Replacing large steel bolts on London’s tallest skyscraper has cost the UK’s biggest structural steel contractor £1 million (about $1.5 million).

Severfield Group disclosed the costs associated with the bolt replacement program on the Leadenhall Building in a trading update Monday (March 23).

Many of the 3,000 steel bolts on the building, known locally as the “Cheesegrater,” were ordered to be examined and replaced after three of the components fractured and fell from the structure in just over two months, according to its developer, British Land.

The first and second bolts, about the size of a human arm, fell in November 2014. The third fell sometime before mid-January. The 734-foot tall building opened in September 2014.

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Three bolts have fallen from the Leadenhall Building in London since November. An investigation has laid the cause of the failure to hydrogen embrittlement. The steel contractor reports it has spent $1.5 million on the replacement program.

An investigation concluded that the bolts had fractured due to “a material failure mechanism called Hydrogen Embrittlement,” the developer announced in January.

No injuries were reported in any of the incidents.

British Land did not say how many bolts will ultimately be replaced but maintains that the building's structural integrity has not been compromised. The bolts—which range from 20cm to 120cm long and have diameters of between 5cm and 7.5cm—connect the nodes on the megaframe, according to The Telegraph.

Bolt Replacement Program

The replacement program that Severfield is conducting with contractor Laing O’Rourke and structural engineers Arup was initiated as a “precautionary measure,” according to British Land.

The project on the 47-story building is expected to last until the end of the year.

Severfield said the parties involved were in discussions to determine the liability for the bolt replacement program, according to the update.

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The 734-foot-tall Leadenhall Building, known locally as "The Cheesegrater," is the tallest structure in London. It opened in September 2014 after almost three years of construction.

Severfield also said that the costs it has incurred were “exceptional” and should not affect the company’s profitability for the current year.

Severfield provided 18,000 tonnes of steel for the building in 2013, according to The Telegraph.

The steel contractor, based in Thirsk, North Yorkshire, was founded in 1978. The company’s portfolio includes stadiums, commercial office buildings, landmarks and industrial buildings.