Introduction to the Analysis of Bolted Steel Splice Connections Under Fire

Bolted steel splice connections play a critical role in the continuity and integrity of structural steel members. During a fire, these connections are subjected to extreme thermal and mechanical demands that can significantly alter their performance. Understanding their behavior under elevated temperatures is essential for ensuring the fire resilience of steel-framed structures.

In this practical example, the beams, bolts, and steel plates are modeled as 3D parts. In the first stage, a heat-transfer and fire analysis is carried out through a Heat Transfer step. The results from this analysis are then imported into the static structural analysis to obtain the stresses and deformations of all parts under fire conditions. All modes of heat transfer, convection, conduction, and radiation, are included.

1. Importance of Splice Connections in Fire Conditions

Splice connections are commonly used in steel construction to:

• Join two lengths of steel members (e.g., beams, columns)
• Facilitate transportation and erection
• Provide continuity in structural systems

In a fire, the global and local stability of the structure often depends on the ability of these connections to maintain sufficient load-bearing capacity despite weakening materials and thermal expansion effects.

2. Effects of Elevated Temperature on Steel and Bolts

Fire exposure causes a significant reduction in mechanical properties:

• Steel members and plates experience reductions in yield strength, ultimate strength, and stiffness above ~400°C.
• High-strength bolts (e.g., grade 8.8, 10.9) lose preload and strength, with pronounced deterioration above ~300–400°C.
• Thermal expansion mismatch between connected parts can introduce additional stresses and lead to prying action or bolt shear amplification.

These changes directly influence connection behavior, failure modes, and overall stability.

3. Thermal and Mechanical Loading Interactions

Analysis must consider the combined effects of:

• Thermal gradients within the connection assembly
• Axial forces and bending moments are transmitted through the splice
• Slip behavior in non-preloaded (bearing-type) bolts
• Loss of bolt pretension in preloaded connections
• Thermally induced displacements, which may introduce additional moment or shear demand

Fire-induced forces can cause connections to shift from ductile to brittle behavior, depending on bolt type and plate configuration.

4. Common Failure Modes Under Fire

Key failure mechanisms include:

• Bolt shear failure (strength loss or increased demand)
• Bearing failure around bolt holes
• Plate yielding or buckling at high temperatures
• Tear-out and net-section fracture
• Loss of frictional resistance in slip-critical connections
• Failure due to combined axial and bending actions caused by thermal expansion

The mode of failure typically becomes more ductile at high temperatures but may still be sudden due to rapid strength degradation.

5. Analytical and Numerical Approaches

The analysis of bolted splice connections in fire involves:

a. Analytical methods

• Temperature-dependent material reduction factors (e.g., EN 1993-1-2, ASCE)
• Component-based modeling to estimate joint stiffness and capacity
• Simplified uniform-temperature models for preliminary checks

b. Numerical simulation (FEM)

• 3D thermomechanical models capturing complex interactions
• Finite element software (e.g., ABAQUS, ANSYS, SAFIR)
• Inclusion of contact behavior, bolt pretension, and nonlinear temperature profiles

c. Experimental testing

• Provides data on real failure modes
• Validates analytical/numerical models
• Identifies behavior not captured by simplified methods (e.g., progressive slippage, prying under fire)

6. Design Considerations for Fire-Resistant Splice Connections

To improve fire performance, designers may:

• Increase bolt diameter or number of bolts
• Use thicker plates to delay thermal deterioration

• Employ slip-critical connections with consideration for pretension loss

• Provide fire protection (intumescent coatings, insulation)
• Ensure redundancy to prevent disproportionate collapse

Conclusion

Complex interactions between thermal degradation of materials, connection geometry, and structural loading govern the behavior of bolted steel splice connections under fire. A comprehensive analysis, combining thermal modeling, material behavior, mechanical loading, and potential failure modes, is essential for designing connections that maintain structural integrity during fire events.