Framing Cape Cod roof.

I am building a large workshop for myself in the Cape Cod style. It will have two dormers on the front and a shed dormer in the rear of the top floor. I plan to build a two foot high knee wall on the front and back of the upper floor, than install the rafters and dormers. I will use engineered wood I-beams for rafters, and hopefully a wood I-beam for the main ridge beam. The dimensions of the workshop are 16 ft across the front and 20 ft deep. The ridge beam will span the 16 ft dimension. I want to have as much head room as possible on the upper floor, hence the knee walls. I do not have architectural plans. I want to have at least an eight foot high ceiling on the upper level.
My queston is this: Am I correct in assuming that I will have to install horizontal members between the rafters to prevent the upper floor knee walls from spreading? Since the rafters will be resting on knee walls, there will not be anything to hold the knee walls vertical without bracing the rafters with horizontal members, front to back. I don't have the exact pitch of the roof calculated yet, but it will be fairly steep, since it will be a Cape Cod. Can I just install the horizontal members at the eight foot or higher level, as long as they are present to tie the rafters together across the 20 ft dimension? Will I have to put horizontal members on every rafter. or can I use another method?

The shop will look similar the one at this link.
http://www.thehousedesigners.com/plan_details.asp?PlanNum=3800