Has a structural engineer ever asked an architect "what limiting temperature should I design to?" Would the architect understand the question? Has a fire engineer ever told the structural team what the building requirements are likely to be, before they start their design? How much could we save on construction and consultancy fees if we did?
Intumescent is a term used to describe products that swell and expand, typically when exposed to the extreme heat generated during a fire. Intumescent coatings are designed to insulate timber or steel during a fire, and prevent their failure. In an industry flooded with technical products and confusing data sheets, the final product decision is often left to the applicators preference. So, what's to know?
To prevent steel failing, you have to keep the temperature down, below the 'limiting temperature' of the structural design. The structural engineer commonly sets this number between 550-750 degrees. Temperatures above the set figure could cause a loss of structural capacity, and possible failure. If they allow a higher limiting temperature, they may require a larger steel member to compensate? At the same time, heavier steel members require less intumescent coating to prevent them heating up, offsetting the cost of steel. This calculation of steel size vs heat is referred to as Hp/A or Steel factor.
Consider a column, it has a sectional area to absorb heat, and a perimeter that could be exposed to fire, it works like this:
The Higher the Hp/A factor, the thicker your intumescent coating will need to be to protect it. Traditionally this leads to textured paint finishes with the 'orange peel' effect. To avoid this appearance, you can design for a low Hp/A and/or specify modern, high preforming, thinner intumescent coatings, that can protect the steel at up to half the thickness of traditional systems.
How to find balance? A preliminary design meeting to discuss the aesthetic, structural and fire requirements of a project, can produce better and more cost-effective buildings. Simply ensuring a 'true' limiting temperature has been calculated could half the thickness and cost of intumescent coating.
With modern and environmentally friendly coatings now available in New Zealand, take some time to specify the right product, and achieve the best results.