採高強度鋼筋之RC 梁與柱之結構行為

Abstract

進行了實驗和一系列分析程序來研究使用高強度鋼筋的鋼筋混凝土（RC）樑和柱的結構性能。 第一部分透過12根足尺RC樑的試驗檢驗了採用SD790高強度箍筋的RC樑的性能。 考慮的設計參數包括剪跨深度比、箍筋間距、箍筋屈服強度、混凝土抗壓強度和縱向受拉配筋率。 實驗結果結合先前研究收集的數據表明，ACI 318-19剪切強度方程中的應力極限可以從目前的550 MPa提高到600 MPa。 甚至可以達到790MPa，但安全水準卻大幅下降。 同時，提出了估計最大剪切裂縫寬度的方程，不僅為本次試驗的測量裂縫寬度提供了良好的預測，也為其他研究提供了良好的預測。 這為裂縫寬度控制設計提供了更全面的工具，因為 ACI 318-19 透過在重力負載條件下的剪切強度設計中施加 420 MPa 的限制來間接控制裂縫寬度。 上述發現使設計人員在設計上具有更大的自由度，因為不同的項目可能有不同的標準（裂縫寬度限制）或優先順序（裂縫寬度控製或僅剪切強度）。 第二部分對六根發生彎曲失效的足尺樑的試驗結果進行了分析。 這些梁採用 SD420W 變形鋼筋、1860 MPa 無應力七股鋼絞線或兩者的混合進行縱向加強。 隨著用絞線取代變形鋼筋的增加，裂紋間距、裂紋寬度和位移增加，而等效剛度、彎曲超強和延展性降低。 傳統的應變相容性仍然適用於具有絞線的截面，儘管與混凝土的黏結力稍低。 Frosch 方程中提出了絞線的黏結係數，以便更好地估計僅使用絞線的截面中的最大裂縫寬度。 提出了三個不同的模型來展示股線中的應力-應變關係。 這些模型成功地預測了彎矩強度並提供了相當準確的彎矩曲率計算。 對於僅使用絞線的梁，提出了 0.0145 和 0.0115 的拉伸和壓縮控制截面的相應應變限制。 使用 ACI 318-19 方程式比使用 ACI 318-14 方程式更好地預測樑的撓度。 第三部分透過兩個全尺寸樑柱節點試體（一內一外）的試驗結果，研究了高強縱向鋼筋SD690和大直徑（36 mm）的黏結滑移。 對梁/柱尺寸及其縱向鋼筋間距提出了標準化設計，以方便實際施工。 然後根據此標準化設計與最小深度要求的結合來確定柱深度，以避免內部接縫中的黏結分裂，並且由於有頭鋼筋而增加了錨固長度，但外部接縫中使用的間距小於規範。 兩個樣本都具有良好的性能，儘管如果期望內部接頭的性能得到改善，則可以使用更高的柱深度。 對於縱向鋼筋 SD690（100 等級），超強係數 1.2 比目前的 1.25 更合適。 第四部分採用雙曲率循環荷載下的四根原型柱來研究高強縱向鋼筋SD690的黏結分裂問題。 使用連續鋼筋（無搭接拼接）的大直徑（32 毫米和 36 毫米）。 所有柱子在強度發展、漂移能力和能量耗散方面都表現良好。 儘管違反了 ACI 對開髮長度的要求 (1.25l_d≤l_u/2), 但沒有發生鍵斷裂。 這是因為 ACI 指引在預測發育長度上過於保守。 為了改進，提出了限制項的上限為 3.0，並且沒有加強等級因子。 此外，透過考慮橫向鋼筋的限制，可以實現預測柱抗彎強度的準確性，因為隨著軸向荷載比的增加，過度保守的程度變得更加明顯。

An experimental and a series of analytical programs were carried out to study the structural performance of reinforced concrete (RC) beams and columns using high-strength steel reinforcement. In the 1st part, the performance of RC beams using SD790 high-strength stirrups was examined through testing twelve full-scale RC beams. The design parameters considered were shear span-to-depth ratio, stirrup spacing, stirrup yield strength, concrete compressive strength, and longitudinal tension reinforcement ratio. The experimental results combined with collected data from previous studies showed that the stress limit in the ACI 318-19 shear strength equation could be raised from the current 550 MPa to 600 MPa. It can even go up to 790 MPa, but the safety level declined considerably. Meanwhile, an equation to estimate maximum shear crack width was proposed, yielding a good prediction not only for the measured crack widths of this test but also for the other research. This provides a more comprehensive tool in the design of crack width control since ACI 318-19 indirectly controls it by imposing a limit of 420 MPa in the shear strength design under gravity load conditions. The above findings allow the designers more freedom in the design since different projects might have different criteria (on the crack width limit) or priorities (crack width control or shear strength only). In the 2nd part, the test results of six full-scale beams that failed in flexure were analyzed. These beams were longitudinally reinforced by either deformed bar SD420W, unstressed seven-wire strands 1860 MPa, or a mix between them. The crack spacing, crack width, and displacement were observed to increase, while the equivalent stiffness, flexural overstrength, and ductility were reduced with the increase of replacing deformed bars with strands. The traditional strain compatibility remains applicable to the sections having strands, although the bond with concrete was slightly lower. A bond coefficient for the strands was proposed in Frosch’s equation to better estimate the maximum crack width in the sections using strands only. Three distinct models were presented to demonstrate the stress-strain relationship in the strands. These models successfully predicted the moment strength and provided a reasonably accurate calculation of the moment-curvature. The corresponding strain limits for the tension- and compression-controlled sections of 0.0145 and 0.0115 were proposed for the beams using strands only. The deflection of the beams was better predicted with the equation of ACI 318-19 than that of ACI 318-14. In the 3rd part, the bond-slipping of high-strength longitudinal reinforcement SD690 and large diameter (36 mm) was investigated through test results of two full-scale beam-column joint specimens (one interior and one exterior). A standardized design for the beams/columns size and its longitudinal reinforcement’s spacing was proposed for more convenience in actual construction. The column depth was then decided based on the combination of this standardized design with the minimum depth requirement to avoid bond-splitting in the interior joint, and with the increased anchorage length due to headed bars but smaller spacing than Code was used in the exterior joint. Both specimens had a good performance, although a higher column depth might be used if an improved performance is expected for the interior joint. For the longitudinal reinforcement SD690 (Grade 100), an overstrength factor of 1.2 was more suitable than the current of 1.25. In the 4th part, four prototype columns under double-curvature cyclic loading were used for examining the bond-splitting issue of high-strength longitudinal reinforcement SD690. The large diameters (32 mm and 36 mm) with continuous bars (no lap-spliced) were used. All columns performed well in terms of strength development, drift capacity, and energy dissipation. Bond-splitting did not occur, although the ACI requirement for development length was violated (1.25l_d≤l_u/2). This is explained by the over-conservativeness of ACI guideline in predicting the development length. For improvement, a higher limit of 3.0 for the confinement term and no reinforcement grade factor were proposed. Additionally, the accuracy in predicting the flexural strength of the columns could be achieved by considering the confinement of transverse reinforcement, as the level of over-conservatism becomes more pronounced with higher axial load ratios.