鋼梁接箱型柱之內橫隔斷裂試驗與有限元素模型分析

Abstract

因鋼箱型斷面柱構件雙強軸的特性，所以在國內鋼建築結構中應用非常普遍。為傳遞梁端彎矩至柱構件，箱型柱內與梁翼同高處須配置與梁翼同厚的橫隔板。內橫隔板與柱板間之銲接常使用電熱熔渣焊(Electro Slag Welding, ESW)之工法，此種焊道施作便利及效率高，目前已大量使用於國內鋼結構製造廠。然而，ESW在施做時的高入熱量恐導致其焊道結晶過大與抗衝擊性質較差，此現象亦常導致梁柱發生非預期的脆性破壞。林克強等人(2008)針對此銲接細節進行22組之梁柱接頭試驗，試驗結果發現，當接頭尚未發展明顯非線性變形前，常在ESW銲道之熱影響區(HAZ)附近發生脆性破壞，其破壞機率約為18%。鄭元良(2011)承接林克強(2008)之試驗結果進行ESW元件試驗，實驗結果顯示ESW偏心與梁翼板高程偏心對於整體反應影響甚大。本研究為避免ESW發生脆性破壞及量化ESW破壞之機制，引用Kanvinde (2004)建議之鋼材斷裂預測模型，利用有限元素模型對試體進行斷裂預測分析。 本研究延續鄭元良(2011)之梁柱接頭試驗，另外進行兩組試驗，藉由變化試驗之載重歷時與ESW之施工儲倉口截面形狀，探討試體之破壞時機並驗證鋼材斷裂預測模型之可行性。實驗結果顯示，標準型載重歷時與增加型載重歷時之破壞時機分別在目標位移角為3%弧度之第2迴圈及第1迴圈，證實累積塑性應變大小確實影響其破壞時機；ESW儲倉口截面由矩形變為與喇叭口形時，破壞時機在目標位移角為3%弧度之第2迴圈位置在ESW之熱影響區，轉變為破壞在4%弧度第1迴圈位置在梁翼焊接之熱影響區，顯示若增加ESW之熔透範圍可增加接頭試驗耐震性能。 為應用斷裂預測模型，本研究藉圓周刻痕拉伸(CNT)試驗與有限元素模型分析，將所得之材料參數對ESW元件試驗與梁柱接頭試驗進行斷裂預測分析。分析結果顯示， ESW之幾何形狀及相對位置對於破壞時機影響甚大，精確之分析模型可有效預測破壞之位置與時間，因此顯示破壞預測分析與CNT試驗之可行性，以及銲道超音波檢測(UT)之重要性。經由實驗與分析所得之結果，本研究進行ESW相關幾何位置之參數分析，藉由ESW銲道之垂直偏心與水平偏心對應不同梁翼板之偏心情況，共建立45組有限元素模型，並預測破壞之時機。分析結果顯示ESW關鍵區域之大小確實為影響破壞時機之主要原因，且當梁翼板與ESW成垂直反向偏心時，破壞時機最早發生，因此擴大ESW關鍵區域可有效的降低ESW破壞之發生，也證實儲倉口截面為喇叭口形可增加其耐震性能。

Steel box columns are widely used in steel building structures in Taiwan because of the strong axes in two directions. In order to transfer the beam end moment to the column, diaphragm plates of the same thickness and elevations as the beam flanges are usually welded inside the box column. The electro-slag welding (ESW) process is usually used in attaching the diaphragms to the column flanges. This ESW process has been widely used in steel beam-to-box column joints in Taiwan because of its’ convenience and efficiency. However, the ESW may increase the hardness of the welds and heat affected zones (HAZs), while reduce the Charpy-V Notch (CVN) strength in the HAZ. This situation could cause the diaphragm to column flange weld to suffer premature fracture before a large plastic rotation is developed in beam-to-box column joints. Results of 22 full scale steel beam-to-box column connection tests conducted in 2008 by Lin et al. show that the probability of ESW fracture at the HAZs adjacent to the ESW is about 18%. Cheng et al. continued the study in 2011 with new tests on full scale beam-to-box column subassembly and ESW components. Test results show that the fractures of the diaphragm-to-column flange welds are sensitive to the eccentricity between the elevations of the ESW and the beam flange. In order to quantify the critical eccentricity and the effectiveness of predicting the fractures, this study adopts the fracture prediction model proposed by Kanvinde in 2004 and utilizes finite element model (FEM) analysis to correlate the test results. In this study, two additional beam-to-box column connection subassembly tests have been conducted with a different loading protocol or the shape of ESW chamber. It is found that the fractures of the aforementioned HAZs can be reasonably well predicted using the proposed procedures on applying the fracture model. Test results show that under the standard and increasing loading protocols, the instances of the HAZ fractures in the specimens correspond to the total beam end deformations of the 2nd and 1st cycles of 0.03 radian, respectively. It is confirmed that the fracture instances can be predicted based on the cumulative plastic deformation in the HAZs. Under the two same standard loading protocol, the flared ESW chamber alter the fracture location from the HAZ in the diaphragm weld to the beam flange weld, and delay the fracture at the 2nd cycle of 0.03 radian to the 1st cycle of 0.04 radian. Tests confirm that the possible fracture of the diaphragm to column flange welds can be mitigated by enlarging the chamber of the ESW. When the fracture prediction model is applied, the material parameters were firstly established from the Circumferential Notched Tensile (CNT) tests and FEM analysis. Subsequently, these parameters were used to predict the fractures observed in the ESW component tests and beam-to-box column connection subassembly tests. The fracture locations and instances can be reasonably well predicted by a suitable FEM model analysis. Thus, the effectiveness of CNT and the fracture model are confirmed. Analytical results also show fracture instances and locations are sensitive to the relative locations of the ESW and the beam flange. Thus, the importance of ultrasonic test in assuring the quality of the ESW is evident. Finally, a parametric study on the eccentricity between the ESW and beam flange locations is conducted using 45 models with various vertical and horizontal eccentricities of ESW with respect to the beam flange. The parametric study results show that the size of critical zone of ESW is indeed the most important factor affecting the fracture instance. When the relative vertical eccentricity between ESW and beam flange is increased, the fracture instances is accelerated. Enlarging the critical zone of ESW effectively reduces the possibility of fracture as evidenced by using a flared shape ESW chamber in the aforementioned test specimen.