Strand layout in prestressed concrete girders: is a partially filled bottom row acceptable/advisable?

Question

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.

Answer 1

Strands in Upper Rows Versus Debonded Strands

You have the right idea that placing strands in upper rows is better than debonding strands. Debonded strands are more time consuming (e.g. cost more) than bonded strands. Also, placing a strand or two in the very top row usually helps to reduce the number of debonded strands needed. It is relatively easy to check if adding those strands will help.

Partially Filling Rows

As long as the strands are symmetric about the centerline, there is no problem with partially filling rows. If you think about it, that is exactly what is being done at the top of the beam in your first example (only four strands at top).

Continuous Strands

I have never seen strands being continued through the ends like you show, so I can't make a comment on those. I'm not entirely sure what the purpose is supposed to be.

Answer 2

@hazzey has answered your main question quite effectively, so I'll just add two comments.

---

While I agree that partially filling rows is perfectly fine, I would just like to add that it may be better to distribute the strands along their rows instead of bunching them all near the centerline (but keep the strands of different rows aligned) as in the figure below. Notice that the orange strands remained at the centerline since I assume they need to harp into the web. You could also bring the outer strands in by one space and then redistribute the strands.

Distributing your strands horizontally along the beam will reduce splitting-stresses at the anchors. It also improves the quality of the concrete bond, but as of a certain distance between strands this loses relevance. That being said, I assume that the actual drawings will display the distance between the strands.

---

While I've never worked with such "standardized" cross-sections, I've never encountered a rule-of-thumb other than "use the cheapest solution to achieve your objective." Cheapest usually means using the least possible total steel area (and therefore force). That implies in jamming as many strands as possible as low as possible (at the midspan, at least) in order to increase the efficiency of your prestress.

In your example, you currently have sixteen strands (6 + 6 + 4). I assume not, but could this be reduced to fourteen (or fewer) on the bottom row only? What about sixteen (or more, so long as the total area of steel is lower) strands of a lesser diameter? Less force but with a greater lever arm. This would be a better solution.

If you can't reduce your area of steel, then you only have two choices: keep your sixteen strands on the bottom and debond; or use your current layout. Debonding strands is more expensive and therefore goes against the "cheapest solution" rule, so your layout is the correct choice.