地下街因空間構造特性,發生火災其濃煙影響人員逃難與救災之危害程度最大,而我國目前地下街空間排煙計畫,大多採用機械式排煙設備,若未能正常作動(如:雪山隧道火災事故),將可能帶來重大的損失與傷亡,故本研究以設置自然排煙塔之可行性進行探討,研究方法為利用FDS火災數值模擬軟體與「建築物防火避難安全性能驗證技術手冊」分析。經由電腦模擬結果分析得知,設置高度7m自然排煙塔煙層下降時間延遲至2.23分;設置高度3m自然排煙塔煙層下降時間延遲至2.17分;皆比現況煙層下降時間1.65分時間長,亦可得知台北地下街增設自然排煙塔能有效提升人員避難時間,故台北地下街增設自然排煙塔之可行性高。上述之設置高度7m自然排煙塔煙層下降時間雖然比設置高度3m自然排煙塔煙層下降時間的久,高度增加2.33倍,時間僅延遲3.60秒(2.76%)效果並無顯著提升,故本研究建議設置自然排煙塔3M,其自然排煙塔高度對於都市景觀衝擊性較小。
Due to the feature of space structure, the high damage level of strong smoke caused by fire will influence on people refuging and disaster rescuing in the underground street. Currently, the space ventilation system uses in the underground street is mechanical ventilation device. Without operating properly, (such as the fire accident inside Hsuehshan Tunnel), it will cause terrible damage and casualty. For that reason, we investigate on the possibility of natural exhaust chimney in the building. We manipulate FDS software as the research methods and “Technical Manual of Structure Fire Evacuation Safety Verification for Building Fire Protection” to analyze the possibility by computer. According to the results, a 7 m height natural exhaust chimney will extent the smoke reduction time to 2.23 minutes and a 3m chimney will extent to 2.17 minutes, which are all longer than the current smoke reduction time, 1.65 minutes. That is to say, adding the natural exhaust chimney in the underground street indeed can effectively enhance the refuging time of people as well as approve that it have a high possibility to build a natural exhaust chimney in Taipei City Mall. Although the reduction time of 7 m height natural exhaust chimney we mentioned above is longer than the 3 m height one, the height of two chimney are 2.33 different and the difference in both extension times is only 3.60 second (2.76%). There is not significant. Therefore, we suggest building a 3 m height natural exhaust chimney. In this height, it has low impact on the city landscape.