Fire and Structural Steel

For over 150 years structural steel has been used in building construction. Structural steel components include open sections, such as angles, channels and “I” shapes that are rolled or fabricated from plate to create structural beams, columns or trusses. In contrast to open sections, a relatively recent structural member is the tube or hollow structural section. And since the 1980s “sheet steel” in the form of loadbearing “C-section” floor joists and wall studs has evolved. Presently numerous types of residential, commercial and institutional buildings are built with C-section steel joists and studs. The steel industry refers to this type of steel as either cold-formed steel or lightweight steel framing (LSF).

Steel, like all materials, loses strength at temperatures in excess of 300C, and at 600C retains around 50% of its room temperature yield strength.1 To protect the structural steel frame or cold-formed steel floor and wall assemblies under fire conditions, building codes have assigned fire-resistance ratings to structural assemblies and components. In Canada fire testing as per CAN/ULC-S101 is used to establish fire resistance ratings. In these tests building construction assemblies are exposed to a standard time-temperature curve that rises rapidly to 840C at 30 minutes and then increases more gradually to 1090C at four hours.

The new 2005 edition of the National Building Code of Canada published by the National Research Council of Canada is written in an “objective-based” format where the intent and objective of each code provision is spelled out. While a prescribed “acceptable solution” can be followed if desired, the objective-based code format also offers designers the option of finding “alternative solutions” (previously known as equivalencies). Objective-based and performance based codes provide more opportunity for fire protection engineers and structural engineers to apply engineered solutions to fire safety and to use advanced calculation techniques such as computer fire modelling.

Active and passive fire protection

When using structural steel, a designer can comply with building code requirements using an array of fire protection techniques. An optimal building design balances the fire safety requirements with the economic and aesthetic specifications of the building.

Fire safe steel buildings use active and passive fire protection systems. An active fire protection system is where sprinklers eliminate the heat source by fire suppression. Smoke or heat detectors generate an alert that initiates the extinguishing action.

A passive fire protection system delays the rate of temperature increase in steelwork thereby providing time for building occupants to exit, combustibles to burn out and fire fighters to arrive to extinguish the fire. Passive fire protection systems basically insulate steel or provide heat dissipation. They can be categorized into four systems:

* Directly applied systems that insulate against heat, such as spray-applied fire-resistive materials and intumescent coatings.

* Membrane systems that provide a thermal barrier against heat, such as gypsum wallboard.

* Water systems that provide a cooling effect, such as water-filled hollow structural sections within steel structural frames to dissipate heat from fire.

* Concrete systems where the concrete encasing of steel slows down the conduction of heat, or where concrete-filled hollow structural sections produce composite behaviour with force redistribution occurring at elevated temperatures.

In 2003 the American Institute for Steel Construction (AISC) published Steel Design Guide No. 19 entitled “Fire Resistance of Structural Steel Framing.” It provides an overview of ways of fire-protecting steel columns, roofs, floors and trusses by using gypsum, masonry, concrete, spray-applied fire resistive materials (fibrous and cementitious), mineral fibreboard and intumescent coatings.

The final chapter of the guide deals with engineering fire protection, recognizing the increased availability of appropriate analytical tools.

The engineered approach

The engineered approach to fire safety has manifested itself in more and more building projects as fire researchers develop a wider understanding how structures respond in fires. Among international conferences taking place there are regularly established ones organized by the Society of Fire Protection Engineers, along with “Interflam” (every three years since 1979) and “Fire and Materials” (10th International Conference in 2005), both organized by Interscience Communications. As well, starting in 2000, “Structures in Fire” workshops organized by universities and research institutes worldwide have been held every two years to create an international forum for fire resistance researchers.

These days engineers who design fire protection for structures can use a range of computer models to help them pursue the engineering approach. Olenick and Carpenter provided a summary of 168 fire-related computer models in 2003. They were in categories such as fire endurance, egress and detector response.2

There has also been a development in a set of sophisticated codes of practice for the fire design of structures, namely Eurocodes. Eurocodes apply to the common building materials of concrete, steel, composite steel-concrete, timber, masonry and aluminum. For steel structures, the code is EN 1993-1-2:2005, Eurocode 3: Design of Steel Structures, Part 1-2: General rules — Structural Fire Design.

The provisions in Eurocode 3 for steel structures and fire deal with the complexity of internal forces induced by thermal expansion, strength reduction due to elevated temperatures, the associated amplified deflections, and other design factors. A book by Franssen and Zaharia published in 2005 offers background material and guidance for the designer when using Eurocode 3.3 Similar engineering guidance is forthcoming in North America where the Society of Fire Protection Engineers is in the process of transforming its “Engineering Guide to Fire Exposures to Structural Elements” to a consensus standard as provision for more activity in performance-based design.

Another advance in the use of an engineered approach to fire design is a new appendix in AISC’s 2005 Specification for Structural Steel Buildings, entitled Appendix 4, Structural Design for Fire Conditions. In line with this development, the technical committee responsible for the Canadian structural steel design standard, CAN/CSA-S16, Limit States Design of Steel Structures, will incorporate a similar appendix as an aid for structural engineers to develop performance-based fire safety for buildings designed with structural steel.

Tests and applications

Another noteworthy development is that research is now being done at full-scale as opposed to single element testing in a standard furnace. Full-scale testing has given considerable real engineering insight into fire resistance. For example, fire research conducted in Cardington in the U.K. at a former airship hangar for the British Research Establishment investigated the holistic behaviour of an eight-storey steel building subject to real fires. Six fire tests were conducted in 1995 and 1996 where floors and columns were exposed to fire attack. In these tests the steel beams and deck supporting a concrete floor slab did not have fire protection and only the columns were protected. A later test in January 2003 looked at a fire in a compartment where columns and beam-column connections were fire protected.

The outcome of the research at Cardington was evidence that less fire protection is necessary for floor beams and steel deck supporting a concrete slab. This outcome is due to load re-distribution through inherent continuity in the steel frame and concrete slab, and due to secondary effects such as membrane action. The results have been manifested in various pro
jects in the U.K., including a hospital building in Nuffield that used 40% less fire protection on steel beams, and in an 11 storey (eight above-ground) office building in London where the majority of secondary steelwork was unprotected. 4 & 5

Last summer, in an article in Advantage Steel (summer 2005, pp. 28-29) published by the Canadian Institute of Steel Construction, R. Bartlett overviewed the use of advanced calculation techniques and computer fire modelling to produce a performance-based design where “unprotected” structural steel was used in a Nova Scotia community college expansion project. In essence, the fire protection engineer used realistic “design fires” to establish the acceptability of unprotected steel.

Finally, there is the September 2005 release of a voluminous final report issued by the U.S. National Institute of Science and Technology (NIST) located in Gaithersburg, MD. Amounting to more than 10,000 pages, the report investigated the collapse of the World Trade Center following the terrorist attacks of September 11, 2001 in New York. The report’s 30 recommendations are in five areas: increased structural integrity, methods of fire resistance design, active fire protection, methods of evacuating a building, and the performance of spray-applied fire-resistive materials (SFRM). A NIST initiative to study SFRM thermal properties and bond characteristics will involve collaboration with the American Iron Steel Institute and the American Steel Construction Institute.

In conclusion, with the advent of a codified procedure in structural design for fire protection design and the availability of sophisticated numerical tools, the possibilities for taking an engineering approach to building fire safety could represent a new opportunity for structural engineers.

George S. Frater, Ph.D., P.Eng. is codes and standards engineer at the Canadian Steel Construction Council in Toronto. E-mail gfrater@steel.org. For fire-related information on steel, see www.cisc-icca.ca

1 Tide, R.H.R., “Integrity of Structural Steel After Exposure to Fire.” Engineering Journal, First Quarter, 1998, pp. 26-38, American Institute of Steel Construction, Chicago, IL.

2 Olenick, S.M. and Carpenter, D.J. 2003. “An Updated International Survey of Computer Models for Fire and Smoke.” Journal of Fire Protection Engineering, Vol. 13, May 2003, pp. 87-110.

3 Franssen, J-M. and Zaharia, R. 2005. “Design of Steel Structures subjected to Fire, Background and Design Guide to Eurocode 3.” Les ditions de l’Universit de Lige, Lige, Belgium.

4 IISI. 2004. “Fire Safe Multi-storey Buildings, Economic Solutions in Steel.” International Iron and Steel Institute, Brussels, Belgium.

5 Lamont, S., Lane, B., Flint, G. and Usmani, A. “Behaviour of Structures in Fire and Real Design — A Case Study.” Journal of Fire Protection Engineering, Vol. 16, February, 2006 pp. 5-35.