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Spanish Energy Giant Chooses SentryGlas® for Atrium Roof

Safety glass with SentryGlas® high strength interlayerTo provide as much natural daylight as possible, Endesa chose a flat glass roof made possible with SentryGlas®.
Safety glass with SentryGlas® high strength interlayerThe 32-metre-high glass roof is a key to the Endesa headquarters atrium's aesthetics and natural ventilation system.

The new global headquarters of energy giant Endesa in Madrid became a landmark building in terms of the use of dynamic new technologies, energy efficiency and environmental responsibility when it was completed in August 2002.

It is the first major European architectural project to incorporate laminated glass with new DuPont™ SentryGlas® ionoplast interlayer - used for a huge glass atrium roof of 3,000 m2 that is essential to the success of the project's spectacular aesthetics and its central atrium's natural ventilation system.

Endesa is one of the largest private electrical groups in the world, with 22 million customers in 12 countries over three continents. It is the Spanish and Latin American electrical market leader.

The London office of New York-based architectural firm Kohn Pedersen Fox (KPF) won the bid to build a new global HQ for Endesa in 1999 and worked with Madrid firm Rafael de La-Hoz Arquitectos as executive architects on the project. The project was managed by the Gerens Management Group.

Project Architect and Senior Associate Partner at KPF, Cristina Garcia, explained: "Endesa wanted to boldly demonstrate to the world its grasp of dynamic new technologies and energy efficiency as well as its commitment to environmental responsibility. Functionally, the new HQ was required to house the activities of various Group companies, previously scattered across several Madrid locations. For the 1,300 people who work in the building, we were asked to design in interactivity, flexibility, a modular structure, high comfort levels regarding climate control and as much natural daylight as possible.

A contemporary reading of the traditional Spanish patio

"In Spain, we have a long architectural tradition of central patios or courtyards for public and private buildings. Airy, shaded courtyards are Spain's solution to living with very hot summer temperatures that frequently rise above 40 degrees C. Using modern technology, we wanted to take the traditional Spanish concept of a big internal courtyard and transform it into a modern, climate-controlled solution for today's business environment.

"The atrium is thus a key element to the Endesa project and has multiple functions. It provides a space of transition and social interaction, as well as acting as a buffer zone between the external environment and the thermally controlled office space. Vertical circulation in the atrium happens through two main steel and glass lift cores at both ends, connected horizontally through a series of external walkways adjacent to the offices."

"Endesa wanted two things that could have been contradictory: as much natural daylight as possible - calling for a glass roof - and a high degree of energy efficiency, achieved in an environmentally responsible manner. To respond to these requirements, we designed a 32 m high, 3,000 m 2 central atrium located between two six-storey office blocks. The resulting covered space with its floating glass roof is designed to bring natural light into the centre of the building. The atrium and its roof form the cornerstone of the project's natural ventilation system."

"The atrium and its roof form the cornerstone of the project's natural ventilation system."

Flat glass roof concept poses structural challenge

A clear, flat glass 'skin' was required to cover this important central space, making it usable year-round. As Garcia commented: "Glass was the only way to get this big, central space to work aesthetically. Yet the glass roof could obviously only be used if it was totally safe. There could be no risk under any circumstances of panes of glass falling out onto the people in the lobby. Overhead safety would require a glass construction of exceptional strength and rigidity.

"Most glass atria are either domed or double point (triangular), which provides built-in structural support. However, the shape of the available site meant that the roof had to be of a trapezoidal shape (a modified square, where one side is 45 m and the opposite side narrows down to 15 m). The optimal solution, architecturally, was a suspended, flat or 'single pitch' glass roof, which would look clean, simple and modern; the aesthetic effect promised to be spectacular."

"As far as we knew, no-one had ever attempted a flat glass roof of this huge size and weight before."

"As far as we knew, nobody had ever attempted a flat glass roof of this huge size and weight before. Laminated safety glass is always legislated by building codes worldwide for overhead glazing; if a flying object hits a glass roof, the PVB interlayer adheres broken glass together, keeping the structure intact until the roof can be safely repaired. Yet the traditional interlayer for laminated glass, polyvinyl butyral (PVB) would have not have had the strength performance needed for this flat construction; it would have deflected under the accumulated load of the roof." (Laminated glass with PVB is often used for architectural glass in earthquake zones, where its deflection properties are highly valued.)

"By contrast, we had heard that DuPont™ SentryGlas® offers five times the strength and 100 times the rigidity of conventional PVB." The low deflection of the glass construction containing the DuPont ionoplast interlayer allowed the architects to design in the flat roof that was required.

The low deflection of the glass construction containing DuPont™ SentryGlas® allowed KPF to design in the flat roof that was required.

Francesc Arbos Bellapart of Bellapart Engineering, based in Olot (Girona province) in Spain takes up the story: "We were assigned the task of fixing the suspended laminated glass roof to a steel grid, 2.5 in height. After months of working with DuPont specialists in Mechelen (Belgium) and Wilmington (USA) on structural strength studies for laminated glass constructions with SentryGlas® ionoplast interlayer, we were confident in proposing a one-pitch (flat) glass roof structure of the type the architects wanted.

"Our primary concern was safety"

"Of course, our primary concern was safety. The Endesa project is seven floors high and the glass atrium roof is positioned 24 metres above ground level. Its area is 3,000 m 2. The glass panels we selected for the best aesthetics and strength are 2.7 m x 1.35 m; the glass roof has an overall weight of 135 tonnes."

Bellapart agreed with KPF that such a large area and weight in the context of a flat, suspended glass roof would require an interlayer with a step-change improvement in rigidity and strength compared to traditional PVB. His company therefore designed a patented, point fixing system to complement the strong new glass construction. He explained: "Our patented point fixing system for the suspension system involved drilling 12 mm holes in the interlayer and 41 mm holes in the glass. From the beginning, working with samples, we were impressed by the way the ionoplast interlayer withstood the drilling.

"Once installed, the glass roof construction bears a host of additional stresses, including maintenance workers walking on it to clean it, heavy winds and snow loads. If individual panes of glass get broken, our testing showed that the interlayer would remain intact and keep the glass panels adhered to the rest of the roof. The interlayer retains its structural properties even when the roof heats up to more than 45 degrees C in the hottest summer weather."

Bellapart Engineering and DuPont conducted a comprehensive range of strength tests on Endesa's laminated roof structure. These included maximum expected load; impact tests (involving a soft body of 50 kg falling 2.5 m onto the glass construction); post-breakage tests and temperature tests - primarily to satisfy fire regulations - where the glass construction was shown to withstand temperatures of up to 80 degrees C caused by hot smoke for 15 minutes.

Steve Bennison in Glass Laminating Products in Wilmington (USA) said: "DuPont supplied strength modulus and stress-strain information for SentryGlas® that contributed to Bellapart's own testing regarding the design of a low-slope (flat) roof. The enhanced stiffness of the ionoplast interlayer results in lower laminate deflection and greater laminate strength, especially in point-supported structures where bending stress states dominate. Additionally, the increased strength modulus of the ionoplast interlayer over PVB leads to greater post-breakage stiffness of glass constructions containing SentryGlas®. This represents a significant structural advantage over laminates made with PVB, particularly in point-supported structures."

Bellapart used a DuPont Engineering Polymer, Delrin® acetal resin, for the bushes that buffer the fixings from the glass construction. Delrin® was selected for its strength, low creep and dimensional stability.

Bellapart summed up: "The bottom line is that safety has no price; we can confirm that the roof is in a class of its own when it comes to post-breakage stability and stiffness."

"The glass roof is in a class of its own when it comes to post-breakage stability and stiffness."

Managing Director of laminator Rioglass of Rioja in northern Spain, Alberto Puente, said: "Most of our specialty laminated glass to date has been supplied to upmarket car manufacturers like Lotus, Lamborghini or Caterpillar, so we are used to working with demanding customers! The Rioglass Glass XXI architectural division has recently invested in full convection tempering furnaces and special, tailor-made laminating facilities that result in top quality glass lamination. Since this is the first time that SentryGlas® has been used in Europe, we spent several months working in close cooperation with DuPont specialists Bjorn Sanden and Luc Moeyersons (Mechelen) and Steve Bennison (Wilmington) to learn about the autoclave requirements and other aspects of working with the new interlayer."

"Magnificent resistance properties"

As a seasoned professional in the lamination business, Puente was impressed with what he saw during testing: "SentryGlas® has magnificent resistance properties; it is extremely strong. It passed stringent stress and post-breakage tests with ease. Its homogeneity as a laminating material is outstanding. In our view, because of its strength, rigidity and stress resistance values, it is an ideal material for horizontal glazing applications in the new millennium, particularly when laminated tempered glass constructions are used. The 18.28 mm interlayer we supplied consists of two tempered glass panes of 8 mm each, laminated together with a 2.28 mm interlayer of SentryGlas®.

"We conducted a very striking test with Francesc Arbos Bellapart to demonstrate the strength of this particular construction. We took one of the panes of laminated glass with SentryGlas® destined for Endesa's roof and deliberately smashed both tempered glass panes. Then Frances, who weighs 85 kg, climbed onto the glass construction, which continued to bear his weight due to DuPont's ionoplast interlayer even though the glass was smashed - and no glass fallout occurred!"