The Miami-Dade County Courthouse on Flagler Street was built in 1925 and in late 2013 minor erosion of the concrete basement columns was spotted.
Experts say it's because of water intrusion and salty ocean air.
"This work of reinforcing the columns is part of the normal process that we would take in securing and improving a building of this age," said Lester Sola, director of internal services for Miami-Dade County.
The courthouse is still up and running even though it's now undergoing a 45-day evaluation.
Sonar testing of the steel beams in the concrete columns will be done and concrete samples will be taken.
The estimated cost of repair won't be known until then, although Sola said more than $30 million will be spent on improving the building.
The courthouse will not close for repairs and reinforcement of the columns will take about one year.