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Title: Effect of Reinforcement Layers, Side Cover, and Bond Stress on Curved-Bar Nodes

Author(s): Hwa-Ching Wang, Christopher S. Williams, and Gary J. Klein

Publication: Structural Journal

Volume: 119

Issue: 3

Appears on pages(s): 277-290

Keywords: bend radius; curved-bar node; design code; frame corners; knee joints; strut-and-tie method

Date: 5/1/2022

Abstract:
A curved-bar node is formed in a strut-and-tie model at locations where two ties representing the bend region of continuous reinforcing bars are equilibrated by a strut or the resultant of multiple struts. The most common example is present within a strut-and-tie model of a knee joint subjected to closing moments. In ACI 318-19, design provisions for curved-bar nodes were introduced based on expressions derived by considering equilibrium of the assumed nodal region and verified with a limited number of experimental studies available at the time. It was recognized, however, that additional investigations were needed to understand the applicability of these expressions for various cases. An experimental program consisting of tests on 20 knee joint specimens was conducted to study the influence of multiple layers of longitudinal reinforcement, clear side cover, and bond stresses within the bend region of the longitudinal bars. The results demonstrate that curved-bar nodes with two layers of longitudinal reinforcement can be designed following the expression for the minimum bend radius of reinforcing bars in Section 23.10.2 of ACI 318-19. Furthermore, the results indicate that no changes to the current provisions regarding clear side cover are needed. However, the code provision related to the development of bond stresses appears to be unnecessary based on the tests.


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