Algo Centre Mall Collapse
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
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This week in engineering news, NASA found a large amount of water in the atmosphere of an Exoplanet.
● 700 light years away, towards the constellation of Virgo, scientists using the Hubble and Spitzer space telescopes, and more recently the James Webb Space Telescope, found the remnants of water, methane, and carbon dioxide in an exoplanet. They believe the planet is a gas giant, a quarter of the mass of Jupiter, bigger in radius than Jupiter, and with 3x as much water as Saturn.
● The planet's name is WASP-39b, which needs work. While it is similar in mass to Saturn and similar in size to Jupiter, it is drastically different.
○ The planet is believed to rotate around its star once every 4 days, is 20 times closer to its star than Earth is to the Sun, and it’s tidally locked meaning it always shows the same face to its star.
○ The exoplanet's dayside temperature is about 776C, and strong winds move heat around to the nightside and keep it at a similar temperature.
○ WASP-39b also doesn’t have rings or high altitude clouds like Saturn.
● The scientists thought they would find water, but they found much more than expected. Suggesting the planet was formed far away from its star and was hit with a lot of icy material.
● The discovery of water, and learning more about how planets form suggests that formation is more complicated and confusing than previously thought.
● If you want to read more about WASP-39b, check out the links on the web page for this episode at failurology.ca
Now on to this week’s engineering failure; the Algo Centre Mall Collapse.
● June 23, 2012 at 2:18pm, when the rooftop parking deck of the Algo Mall in Elliot Lake, Ontario collapsed onto two floors below, tragically killing two women and injuring 19 others.
○ This isn’t the first time we’ve talked about rooftop parking collapse on this show. Check out ep 23 about the Burnaby Supermarket roof collapse.
● The collapse was the result of a sudden failure of the connection at one beam and one column of the steel structure below the parking deck. But this failure is more than just a structural failure.
● Elliot Lake is located in Northern Ontario, north of Lake Huron, between Sudbury and Sault Ste Marie. It was once known as the uranium capital of the world, but is now known for affordable retirement living and waterfront cottages. It has a population of around 11,000 people.
● I want to read a paragraph from the attorney general's inquiry report because it really just sums it up well.
○ Although it was rust that defeated the structure of the Algo Mall, the real story behind the collapse is one of human, not material, failure. Many of those whose calling or occupation touched the Mall displayed failings – its designers and builders, its owners, some architects and engineers, as well as the municipal and provincial officials charged with the duty of protecting the public. Some of these failings were minor, some were not: They ranged from apathy, neglect, and indifference through mediocrity, ineptitude, and incompetence to outright greed, obfuscation, and duplicity. Occasional voices of alarm and warning blew by deaf and callous ears. Warning signs went unseen by eyes likely averted for fear of jeopardising the Mall’s existence – the social and economic centre in Elliot Lake.
● The failure was the result of continued ingress of water and chlorides from the parking deck which has severely corroded the steel structure for decades.
● Despite many complaints about the leaking deck and falling ceiling, the owners failed to install an adequate waterproofing membrane on the parking deck surface to mitigate the ingress of water.
● This problem was missed by everyone. Owners, municipal authorities and structural engineers over the 31 years that the mall operated.
● We’re getting a bit ahead of ourselves here, and are going to circle back to talk about the mall construction in a minute. But before we take a deep dive into Algo Centre, let’s talk a little bit about building envelope engineering.
○ The building envelope are the components of the building that separate the inside from the outside; the exterior walls, the roof, the windows, etc. In the US it is called building enclosure, but I prefer envelope so we are going to use that term in today's episode.
○ Envelope engineering is a newer discipline, coming to light in the late 90s in places like BC, but more like the 2010s in Alberta. There is no formal program for building envelope engineering, at least not like there are for mechanical, electrical, chemical, and other disciplines. In my experience, most envelope engineers start out in civil engineering and “accidentally find their way to envelope” although this is not the only path.
○ The envelope engineers often work with the architect to design systems and strengthen installation details to control four barriers; air, liquid water, heat, and water vapour. As mechanical engineers, we also work closely with envelope engineers because leakage points, envelope performance, and thermal bridging all impact the mechanical system design and required capacities. I’m just going to say that thermal bridging is a fascinating rabbit hole, but today I’ll just say that it’s the path of least resistance for heat loss. For example you can insulate a wall to high heavens, but if there is metal in the wall that passes all the way through that wall, you are providing a path for heat loss. This is not as critical in temperature climates. But in Alberta where it gets as cold as -40C, this can be a big deal.
● Like a lot of small towns, the mall isn’t just for shopping, it's a gathering point. Algo Centre housed federal and provincial offices, library, local members of provincial parliament offices, an 80 room hotel, food court, and numerous stores.
● The mall was built in 1979 for a cost of $10 million and included rooftop parking. From the day it opened, the roof leaked.
● The mall was a three-storey L shaped structure; upper mall level, lower mall level and pick up level. The hotel rose 4 floors above the rooftop parking plus a mechanical room on top.
● The structure was a hybrid of steel frame from steel beams and columns and 75mm (3”) deep, 1.2m wide (4ft) precast concrete panels arranged side by side to make up the slab. The lengths of the panels varied but were typically around 9m (30ft). The precast panels are made off site and have hollow cores to reduce weight.
● The waterproofing issues were two fold.
○ First, the rooftop parking area lacked adequate waterproofing to prevent ingress of water to the structure.
■ As a cost saving measure during original construction, the owner decided against a hot rubberized asphalt membrane and chose one that included a non-composite concrete topping. The topping cracks were to be filled with polyurethane and a sealer applied to the entire surface.
■ The Ontario Building Code 1975, while the waterproof system was not clearly outlined, did read that the “roof shall be installed so as to, shed or drain water effectively”.
■ In 1991, options were reviewed to apply a membrane, but they were rejected. Which takes us to issue number two.
○ The second, and probably more prominent, issue related to waterproofing was the specified load capacity of the concrete panels exceeded the highest capacity that the panels can handle.
■ The structural design is such that the engineer of record provides a load capacity rating for the floor and then the floor components are designed by a separate contractor to meet the required capacity. This process is common in wood frame structures where the structural engineer of record specifies the joist ratings and the joist manufacturer designs the joists systems.
■ The structural engineer for Algo Centre specified a 5.75 kPa (120 lb/sq ft) rating, but was told by the panel suppliers, and this is recorded in the tender minutes, that the panels couldn’t support this load and it had to be augmented by a composite concrete topping. The engineer said they had to meet the load without the topping, the owner awarded the contract, and a system was built and installed that didn’t meet the project specifications.
■ Published design data from the panel manufacturer noted the slabs were 38% under the specified load.
■ When this information surfaced in the early 90s, it prevented any engineer from accepting added weight to the slab to accommodate a waterproof membrane or other topping. Despite these issues, which certainly facilitated the ingress of water, the panels themselves did not play a direct role in the collapse.
■ Of note, despite the concrete panels being under-designed for the specified loads, the steel structure was adequately designed for the loads, including the connection that eventually failed in 2012.
○ Ok the weird part about this whole thing is why did they keep the rooftop parking open after all of this was discovered. I’m not a structural engineer, but based on some light research, it appears the 5.7 kPa (120 lb/sq ft) was to account for vehicles in the loads. So if they removed parking from the roof, could the load have been reduced such that a membrane could have been installed?
● The building underwent condition assessments over the years; specifically associated with a sale of the mall. The assessments occurred in 1999, 2005 and 2009 by three different firms. Only the first report outlined the leakage problem but recommended managing it by sealing cracks. The second and third reports don’t recognize the leaks or offer solutions.
● The investigation found that the corrosion, which was dependent on leakage rates and de-icing salt, was approximately 0.1 mm/year, consistent with marine climates. Despite the level of corrosion being categorised as “severe” to “very severe” in the immediate investigation, it either went relatively unnoticed or was not monitored further to prevent failure.
○ They also found extensive corrosion throughout the mall with most locations classified as “poor”. The top flanges had more severe signs of corrosion than the bottom flanges or the web. The beams are I Shaped, capital I that is. With the top and bottom sections being the flanges and the web being the vertical section.
○ The connections were badly affected as well, with more than a third of the connections having severely corroded welds or bolts.
○ Chloride levels, from the de-icing salt, were also well in excess of the threshold defined for corrosion.
● The forensic evidence suggests that the failure took place in two stages, over several months. The first being the failure of the weld, possibly after being subjected to a heavy load of short duration. This wouldn’t have broken the connection entirely, but sheared a significant part. From there the corrosion continued until the residual capacity depleted even further.
● Fun fact - the capacity at the time of collapse was about 13% of the capacity when it was built. 13% !! A small car passing over the joints moments before the collapse was enough to exceed the threshold for shear failure.
● An urban search and rescue crew arrived at the mall by 2am Sunday morning, about 12 hours after the collapse, and got straight to work. Rescue work was suspended early Monday morning due to the danger of additional concrete falling on the trapped survivors and rescuers. Late Monday night, the search resumed, using heavy equipment to remove the unstable escalator structure and part of an external wall. Unfortunately, two victims were found 4 days after the collapse. They were believed to be an employee and a customer of a lottery kiosk located under the collapsed roof section. The inquiry into the collapse found that while the customer died instantly, the employee may have survived for up to 39 hours after the collapse and would have been found if not for the suspended rescue operations. Which is really, really sad.
● Both the original structural engineer and architect testified that they did not agree with the decision to make the roof a parking area.
● The mall owner’s son also admitted that they had pressured engineers to remove information on the roof leaks and structural steel corrosion from reports, and that they had faked repair invoices, to mislead the bank about the condition of the building.
● The engineer who declared the building structurally sound a few weeks before the collapse was indicted with two counts of criminal negligence causing death and one count of criminal negligence causing bodily harm; although the trial found that there was not enough evidence to find him guilty beyond a reasonable doubt. He was aware that the roof was at risk of caving in, but altered his report to not jeopardise the building’s refinancing. His licence was also suspended when he performed the inspection and he lost it completely following the collapse.
○ We talked about this alot in the last episode when we covered the OIQ losing its ability to self regulate. This engineer knew there was a danger of the roof collapsing, and a danger to the public, but he altered his report at the request of his client. He was not acting with professional ethics, nor did his actions seek to protect the public from harm. I’m glad his licence was revoked completely.
● The inquiry also heard that the rooftop parking area was also often used to bypass a set of traffic lights in town, adding to the traffic and stress placed on the structure.
● The mall was demolished in 2013 and a new mall, without a rooftop parking area, was built in its place, opening in spring 2016. With the mall being such a large part of the town’s economy and access to essentials, its collapse and closure had a big impact on residents. Sudbury and Sault Ste Marie, the cities, were 100km each from Elliot Lake. A temporary grocery store opened in October 2012 and the new grocery store was the first store to open in the replacement mall.
So there you have it, the Algo Centre Mall Collapse. A poorly located rooftop parking lot, with an inadequate waterproofing system, led to decades of corrosion and the ultimate collapse of the mall roof.
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