By Mauro Eugenio Giuliani, Dr.Eng
In Rome, the new headquarters of the BNL-BNP Paribas Group banking organization is squeezed onto a narrow stretch of land between a railway station and a set of roadways. Yet the site helped yield a stunning design that features a long, thin structure clad in a mirrored facade, creating a strong connection to—and even a literal reflection of—the surrounding urban environment.
FROM CERTAIN ANGLES, the new headquarters building of the BNL-BNP Paribas Group in Rome almost disappears against its surroundings. On a beautiful day, for instance, the mirrored facade of the banking group's new structure might reflect a bright blue sky and puffy white clouds. Or on a day that threatens rain, the building can present the overcast shroud of gray. It can even show the urban neighborhood that surrounds the site in a manner that makes the headquarters building appear almost transparent—amazing for a structure that measures more than 200 m long and more than 60 m tall. What it does not reveal, however, is the structural engineering beneath that reflective cladding.
Extending lengthwise along a roughly north-south axis, the new BNL-BNP building is squeezed into a narrow strip of land between a roadway, a tunnel, and the Roma Tiburtina railway station on its west side and a roadway at the edge of the Pietralata urban district on its east side. In plan, the structure measures approximately 41 m across at the northern end, its widest point. At its midpoint and southward, the structure is roughly 17 m wide for an extended section, then tapers along its eastern edge to a narrow, angled tip. That southern tip also features an overhanging 27 m long cantilevered section known as the "prow." The midpoint of the building, which connects these two sections, includes a vertical cutout that in plan partially encircles a historical cistern located immediately adjacent to the building.
The building was designed by the Milan-based architcture firm 5+1AA, which has since been renamed Atelier(s) Alfonso Femia. The Genoa, Italy-based firm is led by Alfonso Femia, who served as the architect of record for the project. Redesco Progetti was the structural engineer. Parsitalia Group, of Rome, was the general contractor on the €83-million (U.S. $103.6-million) project.
A series of spatial and programmatic constraints helped determine the arrangement of the building's volumes, while simultaneously requiring the designers to conduct a novel, even radical, reimagining of traditional structural solutions.
Seeking to generate large areas of floor space from the long, narrow plot while respecting the planning restrictions that limited the height of the building, the architects envisioned a massive structure that resembled a long, slender curtain extending lengthways, roughly parallel to the railway line. The result was a single unit, like a tower laid on its side—though one in which the ratio of height to width is exaggerated, particularly in the prow. Inside, the large, open spaces required floor structures that could cope with long spans, while height constraints meant that any building facilities and systems had to be incorporated within the depth of the floor systems so that the necessary number of floors could be included in the design.
The 72,000 sq m building features four underground and 12 aboveground levels. The former contains 23,000 sq m split among parking, storage, and mechanical systems. The latter contains 45,000 sq m that includes offices, a 300-seat auditorium, a 1,000 sq m training center, a 1,800 sq m company restaurant, and 750 sq m of childcare space. The roof structures encompass the remaining 4,000 sq m and contain additional technical equipment, solar power arrays, and terraces.
Among the more singular features of the building is an open area known as the "bridge" located at the midpoint of the building. Including a cutout directly through the structure, this bridge enables those outside the building to look through it. The opening also frames the adjoining masonry tower, a historical structure that is part of an existing cistern protected under the local municipality's office for architectural and artistic heritage. Also notable are external emergency stairs that extend irregularly from the Pietralata side of the facade.
The structural challenges posed by the design—from the configuration of the internal spaces to the physical constraints of the site—were defined by the need to minimize the size and number of the structural members to a series of clearly delineated, primary elements.
The geotechnical characteristics of the site informed the selection of foundation piles as a means of limiting the differential settlement of the structure's three volumes (the northern end, the midpoint bridge, and the southern end) to acceptable parameters. To address the foreseeable load variation generated by the vertical structures, circular-section piles of either 1,000 or 1,200 mm in diameter were selected and sunk to one of two depths. On the western side, where the site abuts the road tunnel, the pile system was designed to transfer loads at a depth at which it would not cause noticeable subsidence in preexisting structures. A perimeter reinforced-concrete retaining wall system was used, with most of the piles located within its boundaries. A continuous concrete raft is anchored to, and tops, the reinforced-concrete piles.
In the basement levels, the building features flat reinforced-concrete slabs that best serve the auxiliary nature of these lower levels. Concrete suited the load-bearing and fire-resistance requirements, as well as the distance spanned by the horizontal elements. For the aboveground levels, a composite steel-and-concrete structure was chosen; this system is lighter in weight than an all-concrete version yet has sufficient capacity to span large open spaces and performs well during potential earthquakes.
The building is constructed on a step such that the eastern side has a much lower ground level than the western. The basement comprises four floors that extend from 6.3 m below ground to 6 m above ground on the eastern side. Reinforced-concrete columns are used, and the largest open spaces measure 12 by 9 m. The key parameter for these levels was the depth of the floor systems: a structural minimum was fixed at 34 cm for the basement, while a maximum thickness of 40 cm was imposed for the floors at 6 m or more above ground level because they are subjected to both dead and live loads of a larger magnitude.
Despite the use of concrete underground and the composite steel-and-concrete system aboveground, the entire structure—lower and upper levels—was designed for maximum repetition and large-scale production. For the upper levels, continuous columns formed from commercially available steel H sections with cross sections not exceeding 450 sq mm were adopted.
Horizontal stability is provided by reinforced-concrete cores, which house stairs, elevators, and vertical ducts. The locations of the circulation elements, however, were determined by strict architectural constraints, not by structural requirements. So the lateral load-resisting system—provided by the building's structural steel frame—had to be adapted to the cores' locations as dictated by the architectural layout and was designed to accommodate the wind and seismic actions without introducing additional stabilizing elements.
The building's overall structural frame was designed to accommodate the internal space planning and the modulation of the building's striking mirrored facade. It comprises, for the most part, a horizontal grid of 12 by 9 m bays, with story heights limited to 3.79 m. Given that these dimensions are rather large for an office building, the composite solution was designed to provide stiffness while remaining lightweight, allowing the passage of ducts, cables, pipes, and other conduits, and also minimizing the quantity of steel used and accelerating the rate of construction.
For each of the building's upper stories, a floor that follows the typical 12 by 9 m grid is formed from steel beams supporting profiled panels and is finished with placed concrete. The dimensions of the beams are influenced by two antithetical requirements: a limit on the total thickness of the structural elements of 72 cm, inclusive of flooring and dropped ceilings, and the need to confer rigidity to the composite beams such that they fall within acceptable limits of deformation and vibration frequency for a building of this type. HEB 600-type steel girders were used for the 12 m spans, with a flat strip welded to the lower flange across the longitudinal axis of the beam to increase the sectional capacity. The web of the beam incorporates a series of large openings to allow the passage of ducts, pipes, cables, and other system conduits. The 9 m spans between the beams are bridged by a ribbed floor of concrete poured directly onto profiled metal sheeting, with ribbing at a depth of 201 mm.
Continuity of the ribs was achieved by passing rebars through customized holes in the beam webs, with the concrete coming into direct contact with the beams. The panels rest on dedicated flanged supports welded to the beams. This method of constructing the floor elements does not require secondary beams and permits the integration of service ducts along the gaps between the ribs of the paneling, which are aligned with the holes in the beams. The result is a truly innovative system that is extremely light and that integrates the building facilities seamlessly.
The building's central cores, which house the vertical conduits that allow the building to function, are the sole means of conferring the building's horizontal stability. They are made from reinforced concrete, with thicknesses ranging from 20 to 40 cm. The position of the cores is dictated by the building's elongated, narrow footprint and by the requirement to place these elements on only one side of the block, thus dramatically affecting the structural behavior under seismic actions. Built to precisely calculated dimensions, the cores are subjected to bending and shearing stresses and to warping. Their behavior has been verified under normal operating conditions, when the structural joint between the main units is in operation, and under seismic conditions, when the joint is locked.
The structural joint separates the north block from its southern counterpart about halfway along the building's length. The ideal position, structurally and architecturally, placed it in line with the bridge section. The joint is placed in every floor from the higher of the subterranean levels-3.2 m below the ground-all the way to the roof structures. (The stories located above the building's bridge section are supported by a roof-level truss system, from which four upper floors and the roof itself are suspended.)
The structural joint was required because of the length of the building. It relieves stresses created by concrete shrinkage and thermal expansion and contraction. At the same time, the building had to react as a single monolithic unit against seismic actions. To resolve this issue, special viscoelastic couplers, known as shock transmitter units, were installed at the joint to accommodate gradual motions while impeding the rapid movements possible during seismic activity. The shock transmitters are functionally similar to hydraulic jacks: one cylinder is placed in another, with a viscous but incompressible fluid in between. A small hole allows the fluid to slowly pass from one chamber to the other, thus allowing for slow movements (shrinkage, thermal movements, etc.). However, in case of such rapid movements as those caused by a seismic event, the fluid does not pass through fast enough, and the jack almost immediately locks so that the building responds as one structural unit.
In the entirely concrete basement levels, the units are housed in dedicated niches in the slabs, while in the upper stories they are situated between the two beams of the joint in line with the floor ribs. The ribs are rigid along the building's axis and are therefore able to counter and redistribute the forces that the coupling transmits within the material of the floor itself.
The prow is one of the most distinctive features of the building. This element cantilevers 27 m, as mentioned, forming an asymmetric structure of pronounced three-dimensionality at the southern end of the building.
While the western side of this section of the building is straight, it tapers inward on the Pietralata side to reach its point. The closest element that provides additional rigidity to the prow is its reinforced-concrete core, which, as mentioned above, sits off-center with respect to the floor plan. The system's equilibrium is ensured by a space frame formed from latticed structures located along the building's floor planes and the facade panels. This system is visible, especially through the glass parts of the facade. This structure is joined to the core using a system of posttensioned rods in the reinforced-concrete walls. The compression generated by the lower chord of the lattice structure in the plane of the facade on the tapered side is transferred to the reinforced-concrete slab of the floor located 6 m above ground level. The transfer occurs via a special node that links to a transverse posttensioning system in the floor that is in line with the transverse component.
The external emergency stairways, with their deliberately irregular rhythm, are an integral part of the design of the facade on the Pietralata side. The all-steel structures are based on a system of cantilevered beams. Where possible, these are effectively an extension of the beams in the floors of the building, while in other cases, they are fixed to the facade via load-transfer systems. The result is an ideal marriage of the demands of form and function of the stairways.
Standard computer-aided design (CAD) processes were deemed adequate for determining the general 3-D configuration of the three volumes that compose the building, without the need to generate geometric models based on mathematical computations. In terms of structural verifications, the processes of determining the seismic response of the building as a whole, and the subsequent analysis, proved particularly complex. Earthquake forces in Italy must be calculated according to the National Annex of the Eurocode, and a modal response analysis for the building was performed using Eurocode 8: Design of Structures for Earthquake Resistance (European Committee for Standardization, 2004) for seismic issues. The peak ground acceleration for this site is on the order of 0.4g, and the definition of the relevant response spectra for the different limit states depends on many parameters, including, for example, building use and category, soil type, return period, and structural organization.
In terms of more localized phenomena, the unusual characteristics of the beams used in each level necessitated more focused analysis. In bringing the project forward, the coordination of different disciplines was of fundamental importance. The building facilities and systems required particular attention, given the degree to which they were to be integrated with the structural framework.
Analysis was carried out using complete, linear, 3-D modeling, both static and dynamic. Localized models were employed for specific elements and sections, such as the prow, the nodal sections connecting the building's structural steel frames to the cores, areas fitted with posttensioning systems to redirect forces, and so on. Particular attention was paid to the construction system for the different floors, and the unusual characteristics of the beams required a nonlinear analysis. After the finalization of the design, with the project under way, real-world physical testing was carried out at the facility at which the metal components were produced. One of the floor beams underwent stress testing to the point of collapse, the results confirming the accuracy of the previous calculations.
Design development and production of working drawings and details were carried out under the supervision of the executive architect responsible for cross-disciplinary data coordination, using a 2-D CAD environment with advanced drawing modules for the rebar detailing and scheduling.
As often occurs in large, complex buildings that eschew overtly apparent or grandiloquent representations of structure, the underlying system of this building is not immediately perceivable to visitors or casual observers, regardless of whether they are inside or outside the building. Indeed, the structural systems are masked almost entirely by the very size of the building and the nature of its envelope, with the two exceptions being the prow and the central bridge section. And those exceptions have their own hidden structural solutions. But what is apparent are the benefits of careful analysis and customized structural engineering in not only resolving challenges but successfully delivering a structure that meets the client's needs and enhances its surroundings.
Mauro Eugenio Giuliani, Dr.Eng., is the general manager of Redesco Progetti, in Milan.
Client BNP Paribas Real Estate Property Development, Issy-les-Moulineaux, France
Architect Atelier(s) Alfonso Femia (formerly 5+1AA), Genoa, Italy; Milan; and Paris
Executive architectural project, coordination, and works supervision Starching, Milan and Rome
Structural design Redesco Progetti
Milan Mechanical, electrical, and plumbing engineer Ariatta Ingegneria Dei Sistemi, Milan
Tenant BNL-BNP Paribas Group, Rome
General contractor PGC Parsitalia General Contractor, Rome
Metal structures MBM, Caselle di Sommacampagna, Italy