含剪力接頭合成梁於火害中之行為

Abstract

鋼構造建築物受火害時，因鋼材於高溫下材料強度劇降之性質，對其防火性能與行為的了解極為重要。建築結構中之小梁因受到樓版與大梁束制的影響，受高溫時將因熱膨脹而承受軸壓力，冷却時則因收縮而承受軸拉力。此一額外作用力可能對剪力接頭之剪力板及螺栓造成破壞。對應於不同型式之剪力接頭，此溫度荷載所造成之額外作用力亦不相同，進而造成破壞模式與防火時效之差異。本研究藉由兩組大尺寸之火害實驗，探討含剪力接頭合成梁於高溫下之變形行為、溫度變化、破壞模式及防火時效。試體以兩種不同型式之剪力接頭做為研究參數，主要差異為小梁端部翼板是否有裁切，並依鋼構造合成梁規範設計剪力釘、鋼筋與樓版，剪力接頭處之螺孔則採用擴大孔進行設計。實驗規劃依據「建築物構造構件耐火試驗法」CNS12514-1、CNS12514-6，以載重塊放置於樓版上方以模擬小梁載重比為0.6之使用性狀況；並依標準升溫曲線進行加熱，小梁與鋼承板直接受火，大梁則採防火被覆。試驗進行至121分鐘，試體雖未達標準定義之性能基準，基於安全考量遂終止試驗。實驗結果顯示鋼梁撓度隨著升溫幅度增加。升溫初期，受火梁撓度變化較快且呈線性遞增。升溫中期，鋼梁已失去大部分承載能力且喪失結構行為，故撓度不再增加，整體行為改由樓版控制。升溫後期，樓版由撓曲機制逐漸轉換為薄膜效應，樓版裂縫持續發展且撓度持續增加。此階段由於樓版之薄膜效應提供了試體的承載力，使得整體結構保持穩定不致破壞；且因擴孔設計大幅的降低了軸向作用力，鋼梁之局部挫屈與塑性變形並不明顯，剪力板及螺栓無撕裂及剪斷之行為。總體而言，本研究試體於標準升溫下可達到2小時防火時效，而不同型式之剪力接頭則因擴孔設計及樓版薄膜效應之影響，於整體結構行為、溫度分布、破壞模式及防火時效皆無顯著差異。

The understanding of fire resistance and behavior of steel structural buildings in fire is extremely important because the material strength of the steel reduces greatly at elevated temperatures. Due to the restraining effects of the slab and girders, the steel beam in structural buildings will be subjected to axial compression caused by the thermal elongation in temperature rise and, however, the steel beams will be tensioned due to thermal contraction in the cooling phase. The thermal loading may result in failures on the shear tab and bolts of the shear connection. Corresponding to different types of shear connections, forces resulted from this thermal loading may differ and lead to different failure mode and fire resistance. In this study, two large-scale fire tests were carried out to investigate the deformation, temperature variation, failure mode, and fire resistance of the composite beam with shear connection under elevated temperatures. Two types of the shear connections, coped and un-coped beam ends, were used as the parameter of the specimens. In accordance with the specifications of the composite beam, the specimens were designed to have shear stud, reinforcing bar, slab, and oversized bolt-hole. Experiments were conducted following CNS 12514-1 and 12514-6 standards. Loading buckets were placed on the slab to simulate a load ratio of 0.6 in service condition of the steel beam. The furnace temperature followed standard heating curve. The steel beam and metal deck were exposed to fire while the girders were fire-protected. The tests terminated at 121 min considering the safety although the specimens didn’t reach the failure criteria. The test results showed that deflection of the steel beam increased with the rise of temperature. During the initial heating phase, the deflection varied quickly and increased linearly. In the middle heating phase, the change in deflection slowed down and stabilizing due to the loss of the steel beam strength and then the slab dominated the entire structural behavior. In the final phase of heating, the bending mechanism of the slab gradually shifted to membrane action mechanism, accompanied with more cracks on the top surface of the slab and large deflection of the steel beam. Because the membrane action of the slab provided the specimen load-carrying capacity, the whole structure remained stable. Meanwhile, axial force of the steel beam was speculated reducing greatly due to the oversized bolt-hole; thus, local buckling in the steel beam, rupturing of the shear plate, and shear failure of bolt were not observed. Overall, test results demonstrated that, exposed to standard fire, the specimens reached two hours fire rating. Different shear connection had insignificant effect on structural behavior, temperature distribution, failure mode, and fire resistance, attributed to oversized bolt-hole and membrane action developed on the slab.