Application Technology

In most cases when bolts come loose, they can be tightened again. But what if they are impossible to access and must maintain a specific preload? A glass roof inside a newly developed quarter in the heart of Hamburg, Germany, recently presented developers with these challenges and secure bolting connections from Nord-Lock Group were the chosen solution. 

Next to the Elbe River in Hamburg, integrated into the urban development project HafenCity, sits the Westfield Hamburg-Überseequartier. Walking through the Hamburg-Überseequartier, visitors are immediately struck by its unique architectural character. However, standing out is an inner glass roof, designed by renowned German star Architect Werner Sobek, assembled with thousands of unique panels.  

The glass façade specialists for steel constructions, Roschmann Group, whose work includes the headquarters of the International Olympic Committee in Lausanne and the Eiffel Tower’s visitor centre, were tasked with turning Werner Sobek’s vision into reality. Other than the uniquely shaped panels, the roof structure is made up of hollow sections, which is ideal for wide spanning structures where strength, stability, low weight and aesthetics are of high priority. After initially having considered welded notes as the fastening solution, Roschmann realised the challenges it would imply and decided to throw bolted connections into the mix.  

“Using welded nodes exclusively would have been highly complex and time-consuming, requiring a lot of assembly work on the construction site and auxiliary connections,” explains Michael Skopp, head of research and development, and René Hesselbarth, project manager at Roschmann.

There were several advantages to using bolted connections. Firstly, the time effectiveness it brought was key for Roschmann in acquiring the project. Secondly, when not every single connection had to be assembled on-site, separate sections of the roof could be constructed offsite, which made for smoother logistics and coordination. However, the most advantageous upside of using bolted connections was flexibility.  

“The roof segments weigh several tonnes and had to be lifted into place using a construction cane. With welded connections it is no longer possible to check and correct the positioning of the segments once they have been assembled. However, with bolted connections we were able to rework and fine tune the positioning, which gave us crucial flexibility,” says Michael Skopp and René Hesselbarth.

The ability to readjust and torque to exact predetermined preload values was also necessary for the roof to be able to move slightly. For that end, Roschmann turned to The Center of Competence for Tubes and Hollow Sections (CCTH), an engineering company specialising in hollow section applications, to calculate the preload that would allow for it. 

“The entire structure relies heavily on highly specific, predetermined preload intervals,” explains Dr-Ing Stefan Herion, managing director at CCTH. “So, because we had to allow for slight movements in the construction, we had to preload the bolt with just the right levels.”

After having identified the necessary preload force, ensuring that the nodes in the roof construction would bear as they should, while also allowing for some degree of movement, the next step was to ensure that the preload was applied accurately. Therefore, tightening tests were carried out by the Munich University of Applied Sciences.

Also working together with Roschmann and CCTH to identify a solution that would maintain the correct torque and preload of the bolted connections was manufacturer August Friedberg GmbH. The German company provides bolting solutions to numerous industries, which in this project  consisted of a highly sophisticated lubrication system.

“Our job was to find a lubricant that would make it possible to achieve a certain preload in the bolting solution, which would also remain stable over time with low scatter,” highlights Dr-Ing Selcuk Güres, head of research and development at August Friedberg. The company used the newly developed AF Pro lubrication, which, in addition to meeting all the requirements related to torque and preload behaviour, is also able to withstand environmental challenges related to Hamburg’s humid, rainy sub-oceanic climate. Moreover, since the number of nuts and bolts exceeded 5,000, in a range of different sizes, standard partial lubrication of relevant head and thread surfaces would have taken up too much time. Therefore, August Friedberg also came up with a special, automated solution that lubricated the entire bolt.

Bolting out of the ordinary

The bolting challenges in the roof construction were, for various reasons, unique and tricky. As previously mentioned, the calculated preload necessary to allow the roof to be flexible enough to allow for small movements did not fall within the regulative levels – as its neither preloaded nor non-preloaded.

Another detail making the bolting challenging, and not compliant with regulated standardised protocols, was the combination of parts, such as bolt and washer, assembled inside the hollow sections, also called ‘blind hole connection’, meaning that the bolted connections are not visible. In the roof construction, the connections were covered with clipped plates.   

“The blind hole connection was crucial for the aesthetics of the roof. Standing underneath, you cannot really see how the whole structure is fixed together since there are no signs of welding or visible bolts or nuts,” underlines Stefan Herion at CCTH. The combination of not fully preloaded bolted connections, and blind hole connections using a single longitudinal bolt, was an unprecedented construction challenge to Stefan Herion and his colleagues.

Taking a closer look at the blind hole connection in the roof construction – on the opposite side of the hollow section – the bolted connection anchors into a solid material structure, rather than another hollow section. For Friedberg, to understand the relation between torque and preload, and how to optimise the use of lubrication, this meant extensive testing with all components to be used in the final roof application.

So, from all of this follows a critical question – since the bolted connections are located in sealed blind holes directly underneath massive glass panels, how does maintenance work? The answer to that question is – it doesn’t. With no possibility of accessing the bolted connections, Roschmann needed to ensure that preload would remain within the calculated intervals. So, essentially a maintenance-free bolting solution was needed that could handle the roof’s non standardised, not fully preloaded connections, determined by CCTH and Roschmann. 

“My first encounter with Nord-Lock washers was watching a video of the junker test, which clearly demonstrated the washers’ capability of maintaining preload levels under severe vibrations,” comments Stefan. Since then, Stefan has collaborated with Nord-Lock on numerous construction projects and, each time, become more reassured of the washers’ capability of safeguarding the integrity of complicated and critical constructions. In this project, which clearly did not follow standard protocols and made necessary a very specific preload, the bolt securing method needed to make absolutely sure that it would remain stable once installed.  

“The discussion was a rather quick one,” adds Michael Skopp and René Hesselbarth. “What we needed was a solution that would maintain a preload high enough to press surfaces together. Just a little loss in preload would result in splitting and potentially serious negative consequences. Using Nord-Lock washers was also the only solution that allowed for untorquing, which was crucial to set the preload just right and assemble the roof correctly.”

Identifying just the right bolting solution, including preload levels, lubrication, and securing method, was key in turning Weber Sobek’s vision into a ‘real life’ safe and robust construction set to last for decades.