How to consider different notional size for creep calculations for plate elements in orthogonal directions?

Question:
The plate deck elements will be experiencing creep and shrinkage in two directions.  The notational thickness under CEB-FIP considers the perimeter around the plane perpendicular to the expected effect.  In the case of prestressing/post-tensioning in two directions, that perimeter and area will be different and therefore the creep and shrinkage effects will be different.  The Midas input appears to only allow for a single notational thickness to be entered and therefore the effect is applied equally in both directions.  

---

Answer:
This is more of an engineering limitation of this method of applying the creep.  However, there is a simple solution to this.
Generally, what engineers do in this situation is a bit of approximation, and the notional size for a deck is considered as its thickness, the justification for which is as below.