철근의 마디형상을 고려한 수정 정착설계식 제안

Modified Development Design Equation Proposal on the Rib-Shape of Reinforcement

Continued efforts to improve the bond strength were continuously achieved in order to prevent the increase of development and splice length by the development of high strength reinforcement. High relative rib area reinforcing bars by increasing the relative rib area were developed to improve the bond strength, however there is a limit to increase the bond strength because the increase of the rib height can cause the pullout failure. A few domestic researcher suggested new deformation pattern of bars with high and low height tranverse ribs, and these alternating ribs are expected to help limiting local shear failure and increase the bond strength significantly. They verified its bond performance by various experimental research results. But it is very difficult to reflect the bond strength increase in the design equation because most development design equations did not include the variables which reflect the deformation pattern of the bars. This study suggested the modified development design equation to consider the bond strength increase of the new deformation pattern of bars. Previoue research results and proposed design equations were synthesized and analyzed to draw a applicable equation for it. Proposed equation was based on the equation of ACI 408 committee, and replaced the relative rib area $R_r$ to modified relative rib area ${R_r}^{\prime}$ using the weighted average of high and low rib height as 0.8 and 0.2 each. And the analysis of previous experimental results indicated that the restriction value of $(c{\omega}+K_{tr})/d_b$ was too conservative, and it is possible to extend it to 4.5.