高強度鋼箱型柱翼厚與橫隔梁翼交疊高對電熱熔渣焊破壞時機影響

Abstract

箱型柱因具有雙強軸特性，成為臺灣鋼結構產業中常見的柱構件。梁柱抗彎接合須在對應梁翼高程處焊入至少與梁翼同厚度之橫隔板以傳遞梁彎矩，常見之焊接方法為在橫隔板與柱翼板間施作電熱熔渣焊(Electro-Slag Welding, ESW)。此焊接方法效率雖高，但其伴隨之高入熱量將影響焊道周圍鋼材之結晶性質，在電熱熔渣焊道或梁翼板發生偏心的情況下，ESW周圍之熱影響區(Heat Affected Zone, HAZ) 較易因應力集中現象而產生脆性開裂。本研究以SM570M-CHW高強度鋼材作為梁柱構件，探討ESW在受不同幾何配置以及母材厚度影響時之耐震行為，並採用有限元素模型分析與Kanvinde及Deierlein兩位學者於2004年提出之破壞預測模型(SMCS與DSPS，本研究簡稱為α Model與α Cyclic)預測梁柱接頭受反覆載重下之破壞時機，探討在不同柱翼板厚度與梁翼板偏心量下ESW是否會在達到4%弧度層間位移角前發生開裂以及其開裂時機。 本研究以胡祐瑋於2018年進行之兩組實尺寸梁柱接頭反覆載重試驗結果為基礎，另外設計兩組實驗配置並進行反覆載重試驗。四組實驗結果顯示，當梁翼板與橫隔板厚度皆為36mm，且在梁翼偏心量達一倍橫隔板厚度時，柱翼板厚度為25mm之試體於3%弧度層間位移角時即產生ESW開裂，而柱翼板厚度提升至45mm時，ESW於6%弧度層間位移角仍未產生開裂；當梁翼偏心量增加至45mm時，柱翼板厚度為25mm之試體ESW開裂時機提前至2%層間位移角，而柱翼板厚度為45mm之試體ESW仍可於層間位移角達6%時不產生開裂。 為獲得破壞預測模型中之材料韌性參數，本研究針對SM570M-CHW鋼材對應之ESW與HAZ區域製成圓周刻痕試棒(CNT)並進行反覆拉伸試驗，配合ABAQUS有限元素模型分析結果計算出α Cyclic中的材料參數λ以進行破壞時機預測。針對四組梁柱接頭試驗之破壞預測結果顯示，所預測之破壞時機皆早於實際破裂時機，代表α Cyclic預測模型偏向保守。本研究亦以柱翼板厚度、梁翼板厚度以及橫隔梁翼交疊高度為主參數進行參數研究，探討不同配置下對ESW破壞時機之影響。分析結果顯示，當柱翼板厚度與橫隔梁翼交疊高度提升，以及梁翼板厚度減少時，ESW周圍應力集中現象有明顯減緩之趨勢，並可降低ESW開裂之機會。本研究建議柱翼板厚度宜大於或等於橫隔板厚度，且橫隔梁翼交疊高度不宜低於四分之一倍之橫隔板厚度，以避免ESW之脆性破壞。

Steel box columns are widely used in steel structures in Taiwan because both two axes are equally strong. In the moment connections, diaphragm plates are welded inside the box column at the same elevations of beam flanges in order to transfer the beam end moment to the column. Electro-slag welding (ESW) process is commonly applied to attach the diaphragms to the column. The ESW process provides welding efficiency and convenience. However, the high thermal input during this welding procedure results in heat affected zones (HAZs) with an increased hardness and reduced Charpy notch strength. The HAZ may suffer severe stress concentration and fracture in the case of ESW or beam flange eccentricity. In this study, two full-scaled SM570M-CHW high strength steel welded beam-to-box column moment connection specimens were tested, and the key parameters are column flange thickness and beam flange eccentricity with respect to the diaphragm. This study applies finite element model (FEM) analysis as well as stress modified critical strain (SMCS) and degraded significant plastic strain (DSPS) models proposed by Kanvinde and Deierlein in 2004 to predict the fracture instance of ESW under cyclic loading. Two welded beam-to-box column moment connection tests were conducted by Hu in 2018. The test results show that the connection with the 25mm column flange thickness and a 36mm beam flange eccentricity (equals to the diaphragm and beam flange thickness) failed at the 3% IDR cycle. On the contrary, the other connection with the 45mm column flange thickness and the same eccentricity went through 6% IDR cycle without ESW fracture. In the present study, the same column specimen was used. When the beam flange eccentricity was increased to 45mm, the connection with the 25mm column flange thickness failed at the 2% IDR cycle, and the connection with the 45mm column flange thickness still went through 6% IDR cycle without ESW fracture. In order to compute the material parameter λ in the DSPS model, this study conducted circumferential notched tensile (CNT) coupon tests at the ESW and HAZ regions. The finite element models were constructed to analyze the response of the CNT specimens. After conducting regression analysis on the material parameters, this study applied DSPS model and FEM analysis to predict the crack instance of the aforementioned four moment connection tests. The analysis results show that the DSPS model tends to be conservative. This study also carried out parametric study, focusing on the effects of the column flange thickness, the beam flange thickness and overlapping distance of beam flange and diaphragm on ESW fracture. Results show that increasing the column flange thickness, or the overlapping distance of beam flange and diaphragm and decreasing the beam flange thickness reduce the stress concentration near ESW. In order to avoid the ESW fracture, this study recommends that column flange thickness be equal to or larger than diaphragm or beam flange thickness; and overlapping distance of beam flange and diaphragm be larger than one quarter of the diaphragm or beam flange thickness.