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Msheireb: How We Developed A Unique Cable Clamp Solution For A Kinetic Roof

By May 6, 2021October 14th, 2021LEARN

PFEIFER Structures doesn’t just fabricate and install amazing structures; we have a team of the most amazing designers and engineers in the construction industry. Sometimes, however, flaws in design can occur. The client doesn’t care how a flaw occurred, but how it will be resolved. Below in a case study of how one little cable clamp almost ruined a kinetic roof, and how the PFEIFER team created a perfect solution on-the-fly.

Baharat Square, located in Downtown Msheireb in Doha, Qatar, is the largest open-air covered square in the Middle East. The square features the biggest kinetic roof in the region, with 30 axels and over 1,000 panels. The kinetic canopy covers an area of about 3,500 m² (37,674 ft²) over the new plaza. The greatest challenge of the project was about a cable clamp the size of a sheet of paper.

During the construction of the roof for Barahat Square, it quickly became clear that the stability of the main cables would be one of the most difficult challenges to solve. Due to the weight of the panels, the wind forces, and the relatively large spans, there were twists between the main cables that could lead to dangerous situations. To minimize these twists, two tie-bar systems were installed (see Picture 1).

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Picture 1: Barahat Square canopy cable net with tie bar system in traverse direction under construction

These tie-bars connect the main ropes to each other in the transverse direction and increase the rigidity of the system. The tie-bars are attached to the main cables by means of cable clamps. The difficulty lies in the fact that the tie-bars must be regularly passed over by the individual panels and their sliding blocks when the roof is opened and closed without them being damaged or suffering increased wear. In addition, the panels and sliding blocks must be able to move freely in different directions, while at the same time be firmly attached to the main rope (see Picture 2).

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Picture 2: Possible system movements and critical areas

Therefore, we had to develop a rope clamp that was stable enough to absorb the forces that occur, but also delicate enough to be run over without causing damage. A clamp was welded to meet these requirements, while also trying to keep costs as low as possible. However, it quickly became clear that the requirements for the clamp, in terms of the necessary forces, could not be realized with conventional steel, nor as a welded part. A clamp with the specified dimensions was therefore milled from the solid using high-strength steel (S690 fine-grained steel).

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Picture 3: Cable clamp development (welded part)

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Picture 4: Cable clamp devleopment (milled)

An initial test phase was started, during which the clamping forces in particular were tested. However, these tests showed that the required clamping forces could not be applied and that the clamp would slip on the rope. We needed to increase the coefficient of friction. The increase in the coefficient of friction could be realized with the aid of sand. For this purpose, the rope contact points of the finished milled clamp were painted and then sprinkled with fine-grained sand (see Picture 5 and Picture 6). After drying, the excess sand was removed and the contact surfaces were sealed. This significantly increased the coefficient of friction of the rope contact surfaces of the clamps. This was then also confirmed in a further series of tests. Here, two clamps mounted on ropes were moved against each other with the aid of a hydraulic cylinder and the force that had to be applied was measured. The results were now satisfactory and production could be started immediately to test the clamps under real conditions (see Picture 7).

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Picture 5: Cable clamp development (sanding)

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Picture 6: Cable clamp development detail (sanding)

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Picture 7: Cable clamp development (finished clamp lower part)

In addition to the development of high-strength, traversable rope clamps, suitable sliding blocks for the movable panels had to be designed. These sliding blocks needed to meet both the requirements for mobility and attachment to the rope, as well as traverse the newly developed rope clamps without significantly increasing wear. Special attention was paid to the geometry of the sliding block as well as its material. After further tests, the choice fell on a plastic material (with dry lubricant modified PA 6) that stands out for its particularly good sliding properties. The material is UV-resistant and retains its properties even at extremely high temperatures. This is an important factor, especially in view of the geographical location of the Emirate of Qatar. Summer temperatures in the desert state can quickly rise to over 50°C (122°F). Steel ropes exposed to direct sunlight can heat up to over 85°C (185°F).

The geometry of the sliding block was chosen to create a kind of inlet trumpet in both directions. In the center, however, the sliding block almost completely encloses the main rope. The inlet trumpet ensures a continuous, guided feed of the main rope. To ensure the strength of the sliding block, the central part of the block was enclosed with aluminum. This aluminum support almost completely encloses the block and is necessary to prevent the main rope from breaking out, even in the event of severe twisting and the associated forces. Extensive endurance tests were also carried out on this component in order to confirm the durability of the system and to assess the wear that occurs above all when passing over the newly developed rope clamps.

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Picture 8: (series) Sliding block with inlet trumpet

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These endurance tests were successful! After completion of the assembly of the movable roof of Barahat Square, including the tie-bar system, the traversable rope clamps and the specially designed sliding blocks, the first test runs of the real roof could now begin.

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Picture 9 (series): Endurance test cable clamp and sliding block

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Picture 10: (series): Test result: slightly wear after endurance test

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Picture 11 (series): Test result: slightly wear after endurance test

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The positive test results of the individual components were also confirmed in the tests of the overall roof. Both the tie-bar system and the sliding blocks have been working perfectly now for more than 7 years without any problems. The maintenance carried out twice a year shows no issues or more than the usual slight wear.

PFEIFER Structures is more than just a subcontractor; the PFEIFER Structures teams are problem solvers.

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