IBE Project Featured in Civil Engineering Magazine

June 2009

Click here for PDF of article

Article text from Civil Engineering:

"Paris Skyscraper to Rival Eiffel Tower

The business district know as La Defense, on the northwestern outskirts of Paris, is probably best known for its most prominent structure, la Grande Arche. Now another iconic building is planned for the area that promises to be noteworthy not only for its distinctive appearance but also for its height. At 300 m, it will be the tallest building to be constructed in Paris since the Eiffel Tower.

One of several new skyscrapers planned for the district, the tower--to be named le Phare-- is far from a conventional office building. One factor that influenced the structure's unique design is the irregularly shaped and densely crowded site, explained Charles Lamy, the project manager for the structure's architect, Morphosis, which is based in Santa Monica, California. On one side, a major highway abuts the site, and below grade lies a busy train station. Further complicating matters, a large pedestrian bridge bisects the site, slicing straight through the building's footprint.

To accomodate the existing structures, the new building's base will resemble a tripod. Two legs of the tripod will contain occupied space, while the third, which will consist of a grid of diagonal steel beams, will function only as a structural support. This arrangement will create a 24 m wide, 30 m tall void at the base of the building through which pedestrian traffic can continue to flow. A bank of escalators will convey visitors from a ground-level pavilion to the building's main lobby, which will be located on the ninth floor. There the legs will merge in a single, asymmetrical tower that will first widen before narrowing toward the top.

Because one side of the building will be directly above the train station, transferring vertical loads to the foundations will be a complex undertaking, says Jean-Marc Jaeger, who heads the complex structures division of SETEC TPI, the Paris-based structural engineering firm that is working on the project. Most of the building will rest on a 2.8 m deep concrete raft founded on a layer of limestone located some 20 m beneath the ground elevation. Vertical loads from the portion of the tower that will be situated above the train station, however, will be transferred to a small number of piles that will descend through the station.

With respect to structural framework, the building is a hybrid, notes Jaeger. The structural framing for most of the building will consist of a central concrete core, a facade of concrete columns and beams, and concrete floors, all of which will work together to stiffen the structure. The portion of the building above the train station, however, will be framed in steel to minimize its weight. The grid of diagonal steel beams, which wraps around the main lobby, helps to limit wind-induced acceleration at the top of the tower., Jaeger says.

Sustainability is a major goal of the design team, says Peter Simmonds, a senior associate for IBE Consulting Engineers, which is based in Sherman Oaks, California. IBE is not only responsible for the project's mechanical engineering aspects but also is leading the sustainable design effort, Simmonds explains. For example, a wind turbine farm on the roof is expected to provide a generating capacity of more than 200 kW, and the building will also feature a highly efficient chilled-water plant.

Perhaps the most visible way in which the building is designed for energy efficiency has to do with its facade. In fact, the tower will employ several facade systems, each expressly designed to conserve energy on a particular side of the building, says Mitsu Edwards, a project director for RFR Ingenieurs, the facade engineering consultant, which is based in Paris.

The sheer north side of the building, for example, will receive relatively little direct sunlight, so its glazed glass facade has been designed to expose the interior to as much natural daylight as possible, Edwards says. The glazed facades of the east, south, and west sides of the building, however, will be wrapped in a second, curvilinear skin, which will take the form of a stainless steel mesh designed to block much of the direct solar radiation while preserving the views from inside the building. The mesh screen, which will consist of ribbons of diagonally oriented scales, will double as a safety barrier for the maintenance walkways that pass between the two layers of the facade.

The project is part of a wider effort by the Etablissement public pour l'amenagement de la region de la Defense, the French agency overseeing the redevelopment of la Defense. When it opens, the 175,000 m2 building will add nearly 130,000 m2 of office space to the growing district. Depending on when construction begins, Lamy says, the project could be complete as early as 2014."