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LWF PROFESSIONAL BULLETIN Atria In Buildings Introduction The Roman Empire was the first civilisation to introduce atria into buildings. Some 2000 years later the British Standards Institution published a code of practice for the incorporation of atria in buildings as part of the BS 5588 series of Standards. Category C - Occupants who constitute a sleeping risk, and BS 5588 Fire precautions in the design, construction and use of buildings, part 7: Code of practice for the incorporation of atria in buildings took 10 years to write and was finally published in 1997. Whilst concentrating primarily on the subject of life safety, the code also gives limited guidance on measures that may be taken to reduce the potential for loss or damage to the contents and finishes in the event of a fire. The rather loose definition of an atrium used in the code is as follows; Atrium (plural Atria). A space within a building, not necessarily vertically aligned, passing through one or more structural floors. Enclosed lift wells, escalator wells, building service ducts and stairways are specifically excluded from this definition. The stated aim of the code is to ‘ensure that the incorporation of an atrium into a building does not present an increased risk to life as a result of fire and smoke spread’ and it is only concerned with the additional measures that may be necessary to compensate for any increased risk resulting from the inclusion of the atrium. It does not provide a fire engineered solution for any particular design and does not replace the recommendations contained in Approved Document B - Fire Safety, issued in support of the England and Wales Building Regulations. The code deals only with those aspects of the design, construction and management that relate specifically to the presence of an atrium and any other aspects not atrium-related remain the subject of other published codes and technical guidance. The code bases its range of design solutions on four different building occupancy types. These are; Category A - Occupants who are awake and familiar with the building layout, Category B - Occupants who are awake but unfamiliar with the building, Category D - Occupancies with a significant number of people requiring medical or nursing care. There are certain types of building to which this code does not apply. These include; 1) Prisons or other buildings intended for the confinement of persons, 2) Auditoria of theatres etc. to which part 6 of BS 5588 applies, 3) Shopping malls to which part 10 of BS 5588 applies, 4) Certain small premises covered by part 11 of BS 5588, and 5) Buildings in Occupancy categories A & B containing a 2 storey atrium, one storey of which is at ground level, provided that the building is designed for simultaneous evacuation. As always, there is no legal obligation to adopt the recommendations contained in the code if it can be demonstrated that an equivalent level of safety can be achieved in another manner. This opens the way for a fire engineered solution for which it may be necessary to obtain the services of a qualified and experienced fire engineer. Problems Associated with Atria in Buildings The presence of an atrium in a building can provide a route by which fire and smoke may spread through the building with greater ease than in an equivalent non-atrium building. The quantity of smoke produced by a fire is a function of both the energy possessed by the fire and the height through which the rising fire plume can rise. An atrium, resulting as it does from the absence of one or more floors, provides for an unusually high fire plume and, consequently, an increased volume of smoke and fire effluent. Because of this open spatial planning, the fire effluent can spread through the building and may cause damage that is out of all proportion with the actual size of the original fire. load within the atrium and the adoption of an appropriate evacuation strategy for the building. It is obvious that the unhindered spread of smoke from floor to floor will greatly increase the numbers of occupants that may be put at risk during the initial stages of the fire. The time available for the occupants to escape from the floors affected will be significantly reduced and also the conditions with which the fire crews will be presented on their arrival will be considerably worsened. The first method used is a decision tree process that takes the user through a series of questions and recommendations for determining the most appropriate design solution. Each decision tree comprises of two elements; As the smoke from a fire rises through an atrium, it will entrain large quantities of cool air from within the atrium. This reduces the temperature of the smoke plume but increases the mass of smoke produced. As it continues to rise, the plume will cool until its buoyancy is equal to that of the surrounding air. At this point stratification will occur and this may result in a semi-stable layer of air forming some distance below the roof of the atrium. This relatively cool layer of smoke may then spread horizontally into any storeys that are open to the atrium. 2) A recommendation box which provides a solution for that aspect of the design. It is obvious, from the above description, that enclosing the atrium using an imperforate screen will reduce the likelihood of any smoke spreading into the storeys remote from the floor on which the fire originated. For aesthetic and functional reasons, however, it is often required that the atrium be either open to the associated floor areas or, if enclosed, for the enclosing structure to be largely of glass. If the fire is large enough, the temperature of the smoke plume is likely to cause the failure of most nontoughened glasses and if the temperature continues to rise, the heat build-up within the atrium may eventually lead to the failure of any non-fire resisting glass associated with the atrium. However, the provision of fire resisting glazing around the atrium may not be financially viable nor entirely justified when the actual risk is assessed. The code, therefore, provides guidance on other methods that can be adopted in order to minimise the flow of hot smoke and gases from a fire, either in a floor associated with the atrium or in the atrium itself, from spilling into other floor areas remote from the atrium. The suggested methods make use of, what is termed ‘smoke retarding’ construction, fire resisting construction and ventilation and smoke extraction from each floor along with certain specified management procedures such as restriction of fire Application of the Code The code makes extensive use of three graphical methods for illustrating the recommended design solutions. 1) A question box containing a question fundamental to the design, and The user will be guided to further questions and recommendations as they move progressively through the decision tree process. Following on from the decision trees are a number of Tables that identify a range of design solutions appropriate to each occupancy category. The decision trees and associated tables are related to the main technical issues that need to be resolved in order to arrive at an appropriate level of fire life safety. These are; • The evacuation procedure, • the fire detection and alarm system • the degree of separation between the atrium and associated accommodation • the type of smoke management system to be provided • the use of the base of the atrium, and • the provision of an automatic fire suppression system. At the end of the design process, the user is directed to a series of ‘exemplar; buildings contained in Appendix A to the code. These are simplified generic sketches that identify the appropriate design solutions applicable to the different categories of occupancy and atrium types. The layout of the code may actually encourage ‘cherry picking’ from the range of suggested solutions, but it must be emphasized that the decision tree process is fundamental to the design process and should be followed through from beginning to completion and solutions should not be adopted that do not form part of the decision tree. This does not, however, make the adoption of a fire engineered solution any less acceptable. ARC 5 – Atria in Buildings Conclusion The code of practice is an easy to use and fairly comprehensive guide to designing the fire safety aspects of buildings containing an atrium. The design process is represented graphically in a series of decision trees and tables supplemented by a number of simplified building exemplars. Whilst it does provide a range of possible solutions for relatively straightforward cases, the code does not provide guidance in more complicated or novel situations, such as buildings that are designed to include more than a single atrium. The problems associated with adhered plumes, that is, plumes that originate within the floor area associated with an atrium and which emerge into the atrium as a spill plume and rise up through the building adhering to the walls of the atrium, are not dealt with at all. The code does make a substantial contribution to the problems of fire safety design in modern commercial buildings. It does not provide all of the answers but it does provide the building designer, regulator and fire engineer with a collection of design solutions that may be appropriate in all but the most complex of building designs. Lawrence Webster Forrest Limited Copyright Lawrence Webster Forrest Ltd ARC 5 – Atria in Buildings