I recently mentioned how you can think – hypothesize, about the cause of a failure or accident based on very little evidence, then modify your thoughts as more comes in. (Ref. 1) This is the nature and technique of forensic engineering investigation. Some of the evidence can be as brief as a chance remark years ago, as happened to me. Following is another example of this process.
(Counsel benefits from a process like this – ideally when the merits of the case are assessed. But also when you think you have enough technical evidence to go forward and want to cut costs by stopping the forensic investigation)
There are a lot of multistory buildings in the Halifax area. I learned that at least one is defective because the floors are sagging – the floors are “wavey” to use one person’s description. A defect is a failure in engineering.
The floor in one of the rooms on the 10th floor slopes down 1.5% to 2.5% from the end to the middle. It sags in the middle. The room is 22 feet long by 12 feet wide. That means the floor sags 2 to 3 inches. That’s a lot for a commercial building. It’s far more than a construction tolerance of 1/4 inch. You can see the slope in the length of the conference tables. I measured the floor with a tape and a digital level.
In another large, square room chairs with casters roll to the left side of the room when you’re sitting in one. That happened to me. The floor slopes down to the left in this room compared to the middle in the other room. At least on the left half of the room where I was sitting. Quick measurements in three places in the room and also in the reception area indicated slopes of 0.1% to less than 1.0%. Staff in the office on this floor report that the floors slope in all the rooms.
I saw that the floors were not level on the 12th floor of the building. Office staff were not conscious of this but they did say that previous tenants reported that the floors were not level. I also saw the floor sloping down from a concrete column in an office on the 4th floor.
A staff member familiar with four floors in the lower part of the building reported that the floors were “Wavey. Not very, very, very level. We have to level when we do renovations”.
The building is made of concrete – concrete foundations, columns and floors.
The foundations are supported on bedrock which is very strong. I learned this from a friend who saw the foundations being constructed when his company worked on the building site. He also said multistory buildings like this are erected quickly so they can be rented as soon as possible and make money.
The construction technique used to erect multistory concrete buildings is sensitive to construction schedule.
A concrete floor in a multistory building is constructed by placing concrete in forms that are supported on jack posts. The jack posts are in turn supported on a previously constructed concrete floor below. The floor below will also be supported on jack posts below it. Jack posts are steel posts whose length can be adjusted – jacked up
You can see this construction technique in different places in Atlantic Canada – a number of jack posts at each floor level – usually three or four levels, below the floor under construction. There are at least two buildings under construction now in Halifax using this technique, one on Jos. Howe Drive and the other on Young Street.
The technique involves removing the jack posts from the lowest floor and leap-frogging over the upper floors to support the forms for the next floor under construction above.
I knew the construction manager who directed the work crews erecting the defective building years ago. He told me one time – the chance remark, that he was on a very tight schedule to construct the building – had to get it up in a hurry. Just like my friend said for multistory buildings, in general, but this one sounds like it was rushed even more.
The jack posts are removed from the lowest level when the strength of the concrete forming the floor being supported just above is high enough. The strength of concrete floors is specified by the design engineer according to the planned construction and use of the building. Concrete sets up – gains in strength, over a number of days from the wet concrete when placed in forms to the rock-hard concrete later. The quality of the concrete delivered to a building construction site is checked by testing companies to ensure it will set up to the design strength.
Analysis of the cause of the “wavey” floors
The defective, multistory building had five components when it was under construction:
- Concrete columns
- New concrete floor
- Forms temporarily supporting the new floor
- Jack posts supporting the forms
- Recently constructed floors supporting the jack posts
The limited information for a hypothesis in this case is:
- Floor condition: – Sloping and sagging but usable
- Building construction: – Simple concrete columns and floors
- Construction technique: - Construct a new floor by placing concrete in forms supported on jack posts resting on recently constructed floors below.
- Construction schedule: – The multistory building was put up in a hurry
- Concrete strength: – Concrete gains its full strength over time
- My experience during construction of one multistory building and examination of another during construction
Analysis of the limited information suggests the following possible causes associated with one or the other of the building components. The causes are listed beginning at the top surface of the new floor:
Cause #1. The floor forms were constructed level but the concrete was not placed and troweled level by the concrete finishers
I believe the concrete was placed and troweled level – or to the level of the forms, an important qualifier. I’ve seen concrete finishers at work often enough. They are proud of their craft. And besides, the concrete form they must place and trowel the concrete to is right there in front of their eyes a few feet away. It would be difficult to make a mistake.
Cause #2. The floor forms were not level because mistakes were made in measuring the position of the new floor on the concrete columns. These marks are the starting point for leveling the forms
Similarly, it would be difficult to make a mistake measuring the position of the new floor on the concrete columns. This is a simple measurement with a tape. I can imagine it being checked and rechecked. “Measure twice, cut once” like a carpenter does.
Cause #3. The floor forms were not level, either because of the leveling method or because the posts were not jacked up properly
The jack posts would be placed according to the level of the forms and adjusted up or down a little as required by the form leveling technique.
Based on what I’ve seen on construction sites, I can easily imagine a carpenter’s level with a spirit bubble being used to check the level of the forms and the need to adjust the jack posts. Cheap and quick on a job that’s in a hurry, also inaccurate. Inaccurate in different directions too depending on where you put the carpenter’s level. This would result in different slopes to the floor forms – and different slopes and sags to the finished concrete floor like I saw in the defective building.
What I’ve seen - the plumb of concrete columns being set with a carpenter’s level - floor after floor after floor on one 20 story building. Very crude. It’s not too great a leap of faith to believe that the floor forms were set “level” in the same way in the 20 story building.
Cause #4. The forms sagged when the heavy concrete was poured because the distance between the jack posts was too great
This might be possible but unlikely because jack posts would be placed at the construction joints between concrete floor forms. The forms themselves would be more than rigid enough to support a layer of concrete a few inches thick. The forms are likely to be reusable – certainly from floor to floor, but also from job to job.
Cause #5. The floors sagged because the jack posts were removed before the concrete set up and was strong enough
This is possible. I can’t dismiss it. Particularly if low strength concrete was accepted at the construction site and there were only three floors of jack posts in place. However, I might expect sloping and sagging to be more broadly distributed across the new floor rather than quite variable like in the defective building.
The floor in the long, narrow room on the 10th floor that tweaked my interest sagged 2 to 3 inches over about 10 feet. And the slope was in a different direction in the square room about 25 feet away from the narrow room.
I also can’t imagine low quality concrete being accepted at a construction site – truck load after truck load and floor after floor.
But this cause is possible because I just don’t have enough information on how deflection three or four floors down would affect a new floor way up above.
Cause #6. The floors sagged because an inadequate number of the lower floors were supported with jack posts beneath the upper floor that was under construction
This cause might be possible if jack posts were placed at only two levels rather than the three or four that seem to be normal. I see four in the two buildings I drove by recently. It seems like a risky decision for a construction manager on a very tight schedule and in a real hurry to get the building up even if he’s prepared to accept low strength concrete. I also don’t know as mentioned above on how deflection two or three floors down would affect a new floor.
What do I think is the likely cause of the “wavey”, sagging floors based on the limited evidence?
I think – my initial hypothesis, that the floors slope and sag – because the forms were not leveled properly - Cause #3 - in the rush to get the building up.
The following is what we do in forensic engineering when we think about the cause of a failure or personal injury for Counsel and the justice system:
- Gather the evidence as limited as that might be and from whatever source,
- Analyse it – carefully study each piece of evidence, note it’s nature and significance, how each piece relates, where each piece leads and what the whole tells us,
- Identify and list possible causes, and related technical issues
- Factor in our experience,
- Think about and hypothesize cause – come down on one cause or the other,
- Go gather more evidence
- Analyse it – etc. etc.
- Check if the initial hypothesis stands up to the new evidence,
- Accept the hypothesis, modify it, or reject it completely and start over.
More evidence in the case of the defective, multistory building would be a precise elevation survey and contouring of many or all of the floors in the building. Basically quantify the nature and extent of the problem, the defect whose cause you must determine. But this is not likely to happen because the building although defective is functioning quite okay. I’m sure there are others like it in Atlantic Canada.
- Bridge failure in litigation due to inadequate bracing – City of Edmonton. But, inadequate for what? Posted March 15, 2016