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Not far from the Kuwaiti capital, the Jaber Al-Ahmed International Stadium is under construction, on a surface area of 400,000 m². For the “skeleton” of the stadium, consisting of 52 wall piers with inclined cantilevering stepped beams, Doka is supplying formwork solutions that need no falsework support. A special feature of this site is the climbing cantilever formwork, specially developed for the stepped beams. Compared to supporting these beams completely from ground level, this solution delivers huge savings in terms of both equipment and labour time.
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The region’s traditional timber-built merchant ships gave the architecture firm of Weidleplan Consulting GmbH its inspiration for the design of the stadium. With a 250 m diameter, this undulates elegantly upwards on two opposite sides. Its grandstands can seat 60,000 spectators, whose arrival at the stadium will have been made easier by its 6000 car-parking spaces – or even by its helipad! The contracting JV, consisting of Mohammed Abdulmohsin Al-Kharafi & Sons W.L.L. and Philipp Holzmann, started work on the stadium in the summer of 2004, and it is scheduled to open in December 2006. The structure shell will require the casting of 52 hollow wall piers from in-situ concrete. Stepped beams cantilever out as much as 50 m from the tops of the walls, rising to a height of 65 m. They carry the roof construction and the grandstands, and are stiffened in the horizontal by connecting cross-beams that are also made of reinforced concrete.
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The wall piers are being cast using 8 complete sets of formwork. These are made up of Top 50 large-area formwork on Climbing brackets 150 F. When it came to the inclined cantilevering stepped beams, the contracting JV were looking for an operator-friendly and wind-stable formwork solution. The Doka solution does away with the need for equipment-intensive, high-cost falsework supports for the up to 65 m high stepped beams. Instead, a special cantilever formwork was developed which can be “jumped” in a similar way to a climbing formwork. This permits huge savings in terms of falsework and labour costs.
Unlike a “classic” climbing formwork for vertical walls, however, this system has to sustain not only its own dead weight and the wind loads, but the weight of the fresh concrete as well, and transfer these loads into the previously cured section. In each set of formwork, elements of Top 50 large-area formwork are set up on 2 steel girders. These steel girders are fixed in two climbing-shoes on the underside of the stepped beam. At the top, the whole system is back-stayed with 10 m long tie-rods. The two 36-mm tie-rods, each with a failure load of 100 tonnes, begin at the outermost extremity of the formwork-set and end in the back-stay unit. This is made up of steel girders that are anchored in the concrete at the top of the stepped beam. These girders transfer the 70-tonne load of the fresh concrete into the previously finished section.
Owing to the very great heights involved, all the forming operations have to be ones that can be carried out in complete safety. For this reason, the Doka formwork solution comes with specially designed working platforms. Each set of formwork has two side platforms for detaching or fixing the back-stays and form-ties, and for placing the concrete. Their staircase-like platform decking ensures safe working on these steeply inclined platforms. On the underside of the stepped beam, there are 3 different platform levels for operating the outside formwork, detaching or fixing the bottom back-stay point, and for relocating the suspension shoes. On the end face, there is one platform for operating the inside formwork, placing the reinforcement and for pouring.
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Compared to supporting the stepped beam entirely with falsework, the climbing cantilever formwork results in very big savings of time and equipment. What is more, the Doka system is simplicity itself to operate: After the form-ties have been detached, the wall element can be tilted approx. 10 cm back from the cured concrete. The system is held in place by crane, and the tie-rods of the back-stays are then detached. The crane hoists the 14-tonne formwork unit up and along to the next casting section, where it engages in the previously mounted suspension shoes. After this, the back-stay unit is also relocated to the next section and the 10 m long tie-rods are fixed in place. The reinforcement operations on the next casting section can now begin. Once the reinforcement has been placed, the base slab of the stepped beam is cast, and the inside formwork is mounted and tied to the outside formwork. For the inside formwork, the timber formwork beams have been replaced by space-saving steel girders. These leave enough space inside the confined shafts for site-crew to climb through. Finally, the crew cast the walls and the cover-slab in one single pour. In order to prevent strong gusts of wind from impairing the quality of the concrete as it cures, the formwork has a stiffening profile on one side with which it braces itself against the previous section. In this way, the stepped beams grow by one 5 m long segment every week. Up to a length of 38 m, the usual type of reinforcement is sufficient. However, the biggest stepped beams – which have cantilever lengths of up to 50 m – need to be pre-stressed. Suitable jacket tubes for the anchor blocks are installed at the same time as the reinforcement is placed.
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In order to stiffen the stadium construction, all the stepped beams have to be horizontally interconnected. These connecting cross-beams are up to 14 m long and have a Z-shaped cross-section. As the stepped beams have different cantilever lengths and are not all the same height, each of the connecting beams has a different angle and tilt. This called for an efficient and – above all – adaptable formwork solution. Doka has supplied the site with 4 sets of formwork which can be adapted as needed in both the horizontal and the vertical. The 13.5 m long timber-beam formwork elements rest on 60 cm thick steel girders that are mounted between the stepped beams with the aid of special brackets. These girders can sustain 60 tonnes of fresh-concrete load with no need for falsework.
Project engineer Adel Hamdy has been looking after this formwork assignment right from the outset: “On the stepped beams, we’ve been able to manage entirely without time-consuming falsework, thanks to the climbing formwork solution. My crew have been repositioning the formwork-sets quickly and easily. We can rely on Doka’s Kuwait Branch to give us professional support whenever we need it.”
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