Examination of Stress Block Parameters for High-Strength Concrete in the Context of ACI 318 Code


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Title: Examination of Stress Block Parameters for High-Strength Concrete in the Context of ACI 318 Code

Author(s): Sungjin Bae and Oguzhan Bayrak

Publication: Special Publication

Volume: 293


Appears on pages(s): 1-20

Keywords: high strength concrete; stress block; interaction curve; column; beam.

Date: 10/4/2013

In an attempt to provide consistently conservative yet reliable estimations of flexural and axial strengths of concrete columns, various stress block parameters have been proposed within the last two decades. The fact that flexural and axial strengths of many tested high-strength concrete columns were overpredicted by the current ACI 318 stress block parameters is the primary motivation behind all of the proposals for stress block parameters. Chapter 10 (Flexure and axial loads) of ACI 318-11 introduces the concrete stress block parameters and provides design formulas for calculating flexural and axial strengths and bearing strengths. The stress block is also used for various design applications in other chapters of ACI 318. Those chapters include Chapter 18 (Prestressed concrete), Chapter 22 (Structural plain concrete), Appendix A (Strut-and-tie models) and Appendix B (Alternative provisions for reinforced and prestressed concrete flexural and compression members). All ACI 318 design implications stemming from any suggested changes for the concrete stress block parameters needs to be examined holistically. This paper provides a comprehensive examination of various stress block parameters. Flexural and axial strengths predicted by different stress blocks are compared with experimentally-obtained strengths from 224 column tests. Normalized P-M interaction curves are developed for this purpose. In addition, the impact of change of stress block parameters on other design expressions is examined. They include the bonded tendon stress in prestressed concrete and the compressive stress of bottle-shaped struts in strut-and-tie model.