Load & Strength Guide
Understand fastener strength ratings and load capacities for safe installations. Learn about bolt grades, tensile vs shear loads, and safety factors for structural applications.
Quick Answer: Why Load Capacity Matters
Using fasteners with inadequate load capacity can lead to catastrophic failure. Understanding strength ratings ensures your fasteners can safely handle the forces they'll encounter. Key factors include:
- Tensile strength - Resistance to pulling/stretching forces
- Shear strength - Resistance to sliding/cutting forces
- Proof load - Maximum load without permanent deformation
- Safety factor - Built-in margin for unexpected loads
- Grade markings - Visual indicators of strength ratings
General Rule: Always use fasteners rated for at least 2-3 times your expected load (safety factor). When in doubt about structural applications, consult a licensed engineer.
Understanding Bolt Grades
Steel bolt grades indicate tensile strength and load capacity
Grade 2
STANDARD STRENGTHThe most common low-carbon steel fasteners for general purpose applications. No head markings.
Specifications:
- Tensile Strength: 60,000 psi
- Proof Load: 33,000 psi
- Material: Low-carbon steel
- Head Marking: None
Best For:
- Non-structural applications
- Light-duty assemblies
- Cost-effective projects
- Non-critical connections
Grade 5
MEDIUM STRENGTHMedium-carbon steel, heat-treated for higher strength. Three radial lines on head.
Specifications:
- Tensile Strength: 120,000 psi
- Proof Load: 85,000 psi
- Material: Medium-carbon steel
- Head Marking: 3 radial lines
Best For:
- Automotive applications
- Structural connections
- Machinery assembly
- Most general applications
Grade 8
HIGH STRENGTHMedium-carbon alloy steel, quenched and tempered for maximum strength. Six radial lines on head.
Specifications:
- Tensile Strength: 150,000 psi
- Proof Load: 120,000 psi
- Material: Alloy steel
- Head Marking: 6 radial lines
Best For:
- Critical structural joints
- High-stress applications
- Racing/performance vehicles
- Heavy machinery
Fastener Strength Comparison
Quick reference for common fastener grades and materials
| Grade/Material | Tensile Strength (psi) | Proof Load (psi) | Head Marking | Typical Use |
|---|---|---|---|---|
| Grade 2 | 60,000 | 33,000 | None | General purpose, non-critical |
| Grade 5 | 120,000 | 85,000 | 3 lines | Automotive, structural |
| Grade 8 | 150,000 | 120,000 | 6 lines | High-stress, critical |
| 18-8 Stainless | 90,000 | N/A | None or "A2" | Corrosion resistance |
| 316 Stainless | 90,000 | N/A | "A4" marking | Marine, chemical |
| Aluminum | 38,000-58,000 | Varies | Varies | Lightweight applications |
| Brass | 50,000-75,000 | N/A | None | Marine, decorative |
Tensile Strength Comparison — Imperial Grades vs Metric Classes
| Imperial Grades (PSI) | Metric Classes (MPa) | ||||
|---|---|---|---|---|---|
| Grade | Strength | Category | Class | Strength | Category |
| L9 | 180,000 | High Tensile | 12.9 | 1,200 | High Tensile |
| SAE Grade 8 | 150,000 | High Tensile | 10.9 | 1,034 | High Tensile |
| SAE Grade 5 | 120,000 | High Tensile | 8.8 | 827 | High Tensile |
| Stainless Class 80 304 / 316 |
116,000 | Medium | Stainless Class 80 304 / 316 |
800 | Medium |
| Stainless Class 70 304 / 316 |
101,000 | Medium | Stainless Class 70 304 / 316 |
700 | Medium |
| Stainless Class 50 No Markings |
72,000 | Standard | Stainless Class 50 No Markings |
500 | Standard |
| Low Carbon | 62,000 | Standard | — | 427 | Standard |
| — | 58,015 | Standard | 4.6 | 400 | Standard |
Understanding Load Types
Different forces require different considerations
Tensile Load (Tension)
Force that pulls or stretches the fastener along its axis. This tries to pull the fastener apart lengthwise.
Common Examples:
- Bolts holding a suspended load
- Lag screws pulling away from wood
- Anchor bolts in concrete
- Tie-down applications
Key Considerations:
- Use tensile strength rating
- Proper torque is critical
- Thread engagement matters
- Preload affects capacity
Shear Load (Shearing)
Force perpendicular to the fastener's axis that tries to cut or slide the fastener. Often occurs in lap joints.
Common Examples:
- Bolts in structural connections
- Pins in hinges
- Clevis pins
- Lap joint fasteners
Key Considerations:
- Shear strength ≈ 60% tensile
- Single vs double shear
- Bearing on materials
- Bolt diameter critical
Safety Factors & Design Loads
Building in margins for safety and unexpected conditions
⚠️ What is a Safety Factor?
A safety factor (or factor of safety) is the ratio between a fastener's strength and the actual load it will experience. It accounts for unexpected loads, material variations, installation errors, fatigue, and corrosion.
Formula: Safety Factor = Ultimate Strength ÷ Working Load
Static Loads (2:1 to 3:1)
For steady, unchanging loads with controlled conditions. Example: Shelving, non-moving equipment.
Dynamic Loads (4:1 to 6:1)
For changing, repeated, or impact loads. Example: Machinery, moving equipment, vehicles.
Critical Applications (8:1 to 10:1)
For life-safety or catastrophic-failure scenarios. Example: Lifting equipment, overhead installations.
Building Codes (Varies)
Always follow applicable building codes and standards. They often specify minimum safety factors.
Best Practices for Load-Bearing Applications
Essential tips for ensuring safe, reliable fastener performance
1. Verify Fastener Grade
Always check head markings to confirm you're using the correct grade. Counterfeit or mislabeled fasteners are a real concern. Purchase from reputable suppliers and inspect markings carefully.
2. Use Proper Torque
Under-torquing reduces load capacity; over-torquing can strip threads or break fasteners. Use a torque wrench and follow manufacturer specifications for critical applications.
3. Ensure Adequate Thread Engagement
The bolt should engage at least 1.5 times its diameter into threads. For example, a 1/2" bolt needs at least 3/4" of thread engagement for full strength.
4. Consider Fatigue Life
Repeated loading cycles reduce fastener strength over time. For dynamic loads, use higher grades and larger safety factors. Inspect and replace fasteners periodically in critical applications.
5. Account for Environmental Factors
Corrosion, temperature extremes, and chemical exposure can reduce strength. Use appropriate materials and coatings, or increase safety factors in harsh environments.
6. Use Washers Appropriately
Washers distribute load and prevent embedment into soft materials. Hardened washers are essential under Grade 8 bolt heads and nuts. Never omit washers in structural applications.
7. Match Fastener to Application
Don't use high-strength fasteners everywhere - they're more brittle and expensive. Grade 5 is sufficient for most applications. Reserve Grade 8 for truly high-stress situations.
8. Consult an Engineer for Structural
For load-bearing structural applications, building modifications, or life-safety scenarios, always consult a licensed structural engineer. They can perform proper calculations and specify appropriate fasteners.
Frequently Asked Questions
Common questions about fastener strength and load capacity
Need Help With Load Calculations?
Our fastener experts can help you select the right grade and quantity for your specific load requirements. Get professional guidance for structural applications.