鋼筋混凝土版之耐火時效與火害後貫穿剪力強度

Abstract

無梁版系統已經被廣泛使用於許多建築物，施工上較為快速，並可增加使用空間，降低樓層高度，常用於商場、停車場、橋面版等。無梁版系統最大的問題為在承受載重時會發生脆性之貫穿剪力破壞。目前對鋼筋混凝土版貫穿剪力行為研究都侷限在常溫，著重於簡支承版之貫穿剪力試驗資料與強度預測經驗公式發展。 目前國內外著重於鋼筋混凝土梁或柱構件火害研究，但對鋼筋混凝土版柱接頭區遭受火害之貫穿剪力強度行為之研究則甚少。本研究目的為進行試驗與分析，探討鋼筋混凝土版柱接頭區高溫加載下貫穿剪力之耐火時效與火害後殘餘貫穿剪力強度；另外，針對火害後鋼筋混凝土版，進一步提出簡易評估計算式預測鋼筋混凝土版之殘餘貫穿剪力強度。 本研究規劃模擬鋼筋混凝土版高溫加載行為之試驗方法，以混凝土強度、拉力鋼筋比、不同火害面及昇溫時間為試驗參數，依據ASTM E119標準昇溫曲線加熱，藉由耐火能力試驗及火害後殘餘貫穿剪力強度試驗，探討其耐火性能及殘餘強度。觀察試驗過程試體外觀、破壞情形、內部溫度分佈及撓度變化等，探討各參數對耐火時效及殘餘貫穿剪力強度之影響。不同火害面區分為樓版上拉力側受火害與樓版下壓力側受火害。 研究成果發現，高溫對於樓版上拉力側受火害或於樓版下壓力側受火害之耐火時效確有顯著差異；使用不同混凝土強度於不同火害面高溫加載，所造成混凝土表面爆裂、開裂機制各有不同，試體產生變形及破壞情形也有所差異。耐火能力試驗結果顯示，於樓版下壓力側受熱的試體，可以承受耐火時間長達8小時以上，並不會發生破壞；但高強度混凝土試體於高溫試驗過程中產生爆裂現象。樓版上拉力側受熱的試體，於4小時左右發生貫穿剪力破壞；高溫試驗過程中，不論高強度或普通強度混凝土試體均沒有發生爆裂行為，而試體破壞後出現明顯錐形貫穿形狀。火害後殘餘貫穿強度試驗結果發現，殘餘貫穿剪力強度隨昇溫時間增加逐漸遞減。 研究並利用有限元素分析預測試體內部溫度之分佈，將其簡化為等溫線，作為火害後鋼筋混凝土版之殘餘貫穿剪力強度預測之依據。有限元素分析尚能準確的預測試體斷面內部溫度之分佈趨勢。計算鋼筋混凝土版殘餘貫穿剪力強度之分析程序，可合理預測鋼筋混凝土版之殘餘貫穿之剪力強度。 當火災發生於樓版上之拉力側時，殘餘貫穿剪力強度顯著折損，且達耐火能力極限時發生脆性之貫穿剪力破壞，對於建築物結構安全影響甚巨，設計時應予重視，使防火安全設計及結構補強評估更臻完善。

Flat slab systems are widely used in various buildings, such as shopping stores and malls, parking lot, and bridge deck because the flat slab systems can be built quickly, provide more spaces, and reduce floor height. However, flat slab systems under heavy loads usually fail in punching shear. Currently, research on punching shear behavior of reinforced concrete slabs limits at room temperature and emphasizes on conducting the tests, and developing empirical formula to predict punching strength of reinforced concrete slabs supported simply. Currently, domestic and international research of reinforced concrete structures in fire loading is highly focused on beam and column members; however, studies on the fire resistance of reinforced concrete slabs are still quite rare. The objective of this study is to investigate experimentally and analytically fire resistance and residual punching shear strength of reinforced concrete slab-column connections after elevated temperatures. Furthermore, analytical calculation is proposed to predict punching shear capacity of the reinforced concrete slabs after fire. An experimental approach is proposed to simulate fire behavior of reinforced concrete slab-column connections under elevated temperatures and loads. The parameters of this experiment included concrete compressive strength, ratio of tensile steel reinforcement, fire exposed face, and duration in fire. The specimens were exposed to elevated temperatures in accordance with the ASTM E119 standard fire curve. Appearance, failure mode, temperature distributions, and time-deflection relations of the specimens were recorded and used to assess effects of the parameters on fire resistance and residual punching shear capacity of the specimens. Different fire exposed faces were categorized to heating on the tension side of the slabs to represent a fire above the floor slab, and heating on the compression side of the slabs to represent a fire below the floor slab. The test results showed that the fire resistance of the specimens heated on the compression side or tension side clearly differed from each other. The mechanisms of concrete cracking and spalling also varied according to the concrete strength of the slabs and fire exposed faces. The deformation and failure mode of the specimens also differed. The test results of the fire resistance showed that slabs heated on the compression side did not fail up to eight hours. But, the slab with high-strength concrete heated on the compression side would spall. The slabs heated on the tension side, the slabs would fail at around four hours. Neither the normal-strength concrete specimens nor the high-strength concrete specimens spalled during elevated temperatures. The failed specimens appeared to have a cone shape. The residual punching shear capacity decreased with the increase of heating time. Finite element analysis was also utilized to obtain internal temperature distribution of the specimens and the results were simplified to an isotherm to predict residual punching shear capacity of the specimens after fire. Finite element analysis could accurately predict the temperature distributions of the specimens. A proposed procedure to calculate the residual punching shear capacity reasonably predicted the residual punching shear capacity of reinforced concrete slabs after elevated temperatures. The residual punching shear capacity decreased significantly when fire occurred on the tension side of the slab and, consequently, resulted in brittle punching shear failure which would be a serious threat to structural safety. Hence, this concept should be seriously taken into account during the structural design to achieve better fire safe design and structural retrofitting assessment.