Modification to the tensile strength
Figure 167 shows the window that allows a change in the tensile strength depending on the type of load combination. For example, the user may define zero tension for normal cases while higher strength is considered for all other load combinations. If cracking is considered, a distinction is made for the criteria between crack initiation and crack propagation. After a crack is initiated, probably at the upstream end of a joint where stress concentration is minimal, it is likely that the stress concentration would take place at the tip of the propagating crack (USBR 1987). For example, the crack initiation may be defined as 1000 kPa for the tensile strength, but once the crack is initiated, it must be propagated over a sufficient length until a fully compressed ligament is obtained (zero tension condition). The allowable tensile strength for crack initiation and crack propagation are specified for different load combinations: (a) normal operating conditions, Usual cases (b) Unusual Cases (c) Extreme Cases.
Crack Initiation: The allowable tensile strength for Crack Initiation is defined as the concrete tensile strength divided by a user-specified coefficient. Once the crack is initiated, its length is calculated by applying the appropriate crack propagation criterion.
Crack Propagation: The allowable tensile strength for crack propagation is defined as the concrete tensile strength divided by a user-specified coefficient. This value must be less than or equal to the crack initiation value.
Dynamic amplification of tensile strength: During rapid loading caused by seismic movements, the concrete tensile strength is higher than the static loading case (loading rate effect). A Dynamic magnification factor can, therefore, be defined to increase the concrete tensile strength used for crack initiation and crack propagation.