Many innovative designs of steel beams have been developed to allow more efficient construction methods. But designers and specifiers need to be aware of the fire safety issues these can raise, warns Bill Parlor.
Innovations in steel construction include castellated and cellular beams – that is beams with holes in the web that can allow the passage of services through them. But these welcome innovations also bring a number of issues to mind. Do structural engineers really consider the ‘buildability’ of buildings when the basic structural concept is agreed – innovative or not? Do they consider the different follow-on problems from using one type of steel design against another, apart from a simplified concept that services can be put under or through steel beams in a variety of ways. Do they have a sense of the need to maintain the compartmentation of buildings to limit the risk of fire spread through a building, and to minimise the risk for fire spread in the external envelope?
After all, the fire prevention measures apply to the whole building and its continuing use, year on year, so that the occupants can escape in case of fire. Maybe more communication is therefore needed at the structural design stage?
Steel beams are simple things, aren’t they? They support the floor above. Stronger beams allow larger spans, so that the floor below is less cluttered with vertical support columns, and the building is better disposed to diverse uses and earns more income through available floor space.
New challenges
Nowadays, the challenge has been to enable more storeys in a given height of a building. One of the issues with that simple quest is what to do with the building services? If they are located below a beam then the storey height is increased. If they are located and positioned to pass through the beam, the storey height can be reduced by around a metre, so that another floor can be included every three or four storeys. That way, more rental can be obtained and more people accommodated in the building.
So what types of steel beams are we usually taking about? The top and bottom flanges can be the same dimensions or different sizes. Shelf angles may be included to support the floor above. There are ‘simple’ universal beams with a continuous rolled steel profile and also fabricated beams, made from three selected steel plates welded together. Then there are castellated beams, where beams are cut longitudinally in particular ways from different universal beam sizes, and welded back together so that hexagonal shaped holes are formed within the central web of the beam, as in figure 1.
A similar but more complex technique is used to produce cellular beams from rolled sections, or from plate steel, by cutting holes in the web. The resulting central holes may be circular, elongated lozenge shaped by joining adjacent circular apertures, or rectangular. These different shaped apertures provide space for passing M&E services through the web of the beam. The method may yield ‘cellular beams’ with different inside corner details, radiused from rolling or welding.
Figure 1 illustrates how a universal beam may be cut to enable one half to be repositioned before re-welding the two halves together again, such that the new beam depth becomes ‘a + b’. The spacing ‘s’ between apertures is governed by the dimension of the top chord of the aperture. The dimensions ‘s’ and ‘c’ are fundamental to the strength of the steel section. However the residual steel in the vertical web post zone is primarily greater than ‘s’
In Figure 2, circular cellular apertures are shown, spaced at distance ‘x’. This pattern can be regularly repeated along the length of a beam, but circles can be joined to form elongated shapes, or interchanged with rectangular apertures, or mixtures of all three types. The space between apertures may vary considerably to give dimensions ‘x’ or ‘y’ or ‘z’, but it’s important to note that the residual web post zone can vary substantially in width – and this, rather than the bottom flange, may well be the weakest part of the beam in a fire. In general, the castellated beam has a narrower minimum web post; it’s invariably deeper but more slender than the equivalent cellular beam.
Under normal circumstances, the fire protection requirements will depend on the Section Factor of the steelwork in question – ordinarily linked to the ratio of the exposed steel perimeter/cross sectional area of the steel – and the period of fire resistance required. This assumes that the steel, as a universal beam, has a constant cross section along its length. Clearly, this is not the case when castellated or cellular beam structures are concerned, and could differ further when beams with apertures are formed from rolled steel sections or from welded steel plates. So have designers taken stock of these dimensional issues?
Fortunately, some manufacturers of protection systems or castellated and cellular beams, who are members of the Association for Specialist Fire Protection (ASFP) and in conjunction with the Steel Construction Institute, have carried out sufficient fire tests to have created generic or product-specific models. From these, the critical temperatures of the beams, with due consideration to the key dimensions and static/dynamic load, can be determined. This information can be combined with the product-specific performance of a named reactive fire protection system, to determine the fire protection thickness required for a given fire resistance period. One outcome may be to advise the use of a slightly heavier beam to reduce the limiting temperature and the section factor for best economy of the protected steel. The available evidence allows simpler modelling with non-reactive fire protection systems.
It is worth noting that the fire testing and assessment of steel cellular beams is not within the scope of BSI or EN standards. Currently, there is no agreed European approach that deals with the fire protection of these types of beams
Junctions
It is not sufficient to simply satisfy the fire protection requirements of the structure itself; the fire compartmentation of the building also needs to be maintained. This means we have to pay attention to the junctions of a fire protected structure so that the fire compartments – dividing walls and floors and services – can be maintained for all three resistance to fire criteria: structural stability; integrity of the division in fire; and fire insulation across the division. Designers need to consider the fire, thermal and acoustical requirements for buildings via Approved Documents B, E and L respectively.
Should the perimeter steel beams be cellular or not? It is vital that fire cannot easily travel from one floor of the building to another, not least through a window to challenge the external cladding system and the way that the entire cladding system provides fire resisting junctions with the edges of the intermediate compartment floors in the building. This may require a ‘modified approach’ where castellated or cellular beams are used at the perimeter of a building. It is not suggested that solutions are not available, but the associated design will need particular attention.
Cellular beams my be aligned along the line of fire compartmentation in a building, but the structural engineer may not have considered this situation. The architect and contractor now face the problem of maintaining the fire resistance criteria for the entire compartment by continuing the compartment wall for the full storey height, so that the apertures in the steelwork can be disregarded. However, it is not merely that simple, since the cellular beams will also accommodate M&E services.
Where the beams are not directly above a fire compartment wall, it is entirely feasible for any trapezoidal re-entrant deck profile apertures above the beam to be left unfilled, but only for fire periods up to 90 minutes. However, where the beams are above a compartment wall, all the re-entrant profiles must be fire-stopped to maintain comprehensive fire resistance criteria.
Best practice
A new 4th edition of Fire Protection of structural steel in buildings otherwise know as the ASFP Yellow Book, is now available and takes the opportunity to restructure general guidance, as well as provide new test and assessment procedures for passive and reactive fire protection systems. A new section is added for cellular beams protected by thin film reactive coatings – one aspect of the subject under discussion – since spray cementitious and board systems may be equally applicable.
In addition, the ASFP publication Fire stopping and penetration seals for the construction industry (also known as the Red Book) has been restructured to make it easier to use, and it is this document that designers should turn to when considering solutions for sealing penetrations around building services. Both publications are referenced as sources of information in Approved Document B for England and Wales.
The ASFP and its members are pleased to see the innovations from the steel industry, and in particular from the producers of castellated and cellular beams. We now look forward to working with the designers to maximise the potential for these products in such a way that the most use is made out of a building – without compromising its safety or the safety of its occupants in a fire.
Bill Parlor is technical officer at the Association for Specialist Fire Protection. The Yellow and Red Books may be obtained from the publications section of the ASFP website, www.asfp.org.uk. Hard copies are also available by calling 01252 357832.
