Here [1] is a newspaper story from the June 4th, 1937 Milwaukee Journal about the test to demonstrate the strength of the columns to building regulators.
Here's the explanation given for why the columns were stronger than the regulators expected:
Secret of weight carrying ability of the new pillar,
according to Wright, lies in its departure from the
conventional way of building concrete pillars.
Instead of using steel rods to reinforce the concrete,
the architect has perfected a steel mesh core.
“Iron rods in concrete represent the bones of a human
foot. The steel mesh, however, plays the role of
muscles and sinews. Muscles and sinews are stronger
than bones. The concrete flows in unison with the
steel mesh. It ’marries’ the mesh, so to speak,”
Wright explained.