The length of time between an accident or incident and the presentation of any subsequent findings has been a constant source of frustration and annoyance at DemolitionNews.com for many years. And so, in the immediate aftermath of the recent scaffold collapse in Reading, DemolitionNews commissioned a retired but highly qualified demolition engineer to present his analysis of the possible and likely cause of the collapse. DemolitionNews has specifically chosen not to publish the name of the demolition engineer to protect his identity, and would stress that the following is not meant to criticise or undermine specific companies; nor should it be seen as an attempt to pre-empt or influence the findings of any investigation or subsequent prosecution.
It was frightening to see a major scaffold collapse in a city centre again, that could have resulted in fatalities or serious injuries, it was very fortunate there were only minor injuries.
As a qualified demolition engineer, one has to ask the question: Why? Was it down to...
- Lack of planning /foresight
- Wrong methodology selected
- Plant selection
- Pressure from clients over time and price
I must stress that I am not party to the contract. However, I am willing to question the competence of those involved from the client, main contractor, subcontractor and structural engineers. I am also willing to ask the questions that others in high positions are scared to ask in public, only whisper about in corners at conferences, seminars or other “back slapping” award ceremonies where they tell everybody how great they are.
Only a few companies have had the balls to talk about problems on jobs such as a space frame collapse and large concrete panels that fell from tower blocks. On both occasions, the contractors were chastised by other companies. Worse still, the information that was offered to the industry to help educate has subsequently found its way into the inbox of clients with whom those honest contractors were competing.
Speaking from experience, there isn’t one demolition contractor that hasn’t had an uncontrolled collapse or near-miss event. If they say they haven’t, they’re lying.
An Industry Changed
The structure of the demolition industry has changed over the last 20 years. In London, It has changed significantly.
The Big Six London demolition contractors have in effect become main contractors, only employing managers and other white collar workers, a lot of whom have never had dirt under their nails. Whilst relevantly qualified, they have little experience and don’t have the knowledge to call it when they see something wrong. They are “microwaved“ demolition managers.
There is a large group of subcontractors in London who specialise in top down work who have a lot of men and machinery, which will all be badged up in the companies colours. This was the case in Reading. The high reach excavator that appeared on national and local media may have McGee stickers on, but it belongs to a well-known subcontractor.
This is not always told to the client as they want to give the impression to the client that it’s all their men and machinery.
But if it wasn’t for the likes of Tony Rawlins, Mick Brown, Keith Plenty, Danny Kennedy, Burleys, Dave Lee, H and others - all specialist floor by floor demolishers - buildings in London wouldn’t be demolished.
Most clients are naive when it comes to placing complex demolition schemes. A lot of demolition contractors are taking on cut and carve schemes that, in my opinion, are above their competence and ability. Unless the HSE takes to task clients and principal designers, this will continue.
To the best of my knowledge, Berkley Homes is one of the only main contractors that train their site managers on a three-day demolition course and check references and visit sites. They even backed this up last year on a floor by floor contract. The demolition contractor had subcontracted the job out to a company with little floor-by-floor demolition experience, who then used agency staff to carry out the work. On the first audit, this was discovered and they were removed from the contract.
There are pressures placed upon contractors by clients from day one, all risks asbestos, services being the most common problem. Most clients seem to think that demolition contractors have a magic wand to make such issues disappear. It is not unusual for contract start dates to be moved back; completion dates never do as they’re often enforced by large delay penalties.
Too often contractors have to work around live services – electric, gas and mobile phone masts - and are expected to cut and carve around these like a child with paper and scissors. The difference is that if a child makes a mistake and cuts over the line, they can start again on a new piece of paper.
Utility providers should be taken to task by the HSE, and other bodies to resolve this. The National Federation of Demolition Contractors (NFDC) has been badgering this for years. Yet at the last meeting chaired by the NFDC , most utilities didn’t even send a representative.
Very often the first question asked when I visit site is when will be you finished? Clients and main contractors don’t understand or appreciate the complex works being undertaken. Very often they are more worried about their “considerate contractors” accreditations and are more focused upon the hanging baskets of flowers along the site’s perimeter and other “badges of honour” that they sport like cub scouts.
The first phase of the contract was carried out with a high reach excavator running parallel with the road and the scaffold, top-down, bay-by-bay which is a safer method of doing this work. Also, as this was an office block, the grid layout for the columns and beams would be pretty standard and the load paths easier to understand. As long as no large volume of demolition material were allowed to build up on floors and in the absence of faulty workmanship or adverse alterations, the structure would behave as expected and be brought down in a controlled manner .
In the picture above you can even see a banksman on the scaffold watching out for the driver. This is quite common practice; usually with a radio connection between the operator and the foreman. This can also be done from a Mobile Elevating Work Platform (MEWP) but there isn't always room.
The barrier on the roof in the left-hand corner prevented cars driving in that section, which was previously used as a car park. This sections inability to bear the weight of cars should have led to structural analysis and a specialist methodology to remove it.
The building has been partially demolished before. There are large infill panels of blockwork which themselves would give areas of concern to tying in scaffold , the taxi rank was suspended and looks like it was moved to the other side of the road , the footpath looks non-existent , so all pedestrian traffic was on the other side , otherwise the injury count could have been far worse.
Too many times councils are unwilling to close footpaths – and it can take 8-10 weeks to get permission and to schedule a closure as clients have started the procedure with the council forcing the contractor's hand from the start to come up with an alternative scheme.
There are a couple of single leg RMD props supporting steels that used to support the structure before being partially demolished
The temporary props can be clearly seen before the scaffold was erected.
The hoarding has been erected and the gates between the two buildings, this could be the main reason why it was decided to demolish from the rear and not parallel with the footpath and the scaffold .
Looking at the photographs on social media I have the following points and observations to make.
The high reach excavator was working from the inside to the outside towards the scaffold.
This is very difficult as the driver is working “blind” unless he has a spotter/banksman on the scaffold which, in this case, could have been fatal. A man in a MEWP may have been an alternative,
The part of the building that collapsed was lower than the block behind it, and there was debris on the roof which could have caused the roof beam to fail at its shear point due to overloading.
The reason for this direction of the demolition may have been dictated by the site access for other works. As phase 1 demolition was complete, maybe an alternative temporary site access could have been considered. But again, these are difficult to obtain from local authorities and with the number of utilities in footpaths at shallow levels it’s not always easy to construct a heavy-duty crossing across the footpath,
It would have been a lot safer to demolish this building by closing the road first and then using either a high reach or Brokks as shown in the alternative methods section below.
If this building had been tiered and stepped back, then the high reach working from the back to the front wouldn’t be putting potentially any big loads towards the scaffold.
The part of the building that has collapsed looks like a large plant room with large open spans – would be easier to demolish across the short span. This may have taken more time and cost more money but would have prevented this accident in my opinion.
The weight of the floor slab may have caused the column to fail .
The three smaller yellow arrows show the line that the slab had been munched to before it collapsed. The larger yellow arrow shows the rotational path of the beam as it fell.
As shown by the two photographs the C Jaw has over double the blade length allowing it to cut larger diameter Rebar.
The roof slab of the plant room has rotated through approx. 120 degrees (the large yellow arrow). That is the underside of the roof slab you can see.
The large amount of material on the roof could have caused the back beam to break from its column head and cause the scaffold to spill out across the road.
Materials overloading contributing to failure
The barriers on the car park roof being moved back from the edge should have set alarm bells ringing that this part of the roof wasn’t good for any loads, I have had structural engineers tell me that a bay in a car park was only good for a Brokk 90 – approx. 1.0 tonnes - yet there were three 4x4 vehicles (each weighing approx. 2.5 tonnes) parked in the same bay before demolition, with point loads.
The long yellow arrow shows the area of roof slab and direction that should have been demolished before the large higher level plant room.
This has been tiered back or “stepped “ by Brokks or small diggers – to allow safe demolition
From the rear, scaffold tied in around columns – box ties above and below the slab.
High reach working from the back to the front with “steps” to avoid overload to the scaffold in place
Or pressure from the high reach forcing the scaffold out.
When you look at the scaffold, it doesn’t look well tied into the structure – which was difficult anyway due to the large expanses of brickwork. They may have used snap ties (see example 2).
Even with pull tests, these have a habit of failing due to the strength of the bricks, being weathered over the years, or poor quality workmanship in the 1960s contributing to a combination of bad bricks and mortar.
They also have a habit of becoming loose or dislodged as bricks from above when being demolished fall on to them and dislodge them.
There were no strong winds in the vicinity at the time. Extreme weather factors doesn’t seem a likely cause.
Example 1 box tie – a good practice to tie in above and below each floor, and even though the floor slab by core holes.
Example 2- snap tie being load tested.
I recently spoke to a demolition engineer who was involved in the demolition of Barnsley markets, which involved the demolition of a large 1960s car park. The council structural engineer had signed off that it could be demolished using a high reach excavator. The demolition engineer disagreed as it was adjacent to a live and busy outdoor market, road and rail line, and insisted upon a second opinion.
After several consultations, it was agreed that the car park was very prone to an uncontrolled collapse if it had been demolished using the high reach method. Instead, it was successfully demolished using explosives in a controlled night time blowdown.
This is similar to the Barnsley car park but was significantly back propped and braced to prevent an uncontrolled collapse – expert engineering.
Another alternative method can be to secure a night-time closure and work from the road inward. Again this method isn’t considered often enough, not always practical.
High reach method no scaffold.
The demolition contractor in this case secured a night time road closure and worked the outside bays at night time adjacent to the road, then demolishing the remaining bays during the day. This work has been carried oy successfully at Preston. The use of a dust suppression and debris curtain reduces dust and fly during operation.
Without doubt, there will be more and more car parks to demolish due to component failure, redevelopments due to high street fail in shopping numbers as people switch to retail parks, and online shopping
Pictures of Nottingham cantilever failed whilst in use 7/5/2018
Neither the NFDC or the Institute of Demolition Engineers has issued any guidelines on the safe demolition of car parks despite several collapses;
Pipers Row- Wolverhampton British lift slab collapsed out of hours of operation whilst in use 20/3/97 – there is a HSE report of 116 pages on this
Edinburgh Car Park– overload on floors during demolition works leading to partial uncontrolled collapse.
Southend on Sea car park collapse Feb 2011 – Car park collapsed during demolition works.
Contractors need to be very wary of car parks and they should also be aware that there are many types, each with their own specific challenges:
- British lift slab – the most dangerous car park to demolish very prone to uncontrolled collapse
- Pre cast concrete – again require significant propping
- Tradditional r/c frame – Can be demolished by high reach so long as sequenced correctly
- Steel frame with precast concrete planks or steel deck with concrete slab poured in situ – these have been popping up recently at a lot of train stations.
The views expressed in this report are the personal view of an experienced and accredited demolition engineer. They ARE NOT necessarily the views of DemolitionNews, nor are they designed to pre-empt, influence or prejudice the findings of any official investigation.