Dear Valentina,
this sounds like some linear design formula / code recommended construction guideline, probably to account for some minimum anchorage length?
If so, it probably does not make much sense within nonlinear FEM analysis...

When the bond law is considered along a bar, the slips are calculated and the load transfer can be reproduced, including the anchoring regions. Therefore, there is no need to consider such a formula in your ATENA model (however, on the other hand, you can make a parametric study with ATENA to verify if/under what conditions this formula is a good design guideline...).

If the background behind the lambda is something different, please refer to the exact code and page (copy-paste if possible), such that we can look at its meaning and possible influence in nonlinear FEM.

Regards.