I would like to use open web trusses to clear span 28' for a garage/shop with 12' high 2x6 16oc walls. The roof pitch is 12/12 with the intent to have living space upstairs. To make more room upstairs, I would like to make a 3' knee wall on top of the trusses. Can I do this with a normal ridge board, roof rafters and collar tie, or do I have to use a ridge beam 48' long and only supported at the ends? I want the garage floor completely free of center posts. Other suggestions are welcome. Thanks.

It is something that has to be engineered but I believe you will need the structural ridge beam.
An alternative might be found in hardware for mounting the kneewall from Simpson Hardware Co.