Late in 1951 there was a radical change in the way Smith worked, as he began to number and designate works in series. He went on to make over 16 separate series, comprising as many as 28 works each.
Another development in the early 1950s was his growing preoccupation with the standing figure in works such as the Tanktotems and the Sentinels. It was the soar of the human figure that held him, the uncompromising thrust it makes, the fight it carries on with the force of gravity, wrote Clement Greenberg. The Tanktotems were made using boiler parts and cylindrical steel tank parts which he ordered from a catalogue, providing the convex and concave forms that are the hallmarks of this series.
In 1956 Smith began to paint the surfaces of his sculptures in an expressive style, with notable examples including 5½ 1956 and Tanktotem IX 1960. Around this time he also started using industrial I beams extensively as a component in the Sentinel sculptures, and used wheels as part of the base for the first time in Sentinel III 1957.