Balloon flange girder

A balloon flange girder or (colloquially) balloon topper is a form of vertical I-beam wrought iron plate girder, where the top flange, instead of being a simple flat plate, is extended into a hollow tube. When a girder is subjected to a positive bending moment the top flange acts in compression making a flat plate flange more susceptible to local buckling than the balloon flange is.

This type of girder was rarely used, its only common user being Isambard Kingdom Brunel in the 1840s and 1850s.