Soaring high over the Quadracci Pavilion, the awe-inspiring Burke Brise Soleil kinetic sculpture looks as if poised to take flight over the western coast of Lake Michigan. However, thanks to some amazing engineering and science, the Brise Soleil sculpture and Windhover Hall stay firmly roosted atop the Milwaukee Art Museum Quadracci Pavilion, the 2001 expansion that added over 142,00 square feet of space.

While the Burke Brise Soleil, designed by Spanish architect and artist Santiago Calatrava, conjures images of sails full of wind, the outstretched wings of birds in flight, and the bow of a ship cutting through a wave, it has a much more functional role in that it is also a sun screen designed to manage the level of sunlight entering the cathedral-like glass ceiling of Windhover Hall which helps to manage the interior temperature.

But how on Earth was it created and what keeps it from flying away or breaking apart? Good old-fashioned science is the answer.

Because of the location of the Quadracci Pavilion- a man-made, low-lying landfill area next to the lake- a lot of work had to be done to create a foundation that would be solid, support the weight of the new structure, and prevent water from seeping into the building where the valuable art would be housed. The Milwaukee-based GRAEF engineering firm took on the project and settled on a concrete mat foundation that was built “like a raft,” which the pavilion rests on. The reason for this is because the land beneath the area was unable to hold such a heavy structure. Since the area was man-made it’s not completely homogeneous and part of it is below the water table of Lake Michigan.

Feet on the Ground

The “raft” is made of dense concrete that is reinforced with a grid of metal bars called rebars. This allows for a wider spread of load sharing and is coupled with foundation columns made of similar materials that rise up to the surface.  Essentially, the building floats on the raft because the parking level is below the water table level and the weight of the building is what keeps it all in place. In fact, they even added some gravel to the top of the mat around the building to add weight and keep it in place, as a “safety factor against buoyancy.”

Partial section of a wing (left) and diagram of load transfer of gravity loads (right). Image Source: Milwaukee Art Museum Structural Case Study.

For portions of the main building above ground, GRAEF used concrete because it was a preferred material of Santiago Calatrava, as it was locally made, and is malleable, making for fine curves. To make the concrete elements of the building itself they used wooden molds and created the buttresses and curves and other portions of the framework that weren’t to be made from steel or glass. These sleek curves hide another purpose: load sharing and distribution down to the foundation mat. To create the uniform color across the entire structure, a lot of research was done on coatings. The concrete mix was specially formulated to flow better by way of a smaller aggregate in it. This, and the application of external vibrations while the cement was poured and settled into the forms, allowed for less bubbles to be dealt with in the finishing process.

Reaching for the Stars

The Burke Brise Soleil’s 217-foot wide wingspan is similar to that of a Boeing 747 jetliner. The wings are made of 72 individual steel fins which vary in length from 26 feet to 105 feet. All told, it weighs a whopping 90 tons. The core of the Brise Soleil is a central spine which is directly attached to the steel frame of the glass ceiling. Connected to that main spine are triangle-shaped plates that hold the rotating spines. Those rotating spines are attached to the wings and to hydraulic actuators. The fins in the wings are connected by spacers which help share the weight load and make the fins act as a single unit. It all gives the illusion that the fins are moving separately, but that actually comes from the fact that they all start at different angles, and then all move 90 degrees during the 3.5 minutes it takes for them to open or close. Though sturdy, it still has a weakness: the wind. There are two ultrasonic sensors on the wings that determine wind speed. When the wind reaches 23MPH for 3 seconds, the wings are automatically closed. This is to prevent structural integrity as well as for safety.

The hydraulic actuators, 11 per wing, were built specifically for the Burke Brise Soleil and push up to a stunning 5,000 psi. These hydraulic cylinders are pretty large, consisting of 230mm (9.0inch) bores with 125mm (4.9inches) rods. The system is equipped with the aforementioned ultrasonic sensors and a lightning sensor that is able to predict if lightning may strike nearby and close the wings. The pump system for the hydraulics is a dual-set of 30-hp motors. For redundancy, one set of pumps can move both wings if needed, albeit at half speed. A host of monitoring takes place at all times as well, including pump stroke range, hydraulic oil temperature, and wing positions to keep them within 0.1 degree for even loading on the building. The wings are held open by electrical hold-up valves, so when power is lost, the valves de-energize and the wings close under their own weight, though the system controls the closing speed.

Additionally, if electrical power is lost in the Milwaukee Art Museum, the system has a backup diesel generator and if that is out of commission there’s a secondary back up generator that is propane-powered.

The Crowning Element

According to Santiago Calatrava, “in the crowning element of the Burke Brise Soleil, the building’s form is at once formal (completing the composition), functional (controlling the level of light), symbolic (opening to welcome visitors), and iconic (creating a memorable image for the Museum and the city).” Not only that, it’s an engineering marvel that complements Milwaukee’s lakefront and is truly iconic in status. In fact, in 2004, it won the Outstanding Structure Award from the International Association for Bridge and Structural Engineering.

It’s a breathtaking experience to see the wings while opened, or to watch them open and close. If you want to see that event, the Burke Brise Soleil opens at 10 a.m. in accordance with regular days of operation, closes/reopens or “flaps” at noon, and closes at 5 p.m. (8 p.m on Fridays).

Written by Christophor Rick for the Milwaukee Area Science Advocates

Further reading:

Nichols, A. (2004) Example Case Study: Milwaukee Art Museum. Retrieved from http://www.solaripedia.com/files/979.pdf

Schneider, Richard (2002, Jun. 30) Hydraulic system keeps museum’s wings out of jeopardy http://www.hydraulicspneumatics.com/200/TechZone/Cylinders/Article/False/6519/TechZone-Cylinders

https://mam.org/info/details/quadracci.php

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