Steel bridge maintenance methods

Abstract

To provide a maintenance method for a steel bridge capable of preventing the reoccurrence of a crack generated in a steel material not temporarily but for a longer period of time.SOLUTION: A maintenance method of a steel bridge suppresses the growth of the crack 110 by forming a through-hole 100 at the tip part of the crack 110 to the crack 110 generated in the steel material 90 of the existing steel bridge and includes a through-hole forming step of forming the through-hole 100 by projecting a projection material and imparting compressive residual stress to the peripheral edge and the inner peripheral surface of the through-hole 100.SELECTED DRAWING: Figure 3

Description

【Technical Field】 【0001】 The present invention relates to a method for maintaining a steel bridge that forms a through-hole at the tip of a crack in a steel material of an existing steel bridge to suppress the growth of the crack. 【Background Art】 【0002】 Existing steel bridges may undergo aging deterioration (fatigue) due to vibrations caused by the passage of vehicles, which are heavy loads, and changes in weather such as snow and wind, and cracks (fatigue cracks) may occur in the steel materials used. If such cracks are left untreated, their ends will grow, so it is necessary to take measures as early as possible once they are discovered. 【0003】 Several methods for dealing with cracks in steel materials have been proposed so far. The known ones are described below. 【0004】 There is a method (patch repair method) of preventing crack growth by applying a patch plate to cover the crack. This has a high repair effect and a high effect of preventing the recurrence of cracks. However, it takes a lot of labor to prepare a patch plate of appropriate size and fix it, and the maintenance cost also increases. 【0005】 There is also a rewelding method of removing the cracked part and covering the removed part by welding. However, such a configuration also makes the repair complicated and increases the maintenance cost. 【0006】 When the part where the crack has occurred is a welded part, there is a cutting removal method of removing the crack in the welded part by cutting. Such a configuration has a problem that the parts that can be dealt with are limited. In addition, since the strength of the removed part does not improve, there is a risk that the crack will occur again. 【0007】 There is also a peening method of closing the crack by performing peening treatment along the crack. Although it is necessary to prevent the scattering of dust, it can be expected to improve the fatigue strength of the steel material as well as deal with the crack. 【0008】 Furthermore, there is the stop-hole method, as disclosed in Patent Document 1, which temporarily inhibits crack growth by creating a hole (stop hole) at the tip of the crack. With the stop-hole method, a stop hole can be formed as soon as a crack is discovered, and it is a very useful temporary solution as it does not require much effort or cost. [Prior art documents] [Patent Documents] 【0009】 [Patent Document 1] Japanese Patent Publication No. 2020-7713 [Overview of the Initiative] [Problems that the invention aims to solve] 【0010】 However, the stop-hole method described above is only a temporary solution, and cracks often reappear from the stop holes within a short period of time. 【0011】 Therefore, the present invention aims to provide a steel bridge maintenance method that can prevent the recurrence of cracks that have occurred in steel materials not just temporarily, but over a longer period of time. [Means for solving the problem] 【0012】 The present invention relates to a steel bridge maintenance method for suppressing crack growth by forming through holes at the tips of cracks occurring in the steel members of an existing steel bridge, and is characterized by including a through-hole forming step of projecting a projection material around the tip of the crack occurring in the steel member to form the through hole in the steel member and applying compressive residual stress to the periphery and inner surface of the through hole. 【0013】 In this configuration, compressive residual stress is applied to the periphery of the through hole (stop hole) by the projection of the projection material, thereby effectively preventing the recurrence of cracks from the through hole. 【0014】 Furthermore, prior to the through-hole formation step, the process includes a guide hole formation step in which a guide hole of a predetermined diameter is formed at or around the tip of the crack so as to penetrate the steel material, and the through-hole formation step is configured such that a projection material is projected around the guide hole to enlarge the diameter of the guide hole and form the through-hole. 【0015】 This configuration prevents the projection efficiency from decreasing due to collisions between the projected material and the projected material during the through-hole formation process, as the projected material collides with the steel material and reflects. Furthermore, since the hole diameter expands along the guide hole, it prevents problems such as through-holes being formed in unintended directions or positions. 【0016】 Furthermore, it is proposed that the projection material be shot used in shot peening treatment for improving the fatigue strength of the steel material. 【0017】 This configuration allows for the effective application of compressive residual stress to the steel material. 【0018】 Furthermore, a configuration is proposed in which the process further includes a blasting step to prepare the surface of the steel material and a peening step to improve the fatigue strength of the steel material by performing shot peening, wherein the peening step includes the through-hole forming step. 【0019】 This configuration reduces the labor and cost of construction work related to the steel bridge maintenance method of the present invention. Furthermore, the inner surface of the through-holes drilled during the peening process does not require any finishing treatment, making the process even simpler. [Effects of the Invention] 【0020】 The steel bridge maintenance method of the present invention has the excellent effect of preventing the recurrence of cracks over a long period of time. [Brief explanation of the drawing] 【0021】 [Figure 1] It is a flowchart showing the procedure of maintenance painting of a steel bridge according to an embodiment. [Figure 2] It is a schematic explanatory view for explaining a circulating blasting device according to an embodiment. [Figure 3] It is an explanatory view showing the procedure for forming stop holes for cracks according to an embodiment, where (a) is the state before forming the stop holes and (b) is the state after forming the stop holes. [Figure 4] It is an explanatory view according to another embodiment, where (a) shows the state after forming a guide hole and (b) shows the state after expanding the diameter of the guide hole to form a stop hole. [Figure 5] In another embodiment, it is an explanatory view showing a longitudinal section of a steel material in which a guide hole is formed. 【Mode for Carrying Out the Invention】 【0022】 Hereinafter, embodiments that embody the maintenance method for a steel bridge according to the present invention will be described in detail. Note that the present invention is not limited to the embodiments shown below, and design changes can be made as appropriate. 【0023】 The maintenance method for the steel bridge of this embodiment is carried out as part of repainting (maintenance painting) for the purpose of maintaining an existing steel bridge. First, the maintenance painting of the steel bridge will be described. 【0024】 As shown in FIG. 1, as the procedure for maintenance painting of a steel bridge (steel structure), first, a scaffold is temporarily installed on the steel bridge that is the object of maintenance painting. At the same time, a dust-proof sheet is installed to prevent dust from leaking to the outside, the curing of non-painted parts is carried out, and preliminary preparations (S101) for installing a device for blasting treatment and shot peening treatment are carried out. 【0025】 Subsequently, the type and thickness of the old coating applied to the steel bridge, as well as the condition of the steel bridge, are investigated (S102). Based on the investigation results, the type of grit and shot to be used, the spraying speed, etc., are determined. The grit (non-spherical) and shot (spherical) to be selected are both specified in JIS Z 0311:2004. 【0026】 Then, using the grit determined in S102, a blast treatment is first performed (S103). Specifically, the paint film to be removed from the steel bridge is removed and the surface is prepared. This blast treatment constitutes the blast process in the present invention. Although the paint film removed, rust, etc., and used grit are generated as dust by this blast treatment, since a dustproof sheet is installed in S101, the dust does not leak to the outside and accumulates at the work site. 【0027】 Next, the blasting device that has been spraying the grit is loaded with the shot determined in S102 in place of the grit, making it possible to spray the shot. This process constitutes the shot replacement process. 【0028】 Then, the blast-treated areas where the surface has been exposed are subjected to shot peening using the apparatus with the replaced shots (S104). This process constitutes a peening process that improves the fatigue strength and stress corrosion cracking resistance of the surface. The used shots generated by this shot peening process are left at the work site and accumulate as dust, mixed with the used grit from S103. Incidentally, this shot peening process may be performed on all the surface areas that have undergone blast treatment, or it may be performed partially as appropriate around welded areas or in areas where strength is a concern. 【0029】 Subsequently, the surface of the substrate that has undergone the blast treatment and shot peening treatment is inspected (S105). This inspection includes not only visual inspection but also, for example, comparison with an ISO 8501 blast photograph album or roughness check using a surface roughness meter. This confirms whether any unremoved paint film remains or whether the roughness of the substrate surface is within specifications, and appropriate treatment is carried out for any insufficient areas. For example, in areas where blast treatment cannot be performed, surface preparation is carried out using hand tools. 【0030】 After the surface has been inspected, the final finishing coat is applied to form the final coating (S106). Generally, this coating is applied in multiple layers, such as a primer coat for rust prevention, an intermediate coat to protect the rust prevention coating, and a topcoat coat for the final finish. 【0031】 Once the painting work is completed, an inspection (S107) is performed. This inspection includes not only checking the film thickness after the paint has dried, but also, for example, checking the wet film thickness using a wetness gauge during the painting process. Furthermore, such inspections are performed not only after the topcoat, which is the final finish coat, but also during the undercoat and intermediate coat painting. 【0032】 Once the painting work is completed as confirmed above, the site is cleaned up (S108). Specifically, this involves collecting scaffolding and dustproof sheets, and removing the blast-shot peening spraying equipment, thus completing the maintenance painting. 【0033】 In addition, a dust collection process is performed along with the above procedure (S110). Specifically, the used grit generated in the blasting process (S103), the used shots generated in the shot peening process (S104), and dust containing peeling material, rust, etc. generated in each process are collected while being separated. 【0034】 Furthermore, since both the recovered used grit and used shot are iron (metallic) abrasive materials as specified in JIS Z 0310:2004, they do not break down like almandine garnet or steelmaking slag even when they collide with steel bridges during use, and are therefore reusable. In particular, those made from high-carbon cast steel can be reused about 600 times, making them very economical, and the amount of waste can be greatly reduced by separating them from peeled paint and other foreign materials. 【0035】 The following description will explain a circulating blasting system 1 as an example, which is capable of spraying grit and shots using a common device, and which can also recover and separate used grit, used shots, and peeled paint films. 【0036】 As shown in Figure 2, the circulating blasting device 1 includes a main unit 2 installed adjacent to the work site α of the steel bridge K to be worked on. Furthermore, the main unit 2 is equipped with a pressure hose 4, and a sprayer 3 is connected to the end of the pressure hose 4. Grit and shot are sprayed from the sprayer 3. The main unit 2 is also equipped with a suction hose 5, and the end of the suction hose 5 is positioned at the work site α. This allows dust consisting of used grit, used shot, and foreign matter including peeled paint and rust generated at the work site α to be sucked up via the suction hose 5. In addition, a dustproof sheet (not shown) is stretched over the work site α to prevent dust from leaking to the outside, and blowers and dust collectors are also installed as appropriate. 【0037】 As shown in Figure 2, the main body 2 of the circulating blasting device 1 has a grid hopper tank 10 and a shot hopper tank 20 arranged adjacent to each other. More specifically, the grid hopper tank 10 has the function of storing grid and used grid. The shot hopper tank 20 has the function of storing shot and used shot. Furthermore, the grid hopper tank 10 is connected to a grid pressurizing tank 11 for pressurizing the grid stored in the grid hopper tank 10 to the work site α. Similarly, the shot hopper tank 20 is connected to a shot pressurizing tank 21 for pressurizing the shot stored in the shot hopper tank 20 to the work site α. 【0038】 Furthermore, a dry compressed air supply means 30 is connected to the grid pressurizing tank 11 and the shot pressurizing tank 21 via a dry compressed air pipe 31. This dry compressed air supply means 30 consists of an air compressor and an air dryer for supplying dry compressed air. The dry compressed air pipe 31 is also equipped with a switching valve 32, which allows dry compressed air to be selectively supplied to the grid pressurizing tank 11 or the shot pressurizing tank 21, and allows shot to be loaded in place of grid for injection. 【0039】 Furthermore, the pressure hose 4 is connected to the grit pressure tank 11 and the shot pressure tank 21. With this configuration, the air pressure from the dry compressed air supplied from the dry compressed air supply means 30 causes grit or shot to be ejected from the injector 3 via the pressure hose 4, enabling blast treatment or shot peening treatment to be performed on the steel bridge K to be worked on. The circulating blast device 1 with the functions described above constitutes a blast-shot peening injection device. 【0040】 The dust containing foreign matter such as used grit, used shots, and peeled paint that has accumulated at the work site α is collected and sucked up from one end of the suction hose 5. The dust X sucked up by the suction hose 5 then reaches the sorting room 40. 【0041】 Furthermore, a dust hose 51 is attached to the sorting chamber 40, and a dust collection unit 50, which serves as a peeled coating recovery unit, is connected to the dust hose 51. In addition, an air suction device 60, which serves as a dust suction means, is connected to the dust collection unit 50. Therefore, the dust X can be sucked up by the air suction force of the air suction device 60. 【0042】 Here, a discharge section is provided downstream of the sorting chamber 40, and this discharge section is connected to the dust hose 51 and also to the air suction device 60 via the dust collection section 50. 【0043】 Here, the dust introduced into the sorting chamber 40 from the suction hose 5 moves at a predetermined speed due to the air suction force of the air suction device 60, and foreign matter such as peeled paint and rust, which have a low specific gravity, is discharged along with the airflow. The discharged foreign matter is introduced into the dust collection unit 50 via the dust hose 51 and accumulated in the dust collection unit 50, and is discharged into the waste bag 52 at the desired timing and processed as industrial waste. 【0044】 Furthermore, since the used grit and used shot are iron (metallic) abrasives, they have a high specific gravity. When subjected to a predetermined speed, they collide with a predetermined reflector, and based on the difference in the distance they bounce back, it becomes possible to separate the used grit, used shot, and foreign matter including peeled-off paint. The used grit thus separated is returned to the grit hopper tank 10 located below the separation chamber 40 and reused. Similarly, the used shot is also returned to the shot hopper tank 20 located below the separation chamber 40 and reused. In this way, the grit and shot can be continuously circulated and reused. 【0045】 Other designs can be modified as appropriate, in addition to the embodiments described above. For example, the switching valve 32 may be installed within the work site α so that worker H can switch it, or it may be remotely operable. The injector 3 and suction hose 5 may be separate or integrated (vacuum blast type). The dry compressed air supply means 30 may be provided separately for the grit pressurizing tank 11 and the shot pressurizing tank 21, respectively. The pressure hose 4 may be divided into one for grit and one for shot. Similarly, the injector 3 may also be divided into one for grit and one for shot. Furthermore, the separation chamber 40 does not need to be located directly above the grit hopper tank 10 and the shot hopper tank 20, and its position is not particularly limited. Also, the method for achieving an airtight space within the flow path 41 is not particularly limited. For example, the grit hopper tank 10 and the shot hopper tank 20 may be connected to each other while maintaining an airtight seal, or a dedicated tank for temporarily storing grit or shot may be provided to supply grit and shot to each hopper tank 10 and 20 at the desired timing. It is also desirable to be able to supply new grit or shot if the amount of grit or shot in the circulating blasting device 1 falls below a specified value. Furthermore, the main body of the device 2 may be mounted on a vehicle, for example, to make it movable. 【0046】 Furthermore, in this embodiment, a common projection material having spherical portions while also having corners and ridges may be used without separating the grid or shots. Conditioned cut wire shot can be proposed as an example of a common projection material. 【0047】 The main components of the present invention will be described below. 【0048】 If cracks are found in the steel material during the S102 (construction target survey) described above, stop holes are formed to address the cracks in parallel with the surface treatment of the steel material carried out in the blast treatment in S103 or the shot peening treatment in S104. 【0049】 In other words, when forming stop holes in S103, during blast treatment of the steel surface, at the location of crack occurrence, a grid is projected onto the end of the crack using the projection equipment performing the blast treatment to create a through-hole shaped stop hole. 【0050】 Furthermore, when forming stop holes in S104, during the shot peening treatment of the steel surface, at the location of crack occurrence, a shot is projected onto the end of the crack using the projection equipment performing the shot peening treatment to create a through-hole shaped stop hole. 【0051】 In either case, if finishing treatment such as deburring the edges of the stop holes is necessary after forming the stop holes, it shall be carried out as appropriate. 【0052】 In this way, the process of forming a stop hole consisting of a through hole by projecting a projection material and applying compressive residual stress to the periphery of the through hole corresponds to the through hole formation process in the present invention. 【0053】 The formed stop holes are subjected to compressive residual stress on the hole edges and inner surface, resulting in higher fatigue strength compared to conventional stop-hole methods and effectively preventing crack recurrence. 【0054】 The method for forming stop holes in this embodiment will be described below. 【0055】 As shown in Figure 3, the stop hole 100 is formed at the end of the crack 110 that has occurred in the steel material 90. 【0056】 To form the stop hole 100, any of the following may be used: grid, shot, or common projection material. 【0057】 Here, compressive residual stress is applied to the periphery of the stop hole 100 formed by the projection of grit, shot, or common projection material. This improves the fatigue strength of the steel material 90 at the periphery of the stop hole, effectively preventing the recurrence of cracks. 【0058】 As an example, a 6mm thick steel material (SS400) was prepared as a test specimen, and drilling was attempted using a grid. The test conditions were as follows: 【0059】 The projectile material, steel grid, is the same as that used in construction and conforms to JIS Z 0311:2004. Projection conditions Nozzle diameter 8mm Applied pressure: 0.7 MPa Projection distance 50mm 【0060】 Under the above conditions, a through-hole was formed in approximately 3 minutes. Furthermore, at a position 1 mm away from the formed through-hole, a maximum compressive residual stress of approximately -140 MPa was measured at a depth of 90 μm from the surface. It was confirmed that compressive residual stress was present up to a depth of 800 μm at this location. 【0061】 At positions 2 mm and 3 mm away from the formed through-holes, a maximum compressive residual stress of approximately -170 MPa was measured at a depth of 160 μm from the surface. At these locations, the compressive residual stress was no longer detectable at a depth of approximately 300 μm from the surface. 【0062】 Furthermore, when stop holes were created by projecting shots for shot peening as the projection material, no finishing treatment was required on the inner surface of the stop holes. In fatigue tests, cracks were observed at the edges of the stop holes at a stress range of 100 MPa and approximately 600,000 cycles. 【0063】 Furthermore, the same round-cut wire (particle size 0.8 mm to 1.0 mm) used in construction can be used as the projection material. 【0064】 As shown above, the present invention, which forms stop holes by projecting a projectile, has proven to be highly effective. Furthermore, by using shot for shot peening as the projectile, there is no need for finishing treatment on the inner surface of the stop hole, and the effect is high. 【0065】 Furthermore, the steel bridge maintenance method of the present invention may also include a guide hole formation step in which, before forming a through hole to serve as a stop hole, a guide hole smaller in diameter than the through hole is formed at or around the tip of the crack, penetrating the steel material. In this case, during the through hole formation step, the diameter of the guide hole is enlarged by projecting a projection material around the guide hole to create a through hole that serves as a stop hole. 【0066】 More specifically, as shown in Figures 4(a) and 5, a small-diameter guide hole 140 is first made through the end of a crack 130 in the steel material 120. This guide hole 140 is formed using a known drill or the like. Then, by projecting a projection material towards the guide hole 140, the diameter of the hole is enlarged, and as shown in Figure 4(b), the guide hole 140 is converted into a stop hole 150 as a through hole. 【0067】 Even in this case, compressive residual stress is applied to the edge of the stop hole 150, thereby improving fatigue strength. 【0068】 In this way, by forming the guide holes 140 in advance before forming the stop holes 150, it is possible to suppress the collision between reflected projection material and projected projection material, which would reduce projection efficiency. Furthermore, it is possible to prevent the stop holes 150 from being formed in an unintended direction or at an unintended location. 【0069】 In the steel bridge maintenance method of the present invention, compressive residual stress is applied to the edge of the stop hole, which improves fatigue strength and is expected to ensure safety over a long period of time rather than requiring only temporary repairs. The projection material used is preferably the same shot used in shot peening treatment for improving fatigue strength. 【0070】 In the above embodiment, the dimensions and shapes of each part can be freely selected as appropriate. [Explanation of Symbols] 【0071】 1. Recirculating blasting system 40 Separate room 50 Dust collection unit (removed paint film collection unit) 60 Air suction device (dust suction means) 90,120 Steel materials 100,150 stop holes (through holes) 110,130 cracks 140 Guide hole K steel bridge

Claims

[Claim 1] A steel bridge maintenance method that suppresses crack growth by forming through holes at the tips of cracks that have occurred in the steel materials of an existing steel bridge, The process includes a through-hole forming step in which a through-hole is formed around the tip of a crack that has occurred in the steel material by projecting a projection material, and compressive residual stress is applied to the peripheral and inner surfaces of the through-hole. A steel bridge maintenance method characterized by the following features. [Claim 2] Prior to the through-hole forming step, the process includes a guide hole forming step in which a guide hole of a predetermined diameter is formed at or around the tip of the crack so as to penetrate the steel material. The through-hole formation step involves projecting a projection material around the guide hole to enlarge the diameter of the guide hole and form the through-hole. The steel bridge maintenance method according to feature 1. [Claim 3] The aforementioned projectile is a shot used in shot peening treatment for improving the fatigue strength of the steel material. A method for maintaining a steel bridge according to claim 1 or claim 2. [Claim 4] A blasting process is performed to prepare the surface of the steel material, A peening step in which shot peening is performed on the surface of the steel material to improve its fatigue strength, It further includes, The peening process includes the through-hole forming process. A method for maintaining a steel bridge according to claim 1 or claim 2.

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