Advances in building design and concrete manufacture have led to ever taller buildings being constructed since the Second World War. Shear walls stiffened concrete frames against possible lateral deflection under the strain of wind or earthquake. Another successful system was the perimeter-framed tube: reinforced concrete columns were placed closely spaced round the outside of the building and connected by beams. This allowed the DeWitt–Chestnut Apartments in Chicago to reach 43 stories in 1963 – the same number of stories as the Barbican towers.
The next step was to combine the perimeter-framed tube with a solid-walled interior tube or shear walls to give further lateral stability. Another innovation was to frame the tube with diagonal bracing.
Concrete itself was made in stronger and lighter forms. Blast-furnace slag was used as aggregate in place of stone. This reduced the loads the building columns needed to carry.
Slip-forming was an important development in building technique which led to still higher buildings. Concrete is continuously poured in a circle to form an ever-growing tube. A moving framework is used to keep it in place until it dries. Slip-forming was the method used to put up the world’s tallest freestanding structure, the CN Tower in Toronto, which is 1,815 feet high.
However, reinforced concrete is the basis of most modern concrete buildings. There are two ways of fabricating reinforced concrete. Pre-cast concrete is made at the plant and transported to the building site for assembly. “In situ” concrete is simply liquid concrete poured into forms on site. The “in situ” method was used for the construction of the Barbican. Instead of stone, the architects used crushed Pen Lee granite as aggregate.
In barely 100 years, reinforced concrete has risen from an experimental material to the most widespread material in building construction.