A new high-rise being completed this month in Bochum, Germany, has the unusual characteristic of being built directly atop a World War II-era bomb shelter.
September 25, 2012—A new 89 m tall office tower in Bochum, Germany, that has the distinction of being located atop an aboveground World War II-era bunker will be completed this month. Architect Gerhard Spangenberg of Berlin proposed the building—the area's tallest—in response to the city of Bochum's desire to further develop the site. The result is a new reinforced-concrete building that appears as three offset, six-story, rounded blocks stacked atop the seven-story bunker. Floor slabs that measure a mere 250 mm thick are arranged around a stabilizing central core shaft and cantilever up to 4 m to create the nonsymmetrical, elliptical volumes.
Tests showed that the concrete used in the bunker—originally built in 1942—was in good condition. However, reinforcement was "very scarce" and distributed irregularly, according to Boris Reyher, Dr.-Ing., the project manager for the high-rise and the branch manager of the Berlin office of the structural engineering firm schlaich bergermann und partner. Reyher wrote in response to written questions submitted by Civil Engineering online, and added that the interior walls and floor slabs of the bunker had also deteriorated and needed to be removed.
Further complicating matters, a subway line is located just underground, tangential to the bunker's perimeter wall. Both the bunker and the subway line posed "unusual boundary conditions" for the design, Reyher said. "On one hand, the loads from a 27-story building had to be transferred to the soil without any stress increase in the adjacent subway tunnel structure," Reyher noted. "On the other hand, this had to be managed within the confined space of the bunker and with lots of interfaces between new and old structures."
The design team managed the structural loads by treating the bunker as unreinforced concrete and minimizing the floor plate depths of the new construction. The floors are supported by a circular ring of 15 columns located along the perimeter of the building—atop the original 2 m thick concrete outer shell of the bunker—as well as by the walls of the central core shaft, Reyher explained.
The structural behavior of the new floor plates was improved by "folding the cantilever areas [of the floors] up in the shape of a conical shell and applying posttensioning tendons in the slab edges to produce a compression band counteracting the tensile stresses from the shell action in the folded slabs," Reyher said. "This way, it was possible to realize the 4 m long cantilevers with only a 250 mm thickness," and to keep both the deflections "manageable" and to attach "heavy façade loads at the perimeter," he said. The solution also preserved the open floor plans that the architect desired.
Unbonded posttensioning tendons were added to the circumference of the bunker, both to strengthen the perimeter due to extensive cracking of the existing concrete and because several new openings—including the building's new two-story entrance—had to be cut into the perimeter wall. The bunker's original 2 m thick top slab and conical concrete roof were removed and a ring beam was added to the top edge of the perimeter wall as reinforcement and to support the loads from the columns on the upper floors.
The bunker's original foundations also had to be removed under the bunker's interior, according to Reyher. They were replaced with new pile foundations that were connected to the existing outer shell of the bunker. Because of the subway line, "this connection had to be planned such that the additional loads on the existing bunker walls were transferred to the new pile foundations rather than increasing the soil pressure in the existing shallow foundation of the bunker wall," Reyher noted.
"For the new foundation, a piling rig had to be lifted into the emptied bunker shell," Reyher explained. "The space inside the bunker was just large enough for the rig to maneuver." Twelve bored 1.2 m diameter piles, which extend approximately 30 m to the bedrock below, were then placed, and a 2 m deep new pile cap was cast and connected to the perimeter wall with 15 radial corbel beams that sit in pockets cut into the base of the wall, according to Reyher.
The owner of the building is Exzenterhaus GMBH + CO KG. GuD Geotechnik und Dynamik completed the geotechnical and deep foundation design. Mike Schlaich, Ph.D., is the partner in charge of the project at schlaich bergermann und partner; he oversaw the conceptual and construction designs for the project.
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