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The results o f tests o f geometrically similar tied reinforced concrete columns of different sizes, with size ratio 1:2:4 and slendernesses 16, 32 and 48, are reported. The model columns were made o f reduced-size aggregate with maximum size 1 /8 in. and reduced-size steel bars. It is found that, fo r all slendernesses, there exists a strong size effect in which the nominal stress at maximum load decreases as the size is increased. This contradicts the current design codes. The results are in good agreement with the previously proposed size effect law. The higher the slenderness, the stronger is the size effect.