First published in Construction & Property News, Issue: March 2007.
HISTORICAL BACKGROUND
The fire protection of buildings and the introduction of barriers to prevent fire spread and to secure the stability of building structures is a central theme in maintaining a fire safe environment. This is not a new concept however and the origins of this approach to fire safety span over many centuries. Most notably, the Great Fire of London which took place in 1666 and which destroyed four-fifths of the city was the impetus for building control provisions which have subsequently been enacted through various By-Laws and Regulations. Through such legislation, the requirement to construct buildings using non combustible materials and to contain the spread of fire through the use of fire resistant barriers was established. Similarly, a fire in a cotton warehouse in Tooley Street in London in 1861 in which a number of fire fighters lost their lives, led to limitations being placed on the physical size of fire resisting enclosures, referred to as compartments. The origin of many of these early recommendations can still be seen in a number of current guidance documents.
STATUTORY PROVISIONS
Statutory obligations in respect of Fire Safety are enshrined in Statutory Instruments and in Ireland, these requirements are outlined within the Building Regulations 1997.
On the issue of the containment of fire and protection to the building structure Regulations B3 advises that:
“A building shall be so designed and constructed that in the event of fire, its stability will be maintained for a reasonable period”
It goes on to require that:
“A wall common to two or more buildings shall be so designed and constructed that if offers adequate resistance to the spread of fire between those buildings”
And that:
“A building shall be sub-divided with fire resisting construction where this is necessary to inhibit the spread of fire within the building”
These requirements, which form the basis for many of the passive fire protection measures evident in buildings, are presented in a functional format and this format permits a range of solutions to demonstrate compliance with these Statutory obligations. Appropriate solutions can be achieved through the use of Standard Guidance documents such as Technical Guidance Document B (TGDB) 2006, a range of British Standards or the use of Fire Safety Engineering Techniques.
PROPERTY PROTECTION
TDGB in respect of Fire Safety objectives is clear in that the intent of the document and the guidance offered is for “the protection of life from fire”. Clearly such recommendations will assist in the protection of property, however this is not their primary aim, and as such, Insurers may often require additional measures or higher standards of protection to secure the building asset and to maintain business continuity in the event of a fire.
Insurance requirements place considerable emphasis on passive fire protection measures and the Loss Prevention Council (LPC) offers guidance on measures that may be introduced into the fabric of a building to manage particular risks. Their publication “The LPC Design Guide for the Fire Protection of Buildings” is a concise guide in meeting property protection objectives.
AIMS OF PASSIVE PROTECTION
Passive Fire Protection measures provide a number of benefits in particular however, they assist in maintaining appropriate levels of fire protection to structural elements of the building and in forming effective barriers to prevent the spread of smoke and fire within a building.
Structural fire protection is required to:
Such protection can be provided through the use of a range of applied systems such as intumescent coatings, fire resistant boards, and cementitious sprayed applications or indeed it may be inherent within the structure itself.
Similarly, the sub division of a building into fire compartments is necessary to prevent fire spread from building to building as well as internally within an individual building. The compartment boundaries that are created to form this sub-division offer defined lines of safety for occupants and fire fighters and they assist to contain fire from both vertical and horizontal spread within the building fabric.
APPLICATION OF PRINCIPLES IN PRACTICE
Documentation such as TGDB offers a simple means of satisfying both structural fire protection and compartmentation requirements for a wide range of common building uses. It does accept however that alternative approaches are equally valid and that not all buildings can be made to readily comply with standard recommendations, as such, the use of British Standards of Fire Engineering techniques is an equally valid approach.
The ability of a structural member (beam, column, wall or floor slab) or compartmenting element to resist the impact of fire is determined in terms of its fire resistance and the units of time (30 minutes, 60 minutes etc) is used to define its performance. TDGB offers guidance on what constitutes an appropriate period of fire resistance for elements of structure and compartment construction.
An important point that needs to be recognised is that the periods of fire resistance cited in TDGB relate to the time to failure in a standard test not a real fire environment. British Standard 476 includes a series of test specifications to measure the performance of passive fire resisting elements of construction. In essence, the standard specifies a heating regime (temperature – time curve), refer Figure 1, that a fire resisting component is subjected to with the performance of the element determined on its survival in the test environment. Three performance criteria are measured in the test situation, these are:
FIGURE 1 BS 746 time/temperature curve
With regards to fire resistance of components of structure – TDGB categorises buildings into “purpose groups” and takes account of other matters such as the height of the building or the presence of sprinklers in making recommendations regarding appropriate levels of fire resistance. Similarly with regards to compartmentation, the building use and its form (height) are used as measures in making recommendations for limitations on an individual compartment area and/or volume. These measures, in effect consider the anticipated fire load within a building, and the impact on evacuation and fire fighting that a particular building presents along with the use of sprinkler systems to reduce fire severity.
Compartment construction by necessity will include openings and penetrations generally to accommodate people movement between different parts of the building and to facilitate services distribution. To ensure that the effectiveness of the compartmenting construction is not compromised such openings and penetrations need to be protected to the same level as the compartment construction (i.e. same period of fire resistance). Where openings are required protection is offered through the use of Fire Doors, Fire resistant glazing, Fire shutters, Fire dampers In ducts and the use of proprietary fire-stopping to protect service pipes and cable trays where they penetrate fire resistant construction.
The use of compartmentation is a key strategy in a number of different building types. For instance in hospitals the strategy of progressive horizontal evacuation of patients relies heavily on the use of fire barriers to act as sub-compartments where patients are moved progressively from a fire affected area through a series of fire barriers to a place of safety. Similarly, high rise residential accommodation places consideration emphases on the use of compartmentation to secure the safety of the occupants, thus underlining the importance of effective passive measures and the key role they play in fire safety.
PASSIVE FIRE PROTECTION AND FIRE ENGINEERING
As discussed, TGDB offers guidance on passive fire protection solutions for a range of building types. Not all buildings however can be made to comply fully with standard guidance and with a drive for more innovative, dramatic buildings, alternative techniques are needed to ensure that such buildings receive adequate levels of fire protection.
Fire Safety Engineering offers such a mechanism for providing alternative solutions to fire protection requirements using engineering principles. It was noted that BS476 presents a test method for measuring the fire resistance of components of construction. The test condition however does not replicate the conditions of exposure in a real fire but offers a method to bench mark the performance of construction elements in a standardised manner. Figure 2 indicates the heating regime in the standard test environment and this is overlaid with real fire conditions where a fire develops, grows to a peak heat output and decays as fuel is exhausted. It is clear, that there is a significant difference in the heating profile that an element of structure may be subjected to in a standard test when compared to the impact of a real fire.
FIGURE 2 Comparative Time/Temperature Profiles (BS 476 Test Furnace v’s Real Fire)
Fire Safety Engineering techniques have therefore been developed to relate fire severity in a real fire to the standard heating regime. The technique is referred to as Time Equivalence. The standardised approach takes no account of the ventilation provided in a building but new standards such as BS7974 provide a means for determining fire severity taking account of all contributing factors such as fire load and ventilation openings.
Fire severity is a vital factor in accurately predicting fire resistance requirements of a compartment and a buildings supporting structure and Eurocode (EN199-1-2) presents the following formula to assess fire severity:
BS 7974 offers guidance on appropriate values for γ1, γ2 and γ3 and the period of fire resistance may then be determined from the following formula:
RPS Consulting Engineers were recently commissioned by Coca Cola to prepare a fire protection strategy for their new plant in Lisburn, Co Antrim. Operational aspects of the development presented some difficulties when compared with standard insurance recommendations for compartmentation. Fire Engineering Techniques were therefore used to maximise compartment area in both the warehouse and production area and to more accurately define appropriate levels of fire resistance to compartment boundaries for this bespoke development whilst ensuring that the building was adequately protected from the effects of fire to meet both client and insurance aspirations for the building. A similar approach was adopted at Independent News and Media Groups printing plant in Newry, Co Down.
CONCLUSION
Passive fire protection is a key weapon in meeting fire safety objectives in buildings both in terms of life safety and property protection.
An effective strategy for the protection of the structure and its sub division into fire resisting compartments needs to be considered at the design stage in the project development and this strategy must be brought forward during construction. A high standard of installation on site and the ongoing maintenance of passive fire safety measures is a key feature in securing a fire safe environment in the long term. The consequences of failure in structural fire resistance and compartmentation can have severe implications in terms of loss of life and this underlines the importance in creating and maintaining effective passive fire protection measures.
RPS Consulting Engineers
RPS Consulting Engineers is Ireland’s largest Civil/Structural and Environmental consultancy, employing over 500 staff throughout Ireland. The firm offers a multi-disciplinary and integrated Consultancy Service on an “all-island” basis. Areas in which the firm is involved include Roads and Traffic Engineering, Bridges, Ports and Harbours, Project Management, Health and Safety, Railways, Building Services, Coastal Engineering, Drainage and Sewerage, Electrical Engineering, Renewable Energy, Environmental Consultancy, Geotechnical Engineering, Ground Water Engineering, Hydrogeology, Mechanical engineering, Pipelines, Structural Engineering, Solid Waste Management and Treatment and Fire Safety Engineering.
RPS was named “International Consultancy of the Year” by the Institution of Civil Engineers in March 2004.
About the author:
The author of this article is a Technical Director within RPS Consulting Engineers and is responsible for the delivery of Fire Engineering Consultancy Services.
Their 23 years experience has been gained within an engineering consultancy environment and he has extensive experience in providing fire safety advice to a wide range of complex, innovative and bespoke building developments.
The author is a member of the Institution of Structural Engineers Fire Engineering Study Group where he has acted as a peer reviewer for high level fire engineering publications. They are also a part-time lecturer at the University of Ulster in Fire Engineering Design.
For more information please contact:
RPS Belfast (Elmwood House) – Planning & Development
T: +44 (0) 28 9066 7914