noun
post
POHST/pəʊst/
An upright pole or support fixed in place.
The wooden post supports the two fence rails.

Etymology
Latin postis named a doorpost, giving the concrete image of an upright structural timber. The English sense extends that image to fixed upright supports, including metal ones.
In context
main structure of the bridge is comprised of two main beams each made up of two 2x 12s and they're attached by carriage bolts two carriage bolts on each 4x4 post to make them upright