高強度竹節鋼筋之混凝土劈裂握裹性能與拉力伸展長度

Bond Splitting Performance of Concrete and Development Length in Tension for High-Strength Deformed Bars

10.6849/SE.202203_37(1).0001

林克強(Ker-Chun Lin)；林垣彥(Yuan-Yan Lin)；紀凱甯(Kai-Ning Chi)；莊勝智(Sheng-Jhih Jhuang)；王勇智(Yung-Chih Wang)

高強度鋼筋混凝土 ； 伸展長度 ； 鋼筋強度等級修正係數 ； 劈裂指數 ； 握裹應力 ； High-strength reinforced concrete ； development length ； modification factor of steel grade ； split index ； bond stress

結構工程

37卷1期（2022 / 03 / 01）

5 - 24

繁體中文

規範ACI 318-19之竹鋼筋直線受拉伸展長度公式，增加鋼筋強度等級修正Ψ_g，鋼筋強度等級超過420 MPa時，對於550與690 MPa者，須依規範ACI 318-14的計算長度分別加長1.15與1.3倍。現行規範中鋼筋伸展長度設計公式是基於混凝土劈裂機制推導而得，混凝土的劈裂機制主要受混凝土與鋼筋接觸的性質影響，故本文的主要目的在探討鋼筋強度等級修正Ψ_g存在的合理性。本研究共進行20組梁端的鋼筋握裹試驗試體，伸展鋼筋分別採用420、550與690 MPa三種強度等級之#10鋼筋，同時也變化混凝土強度與劈裂指數兩參數。試驗結果顯示，20組試體中的15組試體發生如預期的混凝土劈裂破壞，試體之劈裂指數甚至達4.93。從試驗握裹應力的分析中也證實，若混凝土強度與劈裂指數分別採用70MPa與2.5的上限限制時，規範ACI 318-14計算所得之直線伸展長度的試驗平均握裹應力比介於1.72至2.4之間，可提供充分的安全餘裕，因此，ACI 318-19之受拉伸展長度公式中，無需額外增加鋼筋強度等級修正增長因子Ψ_g。若將混凝土強度與劈裂指數上限條件分別提升至100 MPa與5.0，可獲得約10%保守餘裕的準確結果，惟若應用於工程實務，應考量適當的安全係數。

For ACI 318-19 Code, a modification factor of steel grade of Ψ_g that is equal to 1.15 or 1.3 for 550 or 690 MPa grade of steel bar exceeding 420 MPa was added to increase straight development length in tension for deformed bar. It is known that the design equation of development length stipulated in the existing ACI 318 Code was obtained based on a mechanism of concrete splitting primarily dominated by contact characteristics between concrete and steel bar. This paper was intended to investigate the rationality of the Ψ_g included in the design equation of development length. A total of 20 beam-end specimens was carried out to study bond behaviors of concrete splitting with a deformed bar in this research. A #10 bars for three various grades of 420, 550 and 690 MPa was used as developed steel bar per specimen. Concrete strength and split index were study parameters as well. Test results indicated that the anticipated concrete splitting occurred in 15 specimens of all 20 ones even took place in the specimens with the split index of 4.93. Based on analysis overcomes of the test bond stress in this research, it was confirmed that under upper limitations of 70 MPa for the concrete strength and 2.5 for the split index, the average bond stresses of the three various grade bars for ACI 318-14 Code ranged from 1.72 to 2.4. The modification factor of steel grade of Ψ_g seemed to not be needed in the provisions of straight development length of ACI 318-19 Code. It was certificated as well that raising the limit limitations of 100 MPa for the concrete strength and 5.0 for the split index, good results with around 10% conservative margin could be obtained according to the provisions of straight development length of ACI 318-14 Code. However, a safety factor should be considered for applications of engineering practice.

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