1 Overview The current railway industry standard "Regulations for the Construction of High-strength Bolt Connections for Railway Steel Bridges" (TBJ 214-92), hereinafter referred to as "Construction Regulations", has been published for more than 20 years. With the development of steel bridge technology, China's railway steel Great progress has been made in the design, manufacture, installation and scientific research of bridges, such as the design and manufacturing technology of the whole node [1], the erection technology of the entire section of the steel truss beam [2], and the wide-span public-rail dual-purpose bridge. The use of the connection between the orthotropic steel bridge deck and the main chord, the screwing process and the improvement of the tools, etc., were unforeseeable when the standards were developed at that time, so it is necessary to supplement and revise them. To better guide the construction. The "Construction Regulations" and the industry standard "Design, Construction and Acceptance Procedures for High-Strength Bolt Connections of Steel Structures" (JGJ 82-91) issued by the Ministry of Construction in 1991 are based on "High-strength large hexagon bolts and large hex nuts for steel structures." , and the technical conditions of the gasket (GB / T 1231-1991) and other national standards. In 2006, the GB/T 1228 to 1231 standard in the national standard product series of the "connection type for steel structure friction type high-strength bolt connection" released a new version, and the Ministry of Construction updated its industry standard accordingly and renamed it Technical specification for high-strength bolt connection of steel structure (JGJ 82-2011). In 2011, the former Ministry of Railways included the “Technical Guide for the Construction of High-strength Bolt Connections for Railway Steel Bridges†(hereinafter referred to as the “Guideâ€) in the preparation plan for the railway construction standards of the year, and entrusted the China Railway Bridge Bureau to revise the 1992 edition of the “Construction Regulationsâ€. The establishment of the Bureau of the Bridge Bureau has done a lot of work on the revision of the current standards. At the end of 2012, it submitted the draft of the "Guidelines" to the Economic Planning Research Institute, and passed the preliminary examination. It was suspended due to the withdrawal and reorganization of the Ministry of Railways, but This work will continue until the revision is completed. 2 Discussion on several issues Due to the large dynamic load on railway bridges, the requirements for structural deformation and fatigue resistance are high. The design principles and methods of railway steel bridges are different from those in the construction industry standards, such as pressure-type connection and bolt welding. The connection is not applicable to railway steel bridges. Therefore, the construction of high-strength bolting of railway steel bridges cannot fully refer to other industry standards. It is necessary to study related issues in light of specific situations. 2. 1 Torque method and corner method (twist angle method) The method of applying high-strength bolts is mainly divided into the torque method and the corner method. In the corner method, the high-strength bolts are also firstly screwed by the torque method. Therefore, some documents refer to them as “torque-turning method†and “construction regulationsâ€. The "torsion angle method" in "" can be understood as the abbreviation of "torque-turn angle method". The torque method is a construction method for tightening the high-strength bolt connection by controlling the torque. Because of the simple operation, this method is widely used in high-strength bolt fastening operations at home and abroad. However, due to the constraints of the manufacturing process and surface lubrication technology, the early high-strength bolt torque coefficient is highly variable and easy to change, making it difficult to accurately control the bolt axial force. In April 1980, the surface phosphating high-strength bolt was successfully developed in the Bridge Research Institute of China Railway Bridge Bureau. After the phosphating treatment, the high-strength bolt can reduce and stabilize the torque coefficient, improve and stabilize the pre-tightening axial force, and the surface is black. Compared with the strength bolts, the torque coefficient is reduced by about 50%, and the average value can be guaranteed to be between 0.110 and 0.150. The standard deviation can be stabilized below 0. 010, and the dispersion rate is reduced by about 70% [3]. The quality of the bolt connection is of great significance and lays the foundation for the torque method construction. The Jiujiang Yangtze River Bridge and other high-strength bolts of railway steel bridges built afterwards were constructed by the torque method. The corner method is a method of tightening the bolts and then tightening the high-strength bolt joints by controlling the relative angular values ​​of the bolts and the nuts. In the late 1940s, the United States began to study the relationship between bolt elongation and axial force by the corner method. In 1960, the norm was formulated. In the 1962 revision, the corner method was emphasized as the main tightening method [4], and then many times. Amendments have been made to improve it. At present, this method is widely used in machinery manufacturing and automobile assembly industries at home and abroad. The initial tightening torque and the final twisting angle are two important parameters of the cornering method. The calculation formula of the final turning angle is given in Chapter 4 of the Construction Regulations, but the parameters in the formula and the initial tightening torque before the corner need to be determined through experiments. It has certain difficulties in application, and it is necessary to increase the content of the corner parameter test to improve the operability. 2. 2 delayed fracture Delayed fracture (also known as delayed fracture) is one of the common diseases of high-strength bolting. This situation is more common in steel bridges built before the 1980s. In the later construction of the Jiujiang Yangtze River Bridge, the bridge research of China Railway Bridge Bureau Through in-depth study on the material of the high-strength bolt, the environment of use, the surface treatment method, the working stress, the under-tightening and over-tightening of the bolt, the strength level and the yield ratio, the institute has obtained the “adjustment of tempering temperature and improvementâ€. The correct conclusion of the geometrical tolerance, based on the theory of fracture mechanics, combined with the measured data to derive the limit of the geometrical tolerance [5], the fracture rate of the real bridge bolt is significantly reduced, which fully proves the scientific and effective control method. . However, in the steel bridges constructed in recent years, the phenomenon of delayed fracture of high-strength bolts has occurred, indicating that some aspects of the control have been leaked. Previous research results and experience need to be absorbed into the standard in an appropriate way to reflect. The delayed fracture of high-strength bolts is mainly related to raw materials, manufacturing processes, shape and position deviation, and quality of screwing. With the advancement of technology, the accuracy of the rated torque of the fixed-torque electric wrench has been significantly improved, and the bolt breakage caused by the deviation of the applied torque has been relatively rare. In terms of raw materials, more research has been carried out at home and abroad in the early stage of development of high-strength bolts, recognizing the influence of vanadium, titanium, chromium, nickel, platinum, boron, nitrogen and other elements on the properties of steel, and verifying the delayed fracture and material of bolts. The relationship between the strengths, such as Japan's F8T bolts (the ultimate tensile strength of 784 ~ 980MPa) since the 1960s, and then continue to increase the bolt strength until the F13T bolts (intensity is 1274 ~ 1470MPa), and then found that the bolts are delayed fracture The phenomenon is serious. After seven and a half years of exposure test, the bolts of FllT (strength is 1078~1274MPa) or more are eliminated [6], and the bolts of F10T (strength is 980~1176MPa) are promoted. China's 10.9S high-strength bolt tensile strength is 1040 ~ 1240MPa, between Japan's F10T and FllT, can be considered to be basically suitable. Numerous examples show that manufacturing processes and shape and position deviations have become the main factors affecting the performance of high-strength bolts in China. Delayed fracture is a hydrogen-induced embrittlement that occurs due to material-environment-stress interactions. The enrichment of hydrogen in the steel material during pickling and electroplating treatment under stress will cause delayed fracture of high-strength bolts. For high-strength bolts, especially bolts of grade 10.9 and above, pickling and electroplating Dehydrogenation should be carried out in time. Measures such as improving the processing quality and avoiding excessive stress concentration when rolling the thread can effectively reduce the sensitivity of the high-strength bolt to delayed fracture. In addition, the tempering temperature is also an important factor affecting the delayed fracture of high-strength bolts. The study of the Jiujiang Yangtze River Bridge shows that for high-strength bolts of 20MnTiB and 35VB, the tempering temperature after quenching should be controlled at (420±10) °C. And (440 ± 10) °C. For the 10.9 and 12.9 high-strength bolts of 42CrMoA material, it has been pointed out that the tempering temperature after quenching is not lower than 500 °C, which can effectively improve the delayed fracture resistance [7]. The bolt shape deviation is also closely related to the delayed fracture. When the high-strength bolt is in service, it is subjected to an additional bending moment force in addition to being pulled and twisted. When the bolt bearing surface is not perpendicular and the bolting is not straight, it is close to the national standard. At the limit of the deviation, the bending stress will exceed the axial stress. Since the additional bending moment of the bolt is inversely proportional to its length and proportional to the moment of inertia, the short bolt and the large diameter bolt are more likely to break when the geometrical deviation is constant. When investigating 127 bolts of the Nanjing Yangtze River Bridge, it was found that except for the length of five bolts that reached or exceeded 90 mm, the rest were all short bolts within 76 mm [6]. Delayed fracture of high-strength bolts during service endangers bridge structure and operational safety and must be highly valued. However, since the delayed fracture has a great relationship with the high-strength bolt production process, the existing product standard system is not sufficient to ensure the bolt performance. Some experts have asked the factory to perform the delayed fracture test, but this involves the test method and the qualification standard. Not easy to solve. Due to the high quality requirements of high-strength bolts, complicated production processes and fierce market competition, most large-scale enterprises have withdrawn from this business. Most of the high-strength bolt manufacturers on the market are small and medium-sized enterprises, which are often omitted to reduce costs. Relaxing the strict implementation of some processes and affecting the quality of the products, the construction unit usually only checks the routine performance during the acceptance, and the delayed fracture problem occurs afterwards. Therefore, attention should be paid to the manufacturer's manufacturing process and product reputation during the procurement. It is not appropriate to consider price as the only major consideration. Since the shape and position deviation is one of the main reasons for the delayed fracture of high-strength bolts, it is also an inspection item that can be implemented by the construction unit. When the high-strength bolts are accepted, the inspection standards, especially the non-perpendicularity of the bolts and the concentricity of the nuts, should be strict. grasp. 2. 3 Other process measures The basic process of high-strength bolt production is: batching→forging→phosphating→dehydrogenation→necking→heat treatment→sanding→inspection→alignment→car support surface, deburring→rolling thread→phosphating→saponification→go Hydrogen→sorting→packaging. The process flow of different production enterprises is different, but the forging process does not allow the use of cold heading; the sandblasting and dehydrogenation processes are important processes to reduce the occurrence of hydrogen embrittlement of bolts, and cannot be omitted; the phosphating process includes degreasing, pickling, and table Adjust the content; all the pickling processes in the process must be dehydrogenated within 2h (temperature 190 ~ 230 ° C, time 6h); descaling, burr process should be mechanical. The test determines that the torque coefficient of the high-strength bolted joint after saponification is easily affected by humidity and decreases with the increase of humidity. When the humidity is greater than 90%, the torque coefficient will drop sharply (up to 0.08 or less, exceeding the standard allowable value. Due to the torque If the coefficient is too low, it will become unstable and the bolt's anti-loosening effect will be deteriorated, which should be avoided.) The torque coefficient of the fully phosphatized oil-impregnated bolt increases with the increase of humidity (still within the standard allowable range). 8] Therefore, high-strength bolts used in wet areas should be treated with a fully phosphating surface treatment. A survey of high-strength bolts on some bridges found that the bolts were mostly broken at the first buckle on the inside of the nut and the boundary between the polished rod and the thread, where there is a large stress concentration. According to the study of Xining Special Steel, improving the shape of the thread groove can reduce the stress concentration. In addition to improving the fatigue resistance of the high-strength bolt, it can also reduce the hydrogen enrichment and diffusion at the thread groove, and it can also be significantly improved. It delays fracture resistance. 2. 4 large diameter high strength bolts As the span of domestic railway steel bridges is getting larger and larger, the thickness of steel beam joint slabs is increasing continuously. The high-strength bolts of the existing specifications have become incapable, and there is a realistic demand for high-strength bolts with greater bearing capacity. Some manufacturers have tried to produce high-strength bolts of M36, M42 and even larger diameters. However, the current national standard "Technical Conditions for High-strength Large Hex Head Bolts, Large Hex Nuts and Washers for Steel Structures" (GB/T1231) is the maximum bolt specification. M30. The development of high-strength bolts involves many problems such as materials, manufacturing processes, product standards, etc., belonging to the upstream chain in the technology chain, while construction belongs to the tail chain. In the "Construction Regulations", the requirements on product requirements only involve pre-tension and related The values, which are again proposed by the design specification. At present, the development trend of “non-standard†is to increase the bolt diameter and increase the bolt strength. Due to the existence of delayed fracture problems, it is difficult to use higher strength materials unless significant breakthroughs are made in materials science or manufacturing processes, so increasing the bolt diameter may be a reasonable option. The current product series standards are basically sufficient for ordinary steel structures, but there is indeed a lag in the development of railway steel bridges. It is necessary to expand the models and specifications of high-strength bolts on the basis of sufficient research to adapt The urgent need for high-strength bolted joints for long-span steel bridges. 2. 5 bolt welding mixed connection The bolt-welding joint is different from the bolt-welding joint. The former refers to the joint of some welded joints, some of which are welded, and some of which are connected by high-strength bolts. The latter is defined in the construction industry standard as :" Consider the form of joints designed for friction-type high-strength bolted joints and fillet welds that are designed to bear the same shear force." In railway steel bridges, bolted-welded joints are usually expressed as: bridges in orthotropic steel bridges. In the fascia structure, the steel bridge deck connection is longitudinal and transverse field welding, and the longitudinal beam (rib) connection and the beam (rib) connection with the main chord are bolted or used between the beam flange and the main raft. The welded web is welded by a high-strength bolt and the welding is delayed by the bolting. From the point of view of the joint force of the bolt and the weld joint, the bolt-welding joint has lower synchronization requirements for the joint force and deformation than the bolt-welding joint, thus simplifying the joint design and facilitating construction. For the bolted joints and joints, the original West Germany test in the 1960s showed that under static load, the allowable loads of the two can be superimposed, and the safety factor is ≥2.5. Under the repeated load, the joints used for bolting are resistant. The strength is higher than the tensile strength of the weld (the weld is arranged behind the bolt) [9]. In China, some studies have pointed out that when the ratio of the strength of the friction type high strength bolt to the strength of the side weld is about 0.85, the strength of both can be fully exerted. Under other ratios, the higher strength has different degrees of waste, and the bolt welding The joint bearing capacity can be approximated by a linear combination of the shear capacity of the two joints [10]. The bolt-welding joints have been widely used in the design of long-span railway steel bridges. What kind of mechanical model and calculation theory is used by the design unit is an important basis for the construction unit to formulate high-strength bolting schemes. Therefore, it should strengthen communication with the design unit. Make the construction process reasonable and safe. In the construction of such joints, due to the welding deformation of the rear welded plate parts, additional internal forces are generated on the previously bolted plates, which in turn affects the force characteristics of the high-strength bolts on the first bolting plates, and it is necessary to combine scientific research and test. The engineering practice of large steel bridges clearly stipulates the high-strength bolts, the welding process of components and the construction sequence. According to experience, if the bolt is welded after the welding, the punching nail of the bridge deck is hard to be smashed by the boring, which affects the construction of the high-strength bolt, and the bridge panel and the chord are integrated, the board surface of the bolted part is difficult to stick. Tight, reduce the connection effect of the friction surface [11]. The construction industry standard stipulates that in the bolt-welding and mixing joints of beams, columns, supports and other components, "the construction sequence should be carried out after the high-strength bolts are first screwed, and then the high-strength bolts are finally screwed." In the recent construction of several steel bridges, in order to reduce the additional force of the high-strength bolts on the web of the beam, the 2~3 rows of bolt holes near the side of the steel bridge panel are filled with the nails first, so that the nails are subjected to the deck welding first. The additional force generated by the deformation, the method of replacing the nail with high-strength bolts after the welding is completed, this experience can be referred to. In some bolt-welding joints, if the welding delays the bolting time is very short, consider the high-strength bolts on the webs of the beams after the flanges of the beams are welded. If the lag time is long and the torque factor will change, this method should not be used. 2. 6 final screw quality check For the final tightening inspection of high-strength bolts, the final tightening inspection of the torque method specified in Article 5.0.4 of the Construction Regulations shall meet the following requirements: 1. Observe the high-strength bolt connection pair after the final screwing. Check whether the relative position of the bolt and the nut marked with paint after the initial screwing (re-threading) is rotated to check whether the final screw is leaked. 2. For the main raft (slab beam main beam) and the longitudinal and beam joints, the total number of high-strength bolted joints of each bolt group is 5%, but not less than 2 sets, and the remaining nodes are not less than 1 set for final screwing. Torque check. The 1996 and 2002 editions of "Code for Construction of Railway Bridges and Culverts" (TBJ 203-96, 2002) are stipulated in Article 11.1 22: After the high-strength bolts are screwed, the inspection shall be carried out according to the following provisions: 1. Full-time personnel shall be established. Check it and check the bolts that were screwed out that day. 2. At the junction of the main girders and the longitudinal beams, the number of inspections for each bolt group is 5% of the total, and there must be no less than five for each main node. However, there was a major change in the “Construction Quality Acceptance Standard for Railway Bridges and Culverts†(TB10415-2003) issued in 2003. Article 12.2.5 of the standard stipulates that high-strength bolting joints must be screwed and must comply with relevant standards and construction. Process requirements. Inspection quantity: All inspections by the construction unit... Inspection method: The construction unit uses a torque wrench or a protractor to check; the supervision unit witnesses the inspection. Subsequent versions basically follow this rule. The understanding of the provisions may be different, but in the actual implementation process, the final tightening torque check is performed according to 100% of the total number of bolts. A 100% final tightening torque check of all high-strength bolts is clearly intended to better ensure quality, but the rule itself is not scientific enough. First of all, the torque coefficient and the torque applied to the high-strength bolts that are applied by the torque method are equally important. The “Construction Regulations†not only specify the mean value of the torque coefficient, the standard deviation, and the measurement method, but also specify the operation before and after each shift. The construction wrench must be torque corrected. When the error exceeds ±5%, the high-strength bolt connection pair that is finally screwed on the wrench is inspected and processed by the inspection wrench. Therefore, the connection quality of the bolt should mainly depend on the torque coefficient. The determination and the torque correction of the construction wrench ensure that the inspection of the bolt after the final screwing is only an aid. Secondly, regardless of whether the "tight-fastening method" or the "snap-reduction method" is used, the tightening state and the torque coefficient of the bolt may be disturbed. Since most of the tightening method is used, the "all inspections" after the final screwing will inevitably cause the bolts. The overall torque is generally high, resulting in a large area of ​​over-tightening. In addition, the number of random inspections has increased to 20 times, time and economic costs have increased significantly. Torque inspection is not only time-consuming but also requires multiple people to cooperate. Except for 2 people applying inspection torque, the rest of the personnel are responsible for card travel, reading, recording, Observe the rotation of the nut, etc., and read the torque value at the moment the nut rotates. When inspecting bolts at high-altitude nodes or corners, it is difficult to ensure accuracy due to poor observation conditions. At present, the high-strength bolts used in large steel bridges are more than 1 million sets, and some are up to 2 million sets [12]. When the construction peaks, tens of thousands of sets of bolts are applied every day. One by one inspection will undoubtedly bring huge burden to the construction unit, and check one. The time it takes to set the bolt is several times that of the screwing time. It is also unrealistic to complete the torque check on the same day for all bolts that are screwed on the same day. Therefore, the final tightening torque check of all high-strength bolts is not only unnecessary, but adversely affects, but to change this situation, it can only be realized by revision of the acceptance criteria. As a comparison, Articles 13.13 of the Technical Specifications for Construction of Highway Bridges and Culverts (JTG/T F50-2011) stipulate that after the completion of the final tightening of high-strength bolts, the quality inspection shall be carried out according to the following provisions: ... 3) The joints of the joints, the main body of the slab and the longitudinal and transverse beams shall be checked by 5% of the total number of high-strength bolted joints, but not less than 2 sets, and the remaining nodes shall be not less than 1 set for the final tightening torque check. "Technical Regulations for High-Strength Bolt Connections of Steel Structures" (JGJ82-2011) Article 6.5.1: The final tightening torque shall be checked by 10% of the number of nodes, and shall not be less than 10 points; for each sampled node The number of bolts should be checked by 10% and should not be less than 2 bolts. 2. 7 screwing and inspection tools High-strength bolts are mainly used with fixed-torque electric wrenches. For bolts that cannot be used with electric wrenches at individual dead corners, they are generally screwed with a pointer type, a digital torque wrench or a fixed torque wrench. Compared with foreign advanced products, domestic electric wrenches have a large gap in reliability and durability. Some key components such as motors and torque output controllers are easily damaged and frequently repaired. Although the price of some imported products is 10 times that of domestic similar products, due to the reliable quality and durability, the comprehensive cost of each bolt is still superior. This situation is worth reflecting. Regarding the screwing and testing equipment, the digital electric wrench and the torque coefficient tester (torque axis meter) developed in recent years are important advances in this field. With the help of electronic technology and sensor application, the digital display electric wrench can display the final tightening torque of the high-strength bolts that are applied one by one when the screw is applied, and store them in the number. In the construction process, the over- or under-screw can be visually determined, which is convenient for timely processing. After the construction is completed on the same day, the final torque data stored on the day can be adjusted for further inspection and analysis. The torque coefficient tester can simultaneously display the output torque of the wrench and the axial force of the calibration bolt used when calibrating the electric wrench, which is convenient for calculating the torque coefficient, and can correct the tightening torque according to the temperature and humidity changes. In terms of inspection equipment, new equipment such as electric torque detection wrenches and portable torque testers have been developed. The electric torque detection wrench has a built-in high-precision torque sensor and angle sensor that automatically displays and stores the test results, and can also import data into the computer for analysis. More importantly, the AC servo motor used in this type of wrench can be started at a very low speed. By setting it, it can stop and display the output torque in time after turning a small angle (such as 0.5 ° ~ 1 °). The buckle torque check has been tested on some large bridge projects [8, 14]. The portable torque tester has a built-in 20 times torque amplifier, which can visually display the bolt torque detection value and store it in real time. The nut angle is precisely controlled by the built-in angle sensor to automatically set the angle of the alarm, which improves the accuracy of the inspection result and reduces the artificial The influence of factors has reduced the labor intensity and is expected to improve the current difficult situation of high-strength bolt inspection work. 2. 8 Ordering and packaging The side of the high-strength bolt connecting the auxiliary washer is provided with a 45° chamfer at the inner diameter to cooperate with the transition arc under the bolt head to reduce the stress concentration, so it must not be reversed during installation; when measuring the torque coefficient, It is stipulated that the side of the washer with the inner chamfer should face the nut support surface, so it must not be reversed during installation to maintain consistency. However, since the outer edge of the gasket is not marked, it is impossible to judge whether the gasket is reversed after the bolt is screwed. If the outer edge of the inner corner of the gasket is permanently marked, it is convenient to check and eliminate the gasket. phenomenon. Regarding the length of bolt repair, the dimension table given in "High-strength large hex head bolts for steel structures" (GB / T 1228-2006), according to 5 mm, when l ≤ 100mm, 10 mm when l > 100 mm In the sub-file, the provisions of Section 3. 0 of the “Construction Regulations†regarding the calculation formula of the bolt length and the provisions of the contract are in accordance with it. According to the experience of the construction of steel girders on high-speed railways in recent years, there may be more than ten or even more than 20 kinds of high-strength bolts of the same diameter on a bridge, and different positions of the same bolt joints may also have different lengths. High-strength bolts of the same diameter are easy to cause construction confusion and increase management difficulty. Most on-site construction personnel recommend that the length of high-strength bolts be uniformly adjusted by 10 mm. For bolts above M20, since the thread length exceeds the thickness of the nut and washer by more than 20 mm, the bolts selected according to the 10 mm bin are technically conditional and can be realized under the existing product standards. Regarding the marking of the top surface of the bolt head, "Technical conditions for high-strength large hex head bolts, large hex nuts and washers for steel structures" (GB / T1231-2006) only require the performance level of the mark and the manufacturer, "high strength for steel structures" The example of the mark given in the large hexagon bolt (GB / T 1228-2006) includes the thread specification and the nominal length of the bolt, but due to the inconsistency between the two regulations, some product identification is incomplete, and the construction unit can request the manufacturer to be in the bolt hexagon. The nominal length is indicated on the head for identification and use. In addition, the high-strength bolts produced by the immersion oil and saponification processes after phosphating have the opposite reaction of the torque coefficient to the humidity change, which causes troubles in the calibration and adjustment of the wrench. It is recommended to use the high-strength bolts of the same project. The same surface treatment process. 3 Conclusion High-strength bolts are important members of steel-structured bridges, and their quality should be guaranteed while ensuring product performance. The Regulations for the Construction of High-strength Bolt Connections for Railway Steel Bridges (TBJ214-92) issued in 1992 played an important role in regulating the construction of high-strength bolts for railway steel bridges and ensuring the quality of the connection. 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