My state (USA - Nebraska) uses "NU" type prestressed girders (though the question should equally apply to all geometric variants of prestressed concrete "I" girders).
There are 6 "rows" available for the prestressing strand. Moving from the bottom row upward, a maximum of 18, 18, 12, 6, 2, and 2 strands can be accommodated in each row.
I understand that theoretically the most efficient design will result when the rows are filled from the bottom up.
Virtually all the plans I've seen fully fill that bottom row. However, I consistently run into cases where I'm not inclined to fully fill the bottom row.
Case 1. I need less than 18 strands total.
Case 2. I need 18 or more strands total, but the principle of "fully fill the bottom row first" requires debonding to satisfy the stress limits at transfer. Whereas, if I strategically partially fill several rows I can avoid debonding.
An example of one of my strategic layouts is below (shown at midspan; the orange strands are harped at 0.4L and 0.6L):
The question initially arose because the standard drawings require that specific strands in the bottom row be extended into the concrete diaphragms at supports (see the top figure). If the bottom row isn't full, how will the contractor decide where to place the strands and which (if any) strands should extend into the diaphragms?
More generally, is there any problem with partially filling rows?
I'm also interested in rules-of-thumb for strand layout.