High-strength bolts are the core load-bearing fasteners in steel structure engineering, and their installation quality directly determines the safety and durability of the entire building. According to engineering quality notification data from the Ministry of Housing and Urban-Rural Development, more than 30% of structural safety hazards in steel structure projects originate from non-compliant high-strength bolt installation.
This comprehensive technical guide delivers a rigorous, end-to-end framework-encompassing strategic selection, pre-fabrication, on-site execution, and QA/QC acceptance-strictly aligned with the updated GB 50205-2020 code. As the industry benchmark for steel structure integrity, it serves as the definitive reference for international engineers and contractors to ensure uncompromising compliance with China's latest national quality standards.
Why Are High-Strength Bolts Used in Steel Structures?
Ultra-high load-bearing capacity: manufactured from high-strength steel, capable of withstanding enormous tensile and shear forces.
Friction-type force transfer: components are clamped together by pretension, and shear resistance is achieved through static friction at the faying surfaces, thereby preventing the bolt shank from being directly subjected to shear and bearing deformation.
Seismic and dynamic load resistance: under repeated alternating loads such as wind or seismic action, the bolts are not prone to loosening and exhibit good energy dissipation and fatigue resistance.
Efficient installation: installation is quick, requires no complex on-site welding operations, and can shorten the construction schedule.
Ease of replacement: should damage occur or retrofitting be required, removal and replacement are highly convenient, facilitating long-term maintenance.
Classification and Selection Standards of High-Strength Bolt
High-strength bolts commonly used in steel structure engineering fall into two main types. Incorrect selection can directly lead to connection failure, so they must be strictly distinguished:
Friction-type high-strength bolts
The default type for industrial plants and public buildings, relying on friction at the contact surfaces of connected plates to transfer loads. They offer low deformation and good fatigue resistance, making them suitable for all dynamically loaded and seismic structural connections. Performance grades are classified as Grade 8.8 and Grade 10.9.
Bearing-type high-strength bolts
These allow slip at the contact surface and transfer loads through bolt shank bearing and shear resistance. They offer higher load capacity but greater deformation, and are only suitable for non-dynamically loaded, non-seismic secondary structure connections. They are strictly prohibited for primary structural load-bearing joints.
Selection core specification:
Bolt diameter
For primary structure connections, M20, M22, and M24 are preferred. M16 and smaller diameters are strictly prohibited for use in primary load-bearing joint connections.
Length selection
After tightening, the bolt should protrude 2–3 threads beyond the nut. Insufficient length cannot ensure clamping force, while excessive length increases material costs.
Matching requirements
Bolts, nuts, and washers must be supplied as a matched set from the same manufacturer. Mixed use of products from different manufacturers or different production batches is strictly prohibited.
Corrosion protection requirements
For outdoor and humid environments, hot-dip galvanized high-strength bolts are preferred. For standard indoor environments, black-finished bolts may be used.
Note:Grade 10.9 bolts must undergo a de-hydrogenation process during hot-dip galvanizing to prevent hydrogen embrittlement. For export projects, Dacromet coating is often a safer alternative.
Three Core Preparatory Tasks Before High-Strength Bolt Installation
Material receiving inspection and control
Each batch of high-strength bolts delivered to the site must be accompanied by complete quality certification documents. Witness sampling and re-testing shall be conducted in accordance with code requirements for the torque coefficient (heavy hex head type) or the pretension (twist-off type).
Bolts may only be used after passing the re-test. Bolts must be stored in a dry and ventilated warehouse. Once the original packaging is opened, the bolts must be used up on the same day. Open-air storage is strictly prohibited. Bolts that are rusted, contaminated with oil or grease, or have damaged threads must not be used.
Faying Surface Anti-Slip Treatment
The slip coefficient of the connection contact surface is a core performance indicator for high-strength bolted connections. Q355 steel requires a minimum slip coefficient of 0.45, while Q235 steel requires a minimum of 0.40. Blast cleaning and shot blasting are code-compliant treatment methods; manual wire brushing for rust removal is strictly prohibited.
Prior to installation, the faying surfaces must be thoroughly cleaned of paint, oil, and loose rust to ensure they are dry and clean. The application of any coating on faying surfaces is strictly prohibited. A slip coefficient test must be performed before full-scale installation commences; installation may only proceed on a large scale after the test results are deemed acceptable.
Construction Tool Calibration
On-site calibration shall be performed prior to construction, and the torque deviation must be controlled within ±5%. Torque verification shall be conducted both before and after each shift. The use of uncalibrated or overdue-for-inspection wrenches is strictly prohibited.
The dedicated electric shear wrench for twist-off-type bolts shall also be inspected on a regular basis to ensure that the spline end can shear off properly. The substitution of a standard wrench for the dedicated shear wrench is strictly prohibited.
Core Construction Procedures and Code Requirements of High-Strength Bolt
1.Bolt Hole Alignment and Installation Specifications
The bolt insertion direction shall be uniform throughout the entire project to facilitate subsequent quality inspection. Driving bolts into holes with a hammer is strictly prohibited to avoid thread damage and deformation of the connecting plate. Where hole misalignment occurs, reaming with a reamer is mandatory; gas cutting or arc burning of holes is strictly prohibited.
After reaming, the hole diameter shall not exceed the nominal bolt diameter plus 3.0 mm. The number of reamed holes at any single joint shall not exceed 25% of the total number of bolts at that joint. The reamed hole wall must be ground smooth and free of burrs, and all metal chips must be thoroughly removed before bolt insertion.
2.Tightening Sequence Requirements
The principle of "symmetrical tightening from the center of the joint outward" must be strictly followed. Tightening from the edges toward the center is strictly prohibited to prevent the connecting plate from bulging in the middle and to avoid incomplete contact at the contact surfaces.
All bolts at a single joint must complete the three stages of snug-tightening, re-tightening, and final tightening within 24 hours. Prolonged intervals between stages are strictly prohibited.
3.Torque Control Standards
Initial tightening
50% of the final tightening torque. The purpose is to bring the connecting plates into full contact and eliminate gaps.
Re-tightening torque
Equal to the snug-tightening torque. This step checks whether any bolts have loosened after snug-tightening and ensures uniform preload distribution across all bolts.
Final tightening torque
Calculated per the code formula Tc = K x Pc x d.
Upon completion of final tightening, heavy hex bolts shall be marked with paint to indicate final tightening status. For twist-off-type bolts, the spline end must be completely sheared off.
Note:
K→ torque coefficient
Pc→the design pretension force of the bolt
d→the nominal bolt diameter
4.Construction Environment Control
Construction is strictly prohibited during rain or when the faying surfaces are damp.
Construction shall be halted when the ambient temperature falls below -10°C.
During construction, applying paint to bolts is strictly prohibited, and bolts must not be contaminated by oil or dust.
In high-wind conditions, temporary securing of bolts must be implemented to prevent dropping.
5 Common Quality Issues and Solutions of High-Strength Bolt Installation
1.Torque Coefficient Nonconformity
Issue description: the torque coefficient dispersion of bolts from the same production batch is excessively high, making it impossible to ensure consistent clamping force.
Causes: bolt corrosion, oil contamination, storage exceeding 6 months, or intermixing of products from different manufacturers.
Solution: all bolts failing the re-test shall be rejected and removed from site. Bolts must be stored in their original sealed packaging and used up within 24 hours after opening. Intermixing of bolts, nuts, and washers from different production batches or different manufacturers is strictly prohibited.
2. Substandard Faying Surface Slip Resistance
Issue description: the slip coefficient test value falls below the design requirement.
Causes: insufficient rust removal grade, presence of paint or oil on the faying surface, or deformation of the connecting plate.
Solution: all affected surfaces shall be re-blasted to achieve the Sa 2.5 rust removal standard. Oil and loose rust shall be thoroughly cleaned using acetone. Deformed connecting plates shall be straightened or replaced. A new slip coefficient test must be performed, and construction may only proceed after the test results are deemed acceptable.
3. Under-Tightening, Over-Tightening, and Missed Tightening of Bolts
Issue description: the final tightening torque does not meet the specified requirements, and some bolts have not undergone final tightening.
Causes: uncalibrated torque wrench, non-standard worker operation, or absence of final tightening marks.
Solution: calibrate the torque wrench before each shift. Torque inspection shall be performed twice - at 1 hour and 24 hours after final tightening. Under-tightened bolts shall be re-tightened to the specified torque; over-tightened bolts shall be replaced with new bolts. Each bolt shall be marked with paint immediately after final tightening to eliminate missed tightening.
4. Incomplete Contact of the Connecting Plate
Issue description: gaps are present between connecting plates, and a feeler gauge can be inserted.
Causes: incorrect tightening sequence, deformation of the connecting plate, or presence of debris on the faying surface.
Solution: strictly follow the symmetrical tightening sequence from the center outward. Deformed connecting plates shall be mechanically straightened. Prior to installation, thoroughly clean all debris from the faying surfaces. Where the gap exceeds 0.5 mm, steel shim plates shall be inserted to fill the void.
5. Thread Damage and Bolt Fracture
Issue description: bolt threads are damaged, or bolts fracture during final tightening.
Causes: driving bolts into holes by force, substandard bolt material, or over-tightening.
Solution: hammering bolts is strictly prohibited; where hole misalignment occurs, ream with a reamer. All fractured bolts shall be replaced, and bolt material shall be re-tested. Final tightening torque shall be strictly controlled, and over-tightening is strictly prohibited.
6 Quality Acceptance Criteria for High-Strength Bolt Installation
Documentation Acceptance
The bolt material certification documents, re-test reports, slip coefficient test reports, torque wrench calibration records, and construction records shall all be complete and in order.
Visual Acceptance
The bolt insertion direction shall be uniform, with 2 to 3 threads exposed beyond the nut. There shall be no missed tightening and no loosened bolts. Final tightening marks shall be clear and complete.
Torque Acceptance
For heavy hex bolts, a random sampling of 10% of joints (with a minimum of 10 joints) shall be inspected. A torque deviation within ±10% is deemed acceptable.
Twist-Off-Type Bolt Acceptance
The spline-end shear-off rate shall reach 99% or above. Bolts whose spline ends have not sheared off shall be subjected to a torque inspection in accordance with the heavy hex bolt standard.
Faying Surface Acceptance
The connecting plates shall be in tight contact. When checked with a 0.3 mm feeler gauge, the insertion depth shall not exceed 20 mm.
Corrosion Protection Acceptance
Upon completion of final tightening, anti-corrosion paint shall be uniformly applied to the bolt heads, nuts, and connecting plate contact surfaces. There shall be no missed coating and no sagging.
High-strength bolt installation is a core quality control link in steel structure construction - there is no room for the slightest negligence. GNEE has established a full-process high-strength bolt installation quality management system. From material receiving, through on-site installation, to final acceptance, every stage strictly complies with national standards. Dedicated quality inspectors are assigned to provide full-time oversight throughout the process, ensuring the connection quality at every joint and safeguarding the structural safety of the building.
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FAQ:
What is a high-strength bolt?
High strength bolts are made from high-strength carbon steel or from tempered alloy steel. The high-strength materials tend to offering significantly higher tensile and shear capacity compared with ordinary carbon steel bolts (e.g., Grade 4.6)
Can High-Strength Bolts Be Welded?
No. In the vast majority of cases, welding high-strength bolts is strictly prohibited.
Reason: the high temperature generated by welding will alter the heat-treated microstructure and compromise the mechanical properties of the bolt that originally acquired through heat treatment, leading to a reduction in strength or brittle fracture.
Can High-Strength Bolts Be Reused?
Friction-type: generally, High-strength bolts that have been pretensioned shall not be reused (per AISC 360-16 and EN 1993-1-8) for reuse after their initial installation has achieved the specified pretension.
Specific code provision: bolts in tension-loaded connections or high-stress locations in particular shall be directly replaced with new ones after removal to prevent fatigue failure.