鋼管混凝土柱受潛變及收縮影響研究

Abstract

鋼管混凝土柱(Concrete-filled steel tubular column，簡稱CFT)高強度、高韌性且具有良好的耐震性能。其核心混凝土若使用自充填混凝土，澆置過程無須振動搗實，可加速施工時程，故CFT柱常成為台灣高樓層建築物結構柱的選擇。 早期鋼管內灌混凝土是為了增加構件的側向勁度，並延緩挫曲行為的發生。然而近幾年，工程師開始在設計階段將核心混凝土納入承受垂直荷載的材料。混凝土一旦受力，潛變就會隨時間持續發展，而柱構件為了保持其斷面力平衡，混凝土會將承擔的部分軸力移轉至鋼管，這些移轉量可能會使鋼管降伏，影響結構物的安全。此外，隨著科技進步，自充填混凝土的強度已可達90MPa，強度越高的混凝土，水膠比越低。過低水膠比的混凝土會產生大量自體收縮，使構件間應力移轉現象愈嚴重。 本研究以有限元素分析軟體ABAQUS建置三維鋼管混凝土柱模型，考慮混凝土潛變收縮行為引致之構件內部應力移轉的情形。另外，台灣粒料強度及彈性模數不佳，造成混凝土配比中漿體量大於國際，長期變形亦受影響。若直接引用國外潛變收縮公式會造成模擬分析失準。因此本研究將整理比較國內外混凝土潛變收縮公式，選擇可適當描述台灣混凝土長期變形的預測式。 模擬結果顯示，在初始鋼骨應力為0.6f_y的情況下，無論荷載是否偏心，考慮混凝土潛變收縮效應的最終鋼應力皆有機會超出《鋼構造建築物鋼結構設計技術規範》檢核標準。而且在使用高強度自充填混凝土及構件高寬厚比的極端情況下，鋼應力可上升0.33f_y，將大幅超過原始設計值。構件寬厚比及鋼管降伏強度顯著影響最終鋼應力，設計時應避免高寬厚比及低鋼管降伏強度的組合。各國鋼管混凝土柱潛變收縮規範仍相當簡略，僅注重短期荷載效應。建議台灣應修訂《鋼骨鋼筋混凝土構造設計規範與解說》並納入混凝土長期變形對於鋼管應力的影響。

Concrete-filled steel tubular (CFT) columns show not only high strength and high ductility but also exhibit favorable seismic performance. Besides, CFT columns infilled with self-consolidating concrete (SCC) shorten construction time because of its self-compacting characteristics. As a result, these kind of structural members have been widely adopted in high-rise buildings in Taiwan. The primary intent of concrete infill is to increase lateral stiffness of member and delay the local buckling of the steel tubular. However, in the recent future, engineers begin to incorporate concrete in materials subjected to axial load during design. Once concrete is subjected to load, development of concrete creep begins. In order to maintain the equilibrium of forces of CFT section, part of axial load of concrete will be transferred to steel tubular which leads to the growth of steel stress. Furthermore, with the ever-changing nature of technology, concrete compressive strength of SCC has reached up to 90 MPa. In general, the higher the concrete compressive strength is, the lower the water cement ratio is. Concrete with low water cement ratio intensifies the rise of steel stress in CFT columns on account of high autogenous shrinkage. A three-dimensional finite element model of CFT column, which takes account of the phenomenon of concrete creep and shrinkage, is developed to evaluate stress transfer between concrete and steel in ABAQUS. According to recent research, there is a characteristic of high amount of paste in concrete mix designs in Taiwan owing to the soft nature of coarse aggregates. It is not appropriate to directly adopt foreign prediction formulas which will lead to underestimate long term deformation of concrete in the CFT columns. Consequently, this study compares different creep and shrinkage prediction formulas for concrete to gain a better result of the research. The analysis results show that under the condition of initial steel stress of 0.6f_y, the final steel stress of CFT column is probably not qualified according to “Design and Technique Specifications of Steel Structures for Buildings” owing to the long term deformation of infilled concrete whether the load is eccentric or not. In the extreme case of high concrete compressive strength of SCC and high diameter to thickness ratio, the steel stress significantly exceeds the original design value with 0.33f_y. Apart from this, combination of high diameter to thickness ratio and low yield strength of steel should be avoided during design due to the considerable growth of steel stress. However, specifications among countries so far merely focus on the short term loading performances of CFT columns, while its time dependent behavior is deficient. It is suggested that relevant specifications should be revised in “Design Specifications and Commentary of Steel Reinforced Concrete Structures” in Taiwan.