Conference paper: Understanding hot-spotting and coat-backs in hollow steel sections under fire

Unprotected secondary steel members connected to protected primary steelwork are common in building structures, but the thermal behaviour of unprotected hollow sections at these connections remains less understood. With current coat-back recommendations largely informed by research into open sections, questions remain around how effectively these approaches limit heat transfer and localised hot-spot formation when hollow sections are introduced.

At the 14th International Conference on Structures in Fire, held in Kingston, Canada, BB7’s David Baron, Director of Advanced Engineering and Innovation, presented a conference paper, co-authored alongside Juan Martinez, Technical Associate Director at BB7, and Lefteris Koutsoloukas, Fire Engineer at BB7, exploring the effectiveness of current coat-back recommendations for hollow steel sections framing into protected primary steel elements.

The paper, Parametric Analysis of Hot-Spotting When Considering Coat-Backs to Steel Hollows Framing into Open Sections Under Fire Exposure, uses two- and three-dimensional finite element modelling with SAFIR to investigate localised hot-spot formation under ISO 834 fire exposure. Through a parametric comparison of hollow and I-sections, the research examines whether established approaches to coat-back adequately account for the different thermal behaviour of hollow steel sections.

Understanding the challenge

Current best practice typically recommends coat-back lengths in the region of 450–500 mm, with FABIG Technical Note 13 providing a default recommendation of 450 mm for hydrocarbon fire exposure and ASFP Advisory Note 21 recommending 500 mm for cellulosic fires. These approaches are widely established in industry practice.

However, the research underpinning these recommendations has predominantly considered open-section secondary members. Hollow steel sections introduce different thermal characteristics due to their closed geometry, potential internal radiation and altered conduction pathways at the connection interface. Despite this, current prescriptive guidance does not explicitly distinguish between open and hollow secondary members when considering coat-back requirements.

This creates uncertainty around whether established recommendations provide comparable protection when applied to hollow sections.

Investigating hot-spot formation under fire exposure

To investigate this behaviour, the research team carried out a parametric heat transfer study using the finite element software SAFIR, considering the temperature-dependent thermal properties of steel in accordance with EN 1993-1-2.

We developed both two-dimensional and three-dimensional models. The initial 2D analysis enabled investigation of the governing heat transfer mechanisms between the hollow secondary section and protected primary steelwork, before a 3D model was introduced to capture geometric effects not represented in the 2D analysis.

The study compared open I-sections and square hollow sections under equivalent conditions, including configurations with identical section factors. This enabled the researchers to investigate the influence of section geometry independently of section factor, before extending the analysis across different hollow section dimensions and coat-back lengths.

The researchers considered coat-back lengths of 500 mm, 1,000 mm, and 1,500 mm across hollow sections ranging from 100 mm to 400 mm in width. Each configuration was subjected to the ISO 834 fire curve for 120 minutes, with both average temperatures within the protected primary element and peak temperatures at the connection interface recorded for comparison.

This allowed the researchers to examine how section geometry and dimensions influence heat transfer into the protected primary element, as well as how changes in coat-back length affect both average temperatures and localised hot-spot formation.

The analysis identified important differences in thermal behaviour between the configurations investigated, raising questions around the application of existing coat-back recommendations to hollow steel sections.

Why this matters for structural fire engineering

Coat-back requirements can represent a significant proportion of the passive fire protection applied to a structure, with implications for cost, programme, weight and long-term maintenance. At the same time, insufficient protection at connections can result in localised heating of primary structural steelwork and potentially influence its performance during fire.

As hollow steel sections continue to be used across modern structural systems for their structural efficiency, aesthetics and constructability, understanding how their thermal behaviour differs from open sections is important when developing appropriate and proportionate passive fire protection strategies.

By applying advanced heat transfer modelling to configurations encountered in practice, the research contributes to an area that remains comparatively underexplored within existing coat-back guidance, while examining whether established industry approaches remain appropriate across different steel section types.

Download the full conference paper

David’s full conference paper provides a detailed discussion of the finite element modelling methodology, parametric analysis and resulting thermal behaviour observed across the different section types, sizes and coat-back lengths investigated.

To explore the findings in more detail, complete the form below to download the full paper: