Which Coverage Area to Use to Flow Sprinklers?

I have a project with deep concrete beams which are 24 inches deep, 4 inches wide, and located 6'-6" center to center.

Building is at least 20 years old. The original contractor, I think, treated the first beam like a wall and put a row of sprinklers on each side of it. The deflectors are 12 inches down. I'm assuming they then “skipped “ the next beam. At the third beam he put another double row. And then repeated the pattern.

There are 4-inch deep lights in the center of each bay. They are sticking a layer of sheet rock on all sides and the ceiling.

The sprinklers are 15 feet apart. It looks like the they are just protecting the bay on each side of the skipped beam with no sprinkler lines. The middle beam obstructs the coverage. This is a light hazard area with 9-ft ceiling height to top plat panel.

Should the sprinklers be calculated flowing 6'-6" x 15'-0", or, flowing 13'-0" x 15'-0"?

I believe a case could be made for both.

It’s an all purpose room in a school basement. I could offer them OH 1 at 6'-6" x 15'-0" without changing spacing on the branches. I could only achieve Light Hazard if 13'-0" x 15'-0" is utilized. The architect shows minor changes for small closets, a few walls, and we're changing the sprinklers to quick response.

Thank You very much.

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