The tower has ten shafts for testing and certifying modern high-speed lifts, a glazed panorama lift for visitors and a fire-brigade lift. The load-bearing structure essentially consists of a slip-formed reinforced concrete tube with an external diameter of 20.8 m firmly embedded 30 metres into the foundation soils to allow this structure to work. A plinth building with radial bulkhead walls contributes to the horizontal stiffening effect of the foundation.
Above 115 m, offices and equipment rooms occupy part of the floor area. There is a void at about 80 m height, which acts as a heat store and contains a pendulum system to counteract building oscillations. This damping system consists of concrete slabs suspended on 9 m long cables. The passive pendulum reduces wind-induced oscillations of the tower shaft but can also be actively excited to simulate different wind loads.
In addition to a fatigue analysis for dynamic excitations, a wide range of critical construction stages had to be investigated, such as the completion of the tower carcass without the plinth building, when the tower was simply built into the foundation soils.
The design of the textile facade was particularly interesting: wind loads, installation and maintenance systems and weathering resistance had all to be investigated and clarified before the PTFE-coated, glass-fibre fabric could be attached to the six steel hollow sections that climb helically up the tower. The helical shape affects vortex shedding and reduces transverse oscillations by 40 %.
Markus Jetter, Head of Product Development for the Multi elevator system
Structure: From what height of building do the advantages of the innovative Multi elevator system, which uses cars with ropeless linear motors and horizontal mobility, come into play?
Markus Jetter: There are a number of different perspectives. When we began development in 2015, we were thinking primarily in the vertical direction. We wanted to offer an economic solution for long travel heights above the 200 – 300 m range, but also for situations beyond the 500 – 600 m limit for conventional elevator systems. We were very surprised when architects told us how many advantages elevators moving horizontally would offer, even with much shorter travel heights.
Structure: How many cars can use the same shaft?
Markus Jetter: Every building is different in terms of travel height, passenger numbers and the required elevator speed, but elevators can operate very well if kept 50 to 100 m apart vertically. A 400 m high building could therefore have 4 – 8 cars per shaft.
Structure: Does this system require a larger shaft cross section?
Markus Jetter: Horizontally moving lifts do not require any additional space. We have no physical separation between the shafts and the cars can therefore travel directly from one shaft into another. Because we use smaller cars, we can even reduce the depth of the space required. Our test installation in Rottweil shows that we have almost a metre spare in standard-sized shafts. We can therefore save on plan area.
Structure: Is the Multi particularly effective as a looping system?
Markus Jetter: Yes, of course. When cars do not travel in a loop, some important advantages are lost. The system is designed to operate with multiple cars. This creates a large matrix of options and building designers bring a surprising number of variants to our attention.
Structure: How are collisions avoided?
Markus Jetter: Based on our experience during the last 15 years with our Twin system, we are further developing sensor and control systems for collision monitoring. These systems continuously monitor the travel direction, speed and separation of the cars.
Structure: Does the Multi system bring any special loading implications for the building?
Markus Jetter: Conventional elevator construction transfers all of the forces involved through vertically stacked guide rails into the shaft foundation. To achieve an unlimited travel height and accommodate building movements at the same time, we avoid stacking rails one upon the other, instead we attach them to the shaft wall in sections incorporating movement joints. Force distribution is therefore better than with conventional systems and our system is able to react very flexibly to building movements. The big problem with conventional elevator systems is that the elevator rope becomes liable to oscillate at a certain length, which does not happen with the ropeless Multi.
Structure: You have been able to investigate this oscillation problem very thoroughly using the test tower at Rottweil. Has the test tower helped in the development of the Multi system?
Markus Jetter: Of course, it has helped in the tests and performance assessment, but it has been mainly useful for conventional rope-wire systems. We designed the tower so that it could be used to test all types of elevator systems. Our ability to dampen the building’s oscillations is of benefit. The oscillation damper is permanently in use. It also protects our offices in the tower from high accelerations. We can excite the tower to specific extents for the tests as well.
Structure: What proportion of the market do you see the new system claiming in 10 or 20 years?
Markus Jetter: We have been particularly encouraged by the level of interest from all over the world. The Council on Tall Buildings and Urban Habitat (CTBUH), for example, a non-profit organisation involved in building design, foresees a great need – not only for new buildings but also for retrofitting elevators in existing stock.
Structure: Does that put you under pressure as developers?
Markus Jetter: We have several project enquiries that are due to be realised in the next few years. To provide the market with what it expects of us, we have had to refocus and set up a new business unit with its own organisational structure for this product.
Structure: What were the greatest challenges in the development of the Multi?
Markus Jetter: The system is based on four engineering cornerstones: first there is the new drive system. We are familiar with linear drive technology from the maglev Transrapid train and have adapted it for the vertical direction. This technology requires lightweight vehicles. Thus we moved into lightweight construction, introducing new engineering methods such as topology optimisation – which is the second cornerstone. The third is the functional safety provided by collision prevention. Passenger safety demands proof of compliance. The fourth cornerstone is a completely new component: the “exchanger”, a shaft-switching system that enables the car to change transport directions. The development of this system required extensive knowledge, experience and research. Last but not least are the complex algorithms for managing elevator calls.
Structure: Does the new system allow customised positioning of the elevators in plan?
Markus Jetter: Until now, the circulation core was an extremely large and expensive building component. The challenge is to keep this core as small as possible so as to gain useful floor area. The Multi has stimulated a great many ideas from building designers, such as placing the elevator shafts towards the outside of buildings or using them as “sky bridges” to connect two, three or four towers.
Structure: Where will the Multi system be used for the first time?
Markus Jetter: The first system will go into operation in the Eastside Tower, Berlin, in 2021.
