Beam Depths

I have a question on mounting heat detectors in a space that has 16’’ beam depths. The room is 20’ x 30’ and needs heat detectors for pre action sprinkler initiating. The code says you can’t mount on the bottom of the beams unless they are within 12’’ and less than 8’ on center. These beams are 16’’ and 5’ on center. I have a feeling I will have to add a detector in each pocket but i’m looking for code to back this up. Any suggestions?