強度混合型斜撐往覆加載行為之有限元素分析

Abstract

鋼結構同心斜撐構架是藉由斜撐構件受拉降伏與受壓挫屈來進行塑性消能，大部分的變形集中於斜撐中間區域。本研究提出強度混合型斜撐，目的是將中間區域替換為不同的斷面、材料與長度範圍，除了讓斜撐有更大的消能區域增加消能能力外，也能改善斜撐中間段斜撐因受壓挫屈後又受拉而形成的大量塑性變形集中，進而增加斜撐之韌性與疲勞壽命。 本研究以實驗結果校正有限元素分析模型，並以分析軟體ANSYS進行一系列強度混合型斜撐受往覆加載的行為分析。為探討不同寬厚比、材料與替換長度的影響，分析的試體包括不同寬厚比的H型斷面斜撐與方管斷面斜撐，而中間段皆以符合現行規範但不同長度的H型斷面取代，材料則包括A36、SM570。分析比較的參數包含von Mises應力分佈與等效塑性應變指數(PEEQ)等。分析結果顯示中間段以高強度等斷面的型式取代的試體，降低塑性變形的效果較不理想，PEEQ降低4～15％，試體的PEEQ大於其他類試體；中間段以不同斷面型式取代的試體，可降低von Mises應力，且PEEQ值比原始未加工的斜撐降低10~40%，此方式有效延遲斜撐的斷裂；對於寬厚比較大的斜撐而中間段以不同斷面型式取代的試體，von Mises應力也可有效降低，而PEEQ可降低40～55％，尤其以原斷面為方管的系列下降最多。分析結果建議中間段之寬厚比小於原斷面即可降低PEEQ，而中間段長度則應超過斜撐全長之55%。若設計參數經過適當選擇，強度混合型斜撐能有效提升斜撐韌性與消能能力，延長斜撐疲勞壽命

Conventional steel brace dissipates energy by its tension yielding and compression buckling. Most of the inelastic deformation of a buckling brace concentrates in the mid-length. This region dissipates most energy in a brace under cyclic loading, but it is also vulnerable to failure modes associated with low-cycle fatigue. In this research, we proposed a strategy to improve the cyclic behavior of the conventional steel braces by a spliced mid-segment with more suitable sectional properties and materials. By doing so, we can prevent the local section from concentration of large plastic deformation caused by cyclic tension and compression. We use ANSYS to conduct the finite element analyses of braces under cyclic loading. The primary variables to be investigated include the cross-sectional shape, material and length of the mid-segment. Numerical speciments include wide-flange and hollow square braces with various compactness ratios. The mid-segments of all the specimens are all made up of wide-flange sections with compactness ratios comforming to the current design code. Based on the numerical simulations we compare the distributions of von Mises stress and equivalent plastic strain (PEEQ) which are used as indices of fatigue life in this study. The analytical results show that it is ineffective to replace the mid-segment with the same section but higher-strength material; the PEEQ is only reduced by 4% to 15% which is less than the other series of specimens. Contrarily, the braces with more compact mid-segment show more favorable distributions of von Mises stress and lower PEEQ; the PEEQ is reduced by 10% to 40%. For the braces with slender original section, when the mid-segments are replaced with more compact sections, the PEEQ is reduced by 40% to 55% and von Mises stresses are also reduced. To reduce PEEQ most effectively, the analytical results suggest that the mid-segment should be replaced with a more compact section and length greater than 55% of the original length. If the parameters are properly designed, the ductility, energy dissipation capacity, and fatigue-life of conventional buckling braces can be enhanced obviously by splicing a mid-segment.