Bolt Tightening and Torque Requirements

Bolted connections hold structural steelwork together — incorrect tightening can cause joint failure and structural collapse.
BS EN 1090 sets the standards for execution of steel structures, including bolt tightening and inspection requirements.
Non-preloaded bolts must be tightened to a snug-tight condition where the plies of the joint are in firm contact.
Preloaded (high-strength friction grip) bolts must be tightened to a specific minimum preload using controlled methods.
Torque wrenches, part-turn method, and direct tension indicators are the approved methods for achieving bolt preload.
Torque wrenches must be calibrated at specified intervals — an uncalibrated wrench does not deliver the correct preload.
All bolts in a connection must be tightened progressively in sequence — not fully tightened one at a time.
The bolt, nut, and washer must be the correct grade matched set — mixing grades produces unreliable connections.
Inspection of bolted connections must be carried out at the hold point before the joint is covered or painted.
Under-tightened bolts allow movement and fatigue failure; over-tightened bolts can shear or strip the thread.

Structural safety	Every bolt in a steel frame carries load — a single poorly tightened connection can cause progressive structural failure.
Controlled preload	Preloaded bolts must achieve a precise tension to develop the friction grip that holds the joint together under load.
Quality assurance	Inspection at the hold point confirms every connection meets specification before the structure is built upon or enclosed.

Do

Don't

Use the tightening method specified for each connection: torque, part-turn, or DTI.
Ensure torque wrenches are calibrated and within their calibration date before use.
Tighten all bolts in a connection progressively in sequence, not one at a time.
Use matched sets of bolt, nut, and washer of the correct grade for each connection.
Present bolted connections for inspection at the hold point before covering or painting.
Follow the bolt tightening sequence specified in the steelwork erection method statement.
Record all torque values and tightening records as required by the inspection plan.
Check that bolts are the correct length — too short gives inadequate thread engagement.
Apply lubrication to bolt threads as specified to achieve consistent torque values.
Report any bolts that cannot be tightened to specification to the structural engineer.

DON'T tighten bolts without following the specified method for the connection type.
DON'T use a torque wrench that is overdue for calibration or has been dropped.
DON'T fully tighten one bolt before snugging up all others in the connection.
DON'T mix bolt, nut, and washer grades — each set must be matched correctly.
DON'T cover, paint, or enclose bolted connections before hold point inspection.
DON'T deviate from the tightening sequence in the method statement.
DON'T fail to record torque values — they are mandatory quality records.
DON'T use bolts that are too short to achieve full thread engagement in the nut.
DON'T tighten dry bolts when the specification requires lubricated threads.
DON'T continue with connections that fail to achieve the required torque value.

See also: Steel Erection Safety | Working on Steel at Height

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