Maintenance regimes are expected to fall under the spotlight following the partial collapse of an NCP multi-storey reinforced concrete car park in Nottingham on Saturday.
A 20m long section of cantilevering slab, along with the attached parapet, collapsed onto the pavement beneath at around 4am on Saturday morning. No-one was injured in the collapse, but cars were left hanging over the edge of the deck and the car park has been closed by owners NCP pending structural investigations.
The Health & Safety Executive is investigating, but senior engineers who spoke to New Civil Engineer said the spotlight was sure to fall on the maintenance regime.
Structural safety campaigner, University of Edinburgh chair of future infrastructure and past ICE president Gordon Masterton said lack of maintenance has been a long running issue with this type of structure.
“It’s not new to have structural problems in multi storey car parks,” he said. “It is sad that we’re still seeing issues which ultimately have to do with the lack of adequate inspection and assessment of the risks to the public.
“All of the elements need to be regularly inspected and assessed for any weaknesses in their initial structural arrangements. We have a right to expect for these things not to happen.”
He said after the failure of Pipers Row car park in Wolverhampton in 1997, where the top slab collapsed onto the one beneath it, a raft of new documents from institutions such as the ICE were published to try to prevent these failures.
Comparing a car park and a bridge structure, he said the two structures served a similar purpose in that they both spanned over a gap and carried vehicles. But he said there was a big difference in the regulations for maintenance and inspections for bridges to those of multi storey car parks.
“With car parks, the ownership tends to be more varied and there aren’t the same obligations for formalised principal inspections and on a cycle which you’d expect for a bridge structure,” said Masterton.
“But a question that could be legitimately asked is, is it now time to make it more rigorous and make it more of an obligation for car park owners to have good practice and follow recommendations?”
Safety body Structural-Safety director Alastair Soane agreed saying: “Structural-Safety repeats its advice that all old car parks should be inspected by structural engineers and regularly monitored.
“The difficulty with cantilevers however, as was the case in Nottingham, is that they can fail without warning. It was, as is so often the case with structural collapses, only a matter of timing and luck, that there were no fatalities or injuries.”
In a statement, NCP said is was undertaking a “thorough internal investigation” and that it would ”cooperate with any investigation conducted by the regulatory authorities”, adding that, at this early stage it was inappropriate to comment further. It said the car park would remain closed until further notice.
From images, work looks to have been underway on an adjacent bay of the structure with scaffolding clearly visible underneath the approximately 1m long cantilevering section. But it is not known if this is connected to the collapse.
The actual mechanics of the collapse are likely to be related to corrosion of the steel reinforcement, said Mott MacDonald head of materials and corrosion technology Paul Lambert.
“Corrosion is often the smoking gun on these type of structures,” said Lambert, adding that the corrosion itself is often due to other factors such as the design.
“These sections are often lightweight, simpler and less reinforced. Often they weren’t intended to last as long as they have,” he said.
“While it may be a structural matter that the collapse was able to occur, corrosion is often the source as to why it occurred.”
He said two factors could have contributed to the corrosion: carbonation on the underside of the slab and chloride attack from the top. On the top surface, he said car parks were particularly vulnerable as most do not have any protective coatings and little effort is made to wash de-icing salts from the surface leaving the concrete exposed to chlorides for long periods of time.
Corrosion of the reinforcement can mean that concrete can be lost from the cross section of the member, or the bond between the concrete and the reinforcement is not as strong. Both have weakening effects on the structure.