Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Fire Safety Engineering of Composite Steel-Framed Buildings
Since 1991 the requirements of the Building Regulations for
Approved Document B states that “fire safety engineering can provide an alternative approach to fire safety” and that “it may be the only practical way to achieve a satisfactory standard of fire safety in some large and complex buildings”. Since then a number of important research projects have been undertaken and the resulting conclusions have provided us with new published design guidance on how to design buildings that meet the functional requirements of the Building Regulations without applying prescriptive standards. One particular area in which this approach is proving to be popular is in the design and construction of composite steel framed buildings where reductions in applied structural fire protection can provide significant cost savings on large development projects. The aim of this bulletin is to look at two of the available guidance documents and discuss the potential benefits of adopting such an approach to building design.
Introduction – The Cardington Fire Tests
In 1996 the UK Building Research Establishment (BRE) conducted a series of fire tests involving an eight storey composite steel-frame building at the facility in Cardington. The building was constructed as a typical multi-storey office building and was tested to investigate the behaviour of this type of structure under real fire conditions. The floors were loaded to simulate what was considered to be typical loads for an office environment. A total of six tests were conducted to investigate various unprotected features of the structure.
The results of these and other similar tests and investigations into real fires have shown that there is a considerable difference between the performance seen compared to that expected from the results of standard fire resistance tests. Standard fire resistance tests of isolated beams can only generally expect to achieve between 15 and 20 minutes fire resistance and as a result it has been normal practice to provide some form of applied fire protection to the structure in order to achieve the required level of fire resistance performance. However in a real fire situation the ability of the structure to redistribute loads has been shown to significantly increase its overall ability to resist fire.
As a result of the Cardington tests and the considerable amount of other work that has been done since, the Fire Safety industry has been provided with a number of new tools and published guidance documents that provide a Fire Engineering alternative to achieving an equivalent fire safe design without the application of prescriptive standards of applied fire protection. A number of approaches have been developed but all attempt in different ways to consider a complete fire safety package and more realistic fire scenario.
BS 7974 provides a framework for undertaking an engineering approach to achieving fire safety within buildings. The guidance describes a complete design process starting with a Qualitative Design Review (QDR). A number of separate documents or sub-systems deal individually with the design approach, analysis and acceptance criteria of the various aspects of fire safety engineering design. Where required information generated by each sub-system is fed back into the design process.
The approach followed within the guidance is based around an assessment of the anticipated fire loads and building occupancy conducted as part of the QDR process. Standard time/temperature curves or experimental investigation can be considered when characterising the fire conditions to be adopted during the design. However the guidance also allows for the determination of fire conditions through engineering calculation.
Maximum fire temperatures can be predicted and a theoretical time/temperature relationship for the fire can be described. Calculations take into account the properties of the enclosure including the thermal properties of the surfaces and ventilation conditions. Fire load density and occupancy can also be considered and it is important that this information is fed into the design process from the QDR.
An alternative approach is to characterise the fire conditions by referencing a set duration of the standard time/temperature relationship. A “time equivalence” value of fire exposure can be generated which is then adopted during the design. The time equivalence value is calculated as a function of fire load density, thermal inertia and ventilation characteristics. A number of safety factors are then applied to take into account building occupancy, fire risk and mitigating factors such as intervention.
It is often possible to show through engineering calculation and modelling that the expected fire conditions will be less onerous than those found in a standard fire resistance test. As such the conservatisms inherent in a prescriptive approach to building design can be reduced while maintaining an acceptable level of life safety. In the case of composite steel construction this can lead to savings in terms of the level of structural steel protection that may be required to meet the functional requirements of building regulations.
The Steel Construction Institute (SCI) has developed a method which allows the removal of fire protection from certain parts of the construction all together. The method has been developed using fire test data from the Cardington tests as well as other tests conducted abroad. The method relies on the assumption that beams are designed to act compositely with the floor slab. This means that floors are constructed using composite slabs with profiled steel decking attached by shear connectors to down stand beams. Sufficient shear connections are required between the beams and the floor slab to ensure that effective membrane action can take place.
The SCI guidance presents a series of design tables for the floor slab and beams of steel framed buildings. The information in these tables has been generated using a structural model developed by Professor Colin Bailey at Manchester Universities School of Mechanical, Aerospace and Civil Engineering. The model combines the residual bending resistance of the beams with the membrane action of the slab to predict the structural response of the building.
In this way it has been shown that the integrity of the structure under standard fire conditions can be maintained for prolonged periods without the need to provide fire protection to secondary beam elements.
The potential cost savings of this approach for large developments is obvious but there are also other design issues that need to be considered:
· Primary beams will likely need to be increased in size to accommodate load relocation during a fire.
· Large deflections would be expected in unprotected steel beams and this can compromise the integrity of compartment walls. Walls crossing unprotected beams would require large deflection allowances or these beams would need to be protected.
· Although no increase in life safety risk is incurred other factors such as property protection and business continuity should also be considered.
The two approaches briefly discussed above are both moving toward a more realistic fire scenario taking into account the complete system of fire precautions that are often present in modern buildings. However each has a subtly different approach.
The BS 7974 method is designed to be part of a fully engineered solution and uses a combination of risk based assessment and engineering calculation to look is some detail at all aspects of fire ignition, growth and development. This data is then in turn used to look at the structural response of the building. Benefits with this approach are based very much on the competence of the fire engineers to correctly assess the risks and model the building as a whole. The method also provides us with a basis on which to analyse large and complicated designs against functional requirements.
In comparison the SCI method is based more heavily on the use of advanced structural analysis backed up by experimental results and a better understanding of how this type of building responds to fire. For this method it is not necessary to consider fire loading and building use in detail so long as the design fits within the scope of the guidance. In this way the guidance is certainly more limited in its use but also makes it easier for designers to use. Data is provided in tables for immediate use.
It is important to consider the application of such methods carefully. Significant reductions in steel protection may be required to make it a worthwhile cost saving as during construction additional specification and logistics issues related to complicated design details can be counter productive. Issues with future building flexibility should also be considered. Compartment size, construction, typical fire load density and occupation risk are considered within the design process and should these change the design may need to be reviewed to ensure that an acceptable level of fire safety is still provided. Property protection and business continuity issues raised by the building occupier or insurer may also need to be addressed.
Despite these considerations the benefits are clear. A fire engineered approach to the design of composite steel structures provides not only flexibility of design but also the benefit of possible cost savings through a lower requirement for applied fire protection. Fire engineering can often add value to large or complicated projects as well as aid architects and designers to better fulfil their creative aspirations.
The LWF Bulletin is designed to give general information on fire safety risk management. Readers should take specific advice when dealing with particular situations. LWF accept no responsibility for action taken as a result of information contained in the document. The information in this document is correct at the time and date of the publication.
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