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for reinforcement location, bar coating, and concrete density are all 1.0. It was also assumed that all of the a factors, z1, and act in Equation 15.20 are 1.0. In addition, it was assumed that the minimum concrete cover to the bars being developed was 40 mm (1.5 in.) and that spacing between the bars was between 1.4db and 2db. Both the EC2-02 and CSA A23.3-94 expressions give shorter development lengths than the ACI 318-02 expression. This is due to the different requirements in the three codes regarding minimum shear reinforcement and minimum bar spacing. On the one hand, EC2-02 requires larger amounts of minimum shear reinforcement (Figure 15.25), but has the same minimum clear bar spacing as ACI 318-02. On the other hand, CSA A23.3-94 requires similar amounts of shear reinforcement as ACI 318-02 (Figure 15.25), but requires a larger minimum clear spacing between bars.

ACI 318-02 and CSA A23.3-94 limit the value of -,/ffi to 100 psi (25/3 MPa) and 8 MPa, respectively, in applying the expressions for determining the development length. EC2-02 cautions designers that due to the increasing brittleness of higher-strength concrete, the tensile strength of concrete required in calculating the bond strength should be limited to that corresponding to concrete with a compressive strength of 60 MPa, unless it can be verified that the average bond strength increases above this limit. It is noted that tests [82] indicate that bond failures in high-strength concrete members are more brittle than those in normal-strength concrete members unless adequate transverse reinforcement is provided. More research is needed to determine the relationship between the amount of confinement reinforcement and the required development length for higher-strength concrete.

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