The forensic engineer will guess – an initial hypothesis, based on a simple analysis of the virtual evidence – that the culprit was inadequate temporary support for the section of the 4th floor where concrete had been placed.
According to news reports – the virtual evidence – the 4th floor collapsed on the morning of December 11 when a section of freshly poured concrete gave way crashing through the floors below. Note that it was a section of the floor not the entire floor, and the concrete was already in place.
The multi-story building is being constructed at 555 Teeple Terrace in the west end of London. Two workers were killed, one a new Dad in his early 20s, and four others injured.
A concrete floor in a multi-story buildings is constructed by placing a horizontal form of a material like plywood at the level of the underside of the floor – the 4th floor in this case. The form is supported by beams and jack posts resting on the completed floor below – the 3rd floor.
(Jack posts are simple steel posts whose length can be adjusted a little – jacked up or down)
Concrete from a large bucket is then placed on the form and smoothed off. The result is a floor – the 4th floor – when the concrete hardens.
The large bucket is filled with concrete from a truck parked on the ground below and lifted into place by a construction crane. These cranes can lift buckets of concrete many stories in multi-story building construction.
The 3rd floor below would have been constructed in a similar way a few days before and still supported by the form, beams and jack posts until the new concrete there is strong enough. And the 2nd floor below that, a few days earlier still.
Structural engineers design the size, strength and spacing of the form, beams and jack posts. They also determine when this temporary floor support system can be removed.
Similar, are the vertical timber forms that were placed to construct the basement walls of your house. These forms were supported by diagonal bracing/posts resting on the ground beyond the basement wall. Simple vertical basement forms, adequately supported, like simple horizontal floor forms, adequately supported.
Some simple virtual analysis
Sometimes construction is moved right along – rushed a bit – to stay on schedule, and mistakes are made. (Refs 1 and 2) Christmas is coming and this floor, the 4th, is the last one.
Most of us can see several places where things can go wrong in multi-story building design and construction like the one in London:
- Too much heavy, wet concrete placed at one location on the form – sort of like a point load in engineering design – that was not provided for in design. Concrete is about 2.4 times as heavy as water
- Too much concrete placed too quickly at one location on the form – like a dynamic load in engineering design
- The bucket load of concrete was lowered too quickly or came loose and fell and hit the form – like a dynamic point load
- The temporary supporting beams and jack posts were not placed properly for the 4th floor. Either too few were placed or they were not spaced properly this time
- In general, construction was rushed in some way for the 4th floor, the last one – let’s get on with it, time is money and Christmas is coming
More simple virtual analysis
Construction workers reported seeing a section of the 4th floor collapse after concrete was placed, the debris fall to the floors below and take these out too. Photographs from people on scene and video from a drone above generally support this report.
The construction procedure is simple – support a form with beams and jack posts then place concrete. Not unlike a horizontal basement wall – sounds funny but the engineering principles are similar. The procedure worked for the other floors, and it was being followed on other construction sites in Canada at the time. Why not this site too?
Wet concrete placed on a surface, a form, flows a little like cold molasses – it has to so the workmen can spread and level it to form the smooth concrete floor. I can’t imagine it as much of point or dynamic load on the floor.
The concrete floor was in place according to reports. I can’t imagine the concrete bucket lowered too quickly and hitting the floor form and smashing it. The bucket was probably down at the truck on the ground or on it’s way.
I’ve done materials testing and inspection on multi-story building construction and that’s what happens – the bucket is lowered after unloading the concrete. It doesn’t hang around up there.
But, if the beams and posts supporting the 4th floor form were placed properly like for the 3rd and 2nd floors – which did not collapse during construction – then a section of the 4th floor would not have collapsed like it did.
My conclusion as to the culprit
That leaves inadequately placed beams and jack posts as the culprit, the cause of a section of the 4th floor collapsing at the multi-story building site in London.
That’s my initial hypothesis as to cause based on a simple analysis of virtual evidence and knowing something about how multi-story buildings are constructed. (Ref. 3)
- Thinking about the cause of “wavey”, sagging floors in a building and how Counsel benefits. Posted April 6, 2016
- Bridge failure in litigation due to inadequate bracing – City of Edmonton. But, inadequate for what? Posted March 15, 2016
- What can you get from a virtual site assessment about the cause of a leaning retaining wall? Posted November 13, 2020
(Posted by Eric E. Jorden, M.Sc., P.Eng. Consulting Professional Engineer, Forensic Engineer, Halifax, Nova Scotia, Canada December 31, 2020 firstname.lastname@example.org)