IS 3757:1985 is the Indian Standard (BIS) for high strength structural bolts. This standard specifies the requirements for high strength structural bolts of property classes 8.8 and 10.9 in the size range from M16 to M36. It details dimensions, material specifications, and mechanical testing requirements to ensure the reliability of these fasteners in structural friction-grip and bearing-type connections.
Specifies requirements for high strength structural bolts for use in steel structures.
Key reference values — verify against the current code edition / project specification.
| Reference | Value | Clause |
|---|---|---|
| Subject | High-strength friction-grip (HSFG) structural bolts | Scope |
| Property classes | 8.8 / 10.9 | Classes |
| Class 10.9 | UTS ≈ 1000 MPa, proof ≈ 900 MPa | Property |
| Use | Slip-critical / friction-grip connections | Application |
| Assembly | Matched: IS 3757 bolt + IS 6623 nut + IS 6649 washer | System |
| Tensioning | Turn-of-nut/torque per IS 4000, verified | IS 4000 |
| Read with | IS 4000 / IS 6623 / IS 6649 / IS 800 | Cross-ref |
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
IS 3757:1985 is the Indian Standard for High Strength Structural Bolts — Specification. It covers friction-grip type and bearing-type high-strength bolts used in structural steel connections — beam-to-column joints, splice connections, bracing, baseplates in industrial / commercial / institutional buildings.
Use it when: - Specifying structural steel connections in high-rise buildings, industrial structures, bridges, transmission towers - Designing slip-critical (friction-grip) connections — where fatigue or seismic load demands no slippage at service loads - Procuring high-strength fasteners for structural use — typically property class 8.8, 10.9, 12.9 (HSFG grade) - Auditing supplied bolts against IS 3757 / ISO 898-1 mechanical property requirements - Replacing failed fasteners in maintenance — match original property class for fit-for-purpose
Use with companion codes: - IS 4000:1992 — Code of Practice for High Strength Bolting in Steel Structures (the design + installation companion) - IS 1367 Part 1-22 — Technical supply conditions for threaded steel fasteners - IS 1364:2002 — Hexagon head bolts, screws and nuts of product grades A and B (general structural bolts) - IS 1363:2002 — Hexagon head bolts, screws and nuts of product grade C (lower precision) - IS 6649:1985 — Hardened and tempered washers for high strength structural bolts
Property class system (per ISO 898-1): - 8.8: UTS 800 MPa, yield 0.8 × 800 = 640 MPa (commonly used in structural connections) - 10.9: UTS 1000 MPa, yield 0.9 × 1000 = 900 MPa (premium structural; common HSFG) - 12.9: UTS 1200 MPa, yield 0.9 × 1200 = 1080 MPa (highest grade; usually for compact connections)
With HDG (hot-dip galvanizing), property class 12.9 is prohibited per IS 1367 Part 3:2002 due to hydrogen embrittlement risk.
Standard hex-head bolt assembly: - Hex head bolt — full-thread or partial-thread per design - Hex nut — same property class (8.8 grade nut for 8.8 bolt) - Washer — IS 6649 hardened+tempered HSFG washer; typically two per joint (one under nut + one under head)
Common sizes: - M16: 16 mm thread diameter; tightening torque ~250-300 Nm (class 8.8) - M20: 20 mm thread; torque ~500-600 Nm (8.8) - M24: 24 mm thread; torque ~870-1050 Nm (8.8); the workhorse structural size - M27: 27 mm thread (less common) - M30: 30 mm thread; for large baseplate / column connections - M36: 36 mm thread; for specialty / heavy connections
Property class markings (Clause 4 of IS 3757): - '8.8' marked on bolt head - '10.9' marked on bolt head - '12.9' marked on bolt head - Manufacturer's identification mark also required
Marking on nut: similar marking system (8 for class 8 nut, 10 for class 10 nut, 12 for class 12 nut).
Surface conditions (Clause 5): - Black — uncoated, as-forged surface (rust-prone; for interior use) - Galvanized (HDG per IS 1367 Part 3 or IS 4759) — for outdoor exposure - Phosphate + oil — interim corrosion protection for storage / transport
Property Class 8.8: - UTS minimum: 800 MPa - 0.2% Proof stress: 640 MPa - Elongation A: 12% minimum on standard tensile specimen - Reduction of area: 40% minimum - Hardness HBW: 200-300 (preferred 220-280)
Property Class 10.9: - UTS minimum: 1000 MPa - 0.2% Proof stress: 900 MPa - Elongation A: 9% minimum - Reduction of area: 35% minimum - Hardness HBW: 320-340 (preferred 325-335)
Property Class 12.9: - UTS minimum: 1200 MPa - 0.2% Proof stress: 1080 MPa - Elongation A: 8% minimum - Reduction of area: 32% minimum - Hardness HBW: 385-420
Chemical composition (typical, per ISO 898-1): - Carbon-manganese-silicon alloy steel - C: 0.20-0.55% (for medium-carbon steels used in class 8.8) - Mn: 0.50-1.65% - Cr, Mo additions for higher property classes (class 10.9, 12.9)
Tests required for acceptance (Clause 8): - Tensile test per IS 1608 on representative bolt from each batch - Hardness test per IS 1500 series; minimum 5 readings - Impact test (Charpy V) for high-stress applications - Macrographic examination of cross-section - Dimensional / thread profile verification per IS 6649 - Visual inspection for defects (decarburization, scale, cracks)
Friction-grip (slip-critical) joint installation — Clause 5 of IS 4000:
1. Surface preparation: - Mating surfaces of connection: clean of loose mill scale, paint, dust, oil - For friction-critical joints: mating surfaces must achieve specified friction coefficient (typically μ ≥ 0.45 for grit-blasted clean steel) - Mating surfaces can be untreated grit-blasted, or with red-lead-treated condition
2. Bolt tightening sequence: - Torque control method: apply specified torque (depends on class + size); typical M24 class 8.8: 750-870 Nm; verified by torque wrench - Direct tension indicator (DTI) method: per IS 4000 Annex; squeeze button under washer indicates tension achieved - Turn-of-nut method: tighten to snug-tight + specified angular rotation (typically half a turn for class 8.8); measures elongation rather than torque
3. Joint inspection: - Verify required tension achieved (sample re-test) - Check washer position (under nut) - No gap between mating plates after tightening - No paint / coating on contact face that compromises friction (for slip-critical joints)
4. Post-installation protection: - Galvanized bolts: no further protection needed (for typical environments) - Black bolts: apply zinc-rich paint or other corrosion-resistant coating to exposed thread / head
1. Using property class 4.6 or 5.6 for structural connections — these are general-purpose grades, NOT for high-strength structural applications. IS 3757 specifies 8.8, 10.9, 12.9 only.
2. Mismatched bolt-nut property class — using class 8 nut with class 10.9 bolt overstrains the nut. Always pair bolt + nut + washer of compatible grade.
3. Wrong tightening torque — over-torquing fractures the bolt; under-torquing gives insufficient friction. Use a calibrated torque wrench; follow IS 4000 torque tables.
4. Skipping the washer — washer distributes load + reduces friction-coefficient variability. IS 4000 mandates washer use; some sites omit them "to save time" — leads to early joint failure.
5. Re-using high-strength bolts — once tightened to high tension and then released, the bolt has been plastically strained. IS 4000 prohibits re-use of HSFG bolts. New bolts for every assembly.
6. Bolted joint without verification — sometimes the only verification is "contractor said they tightened it". Use systematic torque verification on a sample of bolts (typically 10% of every joint) — fast and cheap.
7. Mixing galvanized + bare bolts in same connection — galvanic difference + thread-friction differences cause uneven loading. Use uniform finish throughout one joint.
8. Stocking too long — high-strength bolts in coastal / damp storage develop surface rust; thread engagement compromised. Storage time should be < 6 months; protect with sealed packaging + dry storage.
9. Using HDG class 12.9 bolts — prohibited per IS 1367 Part 3 due to hydrogen embrittlement. Use mechanically galvanized or zinc-aluminium flake-coated 12.9 if galvanic protection needed.
10. No traceability of bolt batches — without manufacturer markings + IS reference + batch numbers, failure analysis is impossible. Mandate clear marking on every bolt + maintain procurement log.
IS 3757:1985 is 40 years old but functionally adequate — the underlying ISO 898-1 property class system is internationally stable, and IS 3757 is essentially the Indian adoption. Minor amendments since 1985 have refined acceptance test methods.
Indian fastener market: - Major manufacturers (Sundram Fasteners, Pranav Construction Systems, Lakshmi Machine Works, Punjab Fasteners): consistent IS 3757 / ISO 898-1 compliance, full traceability, premium pricing. Critical for high-rise / bridge / infrastructure projects. - Mid-tier manufacturers: variable quality. Pre-qualify with sample testing. - Imported fasteners (Chinese, Korean): widely available in Indian market; quality variable; check property markings and certificates.
Procurement reality for structural steel projects: - Class 8.8 M20 / M24 / M30 bolts: the workhorse for most Indian commercial / industrial structural steel work. Available from many suppliers; cost ~₹50-150 per bolt depending on size. - Class 10.9 bolts: for slip-critical connections, fatigue-sensitive applications (bridges, transmission towers, crane runways). Cost ~30-50% premium over class 8.8. - Class 12.9 bolts: specialty; for compact heavy-load connections. ~2× cost of class 10.9; restricted availability.
For specifying engineers: - For routine commercial building structural steel: class 8.8 black or HDG - For bridges, transmission towers, crane runways: class 10.9; HDG or zinc-aluminium flake; explicit slip-critical detailing per IS 4000 - For fatigue-loaded connections: class 10.9 minimum + DTI washers for tension verification - For seismic Zone IV / V high-rise: class 10.9 + slip-critical connections per IS 800
Quality assurance: - Demand manufacturer's certificate per batch including chemistry, mechanical properties, hardness, traceability - Random sample testing at independent lab (1-2 bolts per batch) - Visual + dimensional check at installation site - Documented torque values + bolt-tension verification at installation
Cost reality: high-strength bolts are 5-10% of structural steel project cost; getting them right is critical (joint failures cause structural collapse). Premium IS-3757-compliant supply is worth the extra cost over budget unbranded fasteners.
Future direction: BIS sectional committee MTD 5 has been considering IS 3757 revision to formalize newer property classes (e.g., 14.8 introduced in ISO 898-1:2009) and add explicit slip-critical connection coefficients. No public draft yet.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Tensile Strength (Min), Class 8.8 | 830 MPa (for diameters > 16mm) | 800 MPa | ISO 898-1:2013 |
| Yield Strength (Min), Class 8.8 | 640 MPa | 640 MPa (or 660 MPa for dia > 16mm) | ISO 898-1:2013 |
| Tensile Strength (Min), Class 10.9 | 1040 MPa | 1040 MPa | ISO 898-1:2013 |
| Yield Strength (Min), Class 10.9 | 940 MPa | 940 MPa | ISO 898-1:2013 |
| Elongation after fracture (Min), Class 8.8 | 12% | 12% | ISO 898-1:2013 |
| Bolt Head Marking | Manufacturer ID, 'HSFG', and Property Class (e.g., 8.8S) | Manufacturer ID, Property Class, and System ID (e.g., 'HR' or 'HV') | EN 14399-1:2015 |
| Associated Nut for Class 8.8 Bolt | Property Class 8 | Property Class 8 | ISO 898-2:2022 |