M20 bolt inspection coefficient allowable value plug = 213, set γ0=110, M20 bolt bearing capacity design value Nd plug = Fu plug Πγ0 rod = 122kN. The design bearing capacity of the steel pipe fittings with M20 bolts should meet the Nd rod ≤122kN. The steel pipe with the specification of <60×315 is selected, the tensile force σs=19615NΠmm2, and the tensile design strength of the A3 steel pipe material is 210NΠmm2. For A3 steel <60×315 steel pipe, Fu rod = rod Nd bolt = 118×122=21916kN, the average value of the test failure load of the steel tube base material is Fu rod=206kN, which is close to the calculated value of 21916kN, because the Fu bolt is much larger than the Fu rod. Therefore, when the tensile test is carried out, the base material of the steel pipe is destroyed without bolt breakage. If a steel pipe larger than <60×315 is used, the cross-sectional area of ​​the steel pipe increases under the action of the same Nd rod, and the steel rod member has a high rod, which easily leads to bolt breakage.
Similarly, the tensile test load value of the M16 bolt is Fu embossing = 185kN, and the matching value of the bearing capacity of the steel pipe member is Nd rod ≤ Nd bolt = Fu Î Î æ “ 0 bolt = 80kN, indicating that the bearing capacity design value Nd rod ≤ 80kN For the steel pipe rods, the M16 high-strength bolts can be used to select the steel pipe rods of <48×310 specifications. The test failure load value of the steel pipe rods of this specification is about 134kN, γ0u=1168, bolt ball The structure of the node grid structure can be calculated according to the Fu bolt of the bolt, and the Nd bolt is calculated by the Nd rod=Nd bolt. The maximum value of the design bearing capacity Nd rod of various steel pipe members is determined, and the design is matched with the bolt according to the design. Steel pipe cross section size.
Bolt ball node grid structure design should adhere to the principle of bolt ball screw hole and high-strength bolt with axial tensile test to achieve the bearing capacity of the inspection mark is the bolt to achieve bearing capacity, screw hole is not bad, that is, the bolt ball bearing capacity is higher than high Strength bolt bearing capacity, the "strong bolt ball weak bolt" between the bolt ball and the high strength bolt.
Sealing plate, cone head and steel pipe butt weld tensile test piece inspection mark is strong weld and steel pipe, the test piece steel base material is damaged, and also reflects the principle of "strong weld weak steel pipe" between weld and steel pipe. . The test failure load of the high-strength bolt should be greater than the test failure load of the connected rod. The high-strength bolt is the joint, and the relationship between the bolt and the connected rod should constitute a "strong joint weak rod". Strong bolt ball weak bolts, strong welded weak steel tubes, bolts in weak joints, welds and joints can be regarded as joints. When bolts and steel rods are used as rods, the three cases can be attributed. For strong nodes weak rods.
The design of the bolt ball node grid structure should abide by the design principle of "strong node weak rod" <3>. According to this principle, the grid structure is arranged according to the bearing capacity as bolt ball > high strength bolt > rod. The nodes consisting of bolted balls and high-strength bolts have sufficient safety reserves. When accidents occur, structural damage does not occur on the nodes or connectors.
The phenomenon that the bearing capacity of the bolt found in the bearing capacity test is smaller than that of the rod indicates that the design of the grid structure involved does not comply with this design principle. The designed bearing capacity of the rod is greater than that of the connected bolts, becoming a “strong rod, weak Node (connection). The damage to the grid will begin with the bolted joints.
According to the statistics in Table 1, the <48×315 and <60×315 steel pipe rods are equipped with M20 bolts. In the tensile test, the base material is subjected to tensile yield failure, which conforms to the design principle: the rods are damaged and the bolts are intact. <48×315 and <60×315 steel pipe rods are equipped with M16 bolts and <7515×3175 steel pipe rods are equipped with M20 bolt tensile test pieces. The bolts are broken and the steel base metal is intact, which does not meet the design principle.
Reliability analysis of high-strength bolts and steel pipe members When there are only two basic variables of load effect and structural resistance and they are normally distributed, the reliability index of structural members can be calculated as follows: β=μR-μsσ2R+ Σ2s (2) where: β is a reliable index of structural members; μs, σs are the mean and standard deviation of structural member load effects respectively; μR, σR are the mean and standard deviation of structural member resistance, respectively. The load effect and tensile force of the steel ball rods and high-strength bolts of the bolt ball joint grid are random variables and are considered to meet the requirements of normal distribution.
Reliable indicators of damaged steel pipe rods and bolts The bearing capacity design values ​​and test failure loads associated with steel pipe failure in Table 1 are respectively divided by the cross-sectional area of ​​the corresponding steel pipe members to obtain the design stress and measured stress of the steel pipe. The design stress is used as the load effect, and the measured stress is the structural resistance. The average design stress of the steel pipe is calculated as μRg=18013MPa, the standard deviation σRg=2013MPa, the average measured stress of the steel pipe is μsg=36717MPa, and the standard deviation σsg=4910MPa. From the formula (2), the reliability index βg=4109 of the broken steel pipe member is calculated.
Similarly, the bearing capacity design value and the test failure load associated with the bolt failure in Table 1 are respectively divided by the cross-sectional area of ​​the corresponding bolt thread, and the bolt design stress and the measured stress are obtained, and the bolt design stress is used as the load effect, and the measured stress is measured. For the structural resistance, the bolt design stress average value μRL=64717 MPa, the standard deviation σRL=8214 MPa, the bolt measured stress average value μsL=113614 MPa, and the standard deviation σsL=3319 MPa. From the formula (2), the reliability index βL=5149 of the broken high-strength bolt is calculated.
Reliable index of undamaged steel pipe rods and bolts. Due to the element failure of the steel pipe members and high-strength bolts on the same tensile test piece, the above-mentioned reliable indexes βg and βL are not the steel pipe rods on the same test piece connected to each other. The data of the pieces and bolts are calculated and are not comparable. Therefore, reliable indicators of unbroken bolts and unbroken steel pipe members should be calculated.
Conclusion (1) In order to ensure the safety of the grid structure, before the construction of the bolt ball node grid structure, the bearing capacity test of the rod and the supporting bolts shall be carried out, and the inspection results must meet the relevant requirements <2>. (2) The high-strength bolts whose bolts and steel pipe sections are not matched with the test load are lower than the connected steel pipe rods, and do not meet the structural design principle of “strong node weak rodsâ€. (3) According to the reliability analysis, the reliability index of the bolts when the specimens are broken is less than that when the bolts are not damaged; the failure probability when the steel tubular members of the specimens are not damaged is far less than the failure probability of the steel tubular members. The analytical conclusions are in complete agreement with the statistical results in Table 1.
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