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Specifications for the installation of bolts in steel structures
In steel structure engineering, bolt connection, as the core assembly method, its installation quality is directly related to the safety and durability of the building. The installation specifications for steel structure bolts are not only a set of technical standards but also the cornerstone for ensuring the quality of the project. According to the current code requirements, bolt connections must meet the three major indicators of strength, stiffness and stability. Any oversight in any link may lead to the risk of structural failure. For instance, when the preload deviation of high-strength bolts exceeds 10%, the bearing capacity of the node may decrease by more than 15%. The standard system forms a closed-loop control from material selection to construction techniques: bolts, nuts and washers need to form matching connection pairs, the friction surface treatment needs to reach the Sa2.5 grade rust removal standard, and the installation sequence follows the principle of diffusion from the center to the periphery. This refined requirement stems from the unique force-bearing characteristics of steel structures - when a group of bolts jointly bear the load, the installation error of a single bolt will amplify the overall impact through stress redistribution. Therefore, the seemingly strict restrictions in the specification, such as the hole diameter tolerance ±0.5mm and the final tightening torque ±10%, are actually safety thresholds that have been verified through a large number of engineering projects. In terms of material selection, the matching property of steel structure bolt connection pairs directly affects the performance of the nodes. The specification clearly stipulates that high-strength bolts, nuts and washers must be from the same batch of products, and their materials should comply with alloy steel standards such as 20MnTiB or 40Cr, with a strength grade of no less than 10.9. This strict matching can prevent the attenuation of preload force caused by differences in heat treatment processes. For instance, when bolts from different batches are mixed, the torque coefficient fluctuation may exceed the design upper limit of 0.15. For friction-type connections, the chamfer surface of the washer must be installed facing the conical surface of the nut. By increasing the contact area, stress can be dispersed. Experiments show that this measure can increase the anti-slip coefficient of the joint by 8% to 12%. For ordinary bolt connections, the principle of double washers should be followed: one flat washer should be set on each of the bolt head and the nut side. If anti-loosening function is required, spring washers can be added to the nut side, but the total number should not exceed three to avoid local stress concentration caused by the stacking of washers.It is worth noting that the specification particularly prohibits the mixed use of torque-shear type and large hexagon head bolts at the same node, as the preload control mechanisms of the two types of bolts are different. Mixing them will lead to uneven axial force distribution. Measured data show that the deviation can reach more than 20% of the standard value. The hole position treatment before bolt installation is a key preparatory process to ensure the connection quality. The specification stipulates that all bolt holes must be prefabricated in the factory using CNC equipment. On-site, only reamers or files are allowed for trimming, and the diameter of the reamed holes must not exceed 1.2 times the nominal diameter of the bolts. For instance, the maximum hole diameter of M20 bolts can only be expanded to 24mm. Exceeding this limit may reduce the thickness of the connecting plate, resulting in a decrease of approximately 18% in shear bearing capacity. For hole position misalignment caused by manufacturing errors, gas cutting for reaming is strictly prohibited. It is necessary to use a reamer to trim and remove burrs. Residual iron filings will significantly increase the risk of contamination on the friction surface. The temporary fixation links are equally strict: the number of installation bolts shall not be less than one-third of the total number of node bolts and no less than 2, and the usage of punching nails shall be controlled within 30% of the temporary bolts. This proportional control not only ensures the stability of component positioning but also avoids excessive occupation of hole positions by punching nails, which could affect the penetration of high-strength bolts. In actual construction, the progressive threading method of first threading the bolts at both ends and then supplementing the middle bolt is often adopted. This can reduce the forced knocking caused by the deviation of the hole position and increase the bolt threading qualification rate to over 98%. For inclined components, an operating space of 5° to 10° should also be reserved. Extended sleeves should be used to assist in threading to prevent damage to the bolt threads due to Angle restrictions. The bolt tightening process is a core technical link in the installation of steel structures. The specifications clearly require the adoption of a phased tightening method to ensure the uniform distribution of preload. For ordinary bolt connections, they should be tightened symmetrically from the middle to both ends. The tightening torque should be controlled by a torque wrench, and the exposed threads should be kept within the range of 2 to 3 turns. High-strength bolt connections are even more stringent and must follow a two-stage process of initial tightening and final tightening: the initial tightening torque should reach 60% to 80% of the standard axial force, and the final tightening should be completed on the same day. For large nodes, a re-tightening process is also required. Take the M20 high-strength bolt as an example. Its initial tightening torque is approximately 176.5N·m, and the final tightening torque needs to reach 294.2N·m, with a deviation not exceeding ±10%. The tightening sequence follows the principle of stiffness priority, that is, it extends from the most constrained part of the structure to the free end, and at the same node, it is tightened in a way that spreads from the center to the periphery. This sequence can effectively prevent axial force loss caused by steel plate deformation. Measured data shows that the axial force uniformity of the standard sequence is 35% higher than that of random tightening. For torque-shear type bolts, the final tightening is marked as qualified when the star-shaped head is removed. The number of bolts that remain unscrewed shall not exceed 5% of the total. In construction, the color marking method is often used to distinguish between initial tightening and final tightening bolts. For example, white is used to mark initial tightening and red to mark final tightening to prevent missed tightening. In areas with limited space, an extended socket wrench should be used for operation to ensure accurate torque transmission. The length of the socket generally should not exceed 1.5 times the diameter of the bolt. In the bolt connection of steel structures, the treatment of friction surfaces is a key link to ensure the anti-slip performance of nodes. The standards require that the contact surfaces must be kept dry and clean. Oil stains, rust or paint residues are strictly prohibited. Moisture-proof measures must be taken during construction in rainy days. The treatment methods for the friction surface include sandblasting (shot blasting), wire brush cleaning, etc. Its roughness should reach Ra=25-75μm, and the test value of the anti-slip coefficient must not be less than 1.13 times the design value.For instance, after Sa2.5 grade sandblasting treatment, the anti-slip coefficient of the friction surface can stably reach above 0.45, while when only cleaned with a wire brush, this value may be lower than 0.35. During construction, special attention should be paid to protecting the friction surface to avoid secondary pollution or damage. Any scratches or spatter of welding slag may lead to a 15% to 20% reduction in the bearing capacity of the node. For on-site welded nodes, the specification stipulates that the distance between the welding operation area and the bolt connection surface should be greater than 100mm to prevent the heat-affected zone from altering the performance of the friction surface. The acceptance stage is the last line of defense for the quality control of steel structure bolt installation. The standards require multiple inspections to be carried out at each key node. For high-strength bolt connections, torque spot checks should be conducted at 10% of the number of nodes. When using the torque method for detection, the deviation of the final tightening torque must not exceed ±10%. The acceptance of torque-shear bolts is directly marked by the breaking of the plum blossom head. The proportion of unbroken bolts should be controlled within 5%, and the exposed thread length should be maintained at 2 to 3 turns. The anti-slip coefficient test is a mandatory inspection item. Double friction test pieces are used for loading tests, and the average value must not be less than 1.13 times the design value. The minimum value of a single test piece must also not be less than the design value. For ordinary bolt connections, the key points to check are whether the gasket Settings comply with the double gasket principle, whether the bolt insertion directions are uniform, and use a feeler gauge to detect the gap of the connection plate. If the gap exceeds 1mm, an adjusting gasket needs to be installed. All acceptance data must be recorded in writing, including key information such as torque detection values, friction surface treatment conditions, and bolt tightening sequence, which will serve as the legal basis for project acceptance.