Unreinforced masonry is stone, brick, or cement construction that supports itself, without any other framing to add strength and reinforcement. For example, a building made of only bricks, mortar, and poured cement without any steel or wood frame for support, is unreinforced.
A brick wall with no steel frame will crack in an earthquake very quickly. When a steel frame reinforces a building, that building can flex more without damage. The ability of a structure to deform without being damaged is called ductility. If a wall can be temporarily deformed by the shaking of an earthquake without suffering significant breakage, that wall has a high degree of ductility.
When buying a house or building, it is essential to verify that the building is reinforced, especially if you live in an area that experiences earthquakes. The steel reinforcement that generally frames buildings is called rebar.
Interestingly, wood houses are generally the best-performing structures in earthquakes. Wood has a high degree of flexibility and ductility, so it can shake and move flexibly without breaking. Additionally, wood homes are often made almost entirely of wood, without cement or brick that can crack easily. Steel-framed buildings generally are made of cement or brick, so while the steel provides flexibility, the cement is fairly inflexible and breaks more easily under stress.
An experiment by the Portland Cement Company showed that wood and steel have similar levels of ductility, meaning they both can be deformed to a similar extent without suffering damage. In the Portland Cement Company’s tests, both wood and steel began to suffer damage when 3,500 pounds of force was applied, which surprisingly suggests that wood and steel have almost identical levels of ductility.
Though wood and steel are similar in this regard, there are many ways in which they differ. Tests suggest that wood is more durable than steel in the long run, but that steel withstands sudden shocks more effectively.
Earthquakes affect identical buildings in different ways, based on what type of ground the buildings are on, and on their respective heights. For example, a short building will suffer more damage if the earthquake’s shaking is high frequency (short and fast vibrational waves), while a tall building will suffer more damage if the quake’s shaking is low frequency (longer, slower waves of movement). To understand this, it’s useful to think of a massive ship that is unaffected by short, fast, small waves, while a tiny boat will be highly affected by these small, fast waves.
The performance of buildings in an earthquake can be highly unpredictable; however, masonry buildings without any reinforcement perform poorly in all types of earthquakes. When buying an old masonry building, or a condominium inside of one, verifying that it is reinforced masonry should be one of the first steps in your purchase.