A steel tall building is defined as a building where the main vertical and lateral structural elements and floor systems are constructed from steel.
A concrete tall building is defined as one where the main vertical and lateral structural elements and floor systems are constructed from concrete.
A composite tall building utilizes a combination of both steel and concrete acting compositely in the main structural elements, thus including a steel building with a concrete core.
A mixed-structure tall building is any building that utilizes distinct steel or concrete systems above or below each other. There are two main types of mixed structural systems: A steel/concrete tall building indicates a steel structural system located above a concrete structural system, with the opposite true of a concrete/steel building.
Additional Notes:
1) If a tall building is of steel construction with a floor system of concrete planks on steel beams, it is considered a steel tall building.
2) If a tall building is of steel construction with a floor system of a concrete slab on steel beams, it is considered a steel tall building.
3) If a tall building has steel columns plus a floor system of concrete beams, it is considered a composite tall building.