Steel is a non-combustible material that has high thermal conductivity, and therefore allows rapid temperature increase in fires. The increase in steel temperature reduces its strength and stiffness, which may cause structural deformations and collapse. At about 500°C, the yield stress and modulus of elasticity which are vital material properties of steel are considerably reduced to 50% – 60%. This implies that the strength of unprotected steel is unlikely to be sufficient to resist applied loads in fully developed or post-flashover fires. There are many options available to steel structural fire designers to achieve fire resistance; one way is to apply passive fire protection materials or products on the steel member. The following passive fire protection measures are discussed by Buchanan and Abu (2017) and Purkiss and Li (2013) as typical options applied to steel structures:
i. Board systems;
ii. Spray-on protection;
iii. Intumescent coatings;
iv. Concrete filling;
v. Brick or blockwork;
vi. Concrete encasement;
vii. Water filling;
viii. Flame shields; and
ix. Use of timber.
Blanket systems have also been described in the steel construction information document, (SCI, 2016) as a fire protection method. Another option is to optimise the structural design and leave the steel structure unprotected. In this review, the summarised descriptions of some applied fire protection options are presented. It is noteworthy that, the applied passive fire protection options and alternative structural fire design of unprotecting steel structures are
Board systems
Most board protection systems are made of gypsum plasterboards, rock fibre bonded by resins, calcium silicate, etc. Calcium silicate boards are made of inert materials and designed to protect the steel member without damage during fire exposure; while the insulation performance of gypsum plasterboards is heightened by the water of crystallisation existing in the material which dries up at elevated temperatures. Boards can be screwed or glued to framing to encase the steel. The key advantages of board systems are: there is no surface preparation needed for its application implying that dry fixing and quick ‘housekeeping’ is guaranteed; thickness is also guaranteed, and board systems can have good visual appeal. The disadvantages of board protection are: high-costs; may need highly skilled labour given the difficulty in fixing it around complex details, e.g. steel brackets; boards are rarely used for cellular steel sections, and as such, they have limited external applications.
Spray-on protection
Cement-based, vermiculite and mineral wool are common spray materials. They are low-cost passive fire protection products which are wet sprayed on steel structures as shown in Figure 2.4. They are easy to cover complex details, unlike board systems. Some of the spray products can be used externally and are often applied on non-primed steel. However, sprays can produce much mess due to their wet application as well as restrict other trades on building construction sites. The poor appearance of sprays on steel buildings is also a demerit which is not suitable for aesthetic purposes. Sprays are soft and may need further protection if used on unsecured building sites.
Intumescent coatings
Intumescent coatings are thin film paints developed specially to swell up when heated and produce protection foam to shield the steel structure as shown in Figure 2.5. Several amounts of coats may be needed to achieve the desired thickness. They can provide a nice visual appeal after their application and can simply cover complex details, e.g. bolted connections. The application of intumescent paints can be fast, does not increase the weight of the structure or take up space and can be applied externally.
Figure 2.4. Cementitious spray application (SCI, 2016).
Figure 2.5. Finished application of intumescent coatings and the result of intumescence (SCI, 2016).
However, the demerits of using intumescent paints include the high-cost implication of aesthetic demands or high thicknesses, wet application on site which may restrict other trades, the ageing effect due to adverse climatic conditions and it may be difficult to use where very high FRRs are required. The unavailability of information on thermal
conductivity property of intumescent coatings has been discussed by Wang et al. (2013) and
Zhang et al. (2014), given that most intumescent paints are proprietary products. Hence, its
suitability may not be accurately modelled in a structural fire design. Notably, from the stakeholder interviews carried-out in this research, some fire design stakeholders opine that given unpublished thermal conductivities of intumescent paints, their use may depend on the confidence built over time on using a particular proprietary product. Further to this, the
stakeholders considered this as a limitation of intumescent coatings given the risk that the ‘deemed-reliable product’ can be compromised in a pre-fire event e.g. earthquake (i.e. stickability throughout fire duration due to possible professional/production errors).
Concrete encasement of steel
Concrete encasement is one of the oldest and traditional methods of fire protecting steel structures. The key advantages of encasing steel in concrete are: the availability of materials, concrete is weather and impact resistant and may not require special skills in its application. The fire resistance requirement regarding insulation thickness of concrete is available in prescriptive codes. Concrete can also be subtly reinforced or designed as a composite structure in its application on steel structures for fires. Nevertheless, concrete encasement cost is high, its application can be slow on site, and encasing steel columns can take up much floor space and increase the weight of the steel building.
Blanket systems
This type of applied passive fire protection on steel structures is designed to meet the needs of fire protecting complex shapes and details. Given that blanket systems (shown in Figure 2.6) do not require any surface preparation, quick application can be guaranteed.
Figure 2.6. Application of blanket fire protection on structural steel members (SCI, 2016).
The use of the blanket system is also advantageous in scenarios where dry fixing is needed and can also guarantee that the recommended thickness is applied. However, blanket products
are mostly limited to internal building environment; blankets do not add to the visual appeal of the building as shown in Figure 2.6 and are rarely used, given the limited number of manufacturers of the product.
Unprotected steel
As mentioned earlier, structural fire designs can be optimised to accommodate unprotected steel structures. This design philosophy became more prominent after the Cardington structural fire tests on a multi-storey steel-framed building. Armer and O’Dell (1997) mentioned that the results from the Cardington tests showed that there was no structural collapse of unprotected steel members even at elevated temperatures up to 1000°C. The
attributes of the complex interrelationship of floor/beam systems (having survived the very
high temperatures applied in the Cardington test) are presented by Buchanan and Abu (2017). In the structural fire analysis of a single steel element, Wong (1999) reported that a steel column of very high mass could absorb a considerable amount of heat and may not attain its limiting temperature in post-flashover fires of around 40 min. The prospects of achieving structural fire resistance without applying fire protection materials on steel would imply a reduction in cost and construction duration, and ensuring aesthetic appeal of the steel building. However, the risk perception and confidence of some stakeholders (e.g. building owners, insurers etc.) to accept the design option of unprotecting a building’s structural members are highly tested given inherent structural fire design uncertainties.