free spac octagon

I am looking for designs/specs for a freespan octagon approx. 30 feet diameter. I have seen one that is 20 foot in diameter which uses a king post as a meeting point for the rafters and metal plating to connect the rafters, eliminating the need for a center post. I am having difficulty in finding an engineer with any interest or knowledge of this area. Any point in the right direction or help would be greatly appreciated.

I just finished cutting a crearspan Octagon that was 33' across. I used a collar tie/tension rod system and a boss pin for the hips to join into.