Masonry Infill Walls and Partitions in Framed Buildings

In Italian construction from the post-war period onwards the structure is almost always a reinforced concrete frame, and the walls are infills: they enclose, insulate and divide, but they do not carry the floor. This has produced a stubborn habit, namely treating them as minor work, given to the least skilled labour and defined on site rather than on drawings. The structural reality is different. When the frame deforms under a horizontal action, the stiff infill wedged inside the bay resists: it absorbs force, loads up as a diagonal strut and returns it to the frame joints, which were not designed for it. Earthquake damage always shows this: X-shaped cracks on infills, columns damaged at their heads, and in the worst cases the soft-storey phenomenon, when one storey is open and those above are infilled. The countermeasure is not building stronger walls, but deciding deliberately how the panel interacts with the structure, and executing that decision carefully on site. The second countermeasure concerns out-of-plane overturning: a tall, thin panel pushed perpendicular to its face topples as a block if it is not restrained. This is the mechanism that kills, because the wall falls into the rooms or onto the street. With internal partitions the theme is different and more everyday: weight, acoustics and cracking. A partition built up to touch the underside of the slab with rigid mortar cracks at the first settlement, because the slab deflects and the partition does not. The joint at the top is not a finishing detail: it is what lets two elements with different lives coexist.

It is the concept that changes how you look at a framed building, and once understood it is not forgotten. Under a horizontal action the frame deforms into a parallelogram: the rectangular bay becomes oblique. The stiff masonry panel wedged inside that bay resists this deformation, and to resist it compresses along the diagonal that shortens, behaving exactly like a strut. That strut pushes against the corners of the frame, that is against the ends of the columns, with a concentrated force the bare-frame analysis never anticipated. The typical outcome is X-shaped cracking on the infill and damage to the column heads. Then there is the worst case, the soft storey: if one level is left open — arcades, shops, garages — while those above are infilled, all the deformation concentrates there, and that storey becomes the point at which the building can collapse.

It is the commonest defect and the easiest to avoid, and it springs from a wrong idea of solidity: a wall wedged tight under the soffit looks sturdier, and is in fact the one that will crack. The reason is that slab and partition behave independently. The slab deflects under load, and keeps doing so over time through the slow deformation of concrete: its mid-point drops, slightly but continuously, for years. The partition, stiff and with nothing to deform, if in rigid contact receives that movement as a vertical load that is not its to carry. Not being sized for it, it reacts as it can: it cracks horizontally under the soffit, or crushes the blocks of the top band. The compressible joint solves the problem at root by leaving the slab room to drop, while keeping the horizontal restraint that stops the wall overturning.