Stringer placement... structural difference?

Speaking of the ISO 12215 standard, for scantlings of pleasure boats, which is what I know the best, the core material is totally ignored to calculate the efforts that must support a fiber reinforcement.
The opposite is also true: when calculating a reinforcement of wood, with a light layer of fiber as a sealant, fiber is ignored. The reinforcement must be properly "set" to its panel, but not by tabing. It must be studied in great detail the shear stress in the reinforcement / panel interface. Existing books with practical recipes, do not take into account, do not even mention it.
Sandwich construction : the core material in a sandwich panel, as it is very close to the neutral axis, contributes very little to withstand the tension / compression on the element. Its main effect is that it separates the more resistant layers from the neutral fiber. The choice of core material, imo, depends more on weight savings you want to achieve and compatibility with the other components of the panel. (Do not forget the shear stress itself that should be supported by the core material).

 

Dinapero. If the original stringers uses foam core, then use foam core of the same density. It is there to absorb part of the vertical compression loads as the stringers bend.

If you want to eliminate the core, you must place an additional layer of WR in the 0/90 direction, then the 45/45 degree layup (or biax).

In standard LR software calculations, the user is prompted if there is a core or not and the layers is adjusted accordingly.

In areas where there are highly localized loads, a high density foam core is used. That is why you would often find glass laminated timber in engine beds.