Working on the iconic clock tower Aecom’s senior engineer Matthew Penellum found hidden Second World War damage and some serious technical challenges – all of which made Big Ben’s bongs this new year extra special 

©UK Parliament_Jessica Taylor

It was only supposed to be a light-touch refurb, a “lick of paint and a wander around”. But when structural engineers at Aecom lifted the lid on the Elizabeth Tower, it quickly became clear that this would be much more than that. 

The tower, which houses the iconic Big Ben clockface, had not undergone a proper refurbishment since the 1980s and when the structural engineering firm, appointed to the job in 2014, was finally able strip back the roof and internal decoration, it realised the poor condition of much of the tower.  

The building had been subject to significant bomb damage during the Second World War, but fixes made in the make-do-and-mend days of the 1950s fell significantly below the standards of the 21st century and a lack of useful contemporary records on the bombing posed an additional challenge. 

“There is a huge amount of non-technical literature about the tower […] there is news articles written at the time […] but there were no technical reports produced that would be useful to us,” says Matthew Penellum, a senior engineer on the project. “There are photographs taken at the time showing loads of smashed tiles and the glass had shattered in the south clockface […] but it was only in 2018 as we worked our way up and then progressively back down the tower, stripping things off, that we started to be able to tell a story of what happened”. 

I was feeling slightly nervous on New Year’s Eve, thinking ‘god, I hope the bells ring’

Matthew Penellum, Aecom

One of the major achievements of the project was correcting the historical failures in record keeping, producing technical drawings explaining the story of the bomb impact as they understand it and creating a full 3D model of the tower today with every one of its many thousands of pieces numbered and referenced. 

For more than a year, the roof was wrapped in a large white tent to allow the team to blast off the existing paint using a grit blaster while keeping the hazardous lead paint enclosed. After the paint was removed, engineers would go in to inspect the ironwork. This was itself a complex and unusual challenge – typically specialist painters would dismantle a structure and take it to a workshop to strip and repaint, but this wasn’t logistically feasible given the size and setting of Big Ben. 

The damage they found was significant, with 22% of the cast iron components on the roof having to be replaced with new ones a much higher proportion than on previous roof repairs at the Palaces of Westminster. On one part of the roof on the south side of the tower, many of the cast iron roof purlins supporting the tiles were cracked and had been patched back together poorly. On the belfry, meanwhile, large parts of the stonework had been replaced with a stone of a different, much paler colour from the original – although this error helped engineers work out where the bomb damage had taken place. 

ET WWII Bomb Damage

The south face of the tower was significantly damaged by a bomb attack during World War II

In the clockface itself, a series of cracks were found that suggested that the whole clockface had been “squashed” by the bomb’s impact. “The clockface almost acted like an impact absorber to stop the shockwave and the impacts going down the tower,” says Penellum. 

Some of these defects, however, including the cracks in the clockface, were not repaired, with engineers deciding that the risks outweighed the benefits, with the damage not appearing to have worsened in the 80 years since it was sustained. 

“To repair the cracks that are right in the middle, we would have had to take out basically a whole wedge of the clockface and replace it with a new one,” says Penellum. “There’s just a risk – it’s a bit like, if you take five or six spokes out of a bicycle wheel, will it still work anymore or will it all start to go wrong? In a similar way, if you take a big wedge out and one of those clock faces, it wasn’t clear what would happen to the others.” 

From The Archive logo

From the archives: The construction of the Palace of Westminster, 1847

The extent of the new “Palace of Westminster,” is not understood by those who, passing on their way, gaze on the elaborately carved façade presented to the river, nor fully appreciated by the majority of visitors, even, who penetrate the inclosure [sic], and pass with sight-seeing eyes around its length and breadth. It needs a longer acquaintance, a careful examination of it part by part, an ascent of scaffoldings, inspection of the means in operation, and comparison with other buildings, before the greatness of the work can be perfectly understood.” 

>> Read the full passage here

But they did replace a row of decorative orbs on the belfry balconies, which were in poor condition having themselves been installed as replacements in the 1950s. The cracked cast iron roof elements were also replaced, with the broken purlins sent to a foundry to be melted down and remade. 

According to Penellum, the cast iron foundry used was actually re-opened for the sake of the project and the team also had to return to Victorian quarries – abandoned 100 years ago because it was no longer economical to extract stone from them – for matching stonework. Crafting all that stone was no small task either. “I don’t know what percentage of the stone masons in the entire country were working on Big Ben, but it was quite a lot of them,” says Penellum. “There aren’t that many traditional stonemasons in the country and very rarely would there be a project on this scale where there might be dozens of stone pairs being worked on at the same time all by hand, using traditional methods.” 

He wonders whether the project could indicate the resurgence of these jobs. “There is a lot of buildings like Big Ben that were built in Victorian times,” he says. “It could be that historic crafts and trades now become a viable career choice again” 

Not everything could be repaired or replaced to the standards of the original. Engineers had to design a crash mechanism to protect the mechanical and electrical systems underneath the building in case one of the weights that keep the clock ticking were to fall. Penellum says the impact of such an accident would be something like “a van being thrown off the top of a building” and after doing some calculations they realised that the Victorians’ solution – a five-foot heap of sandbags – did not really help. “You would get huge amounts of force transferred into the structure below and potentially also sideways into the walls,” he says.  

Timeline of works

2014 - Aecom structural engineers appointed to the project and start inspections etc to develop scope of project. 

2017 - Principal contractor Sir Robert McAlpine sets up compound and project officially begins. 

21 August 2017 – Big Ben and the quarter bells silenced at midday to enable the works on the Great Clock itself to begin. 

May 2018 – Scaffold around Big Ben completed and work underway. 

April 2022 – Final pieces of scaffolding removed from around the Tower, revealing the fully refurbished exterior for the first time. 

Sunday 13 November 2022 – with the refurbishment works on the Great Clock mechanism completed, Big Ben chimes the hour to mark Remembrance Sunday – the first time that Big Ben and the quarter bells have been chimed by the Great Clock mechanism since the bells were silenced in August 2017. 

He claims their replacement, a metre-deep honeycomb of steel cells, would reduce the potential force transferred into the structure by a factor of 10. Penellum says the team felt that, given the crash mechanism needed replacing wholesale, it was better to use a solution that was clearly of the 21st century, rather than attempting to create something which mimicked the look of the sandbag system: “[Among] heritage architects, there’s this ongoing debate about this, if you have to replace something, should you make it look as much like the original thing as possible? Or should you replace it with something really modern and new to make it obvious that it’s something different”. 

With the work finally completed, the final pieces of scaffolding were removed from around the tower in April 2022, revealing the fully refurbished exterior. At the beginning of this month, to the joy of revellers and the relief of Aecom engineers, Big Ben rang in its first new year using the Great Clock mechanism since the bells were silenced in August 2017. 

“I was feeling slightly nervous on New Year’s Eve, thinking ‘god, I hope the bells ring’,” says Penellum. “Getting messages from people who know that we’ve been working on this – our family and friends and stuff – saying the clock looks amazing […] it’s quite nice to feel that you’ve done something that feels really quite significant to the heritage of the country”.

>> Read more: Elizabeth Tower – The test of time 

Project Team

Client: Parliament Strategic Estates

Project manager: Lendlease

Principal contractor: Sir Robert McAlpine 

Cost consultant: Currie & Brown

Conservation architect: Purcell

Structural engineer: Aecom

Building services: S.I.Sealy

Scaffolding specialist: PHD