1 Introduction
High-strength bolts are commonly used for the connection, fixing, positioning, etc. of components in steel structures, and the two connected members are tightly clamped by the tightening axial force of the high-strength bolts. Most of the high-strength bolts in the project are in a frictional connection state. By applying a tightening axial force (also called pre-tension or preload) to the high-strength bolts, the connected steel plates are pressed and a frictional effect is generated. When the connecting node is subjected to an external force, the external force is transmitted by the friction between the contact faces of the connecting plates, and the stress flow is smoothly transmitted through the contact surface without stress concentration. Therefore, the friction type high-strength bolt connection requires roughening of the surface of the member, and the axial force of the bolt is required. The tightening axial force of the bolt and the anti-slip coefficient of the friction surface directly determine the bearing capacity of the friction type high-strength bolt connection.
There are two fastening methods for high-strength bolts for steel structure connection: torque method and corner method [1]. The torque method is to tighten the high-strength bolt connection by controlling the construction torque value, and the corner method is to tighten the high-strength bolt connection pair by controlling the relative rotation angle of the bolt and the nut. For the construction of two types of high-strength bolts, the large hexagonal high-strength bolt connection pair can use the torque method or the angle method. The torsion-shear type high-strength bolt connection pair is also the torque method, and the tightening axial force is not obtained. Torque wrenches are used, but are converted into slit diameter controls by the factory during manufacture.
The high number of high-strength bolts used in steel structures is of great value and the reliability and convenience of construction methods are of great value. Because the use of the corner method is relatively cumbersome, foreign studies have used the "direct pull indicator" to control the tightening axial force of high-strength bolts, and have been gradually applied to building steel structures, bridges, power stations, wind power equipment, petrochemical equipment and sports exhibitions. Venues and other aspects. In China, Shanghai Jinmao Tower and Hong Kong Bank of China Building also applied “Direct Pull Indicator†(Jin Mao Building: Size 1 1/2â€, 80,000 sets; Bank of China Building: Specification 1 1/8â€, 200,000 Set) [2].
2 Direct pull indicator and measurement method
2.1 Direct pull indicator
The Direct-Tension-Indicator (DTI) is a special washer with a series of projections concentric with the hole in one plane of the gasket, as shown in Figure 1. There are related DTI standards abroad [3-5].
The principle of applying DTI to control the tightening axial force in the high-strength bolted pair is shown in Figure 2. Tighten the nut. The pre-tension compresses the projection on the DTI through the matching flat washer. The gradually increasing pre-tension continuously compresses the projection, and the gap between the DTI plane and the flat washer is also smaller. Using the corresponding relationship between stress and strain, it can be judged according to the gap value whether the tightening axial force meets the design requirements.
The flat washer used with DTI is a high-strength bolted accessory washer. In addition to its original function, it has two new functions: 1 to ensure that all the projections of the DTI can be completely compressed; 2 its hardness is high The DTI is used to ensure that the reduction in clearance is due to the projection being compressed, rather than the flat washer at the point of contact with the projection being forced out of the pocket. Using the stress-strain relationship, the degree of compression of the projection is used to indicate the tightening axial force of the high-strength bolt. This is the working principle of DTI; how to know the size of the gap is the construction method of DTI. There are two common DTI construction methods: gap measurement and color plastic visual inspection.
2.2 gap measurement method
The gap measurement method uses a feeler gauge to measure the size of the gap. There is a gap in the circumference of the DTI (see Figures 1 and 3) that indicates where the feeler gauge is inserted.
Tests have shown that due to manufacturing variations (form, size, hardness, etc.), even if the specifications are the same, the relationship between the gaps of different batches of DTI and the pre-tension of the bolts is not the same. Therefore, before each batch of DTI is used, the relationship between the gap after the DTI compression and the pre-tension of the bolt must be verified in advance. During the construction, according to the measured gap size, the pre-tension of the bolt connection pair can be known by comparing the previously obtained test data of "bolt pre-tension to clearance distance". Obviously, loading must be stopped when measuring the gap with a feeler gauge. If the gap is large, continue to load; if the gap is less than the specified value, then the specific gap size should be measured with a different thickness of the feeler gauge to determine whether it is over-screwed. This intermittent construction process is difficult to accept, reducing its Practicality. This is the main reason why the "gap measurement method" has not been widely applied on high-strength bolts so far.
2.2 color plastic visual method
In view of the defects of the gap measurement method, a foreign company has improved the DTI. The main points are: 1 to set the recess on the back side of the DTI protrusion and the position corresponding to the protrusion; 2 to suppress the radial side next to the recess The groove allows the pocket to communicate with the edge of the DTI; 3 fills the pocket with a colored gel (see Figure 4a). When tightening the nut to compress the projection, the colored glue in the pocket is extruded from the edge of the DTI through the radial groove. When the extrusion amount reaches a certain scale (see Figure 4b), it indicates that the pre-tension of the bolt has met the requirements. .
This method also pre-tests the relationship between the amount of color gel extrusion and the bolt pre-tension in the laboratory, and the constructor must participate in the test so that they can remember the amount of color gel extrusion when the pre-tension of the bolt design is reached. Although this method eliminates the trouble of measuring the gap in the middle, it is not very reliable by the visual observation of the construction personnel. The construction quality depends entirely on the memory, observation, judgment and responsibility of the construction personnel. In addition, it is subject to manufacturing tolerances as well as the amount of colored gelatin filled. In addition, radial grooves are also a potential source of cracks. Test data from a laboratory in the United States indicates that there is a large error between the amount of extrusion of the colored gel and the pre-tension of the bolt. In fact, color plastic visual inspection is a controversial method in the United States, and has not been officially recognized by ASTM, but this law has obtained patents from the United States and other countries.
3 automatic control washer
In order to meet the needs of China's high-strength bolted joints to develop large-scale and high-corrosion resistance, we have carried out analysis and research on foreign DTI and its construction methods. After many trials and improvements, the trial production succeeded in “automatic control with independent intellectual property rightsâ€. "Automatic Control Washer" (ACW), patent number ZL201320888851.5. Test data shows that it can make the installation of high-strength bolts basically correct, reliable, simple and convenient. It can be said that it is an upgraded version of DTI.
3.1 Control principle
The form and size of the ACW and the principle of pre-tensioning of the control bolt are basically the same as those of the DTI, but the control method of the ACW is fundamentally different from the original method of DTI: whether it is the “gap measurement method†or the “color glue visual methodâ€. DTI relies on manual control; ACW uses a new "force control" control principle that automatically controls the pre-tension of high-strength bolts. The control principle of this will be briefly described below with reference to FIG.
Figure 5 assumes that an ACW has only three projections, and the form, size, and mechanical properties meet the standard requirements. The original height of the projection is h 1 . First, the height of the point B (called the "reference point") is compressed to h 2 by the pre-pressure F. Then, the point A and the point C (called the "control point") are simultaneously compressed to Point B has the same height h 2 and the required force must be 2F. This is the basic principle of automatically controlling the "force control force" of the gasket.
There may be manufacturing errors between each product, such as hardness and protrusion size. This only makes some small changes in h1 and h2, but the two forces must maintain a certain proportional relationship.
3.2 Ultra-thin sensor
So, how do you know that the "control points" (points A and C) are compressed to the same height as the "reference point" (point B)? We attach a test strip (ultra-thin sensor) on a “reference point†plane. In order to make the contact point of the attached test strip not higher than the pressing plane of the reference point, the “reference point†plane of the test strip is pasted. At the same time, a shallow groove corresponding to the size of the test strip is pressed, and the test strip is adhered to the groove (Fig. 6). When the "control point" is compressed to the same height as the "reference point", the detection strip captures this signal and outputs it to the control circuit of the construction tool that tightens the nut, and the construction machine stops working.
3.3 fastening process
The fastening diagram of the ACW is shown in Figure 7. The nut is tightened and the control point of the ACW is compressed until the flat washer contacts the test strip (ultra-thin sensor); the sensor captures the signal and outputs it to the control circuit of the construction tool through the wire to cut off the power of the construction tool and stop it.
With ACW, only the output torque of the construction machine (whether hydraulic wrench or electric wrench) is large enough, and there is no requirement for the accuracy of the output torque. In the past, the output torque of the electric wrench was not large enough, and the booster was needed, and the accuracy of the booster was not high, so the large-size bolts were rarely installed with an electric wrench. With ACW, the wrench only needs to be "unscrewed" and does not need to consider "over-screw" (because it won't over-screw).
In fact, the ACW can be thought of as a simple, quantitative pressure sensor, and once it is made, the pre-tension of the bolts that use it is determined. During construction, the operator only needs to install the high-strength bolt connection pair and ACW on the structure according to the general regulations, start the construction machine and reach the specified pre-tension, and the construction machine will stop automatically. It can be said that ACW enables manufacturers to participate in the installation procedure of high-strength bolts, together with the construction side to control the tightening axial force of high-strength bolts to jointly ensure the construction quality.
3.4 Combined structure
After the completion of the construction, there will be a gap between the DTI and the flat washer (between the protrusions). Although the gap is small, when it is used outdoors, rainwater will penetrate into the interior from the gap. Therefore, foreign regulations must be A nitrile rubber coating is applied around the DTI to seal the gap. In order to omit this step and make the installation more convenient, we also tested the combination of ACW and flat washers (Fig. 8), combining the flat washers in the high-strength bolting pair with the ACW. A non-metallic gasket is used instead of brushing the nitrile rubber coating.
4 test data
4.1 Tightening axial force and standard deviation
The tightening axial force was tested with the M30 high-strength bolting pair and the ACW component test specimen. The test data obtained in several sets are shown in Table 1.
For the tightening axial force of the high-strength bolt connection pair, China's national standard GB3632-2008 [6] has provisions: for the specification M30, the nominal value, minimum value and maximum value of the average value of the tightening axial force are 391 kN, 355 kN, 430 kN, the standard deviation of the tightening axial force is ≤ 35.5 kN. It can be seen from Table 1 that the average value of the tightening axial force is in the range of 391 to 426 kN, and the standard deviation of the tightening axial force is between 13.7 and 18.6 kN, which meets the requirements of national standards.
For the coefficient of variation of the axial force of the high-strength bolt connection pair, the old version of the industry standard JGJ82-91 [7] has provisions: for the specification M16 ~ M24, the coefficient of variation of the tightening axial force is not more than 10%. As can be seen from Table 1, the coefficient of variation is between 3 and 5%, ≤ 10%.
4.2 tightening axial force changes with time
The tightening axial force of the high-strength bolt connection pair will weakly decrease within a certain period of time after tightening, which is commonly referred to as the tightening axial force loss. Baosteel Seamless Steel Tube Plant measured the relaxation of the pre-tension of high-strength bolts [8]. The results are shown in Figure 9. The tightening axial force is the average of 10 bolts. It can be seen from Figure 9 that the tightening of the axial force is faster in the first 30 days, then becomes slower and eventually stabilizes. The initial value of the tightening axial force is 287.7kN, which drops to 284.8 kN after 1 day (down 1.0%), drops to 279.7kN (down 2.8%) after 32 days, falls to 278.7 kN after 49 days (down 3.1%), and falls after 354 days. To 276.5kN (down 3.8%). After 79 days (down to 278.0 kN, down 3.4%) the tightening of the axial force has been slow and finally stabilized.
Using the AC30's M30 high-strength bolted coupling, the loss of the tightening axial force is shown in Figure 10. The initial value of the tightening axial force is 412.5kN (tested at 13:40 on April 19, 2015); at 15 and 17 o'clock on the day, the tightening axial force drops to 411.5kN and 410.7kN, respectively. From April 20, 2015, the tightening axial force values ​​were read at 8:00 am (no break on April 30, May 1, 23, 24, June 6, and 7). The tightening axial force on April 20, 2015 was 408.1 kN, and the last point in the figure was the tightening axial force value of 402.6 kN on June 9, 2015. The trial is still ongoing.
It can be seen from Fig. 10 that the tightening axial force of the high-strength bolt connection pair using ACW drops rapidly in the first 10 days, and then becomes slow and tends to be stable. The tightening axial force decreased by 1.1% in the first 18 hours, decreased by 0.99% in the 25 days from April 20, and decreased by 0.33% in the 25 days from May 15th. The decline in force is getting slower and slower. In the 51-day test that has been carried out, the total axial force loss is 2.4% (in Figure 9, the loss of the tightening axial force after 3.1 days is 3.1%). Comparing the data of Figures 10 and 9, it can be seen that the tightening axial force loss of the high-strength bolting pair using ACW is comparable to the tightening axial force loss of the conventional high-strength bolting pair.
5 Conclusion
The researched ACW with automatic intellectual property rights is based on the control principle of “force control force†and the construction method using “ultra-thin sensorâ€, so that the construction of high-strength bolt connection pair is basically correct, reliable, simple and convenient. . The test results show that the average value, standard deviation or coefficient of variation of the bolt tightening axial force meets and exceeds the standard requirements, and the bolt tightening axial force changes little with time.
At present, we are still doing further tests, such as increasing the number of test strips and appropriately changing the shape of the protrusions, so that the pre-tension of the bolts is more accurate and the construction is more convenient.
In short, the application of ACW is not only to add a new construction method for high-strength bolting joints in China, but also to promote the development and application of large-size high-strength bolting joints and hot-dip galvanized high-strength bolting joints in China.
Acknowledgement: Shanghai Shenguang High-strength Bolt Co., Ltd. provided test conditions and assisted in the test. The author expressed his heartfelt thanks.
references
[1] JGJ82-2011 "Technical Regulations for High Strength Bolt Connection of Steel Structures". Beijing: China Building Industry Press, 2011
[2] TurnaSure's website, http://
[3] ASTM F959M – 13 “Standard Specification for Compressible-Washer-Type Direct Tension Indicators for Use With Structural Fasteners (Metric)â€
[4] ASTM F2437-14 "Standard Specification for Carbon and Alloy Steel Compressible-Washer-Type Direct Tension Indicators for Use with Cap Screws, Bolts, Anchors and Studs"
[5] EN 14399-9-2009 "System HR or HV-Direct tension indicators for bolt and nut assemblies"
[6] GB3632-2008 "Twisted shear type high strength bolt connection pair for steel structure". Beijing: China Standard Press, 2008
[7] JGJ82-91 "Design and Construction and Acceptance Procedures for High-Strength Bolt Connections of Steel Structures". Beijing: China Building Industry Press, 1991
[8] Zeng Yuanhe. 20MnTiB torsion shear type high-strength bolt pre-tension relaxation - Baosteel seamless steel pipe plant roof truss connection measured. Industrial Construction, 1993 (4): 21-26
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