Building in the clouds with precast elements
It will be the world’s highest prefab building, yet there are no tower cranes visible at De Zalmhaven tower construction site. The real construction is hidden from sight in a building unit that slowly creeps upwards. Inside is where they puzzle with prefab elements, protected from the elements outside.
By Edo Beerda, 18 october 2019. Source: news Cobouw
Photo: The blue skeleton of the unit is complete. Just the facade sheeting to be mounted to it and the moveable unit will be ready for use.
Since the first pole was driven into the ground exactly a year ago, the mega project near the Erasmus bridge, has climbed up to the fifth floor. From there, the building will shoot into the air towards the top at 215-metres-high at a rate of one storey per week. “This means 104 hoists per floor, of which 86 consist of precast elements”, explains construction site manager Frans Quataert (BAM). “They have been precisely planned, right down to the last stud.”
In the heavy steel frame built around the base by contractor De Boer in the past weeks, two 40-ton overhead cranes are visible. One will hoist up al the precast elements. The other, the assembly crane, will move the elements horizontally after they have been lifted up. Lorries will deliver their facade sections, columns, slabs, internal walls and walls for lifts and stairwells beneath the crane. The overhead crane has a 190-metre cable to hoist them straight up from the vehicle. At the top, the elements are placed on a yoke, after which they are transferred to the assembly crane and put in place. After the slab flooring has been deposited, the construction unit can be raised up a floor. This makes it possible to work quickly and completely sheltered. The sky-scraper is actually built entirely inside, which can really make a difference, especially at high altitude.
BAM previously used a hoisting unit for the new build of the Erasmus MC. It was a slightly different version, whereby the hoisting unit was supported on the precast facade elements. The unit construction at the Zalmhaven tower looks more like a drilling platform. On the four corners of the tower are four enormous columns, along which the platform moves back and forth. When a floor is completed, the 900 ton-heavy platform is jacked up a storey. From below, a six metre high steel pipe is inserted between two floor jacks. The supports of the construction therefore slowly slide further out. The flanges are connected using enormous nuts and bolts.
BAM had to choose for a construction platform, with the surroundings in mind. If an object were to fall from 200-metres-high, it could land dozens of metres away on the ground. Working in a construction unit prevents this. Furthermore, the noise is also kept inside. “A consequence was just that we were dealing with a design that was difficult to build, because we were to pour all the concrete on site”, explains Quataert. “The constructive design by Zonneveld Engineers therefore had to be completely overhauled.”
The structural engineers at BAM put their heads together with Zonneveld engineers and prefab supplier Byldis to change the original design into a prefab design. In the BIM model it looks like an enormous block into which all the elements neatly interlock. “The red spots are the nodes where all the ends of the reinforcement meet”, says Freek Schaap, the project leader of the construction team. “That’s where we join everything together.” The substructure poured in-situ is put under the greatest forces and has therefore been made to extra robust specifications with 600 millimetre thick walls (C55/67) and columns of concrete quality C80/95.
Byldis, the independent industrial construction components division of Hurks, supplies a rich selection of precast elements for the fifth up to the 54th floor: internal walls in 300, 400 and 500 millimetre thickness, facade elements and lift walls of 300 and 400 millimetres and balconies and slabs of 100 millimetres. There are 770 complete facade elements in total, 1140 internal walls, 400 balcony plates and 250 stairs and platforms. The upper three storeys (the ‘crown’) are finished with a lightweight steel construction and sheet steel concrete floors. The crown will house several penthouses and a panorama restaurant.
There is no space for storage on the tight construction site. All precast elements are therefore delivered just-in-time. Trucks are able to collect their load near the Keileweg at a building hub, far outside the bustle of the city. Supporting facade elements from Byldis are put into place including natural stone facade cladding, insulation, steel profiles, doors and glass. Schaap: “Of course, the facade must be finished and watertight in one go. Once the hoisting platform has moved up, it cannot go back.”
An extra challenge comes with the entirely glass corners of the building. For this, Byldis developed three dimensional elements, entirely cast as a form-retaining whole in Veldhoven. In Rotterdam they are hoisted in one go to the corners of the tower.
When a floor is completed, the 900-ton platform is pushed up one storey higher. The higher the building, the longer the hoisting takes. This is why it is important to stow as much as possible into one hoist upwards. For example, eight complete facade elements and eight sets of balcony railings are raised up in one go. Sometimes just one element is lifted up: the heaviest walls weigh an impressive 36 tons.
The whole project demands an enormous amount of organisation in advance. Builder, architect, structural engineer, and suppliers work together according to a strict BIM protocol. The 3D model was later linked to the planning so that it was clear exactly where each construction component should be and when. “A video was made of a week’s cycle showing exactly how construction should happen”, tells Schaap. “The high level of organisation prevents parties getting in each others way and causing congestion at the hoisting point. It is like musical chairs, but then very well directed.”
Once the 58th floor has been reached, the roof has to be removed from the platform in order to build the crown. This consists of a steel construction, finished with a mast. When the roof is removed form the platform, it becomes very easy to lower it down past the corner elements. Disassembly finally takes place on the sixth floor with mobile cranes.
Working with prefab plus a construction platform is more costly than a construction poured in-situ. But on the other hand, it generates gains in safety, environmental nuisance and construction speed. Instead of around four years, the project can be realised in three and a half years. Furthermore, planning is made easier due to the high level of industrialisation. Project director Bernd Schönherr was clear about it at the start of construction. “Now we know for certain that this building will be completed in the first quarter of 2022. Even if we have four Elfstedentochten.”
Tubex poles as basis
To spare the surroundings, BAM chose soil displacing screw piles for the foundation work. A special crane brought the 163 gigantic piles to the desired depth using grout injection, with minimal noise and vibration nuisance for the surrounding residential towers and offices. The 66 metre long Tubex piles went into the ground in two pieces. After screwing the first 33 metre section, the next piece was welded on. After completion, the piles were filled up with concrete. The diameter of the piles is 762 millimetres, with a point of 950 millimetres.
A 2.50 meter thick foundation sheet measuring 38 by 38.4 meters was poured on top. The first five floors were also poured in-situ. Construction of the two flanking towers (70 meters) of the Zalmhaven complex is now also underway. These are cast in-situ.
Opdrachtgever: Zalmhaven CV (AM i.s.m. Amvest Vastgoed)
Architect hoogbouw: Dam & Partners Architecten
Architect laagbouw: KAAN Architecten
Prefab beton: Byldis
Ontwerpend constructeur: Zonneveld ingenieurs
Coördinerend constructeur: BAM Advies & Engineering
Installaties: Techniplan Adviseurs
Bouwfysisch advies: Peutz
Opleverdatum: eerste kwartaal 2022
Aantal appartementen: 475, waarvan 260 in de toren
Read more here.