Passive Fire Protection | Cell Beams Guidance

Intumescent Fire Protection for Cellular Beams

What are cellular beams and what are their advantages?

Cellular beams are structural steel beams that are deeper than normal rolled sections and have holes cut into their webs. These holes allow the passage of services such as ducts and cables. This can often reduce the height required for each floor of a building and so improve the cost efficiency of a design.

The other main advantage of cellular beams is that the increased depth means that they are significantly stiffer than plain beams for no additional weight. In practice this means that the span of the beams can be much greater than conventional beams, often increasing column grid patterns up to 18 m and beyond. This improves the utilisation of floor area in buildings and also reduces the cost of the structure.

How have cellular beams been protected with intumescents in the past?

Cellular beams are essentially a clever refinement of the old castellated steel beams. Common practice in the fire protection industry was to use a rule of thumb known as the "20% rule" to protect for castellated beams, and by default cellular beams. This practice arose from limited fire tested carried out in the early days of applied fire protection which showed castellated beams exhibited a slightly higher heating rate than plain beams.

According to the 20% rule the section factor of the original parent section was used to calculate the thickness of intumescent required and then this was increased by a nominal 20% to allow for this empirical higher heating rate.

Why the 20% rule became outdated

Over the last 20 years fire testing has become far more sophisticated and expectations of accuracy and efficiency for passive fire protection products have become much higher. Rules of thumb are no longer adequate.

About 5 years ago it also became apparent that two key assumptions behind the 20% rule were at least partially incorrect. Firstly, cellular beams carry greater loads than castellated beams and so the critical temperatures in the webs of the beams are much lower. Therefore it is far more important that applied fire protection keeps this area of a cellular beam cool. Secondly, intumescent coatings behave differently to older, less sophisticated forms of applied fire protection. On the web of a conventional beam the expanded intumescent char generally fills the web and forms a continuous layer of protection. On a cellular beam this layer of char is broken up by the web openings and this increases the effective section factor of the remaining parts of the steel web (web posts).

In addition new fire testing showed that certain intumescent formulations could suffer from cracking and shrinkage on the web posts. On highly optimised cellular beam designs with very narrow web posts this could result in much higher steel temperatures, and therefore earlier structural failure, than predicted by the 20% rule.

As a result of this new work the Steel Construction Institute withdrew its support for the 20% rule and imposed a complex set of rules for determining the critical temperature of cellular beam web posts. They also created a set of generic data for intumescents that allowed all manufacturers to modify their plain beam data and calculate a coating thickness to protect most designs of cellular beams. Since, in the absence of much fire test data, it was necessary to be conservative these calculations produced extremely high thicknesses – in many cases 40 – 60 % higher than that for an equivalent plain beam. This document was known as RT983, later updated to RT1085.

Nullifire Fire Testing

Once the implications of the new knowledge about cellular beams became apparent Nullifire launched a fire test programme. Following a series of successful fire tests in the Nullifire furnaces independent testing was carried out at Warrington Fire Research Centre (WFRC).

These tests covered the Nullifire product range and created enough data to allow the SCI to write a specific report for Nullifire products on cellular beams. It was found that for Nullifire products the additional coating required to protect most cellular beams was around, or even less than, 20% in most cases. Only in rare cases, where the structural design of the cellular beam was quite extreme, for example with very narrow web posts or highly asymmetric heavy bottom flanges, was more than 20% required. In all cases the SCI assessment gave a substantially better outcome for Nullifire products than the generic solution published in RT983 and RT1085.

Association of Specialist Fire Protection

The ASFP Yellow Book is a primary reference tool in the specification of intumescent coatings and many ASFP members are intumescent manufacturers. Once it became apparent that the 20% rule was not adequately supported by test data and did not meet modern expectations the ASFP supported the SCI guidance document.

However, it was also clear to members that RT983 was flawed: most importantly it was based on fire test evidence on only one intumescent material when it is well known that the performance of intumescent coatings is highly product specific. In addition, no standard test method existed for testing cellular beams and the original SCI method needed refinement.

To deal with these issues the ASFP formed a technical working group to write a new standard for fire tests on cellular beams. This was intended to allow manufacturers to generate accurate, product specific test data for their products. The fire tests carried out by Nullifire at WFRC provided key information in the design of these tests. The ASFP test method was issued as a draft for development in September ‘04 and Nullifire and several other manufacturers have now tested some, or all, of their products to it.

Current Best Practice

There are now 3 main routes to determining an intumescent thickness for a cellular beam:

  1. Use the SCI RT1085 document to provide a conservative generic solution. To do this an approval for plain beams is required for the required time period of protection. This must be a multi-temperature analysis to cope with the lower critical temperatures for cellular beams.
  2. Use a product specific SCI document based on data from the ASFP test method. This will provide a more accurate, and therefore more competitive solution. This route is relies upon modifying a plain beam approval so a multi-temperature analysis is required.

    3. Both these methods require detailed information on each cellular beam as listed below

    • parent section or plate thicknesses
    • depth of expanded section
    • web post width, cell diameter and spacing
    • fire protection period

    In addition, to provide the best result and avoid making worst case assumptions:

    • steel grade
    • loading of the steel

    Best practice is to supply this information against each loading to allow checking that it is correct and to provide evidence that the calculations have actually been done.

    Route 3 is a recent addition from work carried out by Westok Ltd. and the SCI and is specific to beams made from hot rolled beams designed on the Westok software.

  3. By this new method each design of beam has critical temperatures for the web posts and bottom flange calculated by the design software. This information should be communicated by the beam designer and structural engineer on the steelwork drawings. It is then possible for Nullifire to use the information from the ASFP test results to calculate product specific critical temperatures and so arrive at a thickness of intumescent for each beam design. This method is the most highly optimised and will produce the most efficient result.

Nullifire will use route 3 where the information is available for Westok beams and route 2 in other cases.