| Author: | Yue, Tsz Kit |
| Title: | A study on supertall building fires and associated protection |
| Advisors: | Chow, Wan-ki (BSE) Fong, Nai-kong (BSE) |
| Degree: | Ph.D. |
| Year: | 2020 |
| Subject: | Tall buildings -- Fires and fire prevention -- China -- Hong Kong Tall buildings -- Safety measures Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Services Engineering |
| Pages: | x, 146 pages : color illustrations |
| Language: | English |
| Abstract: | Supertall buildings have been built for decades and it is anticipated that more and more skyscrapers will be built in the future. Despite the generic fire safety concerns inherited in supertall buildings, researches have pointed out additional fire safety issues that require special attention. Since additional fire safety issues in supertall buildings, e.g. effect of ambient wind to vertical fire spread and development, were not studied in depth previously, research questions are raised from relevant literature to facilitate the discovery of research gaps. To start with, this thesis reviewed literature on fire safety strategies and fire safety codes for supertall buildings in various countries and regions. Definitions of supertall building in various codes are presented. Moreover, potential problems in rescue and evacuation in supertall buildings in densely populated cities such as Hong Kong are researched and discussed. In addition, Hong Kong local codes in relation to the provision of refuge floor are analyzed in depth. Brief information on supertall buildings in various countries and regions are also listed. Issues related to the fire safety of supertall buildings in Hong Kong are highlighted and practical solutions to enhance fire safety are then presented. To make the study more comprehensive, a number of issues related to the safe use of supertall buildings were also discussed in this thesis. Firstly, fire safety of supertall building from toxicity perspective is discussed. Secondly, the problem of high fire load density in supertall buildings and its implications are reviewed. Thirdly, the use of timber of high-rise building construction and its associated fire risk are discussed. Fourthly, possible explosion hazard due to the use of clean refrigerant in supertall buildings is investigated and explored. In the discussion, potential research gaps, such as the applicability and reliability of existing fire safety measures of supertall buildings, psychological responses of evacuees in supertall building in case of fire, and the relationship between the design of supertall buildings and the impact on the health of emergency responders, are identified. As refuge floor is a mandatory requirement for tall and supertall buildings in Hong Kong, this thesis hypothesized that the functional performance of refuge floor in evacuation and fire intervention would not be differed in different building types or occupancies, i.e. "acting as a safe place for short break since it is difficult for most people to walk down a tall building in one go" as stated in the local code. To test the hypothesis, this thesis intends to set up an objective benchmark on the functional performance of refuge floor. In the benchmarking process, this thesis found that area of external wall opening and the provision of drencher may be key attributes to the safe use and tenability of refuge floor. To test the hypothesis in detail, key parameters in cross-ventilation and the provision of major fire protection systems on refuge floors of 51 tall residential and non-residential buildings of Hong Kong are surveyed in this thesis. In addition, a new fire safety benchmark, i.e. ratio of external wall opening areas to wall areas of refuge floors (WOAR), is introduced in this thesis and the distribution of these ratios in different type and height of buildings are analysed. In the survey of this thesis, it is noted that refuge floors without drencher protection usually have a larger WOAR than those with drencher protection. This finding may reflect that it would be easier for "performance-based fire engineering study" to claim for omission of drencher in the old days if better cross-ventilation (i.e. larger WOAR) is provided on refuge floors. This finding seems to echo the hypothesis of this thesis that tenability of refuge floor could be increased by better cross-ventilation (i.e. larger WOAR). To quantitatively study how ventilation provision is affected by WOAR, numerical simulations on the effect of natural ventilation to fire growth on the refuge floor are conducted in this thesis using empirical wind data collected from the tallest building of Hong Kong. In the simulations, an office layout was adopted on both upper and lower floors for the sake of illustrating the possible smoke and heat spread from lower floor to upper floor. Open-source CFD package Fire Dynamics Simulator 6.6.0 (FDS6) developed by NIST for simulating fire and smoke movement is used in this thesis. Combustibles with fire load density of about 1,135 MJ/m2 (which is the maximum allowable fire load in office in Hong Kong) are input to the model as fuel. Fire load of same amount are also put on the upper floor to demonstrate whether the plume from the lower floor was strong enough to ignite the combustibles on upper floors (i.e. refuge floor). The computing domain is extended 1.5 m horizontally outside the glass facades so that boundary effect and natural ventilation induced by crosswind could be captured in the model. Subsequently, a total of 10 simulations of different wind speeds and different ceiling geometries are conducted in this thesis. Simulation 1 to 9 have a WOAR of 0.58 while Simulation 10 has a WOAR of 0.14 (Simulation 10 had the same geometry as Simulations 1-9 but the WOAR is only 0.14 because only the centre window out of the three window walls can be broken by fire. Results of simulations indicated that geometry with lower WOAR creates higher fire size than geometries with higher WOAR. Also, for geometry with lower WOAR, fire from the lower floor is strong enough to ignite the sofa on upper floor (supposed to be the refuge floor) even though the ignited sofa could not deflagrate the entire upper floor. However, the scenario is sufficient to illustrate how untenable the upper floor would be. This finding also supports the hypothesis of this thesis that tenability of refuge floor could be increased by better cross-ventilation (i.e. larger WOAR). Although tenability of refuge floor could be increased by better cross-ventilation (i.e. larger WOAR), cross-ventilation also brings an accelerating effect to fire development at higher levels of tall buildings. This is evidenced by the higher maximum HRR in Simulation 7, 8 and 9. Therefore, fire spread from lower floors to upper floors (including refuge floors) through external wall is always possible. If combustible materials are used in building facade just like the case in Grenfell Tower, Torch Tower and Address Downtown Hotel, fire may spread even faster. In summary, this thesis attempts and successfully demonstrated that tenability of refuge floor could be increased by better cross-ventilation (i.e. larger WOAR). Furthermore, this thesis also proves that WOAR and the provision of major fire protection systems could be a reliable benchmark for future quantitative analysis. While in the future, this thesis suggests that correlation between WOAR and tenability on refuge floor could be conducted so as to provide quantitative reference to Authorities Having Jurisdiction in stipulating sufficient amount of opening required on refuge floors. |
| Rights: | All rights reserved |
| Access: | open access |
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