Knee braces are not often present perpendicular to the bent at the center post for the following reasons: 1. Framing is based largely on tradition. The old hay barns did not have a timber running perpendicular to the bent at the top of the center post. The center post usually terminates at the bottom of the tie beam. 2. In new frames where a second floor is present, often the joists run perpendicular to the bents. It is easier to avoid having a joist at the same location as the center post to avoid many timbers comming together at one joint. In this situation there may be no member or only a joist to set the brace into, where at the eaves there is usually a heavier plate to let the brace into.
An engineer can help you make an informed decision on the number and location of braces required. Good Luck!