| 摘要: | 本研究的目的是藉由進行New RC梁受高週次循環載重之疲勞試驗及蒐集先前相關研究[5, 7, 9-11]與本實驗室測試數據[38-42]，以更準確地獲得高強度鋼筋之疲勞S-N曲線，並取得疲勞設計強度（fr,limit），作為往後New RC橋梁結構疲勞設計之參考。
研究中比較New RC梁與普通RC梁在疲勞試驗中的行為，包含荷重-撓度關係、混凝土表面應變、裂縫發展、疲勞破壞情形與鋼筋疲勞斷裂面分析、探討New RC梁與普通RC梁在疲勞反覆荷載過程中勁度EcIe值的變化等。
當中整理所蒐集有關各級鋼筋強度之RC梁疲勞測試數據，以多重變數之線性迴歸方式，求出一修正S-N曲線（公式5-3）。其中，修正式比起原S-N曲線（公式2-5），增加考慮大尺寸鋼筋直徑效應與各級鋼筋強度。從修正式與百萬週次以上產生疲勞之測試值作比較，發現New RC梁以125萬週次代入S-N曲線修正式，可合理推導求得疲勞設計強度公式（詳公式5-4a），以供日後普通RC與New RC梁作具安全性且經濟性之高週次RC梁疲勞設計與分析。
本文亦研究New RC梁疲勞設計參數敏感度分析，結果當應力差幅越低、鋼筋表面竹節幾何特性（r/h值）越大、鋼筋直徑越小、鋼筋等級越大、最小應力越小，對抵抗高週次反覆荷載形式之疲勞較為有利，且鋼筋之疲勞強度會提升。文中亦探討現行AASHTO規範之適用性，發現2020年版AASHTO LRFD橋梁設計[3]之疲勞強度規定（詳公式2-2）只納入fs,min與fy參數是過於簡化，對New RC梁會呈現過於保守之趨勢，其中r/h只考慮0.3是不符合近期鋼筋市場之多元性需求。
最後也探討梁勁度EcIe值隨反覆荷載週次衰減情形。一般而言，RC梁受反覆荷載初期均有大幅度的折減，平均約30萬循環週次後，勁度折減速度才趨緩，其最大折減接近第一週次的30%左右。再者，New RC梁與普通強度鋼筋混凝土梁相比，兩者在中點撓度換算之勁度折減的趨勢大致相同。其中，以New RC梁的勁度折減略大於普通RC梁。有關於New RC梁EcIe值折減問題，值得再進一步作探討。;The objectives of the study is to obtain the more accurate S-N curve by means of conducting fatigue tests on New RC beams under high-cycle cyclic loading and collecting testing data from previous related studies [5, 7, 9-11] and our laboratory′s test data [38-42] and fatigue design strength (fr,limit) for the fatigue design of New RC bridge structures in the future.
The study compares the behavior of New RC beams and conventional RC beams under fatigue tests, including load-deflection relationships, concrete surface strain, crack development, fatigue failure conditions, and rebar fatigue fracture section. It also investigates the variation of stiffness values (EcIe) of New RC beams and conventional RC beams during the process of repeated cyclic loading.
Collected fatigue test data for RC beams with various rebar strengths were analyzed using multiple variable linear regression to derive a revised S-N curve (Equation 5-3). The revised curve, compared to the original S-N curve (Equation 2-5), considers the diameter effect of large-size rebars and various rebar strength grades. Comparing the modified curve with the test values for more than one million cycles, it was found that using 1.25 million cycles in the S-N curve modification formula allows for a reasonable derivation of the fatigue design strength formula (detailed in Equation 5-4a), providing a safe and economical design and analysis method for high-cycle fatigue of conventional RC and New RC beams in the future.
This paper also examines the sensitivity analysis of fatigue design parameters for New RC beams. The results indicate that lower stress range, larger rib geometry (r/h value), smaller rebar diameter, higher rebar grade, and lower minimum stress are more favorable for resisting high-cycle repetitive loading, thus enhancing the fatigue strength of the rebar. Additionally, the applicability of the current AASHTO specifications was explored. It was found that the fatigue strength provisions in the 2020 AASHTO LRFD Bridge Design Specifications [3] (detailed in Equation 2-2), which only include the fs,min and fy parameters, are overly simplified and conservative for New RC beams. The specification′s consideration of r/h only at 0.3 does not meet the diverse needs of the recent rebar market.
Finally, the paper discusses the decay in stiffness (EcIe) values of beams with repeated loading cycles. Generally, RC beams experience a significant reduction in stiffness in the early stages of repeated loading, with the reduction rate slowing down after approximately 300,000 cycles, and the maximum reduction approaching 30% of the first cycle. Furthermore, the stiffness reduction trend, converted by mid-span deflection, is similar for both New RC beams and conventional strength RC beams, with New RC beams showing slightly greater stiffness reduction compared to conventional RC beams. The further investigation on the of stiffness (EcIe) reduction in New RC beams is needed. |
