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I cover a lot of different topics on this blog. It's hard to write daily for a decade and not meander every now and then. But generally speaking, I do try and keep this forum focused on things that are related to city building -- real estate, housing, design, transportation, etc. And I do try and share some of the things that I have learned (and the mistakes that I have made) since I started working in real estate development back in 2007.
But we all have limits, and different perspectives are vital for solving problems. So I'd really like to introduce more Q&A features on this blog, similar to this recent one that I did with structural engineer, James Cranford. If this sounds interesting to any of you, please reach out. I'm open to anyone who touches the built environment: architects, artists, planners (private or municipal), lenders, furniture designers, bollard manufactures ... you name it!
Image: Jason Adam Katzenstein
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.
Balconies are a never ending debate here in Toronto (and in many other places). In some cities, like New York, they don't seem to matter for new housing. Residents seem to be generally content without them. But here in Toronto, we have typically included them in new high-rise housing and there has been a lot of debate and criticism around both their utility (high up in buildings) and their impact to overall energy performance.
I have noticed that we are starting to see fewer balconies on new buildings, and I suspect this might increase with the way that costs are right now. And for some people and some (sub)markets, this will be just fine. But I happen to be a huge fan of outside. As my tanned dad likes to say when asked about the value of outdoor spaces in multi-family housing, "you don't get this dark by staying inside." He is pretty tanned.
I also believe that great outdoor spaces are an important ingredient in shaking off the deep-rooted cultural biases that this city has toward low-rise housing. Since pretty much the beginning, low-rises houses with backyards have been seen as noble, whereas apartment buildings have been viewed as disease-breeding tenements liable to morally corrupt even the best of intentions.
This is one of the reasons why we created the two-storey House Collection of suites at Junction House and why One Delisle is almost entirely formed by its outdoor spaces (both balconies and terraces). We wanted to celebrate multi-family living.
At the same time, I really like this adaptive reuse proposal by Peter Song over at BDP Quadrangle. The idea is to allow people to infill their balconies with more interior space so that our existing stock of housing can become more flexible
I cover a lot of different topics on this blog. It's hard to write daily for a decade and not meander every now and then. But generally speaking, I do try and keep this forum focused on things that are related to city building -- real estate, housing, design, transportation, etc. And I do try and share some of the things that I have learned (and the mistakes that I have made) since I started working in real estate development back in 2007.
But we all have limits, and different perspectives are vital for solving problems. So I'd really like to introduce more Q&A features on this blog, similar to this recent one that I did with structural engineer, James Cranford. If this sounds interesting to any of you, please reach out. I'm open to anyone who touches the built environment: architects, artists, planners (private or municipal), lenders, furniture designers, bollard manufactures ... you name it!
Image: Jason Adam Katzenstein
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.
Balconies are a never ending debate here in Toronto (and in many other places). In some cities, like New York, they don't seem to matter for new housing. Residents seem to be generally content without them. But here in Toronto, we have typically included them in new high-rise housing and there has been a lot of debate and criticism around both their utility (high up in buildings) and their impact to overall energy performance.
I have noticed that we are starting to see fewer balconies on new buildings, and I suspect this might increase with the way that costs are right now. And for some people and some (sub)markets, this will be just fine. But I happen to be a huge fan of outside. As my tanned dad likes to say when asked about the value of outdoor spaces in multi-family housing, "you don't get this dark by staying inside." He is pretty tanned.
I also believe that great outdoor spaces are an important ingredient in shaking off the deep-rooted cultural biases that this city has toward low-rise housing. Since pretty much the beginning, low-rises houses with backyards have been seen as noble, whereas apartment buildings have been viewed as disease-breeding tenements liable to morally corrupt even the best of intentions.
This is one of the reasons why we created the two-storey House Collection of suites at Junction House and why One Delisle is almost entirely formed by its outdoor spaces (both balconies and terraces). We wanted to celebrate multi-family living.
At the same time, I really like this adaptive reuse proposal by Peter Song over at BDP Quadrangle. The idea is to allow people to infill their balconies with more interior space so that our existing stock of housing can become more flexible
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.
It would be a great way to capture additional space within our existing stock of buildings, and I think it would be pretty interesting to see what people ultimately choose when given a binary option: more interior space or more outdoor space. Maybe it would help provide some clarity to the great balcony debate.
Technically, it is my understanding that this is entirely doable.
In the middle of writing this post I shot an email over to one of the best structural engineers in the city (James Cranford, Principal at Stephenson Engineering), and he confirmed that strength is no problem. Typically balconies are designed to accommodate more load than the suites. The thing we'd have to look at is slab deflection, since this is not usually limited on balconies.
The greater challenge is likely to be the overall coordination.
In some cities this sort of thing happens all the time on an ad hoc basis. People just do it and the end result is likely more functional, but the building elevations end up looking pretty schizophrenic. Here you'd need each condominium corporation to bless the change (since the envelope is a common element). And people would also need to agree on what design(s) should be used across the building.
It would require some work, but I think Peter's idea is a really good one. What do you think?
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.
It would be a great way to capture additional space within our existing stock of buildings, and I think it would be pretty interesting to see what people ultimately choose when given a binary option: more interior space or more outdoor space. Maybe it would help provide some clarity to the great balcony debate.
Technically, it is my understanding that this is entirely doable.
In the middle of writing this post I shot an email over to one of the best structural engineers in the city (James Cranford, Principal at Stephenson Engineering), and he confirmed that strength is no problem. Typically balconies are designed to accommodate more load than the suites. The thing we'd have to look at is slab deflection, since this is not usually limited on balconies.
The greater challenge is likely to be the overall coordination.
In some cities this sort of thing happens all the time on an ad hoc basis. People just do it and the end result is likely more functional, but the building elevations end up looking pretty schizophrenic. Here you'd need each condominium corporation to bless the change (since the envelope is a common element). And people would also need to agree on what design(s) should be used across the building.
It would require some work, but I think Peter's idea is a really good one. What do you think?
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