When defining Caltrans circular sections in Section Designer, the user can define hoop or spiral confinement reinforcement and also define casing for the entire section. There are the following options for the confined concrete model:
- Core - this option will utilize the hoop or spiral confinement reinforcement as the confinement reinforcement for the Mander concrete model
- Casing - this option will convert the casing into equivalent hoop reinforcement and use as the confinement reinforcement for the Mander concrete model
- Mander(C) - this option allows the user to specify custom parameters for the Mander concrete model
It is not possible to include both the hoop or spiral confinement reinforcement and the casing as confinement reinforcement for the Mander concrete model, because the standard Mander model, does not explain how to account for multiple diameters of confinement. The user would have to decide what the net effect is, and define their own equivalent material using Mander(C). The standard Mander model only applies to a single perimeter of circular or rectangular confinement.
For example, suppose the casing is quite substantial and the inner hoops are not so big. All of the concrete would be confined by the casing and the inner hoops would not be able to develop significant restraint until the casing failed, at which point the inner hoops would likely fail too. The inner hoops would essentially be irrelevant to the overall behavior. This is the most likely scenario, and the casing model would apply to the entire section. Often casing is added to an existing section that is known to be underconfined.
On the other hand, the casing could be largely decorative and the inner hoops provide the main confinement, in which case using Core1 for the inner concrete would make the most sense.
If the user expects something different, it is up to them to determine the equivalent confinement and apply it to the two concrete regions appropriately using the standard Mander(C) model.
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