As I mentioned yesterday, I am not a structural engineer. However, my friend James Cranford is. He is Principal at Stephenson Engineering and he was nice enough to answer a few of my questions about soft story buildings (storey if you're Canadian).

BD: What is a soft (or weak) storey building? And why is it such an important design challenge, even in a very un-seismic city like Toronto?

JC: A soft storey refers to any level in a building that has LESS capacity than the level above. This means it has both less strength to resist loads and less stiffness so that it will move more than the levels above. Soft stories are one of the most significant challenges that many modern building designers face because they are one of the most likely ways that a building can fail catastrophically if not properly designed. A soft storey failure occurs when the building hinges above the weak level and the columns below can no longer support the load of the building above as they become overstressed and loaded in ways they were never meant to act. This leads to a sudden, often pancake type collapse that is likely to bring down the entire building.

We see potential soft storey issues most commonly in mid to high-rise residential buildings that have either amenity or retail spaces at the ground floor. These are spaces that by nature are large and as open as possible. During design, the structural engineer needs to recognize this and compensate for the lost capacity in other ways. This is usually done through a combination of increasing the capacity of the remaining walls and adding new walls at the weak level that fit with the building layouts.

BD: What does the Ontario Building Code mandate in terms of soft storeys?

JC: The OBC generally does not permit soft stories in any form for buildings where people are likely to live, work or play. In critical infrastructure like hospitals which must remain fully functional in the event of a major earthquake, the OBC goes further and does not permit any ‘lateral force resisting elements’ like shear walls or steel frames to be discontinuous below. This means that if you have a wall on the 5th floor of a hospital, that wall must exist with equal or greater capacity on EVERY level below, without exception.

BD: How much more stringent is British Columbia, where there is greater seismic risk?

JC: The requirements in the British Columbia Building Code (BCBC) are almost identical to those in Ontario in this case. However, the seismic design forces will be much higher based on the potential for much larger earthquakes, so while buildings will generally be designed for a higher seismic capacity, they must be proportioned similarly to prevent soft stories.

BD: Speaking generally, what is usually required structurally in order to retrofit an existing soft storey building so that it can properly withstand things like earthquakes?

JC: The most common way to retrofit a soft-storey is to increase the capacity of the weak level. In smaller buildings this can usually be achieved by adding new ‘lateral-force-resisting elements’ like shear walls or moment/braced frames until the overall storey capacity matches or exceeds the capacity of the levels above. On larger buildings this becomes more complex, as the loads are much higher and simply adding capacity may not be either feasible or practical. Therefore a full structural analysis is usually required to find a solution that can be tailored to the unique structural and architectural conditions. This often involves a combination of increased capacity and the introduction of ductile detailing which will allow the building to dissipate seismic energy. This can be roughly thought of as a ‘bend but don’t break’ approach to surviving an earthquake.

In some jurisdictions, the extreme risk caused by (many) homes built with soft stories has prompted local governments to intervene. The City of San Francisco (as well as many other municipalities in California) have enacted ordinances requiring home owners to assess and upgrade their properties, including single family home with garages a the lowest level, to reduce the risk of soft-storey failure in an earthquake.

BD: Thanks for this, James.

I don't usually do Q&As on this blog, so let me know in the comment section below if you found this one valuable and if you'd like to see more of them.

I am not a structural engineer (or an architect for that matter). But one of the things that has come to greater light as a result of the devastating earthquake that hit Turkey & Syria last month is the number of "soft story buildings" throughout these countries.

Technically, a "soft story building" is exactly what the name suggests. It is a building where one floor is less than 70% as stiff as the floor above it, or less than 80% as stiff as the average of the three floors above it (source).

The typical application of this is a ground floor that has less structure (missing shear walls for example) and is more open. And it is usually done to accommodate things like parking and retail uses, and to, of course, build more cheaply.

However, there is a massive problem in that they are often structurally suboptimal! (Again, not a structural engineer.) This is why we saw so many of the buildings in Turkey "pancake" during its earthquake. The ground floor failed and then it brought down the rest of the building.

I can appreciate that retrofitting older buildings is both difficult and expensive; but it is inexcusable to not work toward that and it is certainly inexcusable to not mandate that every new building meet whatever building codes are required to save lives.