Reducing thermal bridging of a bolted connection - Part 1: stainless vs carbon steel fixings

Introducing a Thermal Break pad into a structural connection typically means introducing a bolted end plate. There are a few considerations (pad thickness, end plate location, fixing geometry) which we’ll look at in other posts but here we explore how to optimise thermal performance of the fixings in a thermally broken connection.

There are two ways to do this:

1. Use stainless steel bolts instead of carbon steel bolts (this post), and

2. Introduce Thermally Broken washers and bushes (Part 2 post to follow).

   
   

Stainless has a thermal conductivity approximately ⅓ of that of carbon steel. So simply switching from mild carbon to stainless bolts can significantly reduce heat transfer through a bolted connection, a reduction calculated to be 26-33% (Source: Larbi et. al, 2016). Thermal modelling from 2016 by another supplier of structural thermal breaks, Fabreeka (see images, source), illustrates this clearly.

Apart from thermal performance, there are other considerations that need to be taken into account before adopting stainless fasteners:

• Isolating the bolts from the plates to avoid bimetalic corrosion risk – especially in corrosive environments (thermally broken washers and bushes can tick this box),

• Same design rules apply as for a typical bolted connection with the correct material properties adopted,

• There are advantages of greater strain hardening effects for bolts in tension, and

• Incorporating design and construction methods to avoid galling. 

  
  

For more information refer to Eurocode 3 Part 1.4 and the SCI Design Manual for Stainless steel 4^th Edition, 2017