I'm in the process of working with a timber framer to build a 28 x 48 tall posted cape with an 8' gabled bump-out. The original timber design was 28 x 44 using four king post bents with principal rafters and common purlins. The two external bays were 14' and the central bay was 16'. My design has the central bay at 20' with the two outer bays at 14'. Can this design be done with a bent system where the bent spacing in the central bay is 20'? Most books I've read (Benson's and others) say 16' should be the max. Thanks for your help.
Otto Parets

you could say that a 16' spacing for timber bents will work for "normal" timber sizes(8x8, 8x10) If you go bigger, your engineer will size bigger than "normal" timbers, costing more and possibly being harder to find.
You could probably stick some extra posts in the walls between bents, but if you have summer beams or joists at 20', you'll find them expensive, and heavy to work with....