Complex sentence: The first propeller wind generators were placed atop brick towers, once high tensile steel was available they were placed atop open steel lattices, as aesthetics and durability became more important they were placed atop steel or reinforced concrete shells.
Split into simpler sentences: The first propeller wind generators were placed atop brick towers. Once high tensile steel was available they were placed atop open steel lattices, as aesthetics and durability became more important they were placed atop steel or reinforced concrete shells.