World's biggest clock tower features Gurit composites

World's biggest clock tower features Gurit composites
There has been an awful lot of overlap of materials knowledge in the last few years between industries, particularly when it comes to composites. This is because these are the materials that everyone is still trying to fully exploit fully and engineers from all industries are coming together to share knowledge in an effort to make that happen.

For example, the aerospace industry is experienced in making strong, light, reliable parts and structures, while the automotive industry has the knowhow with regard to automation and speed of manufacture. The two are learning from one another, as are other industries that are coming in to the frame such as marine and renewable energy. So, when faced with building the biggest clock face in the world – on the second tallest building in the world – it should come as no surprise that engineers turned to composites to make it happen.

Standing 607m tall, the Makkah tower is an impressive structure featuring illuminated clock towers that can be seen for miles around the city of Mecca in western Saudi Arabia. A particularly distinctive feature is that the tower uses fibre reinforced plastic (FRP) sandwich panels to clad the top 200m.

Composite material manufacturer Gurit worked closely with the tower top designers SL-Rasch on the project and turned to the material essentially for the same reasons that those in other industries have: to reduce weight while maintaining stiffness and strength.

This is fairly unusual for civil engineering projects, since usually foundations are just made stronger at early stages of the build to accommodate more weight. However, the design of the upper tower, clock face and upper crescent 'evolved', fairly dramatically, after the initial construction of the lower levels was well underway.

"This lead to concerns about the amount of dead load on the building as the foundations had already been built," says Dr Mark Hobbs, a senior engineer of engineered structures at Gurit. "Initially, it was just a simple small spire on the top, but after building started, the clients decided to add the clocks, the dual spire and crescent all on top of it. This meant there was a significant increase in deadweight on the tower top; about 12,000 tonnes of main steel work in the tower top region alone."

By turning to advanced Fibre Reinforced Plastic (FRP) sandwich panels Gurit was able to more than halve the weight. In addition, the intricate 3D shapes in the cladding would not have been possible with more traditional materials.

The changes meant that some 40,000m2 of FRP composites were to be used as cladding as well as for the largest clock face in the world (43m), and a minute hand that spans 23m. Gurit supplied a range of advanced composite materials, to Premier Composites Technologies based in Dubai, which carried out fabrication and installation of the cladding, clock face, clock hands and crescent.

The clock hands presented a particular challenge due to their long, slender geometry and the potential for high wind loadings caused by its height. The clock hands were manufactured using Gurit's WE91-2 carbon fibre prepreg material and Corecell T-Foam structural core.

Prepreg is traditionally a material used by the aerospace industry and this particular composite was originally developed by Gurit for use on wind turbine blades as it has an excellent stiffness-to-weight ratio, mechanical properties, toughness and low resin uptake.

The project was typified by its need to be completed quickly. The large, pre-assembled sandwich panels facilitated this desire, with some panels as large as 13m by 2.1m. These were preassembled in to 12m by 13m bays and then lifted in to place in a single go.

"One of the most interesting challenges was the crest on the top of the building," says Dr Hobbs. "The original design saw a main steel structure inside the crest which would have composite panels clad over the top. However, during the development of the design there was a desire to free up the space inside to allow it to be used as a meeting or prayer room."

In order to do this, Gurit changed the structural concept completely by removing all the steel work inside the crescent and replacing it with a monocoque shell structure. The new crest would be 21m in diameter and have some 700m2 of surface area. If it could be unrolled it would lay 60m long, 5.5m tall, and 7m wide.

Gurit applied techniques usually applied to the marine sector, using a monocoque shell structure with sandwich panels to form the shell, internal ribs and bulkheads – much the same way in which it would design a boat.

"Our experience in the marine industry did prove useful," says Dr Hobbs. "Changing from the steel structure to the composite shell not only opened up the space inside, but also removed 50 tonnes of steel structure and replaced it with about 19 tonnes of monocoque composite."

The team decided to build the crest in 13 separate pieces that could be pre fabricated and transported by road, and then assembled on site. But joining the preassembled pieces also proved a challenge as it was difficult to get the ideal conditions needed on site to make a laminated joint. As a result, the team decided on bolted joints. Although this increased the overall weight, it was a necessary trade-off.

"When we came to look at the detailed design, another concern was the interface between the composite crest structure to the top of the building, and different ways to attach the crescent to the top of the spire on the building," says Dr Hobbs. "One of the first concepts was looking at something akin to the keel structure on a boat, where there is a framework underneath. We would bolt a piece of composite on top of the steel tower frame and attach the rest of the crescent there."

However, this had a couple of disadvantages. It meant that the crescent couldn't use the full depth of the composite and join for transferring the load between hub section and the mid. It also meant there were some separate cladding pieces that needed to go on after it was installed.

"This gave rise to quite a lot of concern about having a smooth joint between the cladding and the hub section," says Dr Hobbs. "So we moved to a solution that was very similar to what we did on the clock hands. This is having a steel hub around which we assembled a composite structure. The composite structure will be assembled round the steelwork in the factory in Dubai. Then we have a steel connection.

"Eventually we extended the steel strut, the main steelwork, back up inside the crescent. This was a much bigger lever arm to react to the loads and also made a good contribution to the stiffness of the crescent. We have two steel collars which were bolted to the composite structure in situ, so when we put them on, we were doing steel connections on site as well, which was a major advantage."

Additionally the stringent fire safety requirements of buildings could easily be satisfied by Gurit as it has experience working with oil and gas customers to ensure blast protection walls, as well as supplying materials for aircraft interiors.

The composite world has a lot to gain from projects like this that use advanced materials in traditional buildings and structures. The transfer of knowledge, particularly when it comes to joining multiple materials together, often in the field and in far from ideal conditions, is something no doubt Gurit will be able to transfer back in to other sectors.

Hobbs concludes: "This project has made full use of Gurit's wide range of expertise in the technology of advanced FRP composite structures, including materials development, processing, testing and structural engineering."

Author
Justin Cunningham

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