As energy efficiency regulations tighten globally, structural engineers are increasingly tasked with designing steel-to-steel connections that incorporate thermal break pads. These pads prevent cold bridging—where heat escapes the building envelope through continuous steel members (like external balconies connecting to internal floor beams).
However, introducing a non-metallic, compressible pad between two steel plates changes the fundamental behavior of the connection under Eurocode 3 (EN 1993-1-8).
The Shear Transfer Problem
In a standard steel connection, shear forces are transferred through the bolts bearing against the steel plates. When a thermal break is introduced, the low stiffness of the pad allows for micro-rotations and slip. The primary shear transfer mechanism shifts from pure bearing to a combination of friction and bolt bending.
Because the pad acts as a "soft" layer, the bolts undergo additional bending moments. The design shear resistance of the bolt ($F_{v,Rd}$) must be reduced to account for this. The reduction factor, often denoted as $\alpha_b$, depends on the thickness of the thermal pad ($t_p$) and the bolt diameter ($d$):
When calculating the shear resistance of the fasteners, we must verify that the applied shear force ($V_{Ed}$) does not exceed the reduced shear capacity of the bolt group combined with the frictional resistance provided by the preloaded bolts (if used).
Compressive Deflection and Prying Action
Under a bending moment ($M_{Ed}$), the compression zone of the end plate will squeeze the thermal pad. This localized compression creates a rotation in the connection, increasing deflection at the tip of your cantilever balcony.
Furthermore, in the tension zone, the prying forces acting on the bolts are amplified due to the flexible nature of the pad backing. When calculating the tension resistance ($F_{t,Rd}$) of the bolt row, it is crucial to use the modified T-stub method that accounts for the flexibility of the non-metallic layer.
Key Takeaways
- Always account for bolt bending: Do not use standard shear capacities. Thick pads (over 20mm) severely reduce shear transfer.
- Check additional deflection: The compressive strain of the thermal pad will add to the overall deflection of the connected member.
- Use preloaded bolts: To minimize slip, Category B or C slip-resistant connections are highly recommended when integrating thermal breaks.
Looking to speed this up? Our StrucTalogue EC3 connection tool automatically applies stiffness reductions when you toggle the "Thermal Break" option in your connection parameters.