What is the main work that goes on at the u.s. capital building?
The law making of the United States.
I believe it is called "Capitol" rather than "Capital". It houses the legislative branch (Congress) of the US federal government.
Cleaning floors and toilets, emptying waste baskets, dusting, and rarely some touchup painting. Those old (and a few young ones now) fools who wander in and out of meeting rooms thinking they are doing the work they were elected to do are not really doing anything.