For service conditions, RAM Concept assumes that unbonded tendon stresses are not affected by cross section strains.

For accident strength conditions, RAM Concept assumes that unbonded tendons have no stress.

For ultimate resistance moment calculations, RAM Concept 's general approach to unbonded tendon stress-strain curves is detailed in Chapter 41, "Section Design Notes". For Eurocode 2:2004, the maximum unbonded tendon stress (f_pb, called f_limit in "Unbonded Post-tensioning Stress-Strain Curves – Program Implementation") is defined by the equation in TR-43 clause 5.8.5 for both the UK National Annex and the Generic National Annex.

When this equation is used in a cross section that contains multiple tendons, the following terms are used in the calculation:

• l / d = length of an individual tendon divided by its depth
• f_puA_ps = sum of all the individual tendons’ f_pu multiplied by the vector component of their A_ps
• f_ckbd = minimum concrete characteristic cylinder strength multiplied by the compression face width and the depth to the centroid of the vector component tendon area

For Eurocode 2:2004, the value used as a strain reduction factor for unbonded tendons is k = 5d/L

where

L	=	length of the unbonded tendon
d	=	depth of the post-tensioning tendon (measured from the furthest concrete face)

This is equivalent to assuming a neutral axis depth of 0.5 d and a "zone of inelasticity" of 10 times this length.

RAM Concept assumes that each tendon is placed on the more beneficial side of the cross section centroid (the same limiting stress value is used for both positive and negative moment capacity calculations at each cross section). This assumption typically has no impact on the ultimate stress in the tendon as when the tendon is on the "wrong" side of the cross section centroid, the stress in the tendon is less than f_pb, due to the small tension strains (possibly compression strains) in the cross section at the tendon elevation.