January 13, 2020
Epstein's Vice President and Director of Structural Engineering, Tom Suarez, SE, PE, shares some insight on the sophisticated structural systems it took to accomplish the design goals for the Mennica Legacy Tower, a Class A Office building located in the Central Business District of Warsaw.
The recent opening of Warsaws Mennica Legacy Tower marks the end of one of Epsteins most significant structural engineering endeavors in our nearly 100-year history. Faced with the challenge of a tight site and poor soil conditions, Epsteins structural engineers designed a system to accommodate the ambitious scheme set forth by design architects, Goettsch Partners.
Mennica, a development in a joint-venture between Golub GetHouse and Mennica Polska SA, consists of two towers situated around a common landscaped plaza with four levels of underground parking under the full site. The west tower, which is open and fully-occupied by one-tenant, is 10 stories with a roof height of 39.2 meters (roughly 127 feet) while the east tower, which is currently in the tenant fit-out stage, will serve as the new headquarters for mBank, climbs to 34 stories with a roof height of 130 meters (roughly 426 feet).
The site is bounded by city streets on three sides and an existing residential building on the east. To maximize parking, the basement for the complex extends to the property line on each side. Furthermore, subway tunnels run very near the west and south property lines and had to be protected from damaging settlements created by the new construction. To solve this problem, a top down method was used for the underground construction. Therefore, the perimeter slurry wall was first constructed from grade level to a depth of 23 meters (roughly 75 feet). Deep foundations were then constructed from grade to a depth of 34 meters (roughly 111 feet) and cut off at the future level of the lowest basement. Temporary columns were also set on the deep foundations and extended to grade. The ground level floor was constructed first on the un-excavated site and used to brace the slurry walls as excavation proceeded below it using mining techniques as the slab was supported by the temporary columns.
This process continued from the top down with construction of the subsequent basement slabs placed on grade, excavated below, supported on the temporary columns and used to brace the basement slurry walls. This construction sequence was very effective in maximizing the stiffness of the earth retention system and ensured minimal disturbance of the surrounding elements including the tunnels, streets and adjacent buildings.
The foundation system utilized for Mennica includes perimeter slurry walls, deep foundation barrettes, and a thick concrete raft. The slurry walls were used as both temporary earth retention and as the permanent basement walls after excavation was complete. The walls extend deep into the native clay layers, effectively serving as cut-off walls for ground water infiltration to help maintain a dry basement. Deep foundation barrettes, which can be likened to rectangular concrete columns, are used to reduce the settlement of the raft foundation supporting the structures. The raft varies in thickness from under the east tower to the parking areas between the buildings.
The superstructure of the buildings consists of concrete flat slab floors with concrete shear walls at the building cores. To eliminate interior columns for maximum flexibility, the slabs were post-tensioned to achieve 9.4 meter (roughly 30 feet) column free spans. The use of flat slabs with no internal beams also facilitated the overhead routing of building services while reducing the required floor to floor height. The distinctive sawtooth texture displayed in the building facade is created by varying the slab edge profile to mimic this profile.
One of the signature architectural features of the east tower is the outward leaning of the southeast corner of the building. The corner column slopes away from the base of the structure at 2.41 degrees from vertical, resulting in a 4.6 meter (roughly 15 feet) longer slab edge at the top of the building. Another adjustment was made to the location of the south core wall at the lobby level to create a larger first floor space. This cantilever was achieved through a complicated 2 story deep transfer beam in the mechanical levels above the lobby.
The main lobby of the east tower achieves great transparency to the landscaped plaza through the use of a cable supported glass wall. In this system, a series of vertical cables constitutes the lateral support system for the glass wall resulting in a minimal and nearly transparent support structure. The effectiveness of the cables as lateral supports relies on their high tension. To resist the large cable forces, heavy steel beams were placed both above and below the lobby in the ceiling of the garage and the mechanical levels.
Topping out of the structure and completion of the facade occurred in the fall of 2019. With that, the Mennica Legacy Tower has become a beautiful addition to the Warsaw skyline!