Bolt Force Research Articles

Study on structural analysis technique of loess tunnel based on multi-index evaluation

Abstract The geological environment of loess tunnel is very complicated in the process of construction and operation, and the actual supporting parameters and surrounding rock pressure of loess tunnel have great uncertainties, which increases the difficulty of performance evaluation. In this paper, a distributed optical fiber monitoring system for loess tunnel is established by means of field investigation, theoretical analysis and field monitoring, which can monitor the tunnel displacement rate, the initial steel arch stress, the axial force of the anchor bolt, the lining stress and the width of the lining crack. Based on the theory of extension, the long-term performance evaluation model of loess tunnel is established, which comprehensively and objectively considers the influence of various influencing factors on the long-term performance of the tunnel, makes a quantitative judgment on the long-term performance of the typical section of loess tunnel, and provides a theoretical basis for the next tunnel maintenance plan, which is of great significance for the improvement of the maintenance technology of loess tunnel.

Mechanical Properties of Fully-Grouted Bolts Support Based on Compression Tests of Anchored Rock Mass

The mechanical properties of fully-grouted bolt support are critical for the safety of support engineering works. To study the influences of factors including the bolt length and diameter, strength of the rock, and fracture angle on the mechanical properties of fully-grouted bolt support, compression tests were conducted on an anchored rock mass, considering the shortcomings of pullout tests on bolts. The discrete element software PFC2D (4.0) was adopted for numerical simulation and analysis from two aspects, namely, the stress distribution and anchorage force supplied by such bolts. The research found that by increasing the bolt diameter and length as well as the strength of the rock, the maximum anchorage force of bolts increases. Whereas the stress distribution of all bolts increases at first and then decreases along the bolts, and there is only one peak on the stress distribution curves, which also gradually shifts to a greater depth. In a fractured rock mass, the maximum anchorage force of bolts decreases, then increases (and is minimized at a fracture angle of 45°) with the decrease in fracture angle. The influence of fractures with different angles on the stress distribution of bolts is mainly reflected in the fracture zone. The bolt stress decreases abruptly in the zone with a fracture angle of 90°, forming a valley. The bolt stress increases suddenly in the zones with fracture angles of 60° and 45°, thus forming peaks. The bolt stress does not increase or decrease suddenly in the zone with a fracture angle of 30°. Therefore, it necessitates consideration of the influences of fractures on the anchorage force and the selection of bolts of appropriate size during anchorage design. After installation, the bolt stress should be monitored for stability and early warning of anchored rock mass according to changes in the stress provided.

Failure analysis and deformation characteristics of shield tunnel obliquely crossing ground fissure under earthquake

In order to study the deformation law and failure characteristics of shield tunnel obliquely crossing ground fissure under earthquake action, taking the shield tunnel of Xi ’an Metro Line 8 crossing f3 ground fissure as the engineering background, the 1: 20 shaking table model test method was used to analyze the strain of shield tunnel, the contact pressure with surrounding rock soil, the dislocation of segment and the axial force of bolt in detail, and the seismic damage mechanism and failure characteristics of shield tunnel obliquely crossing ground fissure were obtained. The test results show that under the action of earthquake, the shield tunnel has complex three-dimensional deformation characteristics, among which the vertical deformation is the most obvious. The deformation is mainly concentrated in the location of the ground fissure. The tensile strain and contact pressure of the hanging wall of the tunnel segment are greater than the strain value and contact pressure of the footwall. Because the vertical deformation of the tunnel is the largest, the bolts at the vault and the arch bottom are most obviously pulled. Excavation after the test, it can be seen that the tunnel appeared the phenomenon of ring joint opening, lining cracking and other damage. Under the action of the earthquake, the shield tunnel across the ground fissure is mainly subjected to tensile failure. The failure area is within 10 m from the ground fissure in the hanging wall and footwall, and the total length is 20 m. The closer to the ground fissure, the more serious the damage. The research results provide a scientific and reasonable reference for the subsequent construction and disaster prevention and mitigation design of Xi ’an Metro.

Experimental Research on Residual Clamping Force of Friction High-Strength Bolt after Corrosion Loss Simulated by Wire Cutting

Experimental Research on Residual Clamping Force of Friction High-Strength Bolt after Corrosion Loss Simulated by Wire Cutting

The Tightening and Untightening Modeling and Simulation of Bolted Joints

Although bolted joints may appear simple and are easy to manipulate, they are challenging to model and analyze due to their complex structural patterns and statically indeterminate nature. Ensuring the structural integrity of these joints requires maintaining proper bolt preload and clamping force, which is crucial for preventing failures such as overload, excessive bearing stress, fatigue, and stripping caused by seizing or galling. Achieving the necessary clamping force involves carefully controlling the input tightening torque, which is divided into the pitch torque and the friction torques at the bolt or nut bearing surfaces and in the engaged threads. The resulting clamping force is critical for generating the required force within the bolt. However, the achieved bolt force depends on several factors, such as friction at the joint’s contact surfaces, grip length, and the relative rotation between the bolt and nut during tightening. Friction at the contact surfaces, particularly beneath the bolt head or nut and between the threads, consumes a significant portion of the applied tightening torque—approximately 90%. This paper explores the three existing bolt internal pitch, bearing, and thread friction torques that are generated by the external applied torque in a bolted joint, as well as their contributions and variations throughout a loading cycle composed of three phases: tightening, settling, and untightening. An analytical model is developed to determine these torque components, and its results are compared with those obtained from finite element (FE) modeling and experimental testing from previous studies. Finally, this study examines the torque–tension relationship during bolt tightening, offering insights into the required accuracy of bolt and clamped member stiffness. The bolt samples used in this study include M12 × 1.75 and M36 × 4 hex bolts.

FBG-based force sensing with temperature self-compensation for smart bolts

Small variations in bolt component connection can have significant impacts on equipment operating safety and efficiency. A comprehensive understanding of the bolted status supports the equipment optimizing in in-situ health monitoring. Therefore, an improved bolt force measurement method is looking forward. Given the minimally invasive nature, potential for multi-parameter measuring, and ability to operate in harsh conditions, optic fiber sensors present an opportunity for equipment in-situ health monitoring. This paper first strengthened the confidence in embedding optic fiber force sensors within the bolts. Additionally, the FBG temperature self-compensation method is employed and successfully improved the force measurement accuracy, compared with the existing studies. The smart bolt configuration (addictively manufactured) refers to the standard bolt dimensions and integrates a metallized FBG optical fiber with a diameter of less than 0.5 mm. Then, the sensor performance was investigated through a series of routine mechanics tests and reports the force sensitivity of the designed smart bolt is 13.06 pm/kN (for M10 bolts) and 14.59 pm/kN (for M12 bolts), respectively. In dynamic force loading tests, the error of the sensor is within 4.95 %, and the maximum force detection error after temperature compensation is within 8.03 %, indicating an improved bolt force measuring accuracy. The anti-creep and anti-torque interference tests were undertaken to confirm the designed smart bolts are adequate for long-term service. The bolt vibration and connection test results have proved the mechanical solidity and reliability under extreme working conditions. This investigation confirms the viability of installing optic fiber force sensors in a bolt component. Confidence was established that the smart bolts have the advantages of compact structure, improved force detection accuracy, good reliability, and support for modern equipment in-situ health monitoring.

Bolt Force Monitoring Method Used in Maglev Structure Maintenance Based on Radial Strain on the Surface of Bolt Head

Bolt Force Monitoring Method Used in Maglev Structure Maintenance Based on Radial Strain on the Surface of Bolt Head

Numerical experimental study on combined support of bolt-arch in circular roadway

The deep soft rock roadway is in high stress environment, and the surrounding rock is prone to large deformation. Appropriate roadway cross-section form and high-strength support are effective control methods for surrounding rock deformation. Circular roadway is widely used in mine engineering because of its uniform stress and deformation, which gives full play to the characteristics of rock compression and non-tension. With the advantages of high strength, high cost performance and stability, the application of arch frame in deep soft rock roadway is increasing day by day. Based on this, based on the engineering background of deep roadway in extremely soft rock in sea area, numerical experiments were carried out using the FLAC3D bolt-arch fine numerical simulation platform. The progressive failure characteristics of bolt-arch combined support in soft rock roadway are revealed. The influence law of roadway section form, geostress grade and lateral pressure coefficient on support effect is clarified. The results show that: (1) Under the condition of deep soft rock, the surrounding rock deformation of circular roadway is uniform. Compared with the straight wall semi-circular roadway, the deformation is reduced by about 86% at most. (2) The failure of arch frame in circular roadway has a significant influence on the final control effect of surrounding rock. When the ground stress exceeds 15 MPa, the loss of bearing capacity of arch frame is instantaneous, and the deformation of surrounding rock increases suddenly. (3) When the support of circular roadway in deep soft rock fails, the axial force of bolt is very small, and it basically does not play a supporting role. According to the research results, the support optimization design is carried out, and the field verification is obtained.

Exploration of the behavior and evolution features of beam-column joints with T-stub using the structural strain energy method

Based on the structural strain energy method, the work behavior of four T-stub connection joints under cyclic loading is analyzed to visualize their working behavior and failure evolution. The experimental strain data are modeled as generalized strain energy density (GSED) to characterize the structural stress state and the corresponding characteristic parameters, and the Mann-Kendall (M-K) criterion is formulated to determine the performance characteristic points (P and Q) of the joint Mj-Ej curves and to derive the starting point of structural continuity damage. Then, by analyzing the parameters of the joint working state (strain, deformation, joint energy dissipation, connection stiffness, bolt force and pry force), the trends of the joint stressing state curves before and after the critical loading are reflected. Finally, the feature point data interpolation (EDI) method is introduced to effectively expand the joint test data, visualize the change process of the strain/stress field, and deeply reveal the mechanical response mechanism and evolutionary degradation law behind it. The contribution of the above work is to quantitatively identify the P and Q characteristic points in the changing trend of the joint working state, to reveal the mechanical mechanism and the failure degradation law from the perspective of energy evolution, and to visualize the mutation of the field data before and after the P/Q using the EDI method, which is a generalized method that can be applied to the other progressive failure structures.

Numerical study on the dynamic pressure control for self-forming roadways using CHRCT in thin coal seams with thick and hard roofs

Close-hole roof-cutting technology (CHRCT, also called “dense drilling”) has been widely applied in coal mines due to its economic and safety benefits. Inappropriate cutting parameters and support schemes can lead to dynamic pressure disturbances in self-forming roadways with thick and hard roofs. Moreover, fully characterizing the procedure and process of self-forming roadways using CHRCT in the field is difficult, resulting in unconvincing results. Therefore, this study aims to fill the gaps in theoretical knowledge and methodology. First, the dynamic pressure characteristics of the self-forming roadway using CHRCT were investigated, and the dynamic pressure types of the roadway were classified. There are three main types: roof cut off along the coal wall side of, severe deformation, and overhanging roof of a roadway after the second working face mining. The effects of different hole parameters (inclination angle, depth and spacing) on the roof cutting to form a roadway were also investigated. The optimal hole inclination, depth and spacing of 15°, 8 m, and 200 mm were determined through a series of experiments. Then, three support schemes embedded in the roadway were compared in terms of stress evolution, bolt and cable axial forces, roof displacement, and structure. Finally, this study proposes a dynamic pressure mitigation strategy through the optimization of parameters for close-hole roof-cutting and support schemes, monitoring and controlling ground pressure in roadways, and taking auxiliary measures for pressure relief. The results show that this strategy can effectively eliminate the dynamic pressure of the roadway and meet the stability requirements of the full mining cycle. This paper presents a methodology for analysing CHRCT via numerical simulation. Moreover, this approach is of great theoretical and practical importance for dynamic pressure control for self-forming roadways using CHRCT in thin coal seams with thick and hard roofs.

Design method of prestressed anchor bolt system for wind turbine foundation

SummaryThe prestressed anchor bolt system is a reasonable connection mode between the upper steel tower and the bottom concrete foundation for multi‐megawatt wind turbines. This prestressed anchor bolt connection with forged flanges has a similar form to the prestressed high‐strength bolt connection with forged flanges between steel tubes for the tower. However, their mechanical performances have a great difference because of the influence of the stiffness of the anchorage zone in the concrete foundation. Based on the Petersen's method, the engineering calculation method of the prestressed anchor bolt system for wind turbine foundation is derived. The tensile force of the anchor bolt, the anchorage stiffness and clamping force of the base are all deduced according to the theories of mechanics of materials. The numerical models of the segment foundation with the unfavorable anchor bolt and the overall foundation with all anchor bolts are developed for researching the influence of the spatial effect of adjacent segments on the restraint stiffness of the concrete foundation. The numerical analyses of four engineering cases designed by engineering calculation method are carried out for verifying the effectiveness of engineering calculation method. The analysis results show the spatial effect of adjacent segments can be neglected and the Petersen's method can be directly applied for the design of the prestressed anchor bolt system for wind turbine foundation in engineering practice. The engineering calculation method meets the accuracy requirements in engineering practice and can be used to design the prestressed anchor bolt system of wind turbine foundation.

Experimental study on the deformation and failure mode of surrounding rocks of a tunnel in a jointed rock mass

The existence of jointed rock masses in tunnels is a key problem that leads to the deformation and instability of the surrounding rock. Based on the engineering background of the Qingdao Metro, a physical model and a numerical model are constructed to analyze the mechanism of deformation and instability of the surrounding rock of a jointed tunnel using model blocks that conform to the characteristics of joint distribution. The results show that during the whole test process, the cracks between the blocks in the tunnel arch are in the “development − stability − closure − penetration” state. Finally, the block falls in the left area of the tunnel, and the corresponding anchor axial force transits from the linear growth stage to the steady growth stage. After the axial force of the rock bolt suddenly decreases in the loading stage, the bolt enters the stability stage, and finally, S-shaped failure occurs when the surrounding rock is unstable. Under the action of a prestressed rock bolt, the normal stress and radial stress of the rock mass increase, and the difference between the major and minor principal stresses decreases significantly. The radial stress on the surrounding rock surface of the arch is approximately 8 % greater than that in the traditional rock bolt scheme, which improves the strength reserve of the rock mass. In the field test, the prestressed rock bolt can control the deformation of the surrounding rock in the arch within 2 ∼ 5 mm, which provides a reference for the control of the surrounding rock in jointed tunnels.

Evolution characteristics and performance evaluation of extended end-plate joints with single and double side connections under cyclic loading

Experimental studies were conducted on four extended end-plate joints subjected to cyclic loading at the column top, investigating the evolving patterns of the joints' mechanical performance. The paper provides a detailed analysis and discussion of the test joints' failure modes, ductility, stiffness degradation, and energy dissipation capacity. The Mann-Kendall (M − K) trend analysis tool was applied to the mechanical response curves, identifying key performance evolution points (evolution initiation point P and overall yield point Q). The trends in bolt forces, deformations, and strains at critical joints were effectively validated, revealing the transition of the energy system from quantitative to qualitative changes and the component's failure process from stability to instability. Additionally, based on the experimental joints' hysteresis curves and energy dissipation capacity, a theoretical hysteresis model was established to predict the joint's hysteresis curve and cumulative dissipated energy accurately. According to EC3 requirements, joints were classified as partially rigid connections. The experimental results of the initial rotational stiffness and plastic moment were further used to evaluate the calculated values in existing standards EN 1993-1-8, ANSI/AISC 358-16, and GB 51017-2017. The results indicate that extended end-plate connections possess sufficient strength, joint rotational stiffness, ductility, and energy dissipation capacity, making them suitable for seismic moment frames.

Effect of Bolts Overloading in Flange-Type Joints

Abstract The joints of bars composed of flanges and high-strength bolts are currently used in steel structures as simple and economical construction solutions. This paper presents some theoretical aspects regarding the behavior and calculation of joints with flanges and partially pre-tensioned bolts. The overloading of the pretensioned high-strength bolts produced by the tensile effort resulting from the external actions is usually neglected in the technical materials and in the design norms for constructions, as long this tensioning effort does not exceed the pretensioning force of the high-strength bolts. The overloading effort of the bolts can be evaluated theoretically by estimating the relative stiffness of the flanges and bolts and confirmed through experimental tests and computer simulations. This paper also theoretically and computationally analyzes a joint for a tensioned bar made up of pre-stressed high-strength bolts and end flanges.

Impact of small-angle shield undercrossing on existing tunnels

A three-dimensional finite element model was developed to investigate the effects of small-angle tunnels on existing tunnels in terms of surface settlement, arch bottom settlement, and ellipticalization of existing tunnel linings. The model simulated two small-angle tunnels crossing two existing tunnels. The maximum surface subsidence was observed at the recently excavated tunnel centers. The presence of existing tunnels reduced surface subsidence caused by ring-by-ring excavation. Surface settlement followed a “W” curve pattern, with two local minima gradually getting close as the tunnel was excavated. The maximum value of the arch bottom settlement of the existing tunnel gradually approached the intersection of the existing tunnel and the newly excavated tunnel. When the absolute value of the rate of change of the arch bottom settlement was high, the tunnel produced a large inter-ring misalignment. The line connecting the centers of the existing tunnel and recently excavated tunnel was parallel to the major axis direction of the ring after the deformation of the lining on the existing tunnel. Therefore, a small-angle tunnel passing through an existing tunnel may result in misalignment and torsion of the existing tunnel, requiring careful monitoring of the inter-ring bolt force to ensure safety.

Anchorage performance of a new rebar bolt under different surrounding rock strength and borehole depth

The theory and technology of rock bolting are fundamental research topics for strata control in civil and mining engineering. Rebar bolts are commonly used for roadway primary support in underground coal mine. To adapt to deep resource mining, a new left threaded rebar bolt has been developed. Compared to conventional rebar bolts, the result of installation test showed that the new bolt reduced of 41.5% and 57.9% in stirring resistance force and torque, respectively. In laboratory pullout tests, PVC and aluminum sleeves were used to simulate weak and medium strength surrounding rocks. The average peak pullout force, displacement at the peak load and energy absorption increased by 27%, 107% and 108%, respectively, using PVC sleeve; and increased by 113%, 109% and 212%, respectively, using aluminum sleeve. Field tests were conducted under soft coal, hard coal and medium strength rock geo-conditions. Different borehole depths were selected to precisely calculate the average anchorage performance of the new bolt. Results showed that the average peak pullout force of the new bolt increased by 37%, 38% and 28%, respectively, under different surrounding rock conditions. Moreover, based on on-site test results, the pullout curves in field-testing were summarised and classified into 6 different patterns, which were discussed from a viewpoint of causality mechanism. The research findings validate that the newly developed bolt has better anchorage performance compared to conventional rebar bolts, making it a new anchorage material for deep resource mining.

Mechanism of bolt breakage in deep mining roadway under dynamic load and advanced strengthening support technology

To reveal the problem of local instability caused by bolt breakage and failure in deep mining roadway under impact dynamic loads, and explore strengthening support technologies that can reduce bolt breakage and improve the stability of roadway support bodies. This article adopts theoretical analysis, numerical simulation, and on-site industrial experiments to conduct research, investigate and obtain the fracture failure characteristics of bolt in deep mining roadway, quantitatively characterize the deformation pressure of roadway surrounding rock and dynamic load of overburden fracture, establish the mechanical criterion of bolt fracture, and analyze the influence law of different anchoring factors on bolt force under static load and dynamic and static load combination. The results show that when the anchor angle is different, the range under dynamic load increases by 169.3 % compared with static load, when the anchor length is different, the range increases by 863.3 % relatively, and the influence is significant; while the anchor support resistance increases by 11.6 % at different times, and the range decreases by 41.5 % at different pre-tightening torque. Then, based on this criterion, the influence law of bolt failure on the stability of surrounding rock is analyzed, and the optimal support scheme is obtained by simulation research, that is, bolt cable +U steel support scheme. After adopting this scheme for support, the deformation of surrounding rock is reduced by about 60 %, and the maximum concentrated stress of surrounding rock of roadway is reduced by 15.0 %. Based on this, the support parameters of the on-site roadway were optimized and a strengthening support scheme was determined, which includes “roof bolt surface support reinforcement + anchor cable combined with U-shaped steel deep reinforcement, and two strong bolts advanced reinforcement”. The on-site strengthening support practice showed that the bolt fracture rate was reduced by 87.3 %, the deformation of the surrounding rock was effectively controlled, and the stability was significantly improved.

Performance study of eccentrically braced steel frames with core tube flange connection

This paper proposes a kind of assembly brace connections adapted to the core tube flanged column connection joint of the frame-brace structure, starting from the eccentrically braced frame structure in the steel frame system and designing and completing a set of proposed static tests of the eccentrically braced frame with the core tube flanged column connection joint. By conducting in-depth analysis and research at three levels, namely the seismic performance of the eccentrically braced frame system, the performance characteristics of the core tube type flanged column connection joints, and the performance of assembled brace connections, the study has yielded insights into the force mechanism and failure modes of the core tube type flanged column connection joints within the eccentrically braced frame system; the eccentrically braced frame system has been validated to possess good seismic performance by analyzing hysteresis curves, typical section strains, and bolt forces. These analyses have shown that the system can meet the seismic requirements. Moreover, the assembled brace joints were destroyed later than the braces, indicating that the assembled brace connection joints could fulfill the requirement of “strong joints.”

Experimental and Analytical Investigation of Self-Centering Precast Composite Walls with Sloped Plane Friction Dampers.

To reduce the damage to reinforced concrete shear walls in earthquakes and repair costs, a self-centering wall with sloped plane friction dampers (SPFDs) is proposed. In addition to the SPFDs, the proposed wall includes a precast composite wall, steel wall toes, and post-tensioned (PT) tendons. The steel wall toes embedded in the base of the precast wall were used to improve its strength, and the SPFDs installed in the steel toes were used to increase its energy dissipation capacity. To investigate the effect of the initial PT force and prestressing clamping force of the friction bolt on the seismic performance of the wall, quasistatic cyclic loading tests were carried out on three precast wall specimens. The damage to the self-centering walls was slight, the residual drift was small, and the energy dissipation met the specification requirements. The wall with the greater initial PT force showed higher self-centering and bearing capacity, and the wall with higher prestressing clamping forces showed greater energy dissipation capacity. Additionally, a calculation method for the bearing capacity of the precast wall, which was verified by comparison with the test results, is suggested.

Mechanism and control of preload force loss of precast structural joint bolts under freeze–Thaw environments

Bolt-connected precast buildings are increasingly prevalent, especially due to their efficient force transfer and accelerated construction processes. However, in cold climates, special attention must be directed towards the effects of freeze-thaw (F-T) damage on concrete at bolt connection sites. This study conducted an accelerated F-t-test to investigate how concrete F-T damage influences the preload force of the connection bolts. The changes in bolt preload forces across F-T cycles were monitored using advanced fiber optic sensing technology. To mitigate preload force loss, silicone rubber sleeves were introduced into the bolts. Results indicate a correlation between initial preload force and subsequent preload loss, with lower initial preloads leading to greater losses. Additionally, the concrete's frost resistance significantly affects preload force loss, with less resistant specimens showing marked preload decreases. A notable risk of brittle damage arises after 3 to 6 F-T cycles if a 4 mm gap between the bolt and concrete exists, attributed to water freezing and expansion within the gap. Concrete specimens subjected to our proposed mitigation strategies effectively withstand over 140 F-T cycles, substantially lowering crack damage risks.