Fazio, Elissa (2007) Effectiveness of smoke barriers: pressurised stair enclosures. Research Master thesis, Victoria University.

Abstract

Stair Pressurisation Systems (SPS) are a form of smoke control used in many high-rise buildings. They are designed to keep smoke out of the stairwell so that occupants have a safe evacuation route during an emergency. Throughout the years, however, many within the Fire Engineering industry have commented that SPS do not really perform as per their design intent. In order to assess this belief, an investigation was undertaken to assess the effectiveness of a SPS, whereby the effectiveness was assessed by considering the following: if the SPS operates, will the resulting performance be as required by the Australian Standards (i.e. AS1668.1 1998) and when required, will the SPS or components actually operate. To do this, two representative SPS within the Melbourne Central Business District were investigated; one with a Variable Speed Drive (VSD), System 1, typical of systems installed more recently and one without a VSD, System 2, typical of earlier types of SPS. The effectiveness evaluation was divided into two parts. The first considered the influence of variations to wind speed, temperature and leakage (referred to as the E-factors), while the second considered the 'other factors' or F-factors, affecting the effectiveness of a SPS (i.e. those factors that can be influenced by commissioning, maintenance, operation of the system, etc.). For this study, these two parts were assessed in isolation to each other, such that when the E-factors were assessed, the other influences were considered not to exhibit any faults. Similarly, when considering the influence of the F-factors, the influence of the E-factors was ignored. The influence of the E-factors was assessed in terms of achieving the door opening force and airflow performance conditions of AS1668.1. This was done using a computer network program known as CONTAM. Initially a base building model was developed, complying with the performance conditions of the Australian Standard and then variables such as external temperatures, wind and leakage associated with the building elements were varied as part of a sensitivity analysis. The results indicated that in most cases (unless extreme conditions were used), the effectiveness of the SPS was nearly equal to unity (i.e. '1'). The SPS with a VSD also had a slightly higher effectiveness than the system without, when 'Average' leakage conditions were modelled. However, if the stairwell leakage approached the building facade leakage, the effectiveness in relation to achieving the airflow velocity conditions of AS1668.1, for both Systems 1 and 2, significantly reduced. The 'other factors' which may influence the effectiveness of a SPS were those that could be influenced by the design, installation, commissioning, maintenance and operation of the SPS and were assessed after interrelationship diagrams, in the form of fault trees were developed. These diagrams detailed the complex integration of the various components, which make up a SPS and assisted in identifying which components appear to be the most dominant in terms of reducing the reliability of the system. Probability of failure data was obtained through a survey completed by industry personnel intimately associated with SPS. The reliability assessment indicated that the two major dominant factors, in terms of not achieving the performance conditions of AS1668.1, were high door opening forces, followed by low airflow velocities. The importance of high door opening forces is significant, as occupants who cannot enter the stairwell in an emergency, may not be able to evacuate safely. The maintenance requirements associated with SPS as per AS1851.6, are illustrated in this thesis in relation to the fault tree diagrams. The results revealed that some of the dominant factors (such as the VSD, the pressure sensor, etc.), achieved low levels of effectiveness. These factors are only considered during the commissioning stage and there is no formal maintenance for these items. These findings help to reinforce the importance of commissioning as well as maintenance (especially for items currently not maintained) and the need for identified faults to be corrected as soon as they are found, or at least prior to the next maintenance inspection. Assessing the effectiveness of a SPS, with respect to the performance conditions of AS1668.1, found that the factors referred to as 'other factors' have a greater influence on SPS effectiveness than variations due to wind, temperature and leakage changes. Having said this though, the results generated indicate that SPS are not very effective in terms of achieving the performance conditions of AS1668.1 for the whole system (i.e. every door). For example, there is a 1/10 chance that the door opening forces will comply and approximately a 1/2 chance that the airflow velocities will meet the required performance conditions. These low probabilities are for buildings that are being "commissioned" and "maintained", indicating that although factors appear to be identified, they are not being adequately corrected. In summary, if commissioning is performed correctly, and so too the maintenance, the reliability associated with the components can be significantly improved and therefore, so can the overall effectiveness of the SPS.

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