Forgot about the pitch of the sides. I use 22.5 degrees when size is critical, and cover the surface with aluminum coil stock. You can't stand on it very well, but the coal will generally slide down it.
Two of my 9 ton bins have a 45 degree angle, but that was because I have lots of room and plenty of headroom.
If you are the 'Anal Engineer Type', they probably want 56.35 degrees or some freaking number that can't be done easily on site. For hand fed units, I make the bin with a flat concrete bottom and perpendicular sides, well reinforced with a 'drop in' type of entrance. Remove boards as necessary.....
Scott, if you are a real stoker-man Gear Head, have an all steel bin made with 22.5 sides. Use 8' plate stock and 4 by 4 by 3/8" angle. It will withstand a Taliban mine and deliver every piece of coal it holds.
PS: don't go inside it with wet or snowy feet unless you have a good deal set up with your dentist.
You could even have a vibrator installed, hydraulic gates and hydraulic dog house. (for the coal, not the damned dawg)