The Cool Tower, a 150-meter high adventure
Skyscraper construction is exciting, challenging, complex, all of which increases when your high-rise isn’t limited to 100 meters but climbs well beyond that. Its creators discuss why.
The 150-meter high Cool Tower, a white residential tower with a distinctive design, is being built in the Baan quarter in Rotterdam. It was conceived for people who like to have a wide view of the world in a way that you find only rarely. Few dwellings in the Netherlands are over 100 meters tall, and there is a good reason for that: building exceeding 100 meters makes the entire structure more expensive and implementation much more complex.
If you lay out all the objective considerations, the question arises: why build so high then? The creators’ response to that question is unanimous: it is simply fantastic to envisage, design and ultimately build such a tower. To get an idea of how this building originated, designers Richard Wolf and Dirk-Jan Kluft provide a glimpse into everything involved in getting an iconic building literally from the drafting table to 150 meters up in the sky.
Lofty living and feeling safe Dirk-Jan Kluft, an independent consultant, has been operating under the name D. Kluft Holding Ltd. since January 2020. Prior to that he was director for many years at Van Rossum Raadgevende Engineers, a renowned consultancy firm focused on projects with large construction and civil engineering impact.
“Living high up has a special magnetism for people because of the freedom you feel all around you and the completely unobstructed vistas with nothing standing in the way. At the same time, these people want to feel safe in their homes, in that tall building. If there is a strong wind and the lamp on the ceiling sways to and fro a bit or if the building starts vibrating, a person could certainly get jittery. It is plainly a fact that the higher a building is, the more likely it is that severe conditions will generate movement or vibrations. It speaks for itself that we limit that effect to the greatest extent as possible, beginning with the work in the ground.”
168 piles “It is important to bear in mind that a large, 150-meter tall building creates an enormous load on the ground beneath it. For that reason we have to take the building’s settling into account, i.e.: the high pressure compression the ground is subject to which forces out water and air, and the effect of that on the surroundings. For the Cool Tower that could mean that the building might ultimately sink 12 to 16 centimetres into the ground, which would have to take place in such a way that the tower itself and the buildings around it suffer no structural damage. The City of Rotterdam, quite rightly, monitors this very vigilantly.”
“Thus the importance of the foundation is clear: it must “cushion” the settling and support and stabilise the building. Practically the whole of Rotterdam is situated on top of two layers of sand strong enough to support such a building. One is at a depth of 15 meters, on which the Rotterdam, for example, is built. That is fine because although it is an enormous building, there are no buildings adjacent to it within a 20-meter radius that could be affected by its settling. A very thick, weak layer lies under this first sand layer, beneath which lies the second layer of sand at approximately 45 to 55 meters. In contrast to the Rotterdam, the Cool Tower is situated in amongst other buildings, for instance a fire station and the Eye Hospital. In order to keep the effect of settling on the surroundings manageable and to prevent structural damage, it was necessary to build on this second layer of sand. To that end 168 long piles in two sections were bored into the ground. A unique facet in this story is that there is an underpass beneath the tower, rather unusual since a tower normally sits directly on the ground. In order to maintain the distribution of the weight of the tower constant over the 168 piles, there is a concrete slab almost three meters thick under the underpass.”
Columns in the facade “Another distinctive feature of this tower is that it is being built utilising a small concrete core, which extends outward to the facade through concrete walls. There are no concrete gables in the facade, but columns rising 150 meters from the ground.
That means not only that this facade is much lighter but also that one is much freer in designing the living areas: living rooms can be located in the corners so you get incredibly generous, panoramic views. The fenestration in the corners is sealed: you want to exclude every little crack or seam that could start vibrating and thrumming like a string in the wind.”
A crown of balconies “The balconies in the middle of each side form an iconic element of the Cool Tower, with the crown precisely in the centre of the building as eye catcher. The positioning of the balconies one above the other and not at the corners is to better protect them from the wind. Both architecturally and aesthetically speaking, the creation of the crown in the middle is a brilliant idea. And even though it fails to make construction of the building easier, the result is indeed quite extraordinary: at 75 meters high the living areas have four instead of six apartments, with seriously large balconies extending up to 2.5 meters out from the facade. We are talking prestigious and exhilarating living.”
From top to bottom and vice versa Richard Wolf is senior consultant at Wolf Dikken Adviseurs, an independent consultancy in the field of building physics, construction engineering and installation technology.
“Building at this height, i.e. to a height of 150 meters, happens rarely in the Netherlands. From the start the challenges are enormous, and finding solutions to them sometimes requires pioneering work. That requires you to have strong partners, from architect and engineer to builders and contractor, who know how to deal with all aspects of the building ranging from surroundings, the City, costs, to wind hindrance, et cetera.
All of the above has to be with the awareness that every modification or procedure has an immediate effect on the entirety: an additional small window or light switch right away means 265 times an additional window or light switch. Literally everything has to be right, and right from the start. Once construction has begun and it is time to work on the 24th storey, it is too late to pull some extra wiring up through the whole building.”
“A constant issue in high-rise building is what has to go up, what has to come down and how can we do that as efficiently as possible. During construction that pertains to people and materials. In this case since we are going to be building above 100 meters, a construction hoist will not do; it is no longer efficient from 100 meters: it’s just too slow. So we use JumpLifts, which are normally used only when the building is complete, and move them upwards each time with the construction up to the storey on which concrete will be poured. During construction they will be used for hoisting the inner walls, piping, plumbing, cables, lighting, concrete, tiles, et cetera. When construction is finished, they will be converted into passenger lifts.”
Water and heat “In the meantime facilities also have to be installed for power and electricity, clean water and air, which go from bottom to top, as well as for security provisions and wastewater, which go from top to bottom. Clean air in the Cool Tower is supplied from the ground level and distributed throughout the building via advanced ventilation installations. The “used” air goes back out at the top of the tower.
“Clean water goes up in three steps, utilising valves to manage the water pressure so that water comes out of the tap at the same pressure all through the entire building. We also take a bit of overpressure into account, for example in order to make the use of large showerheads in luxury bathrooms possible.”
“The heating and cooling go through the floors by means of a collective thermal storage grid and district heating, where optional separate air conditioning is available for all apartments. One degree Celsius of heating for the entire building takes approximately four days. It requires that much time because, for the sake of energy efficiency and sustainability, the water temperature at the foundation is very low. Once the whole tower is heated, we can maintain a stable temperature and avoid peaks.”
Fire safety “High standards are set for fire safety for such tall buildings. So the fire resistance for the structure must be twice as long as that for a 100-meter building. In order to maintain the situation safe and under control in the event of a disaster, such as a fire breaking out in an apartment, we are installing an intelligent sprinkler system throughout the entire building. In addition, the stairwells create smoke-free fire airlocks: by maintaining a certain amount of overpressure in the stairwells, any type of smoke development will remain in the apartment and the corridor on that storey.
Digital technology “Since on average there is no guarantee of a well-functioning 4G or 5G network over a height of 90 meters, the Cool Tower is equipped with a powerful Wi-Fi network in order to maintain optimum accessibility throughout the building. And not least because you summon the lift using a special app. The Cool Tower has two lifts to 150 meters, one to 30 meters and one more for low-rise. The app calculates which lift offers the user the shortest wait time and then immediately summons that one. Incidentally, anyone who does live in the Cool Tower will have to take one thing into consideration: you have to move house at night since the effect on the lift wait time is too great when moving during the day. Your neighbours would not be pleased.”
Why not 150 meters?
Jan Ultee was the instigator in the development of the Cool Tower. He is a born and bred Rotterdammer, coming up in real estate at a time when skyscrapers were on the rise.
“I have had this project on the back burner since the beginning of the 2000s. About that time we had already begun to buy buildings in the Baan quarter (the Cool Tower is being built on private property) and sometime in 2006 we made sketches of the building, ideas that leaned towards a kind of Gotham City-ish building with connecting walk bridges. Fun, but not realistic. Then came the banking crises and a number of other unpleasantnesses that kept the project from getting off the ground. But in the end I did sit down with the City with a plan and a design. It was during that discussion that someone from the City said: that design goes up to 100 meters, but you could go up to 150 meters: why don’t you?”
“As a result of that meeting I talked to the stakeholders again, and we reviewed the existing design to see if it could be adapted to a greater height. That proved financially untenable, so I sought out an architect who would, with no prior knowledge of a 150 meter tower, consider it. Seeing as you are building something for the ages, it should look good, right?”
Panoramic views are beautiful “In all honesty I have to say that building a tower at these dimensions is not by definition the most reasonable professional decision. The combination of an optimum income from the land and ideal contract sum is clearly involved for high-rise construction. For skyscrapers that means that the most efficient building height is between 70 and 100 meters: lower than that height you make too little use of the square meters and above it technical engineering is too expensive.
“So why did I find it so important to do this anyway? Ultimately, simply because it is spectacular; that you can achieve something so extraordinary. Look, you can always walk down the street and look around. But to savour unparalleled panoramic views and to be able to see the horizon you have to go up, which is not always possible. But if you live at that height you certainly can. I myself will live there, as high up as possible so I can see Scheveningen from my bedroom. When I look at the Cool Tower from the outside, I find the underpass a genuine marvel: standing there, knowing that there is someone at home 150 meters up, it’s just mind boggling.”
