concepts
01 | 2021
7/13

Spectacular precision work

The new hallmark of the Frankfurt skyline can be seen most clearly as the numeral one if it’s viewed from the west. Observed from the east, this symbolic building at least looks like a reflection of the number. But no matter from which direction it’s viewed, the ONE fascinates observers with its seemingly impossible architectural peculiarity: a free-floating overhang at a height of 130 meters, topped by 15 more floors soaring above the center of Frankfurt. At a height of almost 190 meters, the ONE is the sixth-highest building in Germany’s banking metropolis.

This skyscraper with its striking silhouette, which was commissioned by CA Immo, will operate as a hotel and office building as well as having areas that are open to the public. The German Sustainable Building Council (DGNB) has granted it a preliminary Platinum certificate, which is its highest accolade. HOCHTIEF handed over the basic structure of the spectacular new polygon, together with an adjacent parking garage for cars and bicycles, in spring 2021. The company’s managers are having to answer the same question again and again: How is it possible to build a free-floating overhang at a height of around 130 meters?

THE WEIGHT OF 13 FLOORS

Otmar Püchert, the Senior Project Manager at HOCHTIEF, knows the answer. During his more than 32 years in the engineering profession, he has studied many ambitious constructions in detail. But the construction of the ONE near the Frankfurt trade fair complex presented him with an especially spectacular challenge: realizing the architects’ plans for a triangular 8.5-meter-wide building overhang that supports the enormous weight of 15 floors. In order to understand how such an element functions, we need to take a quick look at the history of high-rise construction.

The Chrysler Building in New York, which was constructed in 1930, was the first skyscraper with a height of more than 300 meters. The first generation of skyscrapers, which were built mainly in the USA, were extremely heavy rigid steel structures. Because of their construction material, the range of feasible architectural designs was limited. The revolution in skyscraper construction began with the use of concrete as a building material. As a rule, modern high-rises consist of a massive core of reinforced concrete—the ONE has two of them—to which the floors and the facade loads are in effect “hung on” from the side.

The load-bearing properties of reinforced concrete, which can be flexibly shaped into varied designs, made it possible to create audacious structures with impossible-looking statics, such as the overhang of the ONE. For this building, HOCHTIEF used 65,000 cubic meters of concrete in order to create 49 stories with a footprint of 2,200 square meters. For Frankfurt this architecture is new and exciting. The ONE lends its skyline a bolder note. The HOCHTIEF Project Manager Otmar Püchert says that to his knowledge “no other German building has been built with an overhang at this height under a similar load.”

The architecture of the ONE includes two overhangs above the 15th and 34th floors, which give the building its characteristic look of the numeral one. Whereas the 16th floor—a conference and co-working space—as a terrace created by the tapering-off of the building’s basic framework, an 8.5-meter-wide overhang in the opposite direction had to be built to serve as the base plate of the 35th floor. Because it’s difficult to set up scaffolding at ground level for work to be done at a height of 130 meters, a pre-assembled steel framework was hoisted up along the exterior wall of the building. After it was installed, at times it bore the weight of six floors. The weight of the overhang and the 140 tons of the supporting structure was borne by a small projection on the 30th floor and by tensile and compressive forces that were directed sideways toward the building. It was a genuine miracle of structural calculation as well as the art of assembly.

A NIGHT SHIFT WITH HIGH-ALTITUDE RESCUE WORKERS

Otmar Püchert recalls the nighttime installation of the steel supporting trestle, which projected 11 meters out from the building’s northeast corner. During this operation, all the other work on the high-rise had to be interrupted. Located under the overhang was the exit of the site road, and next to it was the entrance of a heavily frequented shopping mall in Frankfurt’s busy Europaviertel. The nighttime assembly, which required the presence of high-altitude rescue workers, was unavoidable. Also at night, the steel framework was expanded by adding two beam grillage units, and a heavy support frame scaffolding was erected behind a safety zone created by another net-covered scaffold. The substructure for the shell level of the roof above the 34th floor was finished. On September 24, 2020, a special day for the ONE team, the overhanging slab could be poured. From this level the building grew upward for another 50 meters, on top of the overhang as well. Firmly affixed to the shell construction, the supporting structure at times had to carry the weight of the overhangs of six individual floors—until the pressure could be relieved because the building itself could carry the weight. But where do the forces of the overhang actually “disappear” to when the steel framework is removed?

SPECIAL CONCRETE

The answer to that is a scrupulously calculated package of measures. Forces that are directed sideways play an important role; so do the right materials and the way they are processed. A statics expert looking at the 35th floor would immediately spot the additional hoisting walls at the center of the basic structure. In addition, concrete that is more rigid is used at that height. Moreover, the hardening time of this material is constantly recalculated, because at winter temperatures it causes construction work to last much longer than in the summer. HOCHTIEF maintains a special concrete laboratory for this analysis. Samples of various mixtures are placed on vibrating plates at the lab in Mörfelden-Walldorf near Frankfurt and checked after 7, 14, 21, and 28 days to determine their standard strength. Concrete experts generally agree that the strength of concrete “matures” after 28 days. For Otmar Püchert and his crew of approximately 120 people, this is an important figure. It decides when supporting structures are no longer needed, because the new floors, ceilings, and walls of Frankfurt’s gigantic ONE can bear their own weight from then on.

When the latest member of Frankfurt’s skyline is finished at the beginning of 2022, it could become a meeting point and a magnet for tourists. It will feature not only the co-working space and the hotel, which will welcome visitors from all over the world up to the 15th floor, but also the spectacular Skybar at a height of around 185 meters. The ONE will soon be renowned for the spectacular views it offers from the west and the east as well as from the top.

Text: Eric Leimann