Rick wrote:

...at first glance I'd say they calculated the load capacity of the
existing configuration and used that to spec a clear span.

Well, if two 9' 2x10s have S = 3x9.25^2/6 = 42.8 in^3 and M = 1000S
= 42800 in-lb = W9x12/8, W = 3169 pounds, it looks like the 2x4 post
(it might support 5250 pounds) isn't the limiting factor. The uniform
load on an 18' beam would be 2x3169/18 = 352 pounds per foot.

The 1981 NAHB ap note says an 18' 2x10 Hem Fir beam with a 1/2"x9" plate
can support 215 lb/ft of uniform load, so a 0.5x352/215 = 0.82" plate
(or 2 246 pound 7/16" plates) might support 352 lb/ft. They suggest
1/2" bolts every 20" near the top and every 40" near the bottom and
something like 2 2x6 jack posts with 3/4" plywood plates at each end.

Please apply the usual disclaimers.

Nick