高強度鋼梁柱接頭之幾何及梁翼偏心與儲倉口形狀對電熱熔渣銲破壞時機影響

Abstract

鋼箱型柱因具雙強軸特性，廣泛用在鋼結構建築中。為傳遞梁端彎矩至柱構件，須在柱內配置橫隔板。常用電熱熔渣銲（Electro-Slag Welding, ESW）連接橫隔板與柱板，此工法效率高但高入熱量會改變銲道周圍母材的結晶排列。施工失誤或箱型柱兩向梁深不同時，常造成梁翼板與橫隔板的偏心，易在銲道周圍熱影響區（Heat Affected Zone, HAZ）與柱板間的初始縫隙（Initial Slit）發生脆性破壞。將ESW熔填池的截面形狀（儲倉口）從傳統矩形改為喇叭形，以提升韌性容量，熔透範圍增加之優點可勝過入熱量增加之缺點。本研究以SM570M-CHW高強度鋼，製作ESW試體元件以探討儲倉口形狀及柱翼板厚度對脆性破壞的影響。本研究設計兩種儲倉口形狀，以兩種內橫膈板厚度，兩種柱板厚度及不同之梁翼偏心，進行八組ESW元件之單向拉伸試驗。試驗結果顯示，在梁翼厚放大1.25倍且內橫隔板厚等同柱翼板厚時，梁翼板偏心量增加到1倍內隔板厚度，會於ESW處發生非預期之破壞；柱翼板厚度為1.6倍內隔板厚的情況下，梁翼板偏心量能增加到1.25倍內隔板厚度，試體足以乘載強度發展。證實使用較厚的柱翼可使脆性破壞發生在ESW銲道周圍的機會降低。另將儲倉口改為喇叭型、或提升內橫隔板厚度，可使ESW熔幅範圍提升，提供更多韌性，增加對梁翼偏心的容忍度，延後破壞時機。 本研究引用Kiran and Khandelwal於2015年提出之MM-CVGM破壞預測模型，採用楊鈞堯（2021）材料試驗所獲得的模型參數，再針對銲接區域製成之圓周刻痕試棒進行單向拉伸試驗，以獲得MM-CVGM模型中材料的破壞曲線及臨界值，本研究證實MM-CVGM模型可用在ESW元件單向受載的破壞預測。分析結果顯示，SM570M-CHW的HAZ材料與ESW材料相比，HAZ更容易累積塑性應變，但兩者具有相近的韌性容量。另從有限元素模型分析所得ESW元件破壞位置以及破裂路徑，比對試體力學行為與試驗後切片觀察，相互應證MM-CVGM與有限元素模型都具有高準確度。最後建議ESW儲倉口可考慮採喇叭形、且背襯板應加厚至30mm，若柱翼板厚大於1.6倍內隔板，梁翼偏心不宜高於1.25倍內隔板厚；若柱翼板厚等於梁翼板，梁翼偏心更不宜高於0.75倍內隔板厚。

Steel box columns are widely used in steel structures in Taiwan due to the strong axes in two directions. In order to effectively transfer the steel beam moment to the column, diaphragm plates are welded inside the box column at the beam flange elevations. Electro slag welding (ESW) process is commonly used to attach the diaphragms to the column. Although this process provides a high welding efficiency, the high thermal input will change the lattice orientation of the base metal. When there exists construction imperfection or the two framing beam depths are slightly different, eccentricity between beam flange and diaphragm may be presented. This situation could lead to unpredictable brittle fracture in heat affected zone (HAZ) and initial slit between ESW and column plate. The fusion zone can be increased by changing the ESW chamber from a rectangular to a flared cross section. The advantage of a flared ESW section may out-weigh the disadvantage of the increased thermal input. In this study, SM570M-CHW high strength steel is selected for the column plates made into a few of ESW components to investigate the effect of chamber geometry and column flange thickness. In order to quantify the effectiveness of predicting the ESW fractures, this study utilizes finite element model (FEM) analysis together with the micro-mechanical cyclic void growth model (MM-CVGM) proposed by Kiran and Khandelwal in 2015 to predict the crack initiation, location, and crack path in the welded beam-to-box column joints under monotonic tensile loading. Eight ESW Components specimens were fabricated and tested. The key design parameters include two different chamber shapes, two diaphragm thicknesses, two column flange thicknesses with corresponding beam flange eccentricities. Test results show that when the thickness of column flange is equal to the diaphragm and the beam flange eccentricity exceed one time of diaphragm thickness, the component fails. When the thickness of column flange is 1.6 times as diaphragm and the beam flange eccentricity exceeds 1.25 times of diaphragm thickness, the component is strong enough for strength development. It demonstrates the benefits of increasing column flange thickness compensate the increase of beam flange eccentricity. While changing the ESW shape to the flared section, the fractures were highly delayed suggesting that the use of the flared ESW sections can significantly delay the crack initiation and enhance the ductility. This study follows Yang (2021), the results of material property, through micro-mechanical models and FEMA, in order to build the material failure curve and find the critical values in the MM-CVGM. Monotonic Circumferential notched tensile (CNT) coupon tests were also conducted at the related regions to examine the efficiency of using MM-CVGM on ESW components. Analytical results indicate that the HAZ zone in the SM570M-CHW base metal is able to accumulate a larger plastic strain than that in the ESW zone. However, both two regions have almost the same ductility capacity. Test and analysis results confirm that SM570M-CHW can sustain the high thermal input of ESW process. The locations and paths of the fractures were investigated using FEMA. Comparing the analytical results to the test results of the eight ESW components, confirmed that both the MM-CVGM and the finite element model have highly satisfactory accuracy. Based on the research results, it’s recommended that the flared ESW chamber can be implemented and backings should be thickened up to 30mm. When the column flange thickness is greater than 1.6 times of diaphragm, it is suggested that the beam flange eccentricity be no greater than 1.25 times of diaphragm. When the thickness of the column flange is equal to the diaphragm, it’s recommended that the beam flange eccentricity be no greater than three-quarters of the diaphragm thickness.