Flying Carpet

Flying Carpet

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© M . B e l l i n i — R . R i c c i o t t i / M u s é e d u L o u v r e , © 2 0 1 2 M u s é e d u L o u v r e / P h i l i pp e R u a u lt, o p p o s i t e ; L i s a R i c c i o t t i / A g e n c e R u dy R i c c i o t t i , r i g h t

photocredit goes here

photocredit goes here

The structural design of a new wing dedicated to Islamic art at the Musée du Louvre, in Paris, would have been challenging enough given the 800-year history of the facility. But the site selected was a courtyard surrounded by neoclassical facades that could not be incorporated into the structural solution. Moreover, the designers were instructed to make the new structure essentially invisible and to give it a glass and metal roof so thin and lightweight in appearance that it would create the illusion of a flying carpet. . . . . . B y P ierluigi B ucci

D

uring the structural design of the

Musée du Louvre’s new wing dedicated to Islamic art, the multidisciplinary design team of Hugh Dutton Associates (HDA), of Paris, the project’s structural consultant, experienced firsthand the benefits of patience and perseverance, which Muslim scholars regard as key virtues. Appointed in December 2005 by the project’s architects—Mario Bellini Architects, srl, of Milano (Milan), Italy, and Agence Rudy Ric-

ciotti, of Bandol, France—HDA had to design through various iterations the support system and other elements of the undulating steel-framed glass roof that now covers the new galleries.

The new wing opened last September, the culmination of a decade-long project to elevate the status of the Louvre’s extensive collection of more than 18,000 pieces of Islamic art, which date from the 7th through the 19th century. Rather than being just a section of the Department of Near Eastern Antiquities, these works would have a curated space

To prepare the site, the 17th-century neoclassical facades and statuary of the Cour Visconti were restored, and the existing foundations were reinforced, consolidated, and extended, opposite. The new wing was the culmination of a decade-long project to elevate the Louvre’s extensive collection of more than 18,000 pieces of Islamic art to the status of a curated department, above.

Copyright © 2013 American Society of Civil Engineers. All rights reser ved.

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chitect. Next, excavation to a depth of 12 m was carried out beneath the courtyard, all of the soil being removed via a single opening just 2.7 m across. In this way the existing foundations of the original facades could be reinforced, consolidated, and extended via the use of a jet grouting technique to reach a limestone layer at the bottom of the excavation.

Two subterranean levels were constructed in the excavated

most ornate interior courtyards, carefully prepared for its new role.

Lisa Ricciotti/Agence R u dy R i c c i o t t i

Considered one of the Louvre’s the Cour Visconti had to be

of their own, one of only eight such subdivisions within the museum. The site selected for the new Department of Islamic Art was the Cour Visconti, an interior courtyard that previously was not accessible to the public.

Considered one of the Louvre’s most ornate interior courtyards, the Cour Visconti had to be carefully prepared for its new role. After an international competition that selected the architectural team in July 2005 and then the engineers, the project to create the three levels of galleries that would have a total floor space of 3,800 m² moved forward. From January to December 2006, the 17th-century neoclassical facades and statuary of the Cour Visconti were restored under the direction of Michel Goutal, the Louvre’s senior historical monument ar-

it the number of columns in the galleries. Glazed vertical partitions separate the gallery spaces on the courtyard level, which also features glazed facades beneath the signature roof structure.

The three-dimensional, flowing geometry of the glass and metal canopy that forms the roof over the courtyard level galleries is nearly 49 m long in the north–south direction and 32.5 m long in the east–west direction, its surface area being 1,646 m². The design features a double-lattice system of steel tubes of circular cross section supporting a series of glazed panels. The panels are sealed against water ingress and are sandwiched between an exterior and an interior layer of metallic mesh. The exterior mesh is formed from more than 2,300 panels in the shape of isosceles triangles, the equal sides being roughly 1.2 m long, and the panels can be opened for maintenance.

A gold tint to the mesh creates a A gold tint to the mesh bright, translucent structure, the creates a bright and exterior metallic layer filtering translucent structure, daylight while the interior mesh the exterior metallic layer forms the gallery ceilings. layer filtering the dayThe canopy varies in depth, light that reaches the being thicker over the load-bearinterior of the galleries. ing columns that support the roof structure and thinner at the edges, which heightens the effect of undulation. Although the canopy has been compared to a dragonfly’s wing and to a golden, iridescent cloud, the architects expressly asked the engineers for a structure that would suggest a flying carpet. But just as a movie’s special effects are spoiled if the audience can see how the effects are created, the architects gave HDA’s designers a complicated but intriguing challenge: “We want to see the sky and the courtyard facades through the roof,” the architecture team explained. “But we don’t want to see the structure itself—the roof should fly as a flying carpet.”

The task seemed difficult, and the members of the HDA team knew they would have to come up with an imaginative solution.

An early iteration of the roof structure from the competition phase, prior to HDA’s involvement, featured a steel space frame truss system with an unvarying depth of 80 cm. The roof was to be supported by four heavy, straight columns on one side of the space that would provide lateral stability for the whole system. The eight smaller columns on the opposite side would provide only vertical support. The architects, however, decided that this system was not sufficiently innovative. Moreover, the design would have reduced the transparency of the roof and made the structure itself visibly obtrusive.

The challenge for HDA was to find a structural solution that would be cost effective, efficient, and “invisible.” An important consideration was that the external surface shape of the roof was set, although the designers would be allowed to modify the internal skin geometry somewhat. The edges of the roof had to be as thin as possible for the flying carpet allusion, but the roof could not appear to be “sitting on” the

space, the lowest serving as a 3 m tall basement for mechanical systems and featuring a so-called floating concrete foundation system. The 6 m tall upper basement level features exhibition spaces supported on a reinforced-concrete slab. A cast-in-place concrete staircase provides access from this level to the courtyard, the floor of which features glazed openings and a composite steel and concrete structure designed to lim-

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edges of the facades. The roof’s external skin had to be irides- ally, the architects informed HDA that the idea was not feacent to match the images from the competition, while the fil- sible because the fire regulations required that a minimum tered light that reached the interior of the galleries had to be as distance of 2.5 to 4 m be maintained at all times between the natural as possible. At the same time, this lightweight, highly roof edge and the original facades. transparent, and barely visible structure had to accommodate Thus HDA had to begin again, this time with a design that the galleries and their artwork in such a way as to meet the re- replaced the space frame of the original concept with solid I quirements pertaining to solar radiation, shadows, condensa- beams, using the glass pattern triangulation to determine the main directions of the beams. The idea was to create treelike tion, thermal effects, and the comfort of the patrons.

To find a structural system for the roof, Hugh Dutton, column heads using beams of variable depth, thicker beams on the founder of HDA, formed a three-person design team that the support columns and smaller beams elsewhere. The beams would try to comply with the architects’ requests and thus would cross each other in following the pattern of the glass give life to the flying carpet. This compact team included panels and thereby create a single layer of mesh that would Dutton, who was in charge and served as the creative design- be stiff enough to resist the potential bending er; Francesco Cingolani, the firm’s communications director and the project manager for the Louvre operation; and me, an independent engineer and structural consultant. I developed the engineering supports for the innovative solutions and created ad hoc software to demonstrate the new systems. Cingolani acted in a liaison capacity for Dutton and me, providing feedback on the structural solutions I devised in response to Dutton’s propositions. He also adapted the geometry to the structural behavior models each time the new shapes had to be calculated.

It was a trial-and-error process in which each solution was calculated, then considered from the architectural point of view, then modified according to the structural requirements to obtain a more efficient design, then recalculated again, and so on.

Fortunately, this approach matched the HDA philosophy that the structure and the D ancing C olumns form of a building should essentially be one C omputer M odel and the same, exalting both the structural efficiency and the architectural aesthetic. Under this notion, and buckthe engineers and the designers should work together closely ling effects. and should share results in an iterative process to reach the Again, the initial optimal solution. studies were promising During the design of the Islamic galleries, the available and even indicated that parametric software for modeling and calculating multiple this solution could greatly resolutions was not yet very advanced. Thus, much of the work duce the roof depth in some locato optimize the form according to the structural behavior and tions, thereby reducing the overall tonthe architectural scheme had to be done by hand from itera- nage of material and creating an interesting tion to iteration. This was a demanding task that required building that would express its structure through its shape. Uncareful collaboration on the part of the team members. fortunately, the architects felt that this solution would give unEarly on Dutton wondered if the skin itself could assist due emphasis to the roof structure and distract attention from with the structural behavior. For example, if the roof were the artistic collection in the galleries below. The solution had to connected directly to the courtyard facades, he wondered if be discreet and “calm,” the architects explained. those existing facades could also serve as the facades of the Still, this iteration proved useful because it indicated that new galleries. Such a possibility would provide stiffness and the depth of the roof could vary according to the stresses support for the new roof and obviate the need for the col- and thus optimize the form with respect to the structural umns. Although this seemed to be a good solution structur- behavior.

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behavior and the architectural scheme had to be done by hand.

At the same time the engineers and the architects also began to discuss whether, because the structure of the roof was intended to be a free-form surface rather than flat, the interior mesh should be a vertical projection of the exterior mesh or whether the form of the interior mesh should be determined by the nodes. Although the final decision was to make the internal surface a vertical projection of the external one, that determination also raised the possibility of using vertical members of varying length to connect the two mesh layers.

At this design stage it became clear that although several aspects

H u g h D u t t o n A s s o c i at e s

Much of the work to optimize the form according to the structural

of the design seemed independent of one another—including the geometry generation method, the glass dimensions, the insulation and conditioning, the shadowing mesh, the iridescent aspects, and the facades—each aspect actually shared connection points with the structure. Thus changes to any of them would affect the structural design as well.

When HDA started to build three-dimensional computer models of the roof, the glass dimensions and the triangular patterns had to be considered more carefully. Although an earlier, two-dimensional plan had considered glass panels that would all have the same projected shape, the free-form surface of the roof meant that the actual glass shapes would differ for each panel. Because that solution seemed quite expensive, the engineers initially sought to reduce the number of variations in the panels and thus reduce the overall cost.

While this effort produced some interesting results and reduced the number of glass panel “families” to roughly 10, the cost savings would have been negligible. Moreover, the engineers determined that the insulating edge joints of the panels would not align but rather would deform slightly, thereby creating significant challenges. As a result, the idea to have similar glass panel families was abandoned, and the

panels were adapted to the geometry of the surface in order to maintain straight joints.

As the structural analysis process proceeded, Dutton suggested that to minimize both the opaque part of the roof and the size of the cladding system, the glass panels should be fixed directly to the steel member profiles using a silicone gasket channel to ensure proper drainage. Because the depth of the glass edge support was at least 15 mm, the support system had to have a minimum width of 30 mm to accommodate two adjacent panels. When the tolerances were included, along with a gap for a silicone bond, the width of the glass support system reached almost 50 mm. Finally, it was necessary to consider the fact that the adjacent panels in the free-form roof feature concave and convex angles. Together, all of these factors resulted in a joint gap of 73.5 mm; thus, two panels would have sufficient edge support but would not clash.

On the basis of its analyses, HDA ultimately proposed a double-lattice space frame truss system made up of steel tubes of circular cross section with fixed diameters but wall thicknesses that varied from 4 to 12 mm according to the stresses. For example, the horizontal tubes were fixed at a diameter of 60 mm while the vertical members were fixed at a diameter of 50 mm in order to minimize the “structural volume” in the space between the two surfaces. The tubes with the greatest wall thicknesses were located at the connections to the columns to accommodate the high compression forces and to avoid the possibility of compression buckling in the members.

The connections between the vertical members and the interior and exterior meshes, as well as between the members forming the mesh structures, were designed as rigid connections, while the diagonal members were pinned at the ends. The global lateral stability would be ensured by the use of four large columns that would be fully moment connected at the base.

During each step of the design, the support geometry was readapted to reach an optimal solution with regard to stiffness and stress in the members. A new, smooth surface had to be created, and the mesh was reworked as well. More than 20 structural models were created during this lengthy refining period. At the same time, important modifications also were being made to the design by the architects. Of critical importance, the glazing pattern was changed from equilateral triangles with an internal angle of 60 degrees to isosceles triangles with a 45 degree angle, thus creating two main orthogonal directions parallel to the surface edges and a diagonal weaving effect.

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The structure now featured areas that were either thicker or thinner in accordance with its behavior.

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© M. Bellini—R. Ricciotti/Musée du Louvre, © 2012 Musée du Louvre/Antoine Mongodin

© M . B e l l i n i — R . R i c c i o t t i / M u s é e d u L o u v r e , © 2 0 1 2 M u s é e d u L o u v r e / P h i l i pp e R u a u l t

the shortest path to connect directly to the The undulating canopy that havior. The roof is enclosed within a courtyard covers the Islamic galleries columns. in which, except when one particular entrance was inspired by the concept This new structural pattern also presented is opened, the wind load on the facades is genof a flying carpet. Varying in the structural team with an opportunity to erally low. When that entrance is opened, a depth, the canopy is thicker make some important optimizations in the tunnel effect can be generated. For the roof, design. In particular, HDA proposed the elim- over the load-bearing columns the calculation was more complex, since it was that support the roof strucination of four of the original eight small colalmost impossible to obtain the aerodynamic umns, meaning that the roof would now be ture and thinner at the edges. coefficients from the local building codes. Insupported on four moment-connected colstead, the engineers considered the highest umns and four doubly pinned columns spaced at 13.4 m in- dynamic coefficients related to a sloped roof under Eurocode 1, tervals along the long edges of the roof. The roof would also developed by the European Committee for Standardization. feature a central span of approximately 20 m with a 4.2 m With the aid of the mean slope for the different zones of the cantilever on its southern edge and an 8 m cantilever on its roof, the pressure distribution was then adapted, and an overnorthern edge. all wind drag effect also was considered.

During this phase Dutton sought to eliminate all of the A similar process was followed for the snow loading, inefficient members from the structure. Two ways of doing which in this case might be more realistic given that the form this were considered. If all of the vertical members could be of the roof and the surface roughness of the paneled mesh oriented to be in constant tension, this would reduce the sec- might produce snow accumulations, and other factors might tions because there would be no instability effects or slen- amplify the loading. The surface was divided into convex and derness limitations. The second approach was to remove concave areas, and for each zone a load pattern was chosen on the vertical diagonal members that were not fully effective. the assumption that on the “hills” no accumulation would be For the HDA team, which now also included the engineers possible whereas for the “valleys” the accumulation might be Pietro Demontis and Pierre Chassange, the architects Car- an amplified load. A linear load was also applied on the edges la Zaccheddu and Mariangela Corsi, and the designer and to take into account the possible accumulation. technical draftsperson Cathy Shortle, this presented a new The snow was then applied following three different challenge. paths: loading all of the hills, loading all of the valleys, and To understand how this hurdle was surmounted, it is nec- loading the entre roof. The snow was then of course comessary to first consider the loads on the roof and the roof’s be- bined with the wind to maximize the bending effects.

At this stage an earlier idea to use pression regardless of what was done the plenum between the two mesh in the model, indicating that the skins as a cushion was abandoned. beam direction could not always be The design adopted an aluminum determined. mesh for the interior layer, and a honTo compensate for this anomaeycomb system would diminish the ly, a complementary parameter was structure’s visibility. The external introduced into the software model mesh was realized using expanded that accounted for small amounts of aluminum meshes. The self-weight tension and small amounts of comof the roof structure became approxipression under certain load cases. Almately 50 kg/m². though the “small compression” and It then became obvious that the the “small tension” values were submultiple load configurations on jective, their inclusion made it posthe roof meant that it would be too sible to determine a nearly optimal complex to optimize the structure geometry. Ultimately, the engineers by trying to keep all of the diagonal were able to eliminate some of the members in constant tension. Likewise, it would be quite diagonals and transform the truss space frame into a hybrid challenging to find the low-stress elements, and any changes structure possessing both truss and Vierendeel sections. Obthat were made might have changed the stiffness of the roof, viously, a force redistribution occurred in the overall system redistributed the forces and stresses in all of the members, causing certain beams that had been in compression to now and possibly affected the deflections. be in tension, and vice versa. The software model was run a In an effort to resolve these concerns, I number of times as the engineers fine-tuned developed a software model that carried out The roof design features a double- the diagonals, increasing the dimensions of lattice system comprising steel calculations for the overall structure under some and removing others. all of the different load cases and automati- tubes of circular cross section supAs a final touch, 20 mm diameter solid cally produced a report indicating which porting a series of glazed panels. rods were used as the tensioned members, beams were in compression and which were The panels are sealed against wa- and 40 mm tubes were used for the comin tension. In this way the engineers could ter ingress and are sandwiched be- pressed elements. A final nonlinear analysis tween an exterior and an interior change selected beams while continuously was performed to simulate the axial buckupdating the structural model. For some layer of metallic mesh, the interior ling of the thin rods and to evaluate the layer forming the gallery ceiling. reason, certain beams remained in comglobal behavior.

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The structure now featured areas that were The Musée du Louvre’s new be inclined, creating what came to be referred wing dedicated to Islamic either thicker or thinner in accordance with its to as the dancing column effect. behavior, and the weight was reduced to less art was constructed within This idea was quickly accepted by the archithan 45 kg/m² for the roof itself. The HDA the Cour Visconti, considered tects, but for the structural design team it meant crew was quite pleased: the carpet was start- one of the Louvre’s most or- starting the design again almost from zero. nate interior courtyards. ing to fly.

As the redesign progressed, the dancOne aspect of the design, however, contining columns and braces were modified sevued to bother both the architects and Dutton: the columns eral times. The designs were adapted to the new stress were too chunky and rigid compared with the rest of the distributions. The structure was made denser in certain structure’s soft geometry. So Dutton proposed a radical mod- areas while the edges were kept as thin as possible. Ulification to the system: make all of the columns pinned, thus timately, the engineers managed to reduce the colreducing their diameters. The lateral stability would be pro- umn sizes to circular tubes with a diameter of 170 mm vided by three braces formed from inverted V-shaped trusses and a wall thickness of 20 mm for the small columns and located outside of the facade. Moreover, the columns would tubes with a diameter of 400 mm and a wall thickness of [56] C i v i l E n g i n e e r i n g j u n e 2 0 1 3

© M. Bellini—R. Ricciotti/Musée du Louvre, © 2012 Musée du Louvre/Antoine Mongodin

Built as a fortress in the 12 th century, the Louvre spent centuries as a royal palace before becoming an art museum in 1793 .

25 mm for the longer supports. The lateral braces became tubes with a diameter of 219 mm and a wall thickness of 25 mm.

The use of inclined columns also led to critical modifications in the behavior of the roof because unequal load impositions were now causing the edges to deform. This effect produced changes in the facade design, which had at various times included double glass panels, simple glass, glass fins, steel mullions, trapezoidal panels with inclined edges, heated glass, and other solutions. In the end, however, a simple laminated glass of low iron content was selected. The panels are supported only at the bottom and top edges, no mullions being used. The bottom edge is pinned, while the top edge is supported horizontally by the roof and is free to slide ver-

tically, accommodating potential movement by the steel structure.

Additional changes to the design were made by the general contractor for the roof and facade structure, Waagner-Biro AG, of Vienna, Austria. This firm reduced the steel tonnage even more by alternating squared and triangular elements and substituting the 60 mm tubes for the diagonal tensioned rods. It also welded the steel tubes without the use of stiffening plates, a technique that lessened the visibility of the structure through the mesh layers and made the entire roof appear weightless. The final steel tonnage, including the columns and all of the steel connections, amounted to approximately 135 metric tons. More than 1,800 glass panels form the roof, and 2,356 mesh panels constitute the internal and the outer skin.

Built as a fortress in the 12th century, the Louvre spent centuries as a royal palace before becoming an art museum in 1793. Today, according to a January 2012 press announcement from the Louvre concerning the goals of the new Islamic galleries, the venue is a universal museum with a mission to break down barriers, cross borders, and foster a dialogue between cultures and civilizations. Clearly, that is the perfect role for a flying carpet. ce Pierluigi Bucci is an independent engineer and a structural consultant for Hugh Dutton Associates, in Paris.

Bucci

Musée du Louvre, Paris Architects: Mario Bellini Architects, srl, Milano, Italy, and Agence Rudy Ricciotti, Bandol, France Structural design consultant: Hugh Dutton Associates, Paris Engineering and design consultant: Nerco, Paris Geotechnical engineering: Arcadis nv, Amsterdam, the Netherlands Mechanical systems: Bérim, Pantin, France General contractor, roof and facade structure: WaagnerBiro AG, Vienna, Austria P r o j e c t C r e d i t s Client:

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