduced following 872 freeze-thaw cycles. At the upper edge of the RC beam, the ultrasonic wave propagation velocity decreased from 4,000 m/s to 1,500 m/s in some parts. This corresponds to a relative dynamic elastic modulus of approximately 14%. (2) The residual deflection of RC beams with frost damage increased at most by 20% compared with beams without frost damage. The increase in residual deflection was primarily related to the peeling of concrete at the collision site and the opening of multiple bending cracks. (3) According to the existing residual deflection calculation formula, an increase of 20% in the residual deflection corresponds to a decrease of about 17% in the bending capacity of the RC beam. When the relationship between the degree of frost damage deterioration and the impact resistance of RC structures is defined, existing structures subjected to accidental impact force from rockfalls are safer and can be maintained more efficiently.

The Refined Model of Viscoelastic-Plastic Deformation of Reinforced Cylindrical Shells

The paper formulates the initial-boundary-value problem of the viscoelastic-plastic bending behavior of cylindrical circular shells cross-reinforced along equidistant surfaces. The instant elastoplastic deformation of the shell composition components is described by the governing equations of the theory of plastic flow with isotropic hardening. The viscoelastic deformation of these materials is described by the defining relations of the Maxwell - Boltzmann model of body. The geometric nonlinearity of the problem is taken into account in the Karman approximation. The used system of two-dimensional resolving equations and the corresponding initial and boundary conditions make it possible to determine displacements and stress-strain state (including residual one) in materials of the composition of flexible cylindrical shells with varying degrees of accuracy. In this case, the weak resistance of the considered composite structures to transverse shears is taken into account. In the first approximation, the equations are used, the initial and boundary conditions correspond to the relations of the widely used non-classical Reddy theory. A numerical solution of the initial-boundary-value problem posed is constructed using an explicit step-by-step "cross" scheme. The elastoplastic and viscoelastic-plastic dynamic deformation of a relatively thin long circular cylindrical shell is investigated. The structure is rationally reinforced in the circumferential direction and is loaded with an internal pressure of an explosive type. It has been demonstrated that under intense short-term loading even of a relatively thin cylindrical reinforced shell by internal pressure, the traditional Reddy theory does not guarantee that the maximum residual deflection and the intensity of residual deformations of the components of the composition are accurate to within 10% compared to calculations performed by the refined theory. The difference in the results of the corresponding calculations increases with an increase in the relative thickness of the composite shell. It was found that after plastic deformation of a long reinforced cylindrical shell in its residual state, not only appear zones of edge effects, but also a local zone of an intense deformation located in the vicinity of the central section of the shell. The length of the local central zone is comparable with the length of the zones of edge effects. It is shown that the amplitude of the transverse vibrations of the reinforced shell in the vicinity of the initial moment of time significantly (by an order of magnitude) exceeds the value of the maximum modulus of the residual deflection. Therefore, the calculations performed in the framework of the theory of elastoplastic deformation of composition materials do not allow a very approximate determination of the magnitude of the residual displacements and the magnitude of the residual deformed state of the components of the composition of the cylindrical shell.

Study on Springback Straightening after Bending of the U-Section of TC4 Material under High-Temperature Conditions

The thin-walled structures of titanium alloys have peculiar characteristics involving thin curved surfaces, complicated structures, and a poor rigidity. Therefore, bending or twisting distortion frequently occurs in forging, extrusion, drawing, transportation, cooling, and manufacturing. Straightening theory focuses on the straightening curvature or bending moment at room temperature, and a unified analytical model of the straightening curvature, the straightening bending moment, and the straightening stroke, as well as a study on springback straightening under high-temperature conditions, have not been investigated comprehensively. In order to understand the inherent mechanism of springback straightening and quantitative prediction of springback under high-temperature conditions, uniaxial tension tests were carried out to obtain the true stress–strain model of material and stress relaxation under the stress relaxation model. This paper is based on the theory of elastic-plastic mechanics and combines this with the mechanism of stress relaxation to establish springback and residual relative curvature equations of springback. The law of springback straightening is further explored, and springback and residual deflection equations are provided. The results of the study showed that the relative errors of the theoretical residual deflection of the bending deformation and residual deflections obtained by the experiment were less than 20%, with an average absolute error of less than 10%. Therefore, the hardening models adopted can achieve an allowable relative error if hardening parameters are properly selected. The proposed research provides basic data for the prediction of springback straightening, and the design of springback compensation tools can be applied in practical applications.

Experimental Study on Flexural Fatigue Properties of Reinforced Concrete Beams after Salt Freezing

In order to study the fatigue behavior decay law of reinforced concrete structures in cold region under the action of chlorine salt and freeze-thaw, 150-time water freeze-thaw and salt freeze-thaw cycles of reinforced concrete beams were carried out by the quick freezing method, and then the fatigue properties of the test beams were obtained by the four-point bending fatigue test. The fatigue life of the test beam without freeze-thaw is 1,074,282 times, and the fatigue life of the test beam after freeze-thaw is reduced; the minimum fatigue life of fatigue failure is 493,972. The test results show that the residual deflection of the test beam is similar to the relative dynamic elastic modulus, which accords with the damage and failure mechanism of concrete, and the growth rate of residual deflection accords with the law of the block model. The fatigue damage model of reinforced concrete specimens is established, the nonlinear fitting of the damage model is carried out according to the test data, the fitting correlation coefficient is more than 0.98, which indicates that the model can better reflect the damage degree of concrete, and the method of predicting the life of in-service concrete beam is put forward in combination with the concrete damage model.

Bending by explosion of a multilayered concrete beam on a visco-elastic basis

In this work, the problem of bending of a multilayered concrete beam of arbitrary cross-section by explosive loading on a visco-elastic basis is considered. It is assumed that different grades of concrete can be realized in layers in the cross-section. The property of concrete resistance to tension and compression is considered in work. It is assumed that the dynamic loading is caused by consecutive explosion of two charges over the middle of the span of beam. The distribution of bending moments and deflections of the beam at each time is determined. The time of the end of motion and the residual deflection of beam are found.