Report on Equivalent Rectangular Concrete Stress Block and Transverse Reinforcement for High- Strength Concrete Columns
|Publication Date:||1 July 2018|
High-strength concrete (HSC) has advantages over normal-strength concrete (NSC), especially for columns of high-rise buildings, as it reduces column sizes and increases the durability of concrete (Smith and Rad 1989). Moreover, HSC can be advantageous with regard to lateral stiffness and axial shortening (Colaco 1985). Another advantage cited by Colaco in the use of HSC columns is the cost reduction of formwork stemming from reduced cross-sectional dimensions. This economic advantage is achieved by using HSC in lower-story columns and reducing concrete strength over the height of the building while keeping the same column size over the building height.
Increased use of HSC has caused concerns over the applicability of the current building code requirements (ACI 318) for the design and detailing of HSC columns. Those concerns are mainly related to: 1) equivalent rectangular concrete stress distribution; and 2) transverse reinforcement requirements for seismic design. Chapter 22 of ACI 318-14 provides a concept of equivalent rectangular concrete stress distribution (ACI concrete stress block) for design of reinforced concrete columns. In the equivalent rectangular concrete stress block, an average stress of 0.85fc′ is used with a rectangle of depth a = β1c. The 1976 supplement to the 1971 code adopted a lower limit of β1 equal to 0.65 based on research data from tests with concrete strengths exceeding 8000 psi (55 MPa). Several research studies reported that the use of current rectangular concrete stress block expressions of ACI 318 could produce overestimated flexural and axial strengths of HSC columns (Wahidi 1995; Ibrahim and MacGregor 1996; Lloyd and Rangan 1996). As a result, alternative concrete stress block expressions have been proposed (Ibrahim and MacGregor 1997; Bae and Bayrak 2003; Ozbakkaloglu and Saatcioglu 2004; Azizinamini et al. 1994).
Numerous research studies (Sakai and Sheikh 1989; Elwood et al. 2009a,b; Paultre and Légeron 2008) have been conducted in several countries to investigate the behavior of HSC columns, to evaluate similarities or differences between HSC and NSC columns, and to identify important parameters affecting performance of HSC columns designed for seismic, as well as nonseismic, areas. As a result, Chapter 18 of ACI 318-14 has updated the transverse reinforcement requirement to address the concern on the use of HSC columns for seismic design.
This document reports the results of recent studies on the equivalent rectangular concrete stress distribution, or concrete stress block, and transverse reinforcement requirements of HSC columns for seismic design.