Canadian Consulting Engineer

Atriums the Great Indoors

Enter almost any complex in North America and you will find yourself moving into a large open space. Atriums --or atria, if you prefer the Latin --have become the living heart of most commercial and i...

December 1, 2010  By Bronwen Parsons

Enter almost any complex in North America and you will find yourself moving into a large open space. Atriums –or atria, if you prefer the Latin –have become the living heart of most commercial and institutional buildings. We use atriums primarily as access routes to avoid Canada’s wintry blasts and searing summer heat. But they’re also places where we stop to meet friends, eat lunch, or simply sit and gaze at the great outdoors through their expansive plate glass walls.

As Canada’s indoor answer to the European piazza, the atrium is here to stay. But these indoor spaces can be dreary places sometimes. Tinted glass walls cast a brown, muddy pall over everything. Sound echoes off the hard surfaces, and their huge scale can be overwhelming. Their roofs are often weather-stained and leak, and they can also be huge energy wasters.

Today’s architects and engineers are perfecting and improving on these designs. Structural engineers are helping to make atriums brighter and more transparent. Mechanical engineers are finding ways to use their great volumes of air to make building heating and cooling systems work more efficiently.

Less Structure

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Halcrow Yolles consulting engineers have worked on some extraordinary glass structures around the world. The firm is currently renovating the glass roof of the Devonian Gardens in Calgary, and it is just finishing up replacing the glazing at the Calgary Eaton Centre.

One of their most innovative recent atrium designs, says David Thompson, manager of the firm’s building envelope and structural glass group in Toronto, is at the Delegation of the Ismaili Imamat on Sussex Drive in Ottawa.

“The Aga Khan was trying to create something similar to rock crystal,” Thompson says. “He wanted the glass to be translucent and transparent. We used fritted glass and also a veil of white material on the inside to give diffused light. If you look at the roof from different angles you can see through two different layers of glass and then through to the sky. It’s like a white scrim of sorts. From the outside it has more of an appearance of a quartz crystal.” The architects were Fumihiko Maki and Associates in collaboration with Moriyama & Teshima Architects.

Thompson says the general trend is for architects to ask for two things when it comes to atrium design: “more transparency; less structure.”

One way to achieve greater transparency, Thompson explains, is to strip down the layers supporting the glass. “What we’re trying to do is reduce one level of structure. The way atriums used to be designed was with a steel frame, then with an aluminum frame over the top of that, on which they laid the glazing. Today we’re trying to use just a steel frame. We can create a light and nimble steel frame so that we don’t impede the view through the glass.”

In the Calgary Eaton Centre’s arched roof, for example, the glass is hung from tubular steel arches by stainless steel “spider” fittings. “The structure is almost concealed in the joints of the glass,” Thompson says. “The glass roof appears as one monolithic film. It has probably the least amount of steel you would ever see in a glass roof.”

Structural engineers are also pushing the limits when it comes to the size of glass roofs, says Thompson. The Calgary Eaton Centre skylight measures 230 metres long by 26 metres wide, covering 6,500 square metres (70,000 sq. ft.), making it one of the largest continuous skylights in North America. To deal with the thermal expansion and contraction that occurs in Calgary’s extreme climate, the steel has to be discontinuous in order to flex with differential movement. “The whole thing is designed like a movement joint,” says Thompson.

Across the country, in Vancouver, president C.C. Yao, P. Eng., a principal of Read Jones Christoffersen (RJC), also sees a trend for atrium walls to be more transparent with seemingly lightweight supports. “Basically, with glass atriums, the challenge from designers, especially architects, is that they want them to be as transparent as possible.”

At the same time, Yao says, a glass wall has to withstand wind loads, and especially in Vancouver, it has to be designed to seismic standards in order to survive earthquakes.

To avoid having heavy structural supports, Yao explains, the engineers might use cable systems. “Over the last 15 years you can see a lot of cable-supported backing structures to support the glass.” But these systems need pre-tensioning, and intricate connections, and are challenging to design. As a result, Yao says, “They become quite expensive. You only do these for institutional or iconic structures.”

Yao is particularly proud of the award-winning atrium in Building ‘C’ of the Telus William Farrell complex in downtown Vancouver. The fully glazed atrium sits between two adjacent existing buildings and is around eight storeys high. At one end is a glass wall that is 26 metres high and 15 metres wide — very large.

Yet the Telus atrium is supported by a minimal structure. “We used steel rods to hang the glass,” says Yao, “and used cast steel to make the supports in an aerodynamic shape, which is what the architect wanted to achieve.” The system includes full height elliptical steel columns with custom fabricated arms supporting the glazing walls.

Two lightweight three-dimensional trusses span the 15 metres between the buildings, providing the glass wall with additional wind resistance. Meanwhile, sky bridges traverse the space at the sixth and eighth floors, and a steel stair is cantilevered off a single elliptical column. The architects of the project were Busby Perkins & Will and Musson Cattell Mackey Partnership.

RJC is currently working on a new atrium design for the Surrey Civic Centre. “The challenge here is using very slender wind columns,” says Yao. And on another project, RJC is using pre-stressed carbon fibre cables. “The idea is just like a tennis racket,” says Yao. “You tension the cables until they are very stiff so that they support the glass with a minimal structure.”

Structural engineers are pushing the envelope even further. “Traditionally glass is an inlaid material,” Yao explains. “But the trend is to recognize the structural properties of glass and to use it as a loadbearing element. So why do we need that cable? The glass becomes part of the structure.

“The challenge is obvious: glass is strong, but it is brittle. So there has to be enough redundancy introduced into the system. Engineers use composite materials to increase ductility and they perform analysis to ensure that if one glass component is broken due to vandalism, or whatever, that the loads can be transferred by another element.”

Material Developments and Energy Efficiency

Atriums are very susceptible to outside weather conditions, so much effort is put into finding a glass that is as energy efficient as possible. The glass has to keep heat inside the space during cold days, and must ward off solar heat and glare when the sun shines too brightly.

As Thompson sees it, structural engineers are faced with trying to satisfy two different goals: “Right now we’re blessed that architects want clarity in the glass, and the mechanical consultants want energy performance.” But, he points out, “Those are at two opposite ends of the scale.”

Thankfully, he says, “glass coatings have evolved considerably in the last few years to allow us to get better thermal performance.” Halcrow Yolles designers are using fritted glass and low-e insulated units for thermal efficiency. To achieve better clarity they’re using more low iron glass, which doesn’t have the usual green tinge It has traditionally been reserved for applications like display cases.

Manufacturers are constantly coming up with new products. One new insulated glass unit achieves an insulation value as high as R20 (thermal resistance value of U 0.05) compared to R-4 for standard low-e glazing.

Furthermore, mechanical engineers are finding ways to use atri
ums as an integral part of their HVAC system designs, capitalizing on their large volumes of air in order to save energy and improve the indoor air conditions. Hemisphere Engineering has done so at the Calgary Courts Centre (p. 22).

Condensation and Leaking

What about the problems that plagued older atriums? “Condensation is an issue with skylights,” says Thompson. We control it by working with the mechanical engineers to get air movement that washes across the glass. In the open skylights in the large malls the air is moving across them fairly easily, so condensation is next to nothing.”

The performance of the glass from a U-value or thermal transmittance value also helps to reduce the potential for condensation, says Thompson.

And he believes that today’s pared down glass walls and roofs won’t leak like their older cousins did. “By simplifying the assemblies, we’re also making them easier to maintain. From a design point of view they look a lot more complicated, but truly we are simplifying them.”

In the older aluminum frame systems, the joints were hidden, which meant the systems had to be taken apart for repairs. “It was a costly problem to fix them, other than just adding sealant on the outside,” Thompson says.

But with the new skylights at the Calgary Eaton Centre, for example, all the joints are visible from each side, so it is easier to see what needs replacing. “We’re anticipating that these skylights will outperform any of the older skylights,” Thompson says.

Asked whether using vast expanses of glass in buildings can ever be considered a really good idea, energy-wise, Thompson replies: “It is more of a challenge in an extreme climate like Canada. But why should we be hidden inside and not be able to take advantage of the outside environment from a visual standpoint? It’s just a bit more challenging here, that’s all.”

———

Developments in Glass

The trend driven by advances in glass technology is to longer spans, greater transparency and increased integration with building envelope systems.

Methods of combining glass with plastics, resins, fibre meshes, etc. are helping to produce a more ductile glass structure.

Larger autoclaves are enabling the production of larger laminated glass panels.

Advanced coatings include anti-glare, anti reflective, low-e, self cleaning, and restorative/self healing (the appearance of scratches is reduced over time).

Very complex surfaces are starting to be constructed, with high precision and high optical qualities.

Composite construction methods are being explored, such as glass-steel, glass-concrete and glass-wood assemblies.

Structural adhesives are being explored to increase the transparency of edge connections.

For aesthetic purposes, silk screened images are being used on the glass interlayer; also LED lights are being embedded in the glass inter-layers.

— Source: Brock Schroeder, P. Eng., Read Jones Christoffersen

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