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, morphing metal.
Researchers at Virginia Tech are using rubber, metal and temperature to morph materials and fix them into place without motors or pulleys
At the start of the project, the goal was to create a material that could change shape, hold that shape and then return to the original shape.
It had to be soft enough to morph, but strong enough to create machines that could perform a specific function
The team studied Kirigami, the Japanese art of making shapes from paper by cutting (not origami which uses folding). They used this to develop a material architecture from a repeating geometric pattern
For the material, they used a low melting point alloy endoskeleton in a rubber skin
When stretched the material would hold its shape and then heat was applied to melt the metal back into the shape of rubber skin. The temperature required is about 60C which is potable hot water.
Aside from shape shifting, the weakening of metal from bending or breaking can be easily fixed during the melting process.
One of the hopeful applications of this product is to build a functional drone that can morph from a ground to air vehicle.
If you want to read more on the morphing metal, check out the links to sources on the web page for this episode at Failurology.ca
Now on to this week’s engineering failure; the L’Ambiance Plaza.
April 23, 1987 at 1:30pm - the day before this episode comes out but quite a few years before.
16 storey residential structure, with three levels of parking and 13 apartment leves and two offset rectangular towers (east and west) separated by a construction joint at the central elevator lobby.
We don’t do above grade parking in Calgary. At least not often. Also we call them parkades, which is definitely a Canadian word that likely originated in Edmonton, the capital of Alberta. I have had friends from America visiting that had no idea what a parkade was. In America, it is a parking structure, or multi-level parking garage.
The National Bureau of Safety did a formal investigation and published their findings, which we’ll get to shortly. But the industry consensus is that they don’t agree with those findings and they believe the cause of the failure to be something else entirely, which is really interesting. This is the first time we’ve seen this much of a disagreement on the cause of a failure. There is usually a bit of disagreement on contributing factors, but to see disagreement on the root cause, this is a first for us.
The slabs were 175mm thick post concrete flat plates with plastic sheathing post tensioned tendons in each direction.
Lift slab method – the floor slabs were constructed on the ground on top of the lower floor with a bond breaker between each slab. They poured all 16 slabs and once they were cured and the slabs were post tensioned, they were lifted 2 or 3 at a time into place with a series of hydraulic jacks and lifting rods and secured to columns – cost saving and speed of construction are two advantages to this method since 90% of the formwork can be eliminated
The west tower collapsed during construction – when there were three levels of the parkade poured and 3-6 levels of each tower were in place - the building was about 25m above the foundation when it collapsed – 28 construction workers were killed
The east tower also collapsed due to one or more factors
Forces transferred from the west tower collapse
Damage to the post tension cables caused by debris from the west tower
Lateral instability caused by falling debris from the west tower
Investigated by the Center for Building Technology, National Engineering Laboratory and National Bureau of Standards
interviewed survivors and eyewitnesses
reviewed design plans, specs, shop drawings, construction records, project correspondence, and testing lab reports
lab tests from collapsed structure
data from a subsurface investigation after the collapse
computer analyses, which existed in 1987, but certainly not to the extent of today’s programs.
Most probable cause as per the National Bureau of Safety
Was the failure of the lifting mechanism, i.e. the jacks, in the west tower while attempting to place three upper slabs - there was excessive deformation of the lifting angle in the jack head on the most heavily loaded jack. This created a loud noise.
from the deformation, the jack head slipped off of the lifting angles and the slab package fell, collapsing the entire west tower.
Essentially the jack was overloaded and it deformed, when this happened the part of this jack or another close by that were in contact with the slab package slipped, the loads redistributed without the use of the deformed jack but it was too much and the entire package fell and pancaked all of the slabs below it.
This was duplicated in lab experiments, including the loud noise when the jack deformed.
Other investigators of the collapse did not necessarily agree with the National Bureau of Safety findings. This is the first time we’ve seen this much of a disagreement on the cause of a failure.
The lifting manufacturer used different test conditions and found that the lifting mechanism was not the probable cause of the collapse. The lift subcontractor also said that this design had been used in over 100,000 lifts over 17 years.
Also important to note, the lifting assembly didn’t have a 2.5 safety factor that would allow people to work under the slab during lift operations.
The project used a design concept that transferred major design responsibility to the contractor in performance specifications and permitted construction procedures that weren’t backed up.
The structural engineer designed the structural frame, including the lift-slab method, but didn’t include post tension design and details or column connection designs and details, which were by the contractor. The structural drawings also did not include details about large slab openings like the elevators or stairs.
The structural design for the post tensioning calculations and shop drawings were completed by a sub of a sub of the general. So the general contractor hired subcontractor A, who hired subcontractor B to do this design. This isn't uncommon, except that the structural engineer of record’s review of these shop dwgs noted “checking is only for general conformance with the design concept of the project and general compliance with the information given in the contract documents”. And even though the contract documents required the contractor’s design to be performed by a registered professional engineer, it wasn’t. There was no engineering stamp on the shop drawings or post tension design drawings.
In addition to contracting out the post tensioning design, contractor A also subcontracted out the mild steel reinforcement, structural steel and lifting operations to three other contractors.
There didn't appear to be an allowance in the post tension design for large slab openings like the elevators or stairs and this is what others believe to be the probable cause of collapse. In fact, the post tension shops showed the same typical cable size, spacing and layout in those areas as in typical bays that didn't have large openings.
The elevation of the post tension cables within the slab thickness did not adjust to accommodate for the large openings either. So in the mid span along the elevator opening, the post tensioning stresses in the slab added to the loads rather than compensated for them. There was also a lack of rebar in this area, leaving the post tension cables unreinforced.
At column locations, there were questions about local shear and bending strength in connection and transfer of loads between the slabs and columns.
Defects in the post tension cables and rebar mats didn't provide the ability to support loads by “netting” or “membrane action” where if there is a localized defect, the rest of the slab can continue to support without collapse. Like if you have a hammock and one of the strings breaks, the rest of the strings keep you in the hammock. But due to deficiencies here, localized issues increased the risk for brittle or rapid progressive collapse.
Allowances were made in the pre poured slabs for the eventual casting of shear walls. But until those shear walls were poured, there was high compressive and shear stresses in these areas during the slab lift.
The contract requirements limited the advance of the uppermost lifted slabs to not more than three levels above completed shear walls. But they didn’t follow this as welding and shear wall installation lagged behind.
The contractor deviated from plans and specs in a number of areas, they don’t appear to be probable causes, but are still causes for concern. These deviations are as follows.
The footings were resting on fill and disintegrated rock rather than “rock of suitable quality” or lean concrete
Some of the welds were not as strong as the structural dwgs required
The column splice details on the structural plans didn't meet the AISC American Institute of Steel Construction specification or the AWS American Welding Society structural welding code for complete penetration groove welds
And a lack of joint penetration and large amounts of porosity were observed in some of the field welds.
Not a structural engineer, but these “field changes” seem pretty significant, and I wonder if the building was at risk of structural collapse at some time in the future, had it not collapsed when it did.
In summary, while the National Bureau of Safety thought that deformation of the lifting jack caused the failure, the industry and independent analysis consensus is that the failure was caused by inadequate post tensioning and rebar design at the elevator opening in the west tower.
There were some procedural issues that also contributed.
excessive reliance on performance specifications, allowing structural design responsibilities to be split among the engineer-of-record and multiple contractors,
building regulations did not assure competent structural design for complex structures, or for projects where design responsibilities are split between an engineer-of-record and contractors,
inadequate safety factors and other technical provisions in building codes, and
construction phase quality assurance inspection by testing agency technicians in lieu of the engineer-of-record or a substitute engineer with knowledge of the structural system.
So there you have it, the L’Ambiance Plaza structural collapse. Similar to the Hyatt Regency walkway collapse, the structural engineer put a lot of the engineered design of structural components on the contractor. Which is fine in theory, as long as they are stamped and thoroughly reviewed by the structural engineer of record.
For photos, sources and an episode summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to Failurology, so more people can find it. If you want to chat with us, our Twitter handle is @failurology, you can email us firstname.lastname@example.org, or you can connect with us on Linked In. Check out the show notes for links to all of these. Thanks, everyone for listening. And tune in to the next episode where we will talk about the Florida Pedestrian Bridge collapse that occurred in 2018 over a 6 lane open roadway. The bridge was supposed to be a redundant structure, but mistakes during design meant that the critical members did not have backup.
Bye everyone, talk soon!