Their team of architect Bruce Graham and structural engineer Fazlur Rahman Khan designed the building as nine square `` tubes '' ( each essentially a separate building ) , clustered in a 3 × 3 matrix forming a square base with 225-foot ( 75 m ) sides .
Their team of architect Bruce Graham and structural engineer Fazlur Khan designed the building as nine square `` tubes '' ( each essentially a separate building ) , clustered in a 3 × 3 matrix forming a square base with 225-foot ( 75 m ) sides .