Chamberlin Powell and Bon and Ove Arup, the structural engineers, adopted the cross wall construction system for the terrace blocks. (Cross wall construction is also called box frame structure.)

Cross wall construction is a structural system in which the internal partitions dividing up the building between flats become the structural walls. Instead of the external walls carrying the weight of the structure, the internal walls do. The result is that the building is like a number of open-ended boxes next to each other, with others stacked exactly on top. The structure between each box is the main structure. That’s what allows a Barbican flat’s living room windows to run wall-to-wall and floor to ceiling. Of course, not all the internal walls are structural. Most internal walls inside a flat are nonstructural. But the walls between flats and between flats and corridors are mainly structural. To work, all the structural walls have to be exactly on top of each other. That requires the same shape of flats stacked on top of each other on all floors.

The advantages of the system are:

• the load bearing walls can be designed to use materials and labour more efficiently than the traditional forms of construction.
• the system lends itself to repetition and standardisation of structural as well as nonstructural elements, and therefore to prefabrication and to more rapid building methods
• since the external walls don’t have to perform a load bearing function, they can be better designed to meet the requirements of daylight and other functions – aesthetic possibilities are opened up.
• space consuming or protruding beam and column frameworks are not necessary.

The disadvantage – or let’s say, discipline – of the system is that the design has to take account of the fact that internal crosswalls must maintain their same position on all floors.

The cross wall system works by placing the floor slab on top of the cross wall construction. However, in the Barbican, the required area of flats meant that there would be a greater distance than usual between party walls which could act as load bearing cross walls. The maximum distance is usually 35 feet. They could have used thicker slabs, but that would have reduced the height from one floor to the next floor to below 9 feet. So they had to introduce some strengthening beams along the façades of the blocks to support the wider floor slabs. You wouldn’t be aware of them, because they are the walls on which the window boxes are placed just beyond your balcony. They’re not just the low balcony wall you see when you step outside your flat, but also the concrete you see at the bottom of the overhanging balcony of the flat above you.

The design aim with the terrace blocks was to create the effect of a colonnade so that you are walking beneath the building with columns at regular intervals. The ideal arrangement is when the cross walls in the structure above sit directly onto the columns. Sometimes, the cross walls would not be in the exactly regular position and that would have messed up the regular appearance of the columns at podium level by changing the distance between them. In such a case, a table top of thick reinforced concrete slab (solid or coffered) was placed between the columns; then the superstructure of the block at that point was supported by the table rather than the column directly.

The cross wall system carries on down through the the three storeys below the podium.