ACI 441.1R-18 Report on Equivalent Rectangular Concrete Stress Block and Transverse Reinforcement for High- Strength Concrete Columns

ACI 441.1R-18 Report on Equivalent Rectangular Concrete Stress Block and Transverse Reinforcement for High- Strength Concrete Columns.
A parametric study was conducted as part of this report to provide further insight into the diferences among various proposals. Column interaction diagrams were calculated without the strength-reduction factors ϕ to compare ACI 318 stress block with the other code expressions and proposals of various researchers. The column cross section analyzed is shown in Fig. 3.2c, with the bending moment about the Y-Y axis. The column was analyzed for steel ratio of 1 percent and concrete compressive strengths of 4000, 8000, 12,000, and 16,000 psi (28, 55, 83, and 110 MPa). Results of the parametric study are given in Fig. 3.2d. From Fig. 3.2d(a) and 3.2d(b), for concrete compressive strengths of 4000 and 8000 psi (28 and 55 MPa), the nominal column strength obtained by the ACI 318 stress block were noticeably diferent from those of the CSA A23.3-04 and Ibrahim and MacGregor (1997). The stress blocks of CSA A23.3-04 and Ibrahim and MacGregor (1997) resulted in progressively smaller estimates of nominal column strength as concrete compressive strength increased. Other stress blocks resulted in identical estimates of nominal strength to ACI 318 at a concrete compressive strength of 4000 psi (28 MPa), and produced slightly more conservative estimates than ACI 318 at a concrete compressive strength of 8000 psi (55 MPa). For a concrete compressive strength of 12,000 psi (83 MPa) in Fig. 3.2d(c), all stress block expressions resulted in conservative estimates compared with the ACI 318 stress block. The stress block proposed by Bae and Bayrak (2003) produced less-conservative nominal strength estimates in comparison to other stress block expressions, except for ACI 318. For a concrete compressive strength of 12,000 psi (83 MPa), the stress blocks of NZS, Azizinamini et al. (1994), and Ibrahim and MacGregor (1997) produced identical or close to identically the same nominal strengths. For a concrete compressive strength of 16,000 psi (110 MPa) in Fig. 3.2d(d), all stress block expressions yielded more-conservative results than those obtained by using the stress block of ACI 318. However, the model proposed by Azizinamini et al. (1994) resulted in severe underestimates of strength when compared to the other expressions.
4.1—Constitutive models for confned concrete Several researchers have indicated that constitutive models developed for normal-strength concrete (NSC) do not ofer a good representation of the behavior of HSC, espe- cially in the case of columns, where the characteristics of the constitutive model have the highest impact on the calcu- lated response. Therefore, previously developed constitu- tive models have been modifed to refect the diferences in behavior, and several additional analytical models have been developed specifcally for HSC. Ahmad and Shah (1982), Martinez et al. (1984), and Faftis and Shah (1985) were among the frst to develop models for high-strength confned concrete based on tests of spirally reinforced small cylinders. These models incor- porate the efect of confnement through a lateral confning pressure that develops under hoop tension. The models were shown to produce good correlations with tests of spirally confned circular cylinders for concrete strengths of up to 12,000 psi (83 MPa).ACI 441.1R pdf download.