鋼筋混凝土樑受火害後之力學性質數值模擬

Numerical Modeling of Mechanical Properties for Reinforced Concrete Beams Exposed to Fire

徐 瑞 祥(Jui-Hsiang Hsu)

指導教授 ： 林誠興

元智大學/工程學院/機械工程學系/博士(2006年)

摘要

台灣為一位處西太平洋邊緣之島嶼，面積三萬六千平方公里，人口兩千三百萬，然而由於山區陡峭，因此有百分之九十之人口居住於百分之三十之平地，人口密度堪稱世界第一。由於人口居住稠密，經常會伴隨著發生火災意外，造成人員及財產非常大之損傷，然而大部份之建築物均只經過簡單之勘查即繼續使用，對於火害造成建築物之損傷程度並沒有更進一步之了解。
鋼筋混凝土建築物在遭受嚴重火害後是否還能使用，必須對其結構體之安全性進行評估，評估的結果對人們生命與財產的權益影響甚鉅，因此必須發展出一套經濟有效的分析模式以判定鋼筋混凝土建築在火害後繼續使用的可能性。由於結構體之安全性與各桿件之行為有密切之關係，因此欲評估火害後結構體之安全性，必須先行了解鋼筋混凝土桿件在火害後之相關力學性質。
建築物在發生火災後，由於鋼筋及混凝土材料受溫度之影響產生劣化，使得構件之承載能力降低。樑、柱構件斷面內各位置承受之火害溫度不同，鋼筋及混凝土材料之力學性質也會隨著改變，力學的分析將變得極為複雜;再加上火災可能有不同之升溫狀況、延時;樑構件也可能因位置之不同而有不同之邊界情形，如此複雜之狀況實有必要藉由數值模擬來加以探討。
樑構件為承受撓曲與剪力之構件，因此火害後樑構件之撓曲強度與剪力強度之折減對火害後結構體之安全有很大之影響。另外，彈性模數之折減大小亦會影響火害後構架在力矩重新分配時之分配量。因此本研究所探討鋼筋混凝土樑受火害後之力學性質包含有剪力強度、撓曲強度及彈性模數。
本研究先以有限差分法(Finite Difference Method)模擬樑斷面受火害後之溫度分佈，再將斷面分割為M×N個單元，每一單元內視其為等溫、等性質，以塊狀系統法(Lumped System Method)，分別計算鋼筋及混凝土材料受火害後之溫度折减，再根據ACI（American Concrete Institute）之設計規範，模擬計算出構件受火害後之殘餘撓曲強度、剪力強度及彈性模數。本研究提供一個符合力學原理的方法探討鋼筋混凝土樑受火害後相關力學性質之折減情形，結果將有助於瞭解火害對結構物之損傷程度，進而可作為判斷火害後結構物安全性評估之依據。

關鍵字

鋼筋混凝土樑 ； 火害 ； 力學性質 ； 數值模擬

並列摘要

Population densities are typically high in urban areas, including in cities in Taiwan. More than 90% of the buildings in Taiwan use reinforced concrete (RC) structures and on average 10,000 fires occur annually. Following fire damage, whether the RC structures are sufficiently safe must be determined to protect human life and property. The fire safety of any RC structure depends strongly on its resistance to fire, which in turn depends on the combustibility and fire resistance of its main structural elements, beams and columns. No two fires are the same so modeling must be employed to study the behavior of RC beams exposed to fire.
The fact that different parts of the cross-section of a beam are exposed to different temperatures during a building fire makes the analysis in such beam structures a nontrivial problem. In this study, our thermal analysis employs the finite different method to model the various temperature distributions of reinforced concrete beam after sustaining high temperature. The modified version of the ACI (American Concrete Institute) Code then is used to include the effects of temperature on the reinforced steel and concrete materials, and the residual abilities of reinforced concrete beams are accordingly calculated. The analytical results, including the residual ultimate bending moment, residual shear strength and residual elastic modulus, are compared to experimental results in the literature with actual full-scale RC beam fire attack, and consistency between them has proved our method to be an accurate mathematical modeling. Extensive simulations are carried out to calculate a new equivalent beam which has the same residual abilities like the beam exposed to fire.

並列關鍵字

RC beam ； Fire ； Numerical Modeling ； Mechanical Properties

參考文獻

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20. Lie, T.T.; Rowe, T.T. and Lin, T.D., “Residual Strength of Fire-Exposed Reinforced Concrete Columns”, ACI Publication Sp 92-9.