The strains in a cross section can be determined using the "plane sections remain plane" assumption. However, due to the effects of prestressing and the sequence of construction, the strains in the post-tensioning in a cross section are not equal to the strains in the adjacent concrete.

Conceptually, to calculate the strain in a bonded tendon at that corresponds to the adjacent concrete strain is simple:

εp = εc + (εpi – εci)

where

εp	=	strain in tendon
εc	=	strain in concrete adjacent to tendon
εpi	=	strain in tendon at time of bonding
εci	=	strain in adjacent concrete at time of bonding (typically negative)

RAM Concept uses the following procedure and assumptions when calculating the ε_pi and ε_ci values for each tendon in each design cross section:

• All tendon "long-term losses" (relaxation, elastic shortening, creep and shrinkage) occur before bonding.
• The formwork applies an upward force on the concrete exactly the same as the weight of the concrete. The only forces in the concrete are those due to the balance loading.
• The concrete strains can be determined using gross section properties and the "initial" concrete modulus.