When a Caltrans circular section is defined using the Section Designer, a variety of confinement options are available, including casings and hoop or spiral transverse reinforcement. These options are described as follows:

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Confinement options

Confinement for reinforced-concrete sections may be modeled using any of the options which follow:

Core applies the Mander confined-concrete model using either hoop or spiral transverse reinforcement.

Casing also applies the Mander model, though the casing is converted into equivalent hoop reinforcement.

Mander(C) allows manual specification of the constitutive parameters which characterize the Mander confined-concrete model.

Casing with transverse reinforcement

Since the Mander model only distinguishes confined and unconfined concrete, and not multiple concentric regions of confinement, both the casing and the transverse reinforcement may be modeled as follows:

Determine the material behavior of each confined region.
Use the Mander(C) option to define materials which simulate an equivalent effect.
Assign these materials to the corresponding regions of confinement.

Best practice

Structural casing – When the casing contributes substantially to structural behavior, it is often sufficient to model only this layer as the confinement mechanism since the inner transverse reinforcement will only provide significant contribution once the casing has failed, at which point, crushing occurs and the hoops or spiral reinforcement will likely fail as well. In this case, inner transverse reinforcement is essentially irrelevant to overall performance. Such applications may occur when casing is added to an existing section that is known to be underconfined or inadequate.

Decorative casing – When the casing does not serve a structural function, and instead offers only a decorative layer, or formwork for the concrete pour, the inner transverse reinforcement acts as the primary confinement mechanism. In this case it is best to assign the Core option and apply the conventional Mander confined-concrete model in which only the transverse reinforcement provides confinement.

Casing with transverse reinforcement – If multiple regions of confinement are to be modeled, the appropriate material behavior must be determined, then manually defined and applied using the Mander(C) option.

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