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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.
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.
Earlier this week, Slate Asset Management and Forum Asset Management submitted a new development proposal for 100 Lombard Street in downtown Toronto.
At the time of writing this post, the applications (zoning by-law amendment and site plan control) hadn't yet hit the city's website. So here's some information about the project, including its big moves:
This is the first mixed-use residential project in Toronto designed by the Office for Metropolitan Architecture (OMA). The proposal includes residential, office, and retail spaces.
Architecture by OMA and WZMH Architects. Heritage by ERA Architects. Landscape and public realm by Claude Cormier + Associés. Planning by Urban Strategies. Structure by Stephenson Engineering.
The principal architectural idea is to create a vertical urban village through a series of "urban rooms" interspersed throughout the tower. These spaces would serve as amenities for the building and house a variety of different functions. See above rendering.
The proposal introduces three important public realm moves: (1) a new public plaza that pays homage to the site's former neighbor to the east -- Second City; (2) a new mid-block pedestrian connection running north-south from Richmond Street East to Lombard Street; and (3) an outdoor public art gallery featuring oversized art tableaus.
The site currently houses one designated heritage building (86 Lombard Street), and the design contemplates relocating and fully retaining this building on the eastern edge of the site. Once you see the drawings, you'll fully understand why this was the most logical move.
The entire project team is very excited to get this proposal out and into the world. And we hope that you will see it as being representative of our ongoing and lasting commitment to elevating architecture, sustainability, culture, and city building in Toronto.
This week the team submitted a rezoning application for the southwest corner of Yonge & St. Clair (1 St. Clair Avenue West).
More of the details can be found over here on Urban Toronto, but so far the response has been overwhelmingly positive. People seem to appreciate the architecture, the public realm improvements, as well as how we're proposing to deal with the embodied carbon in the existing building.
https://twitter.com/alexbozikovic/status/1471557617789059074?s=20
The application proposes to retain the existing 12-storey office building and both expand its floorplates to the west and build new residential on top. In the middle is a shared multi-storey amenity space that also performs some pretty cool structural gymnastics courtesy of Stephenson Engineering (see above rendering).
This approach created some interesting design challenges for the team. Typically when you're adding onto an existing building, you want to do something new and not try and copy/bastardize what's already there. Oftentimes this means something more contemporary.
The architecture team at Gensler Toronto tried this approach but the podium proportions didn't feel quite right when we did it. So a decision was made to instead pay homage to the existing building's architecture, and then kind of reinterpret it by playing with scale and other details.
This way the original building remains architecturally legible, but the entire podium still reads as one and its proportions feel much better. We hope you like it as much as we do.
Earlier this week, Slate Asset Management and Forum Asset Management submitted a new development proposal for 100 Lombard Street in downtown Toronto.
At the time of writing this post, the applications (zoning by-law amendment and site plan control) hadn't yet hit the city's website. So here's some information about the project, including its big moves:
This is the first mixed-use residential project in Toronto designed by the Office for Metropolitan Architecture (OMA). The proposal includes residential, office, and retail spaces.
Architecture by OMA and WZMH Architects. Heritage by ERA Architects. Landscape and public realm by Claude Cormier + Associés. Planning by Urban Strategies. Structure by Stephenson Engineering.
The principal architectural idea is to create a vertical urban village through a series of "urban rooms" interspersed throughout the tower. These spaces would serve as amenities for the building and house a variety of different functions. See above rendering.
The proposal introduces three important public realm moves: (1) a new public plaza that pays homage to the site's former neighbor to the east -- Second City; (2) a new mid-block pedestrian connection running north-south from Richmond Street East to Lombard Street; and (3) an outdoor public art gallery featuring oversized art tableaus.
The site currently houses one designated heritage building (86 Lombard Street), and the design contemplates relocating and fully retaining this building on the eastern edge of the site. Once you see the drawings, you'll fully understand why this was the most logical move.
The entire project team is very excited to get this proposal out and into the world. And we hope that you will see it as being representative of our ongoing and lasting commitment to elevating architecture, sustainability, culture, and city building in Toronto.
This week the team submitted a rezoning application for the southwest corner of Yonge & St. Clair (1 St. Clair Avenue West).
More of the details can be found over here on Urban Toronto, but so far the response has been overwhelmingly positive. People seem to appreciate the architecture, the public realm improvements, as well as how we're proposing to deal with the embodied carbon in the existing building.
https://twitter.com/alexbozikovic/status/1471557617789059074?s=20
The application proposes to retain the existing 12-storey office building and both expand its floorplates to the west and build new residential on top. In the middle is a shared multi-storey amenity space that also performs some pretty cool structural gymnastics courtesy of Stephenson Engineering (see above rendering).
This approach created some interesting design challenges for the team. Typically when you're adding onto an existing building, you want to do something new and not try and copy/bastardize what's already there. Oftentimes this means something more contemporary.
The architecture team at Gensler Toronto tried this approach but the podium proportions didn't feel quite right when we did it. So a decision was made to instead pay homage to the existing building's architecture, and then kind of reinterpret it by playing with scale and other details.
This way the original building remains architecturally legible, but the entire podium still reads as one and its proportions feel much better. We hope you like it as much as we do.
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