Most field connections in a steel building are bolted, and most structural connections use high-strength bolts. These bolts are not ordinary fasteners - they are tightened to a controlled preload and inspected to a standard, because the safety of the connection depends on it. Knowing the grades, the installation rules, and what to check protects buyers from the most common site mistakes.
What Makes a Bolt "High-Strength"
High-strength bolts are made from heat-treated alloy steel with a much higher proof strength than ordinary bolts. They are identified by their property class - for example, grade 10.9 means a minimum tensile strength of 1000 MPa and a yield ratio of 0.9. In Chinese practice, high-strength bolt assemblies for steel structures are specified under GB/T 1231 (2024 edition); internationally, ASTM A325/A490 and EN 14399 families are common.
Preload: Why It Matters
Structural high-strength bolts are tightened to a specified preload (tension), which clamps the connected plates together. This clamping action is what makes the connection work:
In slip-critical connections, the load is transferred by friction between the clamped plates - the bolt must be at full preload.
In bearing connections, the preload still reduces joint slip and improves fatigue behavior.
A bolt tightened by "feel" or to the wrong torque is a bolt that does not do its job.
Installation and Inspection
Bolts are tightened in a sequence (from the center of the joint outward) and often in stages (50%, then full).
Preload is achieved by calibrated torque wrench, by the turn-of-nut method (a measured rotation from snug), or by direct tension indicators.
Torque wrenches are calibrated; the results are recorded on the erection checklist.
After tightening, the connection is inspected: proper seating, correct washers (hardened, under the nut), correct protrusion of threads, and no re-use of bolts that have been over-tightened.
Acceptance follows the project's standard - in China, GB 50205 covers bolted connection inspection; AWS D1.1 and EN 1090 apply on international projects.
Buyer's Checklist
Confirm the bolt grade and the tightening method are stated in the drawings or specification.
Ask how preload is verified (torque wrench, turn-of-nut) and whether the check is documented.
For slip-critical connections, verify the friction surfaces were prepared as specified (blast-cleaned, no paint on the faying surface).
Order spare bolts with the shipment - site loss and damage are common, and mixing bolt grades is never acceptable.
High-strength bolting is a controlled process, not a handyman job. A supplier that includes the bolt specification, the tightening method, and the inspection requirement in the contract is protecting the connections that hold the building together.