Code of practice for high-strength bolts in steel structures
1992 Edition

According to IS 4000, bolts must be tensioned to minimum values specified in Table 3 to ensure proper joint function. These minimum tensions correspond to proof load benchmarks from IS 1367 (Part 3). For example, M16 bolts of property class 8.8 require a minimum tension of 94.5 kN, while those of class 10.9 need 130 kN. The tensioning process involves either the part-turn method or direct-tension indication, with inspections at snug-tight and final tension stages. Torque wrenches mainly detect significant under-tensioning, ensuring bolts achieve sufficient clamping force to safely resist applied loads.

IS 4000 defines bearing-type joints as connections where fully tensioned high-strength bolts transfer load via shear action and bearing on connected components, allowing some slip under loading. In contrast, friction-type joints rely on bolts tensioned to create high clamping forces so that friction between contact surfaces transmits the entire load without slip. Bearing joints primarily use bolts as shear connectors, whereas friction joints utilize bolts to maintain clamping pressure preventing movement. This distinction is essential for design and detailing per the code.

IS 4000 permits two approved tensioning techniques: the part-turn method and direct-tension indication. The part-turn method starts with snug-tightening all bolts, marking bolt and nut positions, then rotating the nut further by a specified amount based on bolt size and length to achieve full tension. The direct-tension indication method involves tightening to snug-tight, then using a specialized device to reach and verify the minimum required tension. Torque wrenches are only suitable for detecting gross under-tensioning. These methods ensure bolts reach the necessary tension for structural safety.

IS 4000 permits oversize holes in bearing and friction-type joints provided hardened washers cover the holes, with a maximum hole diameter limited to the lesser of 1.25 times the bolt diameter or bolt diameter plus 8 mm. Short slotted holes are allowed in shear-type connections for both joint types with hardened washers; in friction joints, slots may be oriented in any direction, while in bearing joints, slots must be perpendicular to the load with no eccentric loading. Long slotted holes are allowed only in shear connections on alternate plies with special washers at least 8 mm thick, and friction joints require 33% more bolts if long slots are used. Standard hole diameter tolerances are bolt diameter plus 1.5 mm for bearing and plus 2.0 mm for friction joints.

IS 4000 advises inspection at two key stages: snug-tightening and final tensioning. For part-turn tensioning, inspectors verify the specified nut rotation from the snug-tight position using Table 4, with permanent marks on bolts and nuts to monitor rotation. For direct-tension indication, manufacturer's instructions must be followed to confirm minimum tension via the tension-indicating devices. Torque wrenches are recommended only for detecting major under-tensioning. Additionally, bolt extension can be measured with a displacement transducer to calculate tension using calibration curves or the provided formula, ensuring compliance with minimum tension requirements.