Setting sail in the Champs Elysées

Solar Architecture & Energy Efficient Glazing Solutions

Setting sail in the Champs Elysées HUGH DUTTON Hugh Dutton Associates

Surfacade The Publicis headquarters are situated in one of the most visible addresses in the world, at the top of the Champs Elysées, and neighbours the Arc de Triomphe. The company’s recent expansions have made it one of the largest advertising agencies in France and the chairman felt it appropriate to transform the headquarters from the ageing and dated 70’s office building into something more in fitting with the company’s image and unique location.

The company chose Los Angeles based architect Michele Saée’s response to their brief to upgrade the ‘Drugstore’ a café occupying the ground floor of the building. His proposal involved not only a restructuring of the café, but also a manifestation of the Drugstore transformation on the facades of the whole building. His design involved 74

adding a series of wavy glass screens in front of the existing mirror glass curtainwall with a tall sculptural corner piece on the main entrance corner that faces the Arc de Triomphe.

Freedom of Form HDA’s technical design began with a computer graphic model image supplied by the architect Michele Saee. Contemporary computer graphics give a remarkable liberty of form and representation. Materials can be just alluded to, light can be manipulated, forms can be completely free, surfaces can be stretched, and distorted at will. The modern film industry’s reliance on the computer for the image is powerful evidence of this. The freedom that the computer gives is however deceptive. The same liberty cannot easily be found in traditional construction materials.

The challenge for the facades was to realise this fluidity and freedom of form in glass and steel, which by nature, are naturally fragile and rigid.

The design team had difficulty in finding a name for the glass screens. In the end the word ‘Voiles’, the French word for sails, was chosen. The term is evocative of flat curves, wind, and movement.

Such images were considered appropriate.

Planning constraints All facade components must remain within strict legal limits as determined in the building regulations: 1 meter in front of the building property limit on the Champs Elysées and 600mm on the Pressbourg and Vernet Facades.

Anchorage to the existing building The existing building, built after a fire destroyed its Hausmannian predecessor in 1974, was intelligent glass solutions

Solar Architecture & Energy Efficient Glazing Solutions

Concept design image by Michele Saee

evidently not designed to incorporate the ‘voiles’. The entire system is fixed to the existing columns, without touching the existing glass and aluminum curtain wall. The attachments are generally located therefore at floor slab levels on the vertical columns, the columns resolving vertical loads and the floors the horizontal ones.

All work had to be done without interrupting the use of the offices and Publicis’s headquarters above the drugstore which remained occupied during the construction. For example, drilling the concrete columns had to be done in specific time slots in the morning and evening. Busy traffic on the Champs Elysées imposed time constraints for deliveries and hoisting from the street. Most deliveries were done at night and the major piece, the corner spiral, was delivered and lifted into place between 11pm and 4am. intelligent glass solutions

Geometry The glass was supplied from Spain, and the steel from Germany. The surface where the two meet had to be very carefully defined. Not only for practical constructional reasons, but also for contractual and responsibility reasons. One could not be left in a situation where different people were left arguing about who’s geometry was right! One clearly identifiable surface had to be rigorously defined: The interface between the glass and the steel profile was therefore described according to buildable geometries.

The glass surface is inherently flat, but can be heat bent to simple cylindrical and conical shapes comparable to a sheet of paper. A steel cage mould is made and the glass is heated and gravity bent to fit the mould form. Saee’s drawings were therefore reconstructed in

“The freedom that the computer gives is however deceptive. The same liberty cannot easily be found in traditional construction materials”

75

Solar Architecture & Energy Efficient Glazing Solutions

AutoCad to a series of cylinders, cones and flat panes and described in the contract drawings. At the outset, the client was sceptical of the economic viability of the proposal for up to 700m2 of complex curved glass and commissioned HDA for a feasibility study to advise on the constructability and cost. The following ‘catalogue’ of glass panels shown in the image defines the geometry of each panel, according to only six specific curve radii and overall dimensions that could fit in the glass curving ovens. The catalogue was analysed structurally and priced. The project became tangible with this information and the go-ahead was given.

The contractor was required to build a complete geometry formwork jig in his factory to check that both glass and steel components respected this theoretical geometry to within a tolerance of 5mm that could be tolerated by the silicone joint.

Primary structure steel frame

Analysis and Testing The Autocad geometry model was converted to a structural analysis model, where each member is given the appropriate mechanical properties. Steel profiles were dimensioned with this model. Circular section tubes were used as their geometry was the most compatible with the complex curves because the section remains consistent irrespective of the angle to which it is twisted. The analysis model provided specific loads to the existing building that were verified for compatibility.

Given the non-traditional nature of the glass fixing system, a specific campaign of tests was carried out. Together with Apave checking engineers and the CSTB, the statutory approvals authority, the worst case glass panels were identified according to criteria of geometrical complexity and maximum spans. Five panels were therefore identified for testing. These panels were duplicated and fully tested for structural performance, including the warping and distortions of the supporting steel frame.

Wind Loading Recent wind storms in France, and notably at the beginning of 2000 raised concerns about the capacity of the voile system. This was a particular concern for Publicis, and notably because the existing building was not designed to support them. Wind turbulence and aerodynamic effects were therefore tested in a wind tunnel, with specific simulation of the voids between the new intelligent glass solutions

“All work had to be done without interrupting Publicis’s headquarters which remained occupied during the construction”

voiles and the existing curtainwall, as well as the complex curves of the corner spiral.

Structural and Glass Support System 168mm diameter curved steel tubes span between double ‘console’ supports bolted to the existing concrete columns. Extruded stainless steel glazing profiles, called ‘sabres’ span vertically between the tubes, with long cantilevers at the top edges. A highly resistant Duplex steel grade was used to minimise the amount of material used. It is 4 times more resistant than aluminium that is conventionally used to support glass in 77

Solar Architecture & Energy Efficient Glazing Solutions

Typical detail of the glass fixing to the steel sabres

facades therefore with a quarter of the material that would normally be required. Variable shaped ‘fins’ connect the ‘sabres’ to the tubes using from adjustable assemblies. The glass is bonded to aluminium extrusions that are in turn clipped to the ‘sabres’.

Glass: Transparency vs Reflectivity The original computer generated image provided by Michele Saee shows the glass as a magical ephemeral surface, at the same time present and not present (according to Sydney Opera House designer Jorn Utzon from Intelligent Glass Solutions iss.1.2003 when speaking of his love for glass, “It’s there and it’s not there”). We want to see through it with a minimum of obstruction if we are an occupant in the building so that the brightness of the office space is not compromised. But from outside we want the variation of transparency and reflectivity.

All the glass for the ‘voiles’ is a special ‘low iron’ variety, with 98% light transmission specification.

The light penetrating the offices is therefore at a maximum with as little colour change as 78

possible. As the angle at which the glass is viewed changes, moving from frontal to oblique, the transparency diminishes, being replaced by reflection. Also, as the relative light intensity changes, the image of the glass is transformed.

The more light outside, the more reflectivity - the more light inside, the more transparency. So at night, the glass disappears. This was desired because at the outset, the designers and the client were trying to minimise the impact of the screens, wary of the impact of their newness and the audacity of the design. The question at the

time, was would they be accepted? However now after several years since the completion of the project in 2004, during which time the newness has been digested, the client felt the impact was in fact too subtle and the nocturnal disappearance was a lost opportunity. A new lighting study proposed the addition of a mist of dots applied to the glass that is lit at night by LED strip lights concealed behind the structural tubes.

The new lighting proposal gives a nocturnal presence to the screens that recalls the initial ephemeral image proposed by Saee.

Credits Architect: Building Inc. Los Angeles Michele Saee, Rudabeh Pakravan, Brant Gordon

Control Engineers: APAVE (Marc Sassot)

Facade Engineering Design: Hugh Dutton Associès Paris Hugh Dutton, Frederic Bindji, Alberto Rubin-Pedrazzo, Carla Zaccheddu, Nicolas Sterling, Romain Stieltjes

Curved Glass supplier: Cricursa (Spain)

Contractor : GARTNER

Extruded Stainless steel supplier: Hoechst (Germany) Computer Aided Shop drawings : Elsner (Germany)

intelligent glass solutions