Confined masonry vs reinforced concrete frames

Confined masonry has a proven track record of performing well in earthquakes. It is often recommended for multi-story buildings up to six stories high and is included in the seismic codes of several countries.

This building system behaves differently from a reinforced concrete frame , where columns and beams are constructed first and then filled with masonry panels. In contrast, confined masonry is built the other way around. The masonry panels, typically of full-brick thickness, are constructed first using a "Flemish bond" with a toothed pattern at the wall edges. These walls are then encased by concrete columns and beams that are cast around them afterwards, with the toothed pattern strengthening the connection between the confining elements and the masonry.

Interestingly, confined masonry is generally cheaper and less complicated than making a concrete frame earthquake-resistant. While it uses similar amounts of bricks and concrete, it requires significantly less steel reinforcement, which not only lowers the cost but is also better for the environment. The construction process is also easier to explain to local laborers.

Structural behavior

In a concrete frame, the frame carries all the structural loads, and the masonry acts only as an infill panel. Under seismic loading, the frame is designed to resist moment forces, which can lead to bending and deformation at the column-beam connections. The infill panels, however, act as stiff elements that resist the movement of the frame. When combined with the often substandard construction quality found in remote areas, this can easily lead to cracking, displacement between the frame and masonry, and the collapse of infill walls.

In confined masonry, the masonry walls themselves are load-bearing and carry all the loads. The confining ties work together with the walls, which act as shear panels. The toothed pattern enhances the interaction between these elements, allowing them to resist seismic forces in unison.

Important Notes

Confined masonry has an excellent track record of earthquake resistance, and many countries in South and Latin America have included it in their national codes since the 1930s. Recently it is also gaining popularity in the Middle East and Asia, with India publishing its first confined masonry code in 2022. A significant amount of research has been conducted on this method, including post-earthquake assessments and shake table tests, making the knowledge base quite elaborate compared to most low-tech construction types.

However, some published information is contradictory. For example, more certainty is needed regarding the necessity of extra tie beams and columns around openings, the correct weaving patterns for steel reinforcement, and the best tooth pattern and maximum dimensions. For those interested in investigating these issues, we have added a student’s research assignment here.

Recommended reading

IS 17848:2022. Confined Masonry for Earthquake Resistance - Code of Practice. New Delhi: Bureau of Indian Standards.

Smart Shelter Foundation applied the basics of confined masonry for their hostel for blind students in Nepal, and the building performed well during the 2015 earthquakes. See the finished project here.