1566 Flexure and Axial Loads

Figure 15.23 compares the simplified equivalent stress distributions for determining the nominal resistances under flexure and axial load. EC2-02 permits the use of several equivalent stress distributions — the preferred one being the parabola-rectangle stress distribution. In the use of the equivalent rectangular stress distributions prescribed by the different codes, there are significant differences in the treatment of higher strength concretes. CSA A23.3-94 and NZS 3101-95 have departed from the traditional ACI stress block factor a1 of 0.85 to give a1 values that are as low as 0.67 for very high-strength concrete. EC2-02 uses stress block factors that are reduced for concrete having a compressive strength greater than 50MPa. In EC2-02, acc is a factor accounting for long-term and loading effects, and is recommended to be 1.0. EC2-02, however, indicates that this factor may be changed by amendments in a National Annex when adopted by an individual country. Figure 15.24 illustrates the differences in the axial load bending interaction diagrams for columns containing 1 and 4% of vertical reinforcement as the concrete compressive strength varies from 30 to 80 MPa. In calculating the predictions using the EC2-02 code, the traditional value of 0.85 has been used for acc. It is noted that the four code approaches, compared in Figure 15.24, give similar nominal strength predictions for the lower-strength concrete columns. Above a concrete strength of 50 MPa, however, the ACI 318-02 approach predicts significantly higher column capacities than the other three codes for "compression controlled'' columns. This difference is particularly evident for columns containing the minimum amount of vertical reinforcement (1%) permitted by the codes. In view of observations [79] that there is a tendency for splitting and splitting of the concrete cover in high-strength concrete columns, CSA A23.3-94 requires that, for columns with f greater than 50 MPa, the column ties must have 135°-bend anchorages, and the traditional tie spacing limits must be multiplied by 0.75.

Considering the differences in strength predictions and detailing considerations, it is clear that more research on high-strength concrete column behavior is needed. The behavioral aspects that need to be

aif c

aif c

cu c

Rectangular stress block

Parabola-rectangle stress distribution

Strains

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