Non-preloaded bolted connections are widely used in steel structures, particularly in applications where high-strength clamping forces are not required. These connections rely on bearing resistance and shear capacity, making them suitable for low to moderate load conditions. In this blog, we explore the mechanical behavior, strength calculations, and best practices for designing non-preloaded bolted connections.
- What Are Non-Preloaded Bolted Connections?
✔ A non-preloaded connection uses standard bolts without applying tension preload.
✔ The load is transferred through bearing contact between the bolt shank and the hole surface.
✔ Common in secondary structures, access platforms, and light-duty connections.
- Load Transfer Mechanism in Non-Preloaded Bolts
✔ Shear Load Transfer
- The force is transmitted through the bolt cross-section, resisting shear failure.
- The strength depends on the bolt diameter, material grade, and number of bolts.
✔ Bearing Load Transfer
- The bolt presses against the hole edge, creating localized stress concentrations.
- Overloading can cause elongation or tearing of the bolt hole.
✔ Tensile Load Resistance
- In axial loading conditions, the bolt resists direct tension.
- Tensile stress must remain below the ultimate tensile strength (UTS) of the bolt material.
- Strength Calculation of Non-Preloaded Bolted Connections
- a) Shear Strength Calculation
VRd=fu⋅Abγm2V_{Rd} = \frac{f_u \cdot A_b}{\gamma_m2}VRd=γm2fu⋅Ab
Where:
- VRdV_{Rd}VRd = Shear resistance of the bolt
- fuf_ufu = Ultimate tensile strength of the bolt material
- AbA_bAb = Bolt cross-sectional area
- γm2\gamma_m2γm2 = Partial safety factor
- b) Bearing Strength Calculation
Fb,Rd=k1⋅d⋅t⋅fuF_{b,Rd} = k_1 \cdot d \cdot t \cdot f_uFb,Rd=k1⋅d⋅t⋅fu
Where:
- k1k_1k1 = Bearing coefficient (depends on hole clearance and material)
- ddd = Bolt diameter
- ttt = Plate thickness
- fuf_ufu = Ultimate tensile strength
- c) Tensile Strength Calculation
Ft,Rd=fu⋅Abγm2F_{t,Rd} = \frac{f_u \cdot A_b}{\gamma_m2}Ft,Rd=γm2fu⋅Ab
Where:
- Ft,RdF_{t,Rd}Ft,Rd = Tensile resistance of the bolt
- Advantages of Non-Preloaded Bolted Connections
✔ Simple and Cost-Effective Installation
- No special tightening procedures required.
✔ Easier Inspection and Maintenance
- Can be quickly disassembled and reassembled if needed.
✔ Good Load Distribution for Static Loads
- Suitable for non-dynamic, low-vibration environments.
- Limitations of Non-Preloaded Bolted Connections
✖ Not Suitable for High-Vibration Environments
- Loose bolts can lead to connection failure over time.
✖ Lower Fatigue Resistance Compared to Preloaded Bolts
- Repeated load cycles can reduce long-term stability.
✖ Bearing Stress Concentration
- Excessive loading can cause hole elongation and permanent deformation.
- Best Practices for Designing Non-Preloaded Connections
✔ Ensure Proper Hole Clearance → Excessive gap reduces shear strength.
✔ Use High-Quality Fasteners → Strength grade 8.8 or higher improves performance.
✔ Apply Anti-Loosening Mechanisms → Locking nuts or washers reduce bolt slippage.
✔ Follow Standardized Design Guidelines → Use ISO, DIN, or EN specifications.
- Non-preloaded bolted connections provide a simple and effective method for joining steel structures in low-stress environments. By following proper strength calculations, material selection, and installation techniques, engineers can ensure safe and reliable performance in structural applications.