There are 614,387 bridges in the United States and 55,707 of them are thought to be structurally deficient according to the US Department of Transportation (2016). About 188 million people cross “a deficient bridge” every day in the US (also a 2016 figure).
Inspections are often infrequent and only visual, and so MIT Senseable City Lab is currently on a mission to come up with a more scientific approach. They believe that there’s a solution in crowd-sourced data and that it’s possible to create a community-driven maintenance program.
What they discovered through a recent study, called Good Vibrations, is that mobile phone sensors can actually pick up the natural vibrations and oscillations of a bridge. And, that a mobile sensor located within a traveling car is actually 120x more precise than fixed sensors located on the bridge.
Part of the problem is that fixed sensors have poor spatial coverage. They are located in specific locations. Whereas mobile sensors give you data across the entire span of the bridge as someone crosses it. And if you know how a bridge normally vibrates, you can quickly tell when something is off.
I have always found structural engineering fascinating. Structures, along with physics, were some of my favorite classes from high school all the way to grad school. So even though I don't think my personality is ideally suited to engineering, if I were ever to become an engineer, I'm fairly certain that I would need to be a structural one.
For this project the big structural challenge was the large cantilevers that you see above in the tower on the left. As I understand it, there a number of ways to deal with this. One way would be to just design large transfer slabs and/or beams. But given the size of this tower, these would end up being very deep, and so you'd be really compromising the spaces where these structural transfers occur.
How they actually dealt with it is through sloping columns (which you can see in the above photo if you look closely). What these columns do is gradually transfer the loads across multiple floors in the building, until they reach structure that runs all the way down the tower.
At the same time, the spaces underneath the sloping columns are essentially "hung" from above. Meaning the columns are in tension, instead of being in compression, which is typically how columns work. The result is that you get some sloping columns in the suites. But I think that's kind of cool. If you're nerdy enough to care, it tells you how the structure of the building is working.
Obvious disclaimer: I am not a structural engineer. You probably want to consult one if you're looking to do a cantilevered tower with sloping columns.
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.
