Structural Bolt Details: Types, Grades, and Applications
Structural bolts are high-strength fasteners used to connect steel members in buildings, bridges, and industrial structures. They provide strong, reliable, and durable connections in structural steelwork, ensuring safety and stability under various loading conditions.
1. Types of Structural Bolts
a. Based on Strength & Application
| Bolt Type | Description | Common Standards | Application |
|---|---|---|---|
| Hex Head Bolts | Standard high-strength bolts with hexagonal heads | ASTM A325, A490, IS 3757 | General steel connections |
| Heavy Hex Structural Bolts | Thicker and stronger than standard hex bolts | ASTM A325, A490 | High-load steel structures |
| Tension Control (TC) Bolts | Pre-tensioned bolts with break-off splines | ASTM F1852, EN 14399 | Preloaded bolted connections |
| Turned & Fitted Bolts | Precision-machined bolts for exact fit | IS 4000 | Bridges, crane rails |
| Anchor Bolts | Embedded in concrete for securing columns | ASTM F1554 | Foundations, equipment fixing |
2. Structural Bolt Grades & Standards
| Standard | Bolt Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Application |
|---|---|---|---|---|
| ASTM A325 | Grade A325 | 830 | 660 | General structural steel |
| ASTM A490 | Grade A490 | 1040 | 900 | High-strength steel joints |
| ISO 898-1 | Grade 8.8 | 830 | 660 | Bridges, towers |
| ISO 898-1 | Grade 10.9 | 1040 | 900 | High-load structures |
| IS 3757 | Grade 8.8S, 10.9S | 830–1040 | 660–900 | Structural steelwork |
👉 A490 bolts should not be galvanized due to hydrogen embrittlement risk.
3. Structural Bolt Coatings & Finishes
- Hot-Dip Galvanizing (HDG) – Corrosion-resistant (Not for A490 bolts).
- Zinc Plating – Light-duty corrosion protection.
- Black Oxide – Aesthetic finish, limited corrosion resistance.
- Mechanical Galvanizing – Alternative for A490 bolts.
4. Structural Bolt Connection Methods
a. Bearing vs. Slip-Critical Connections
| Connection Type | Description | Application |
|---|---|---|
| Bearing Type | Load transferred by direct bolt contact | Standard steel framing |
| Slip-Critical | Load transfer via friction; requires pre-tensioning | Bridges, seismic structures |
b. Preloaded vs. Non-Preloaded Bolts
| Bolt Type | Tightening Method | Preloading Required? |
|---|---|---|
| Snug Tight Bolts | Standard tightening with wrenches | ❌ No |
| Fully Tensioned Bolts | Torque-controlled tightening | ✅ Yes |
| Tension Control (TC) Bolts | Special TC wrenches break spline at correct torque | ✅ Yes |
5. Structural Bolt Installation & Tightening
- Turn-of-Nut Method – Rotate nut by a specified angle after snug tightening.
- Direct Tension Indicator (DTI) Washers – Ensure proper bolt tension using compression indicators.
- Calibrated Wrench Method – Uses torque-controlled wrenches to achieve desired preload.
- Tension Control (TC) Bolts – Self-calibrated bolts that shear off once the correct preload is reached.
👉 Proper preloading prevents bolt loosening under dynamic loads.
6. Structural Bolt Hole Types
| Hole Type | Description | Application |
|---|---|---|
| Standard Hole | Slightly larger than bolt diameter | General structural joints |
| Oversized Hole | Larger than standard, requires washer | Adjustment and alignment |
| Short-Slotted Hole | Slot parallel to load direction | Allows slight movement |
| Long-Slotted Hole | Extended slot for large adjustments | Bridges, seismic applications |
7. Structural Bolt Testing & Quality Control
- Tensile Strength Test – Confirms load-bearing capacity.
- Hardness Test – Ensures proper material properties.
- Impact Test – Assesses resistance to sudden loads.
- Torque Testing – Verifies proper tightening for slip-critical connections.
8. Applications of Structural Bolts
✔ Steel Bridges & High-Rise Buildings
✔ Transmission Towers & Power Plants
✔ Industrial Structures & Warehouses
✔ Cranes, Rail Tracks & Heavy Equipment
✔ Offshore & Marine Structures
9. Conclusion
- Structural bolts are high-strength fasteners essential for load-bearing steel structures.
- Selection depends on grade, strength, coating, and connection type.
- Proper installation and tightening methods ensure structural stability and safety.
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