Iwan Baan

Last month hundreds gathered to honor Sean Collier at the official unveiling of a memorial, located at the corner of Vassar and Main streets on the campus of the Massachusetts Institute of Technology (MIT), in Cambridge, Mass. The crowd was surrounded by architectural landmarks such as the Ray and Maria Stata Center by Frank Gehry, FAIA, and the Brain and Cognitive Sciences Complex by Charles Correa, Hon. FAIA, but the relatively humble stone monument and, moreover, the life of the young man it celebrated, was the main focus.

Sean Collier was the MIT police officer killed on April 18, 2013, three days after the Boston Marathon bombing. After his death, mourners from the campus, the city, and beyond created a makeshift memorial at this corner, where he was shot in his squad car.

Two years later, these communities rejoined for the dedication of a permanent memorial—a project that defied conventional boundaries in design, engineering, and construction. The crowd, which included Collier’s family and members of the local police forces, quietly explored the structure, a curvilinear vault made from 190 tons of granite.

Perhaps even more staggering than the memorial’s mass are its voids. The expansive openings and passageways created by suspending the granite masonry in compression form five half-arches, or outstretched fingers, that meet at a central vault, or palm. At the dedication ceremony, MIT’s president Rafael Reif said the memorial symbolized a community coming together after tragedy: “We are held together by invisible forces too.”

Collier’s death hit the MIT community hard. Though he had worked on campus for only 15 months, he had already many connections. “Sean never met a stranger,” says MIT police sergeant Cheryl Vossmer. At 27, Collier was not much older than many students. He was a regular at campus pubs, took a dance class at the school, and joined the institute’s Outing Club, becoming an avid winter hiker in the months before his death. “Sean was really nerdy,” says Sara Ferry, a graduate student in nuclear science and engineering. “That was one of the reasons he fit in with MIT students.”

The institute wanted to create a permanent memorial using its own funding combined with private donations. MIT provost Martin Schmidt and director of facilities operations and security John DiFava assembled a committee of students, faculty members, and police officers, which included Vossmer, Ferry, and architectural history professor Mark Jarzombek.

A call for ideas in June 2013 garnered more than 100 responses from the public, but holding a formal design competition seemed inappropriate. “We didn’t want this to be someone’s amazing personal win,” Ferry says. Instead, the committee approached J. Meejin Yoon, AIA, head of MIT’s architecture department. “A lot of people felt that … the designer [needed to] be someone who understands MIT,” Schmidt says.

Beyond architecture, Yoon, also a principal at Boston–based Höweler + Yoon Architecture, is known for mesmerizing light and sound installations, such as White Noise/White Light, an interactive field of fiber optic lights and sound that was displayed at MIT and the 2004 Olympics in Athens. Her work had what Ferry calls “that cool factor”—perfect to preserve the memory of a young and active man.

Designing a memorial can be fraught with contention, but this project was intimate, the product of a tight-knit campus community honoring one of its own. “It was a very emotionally wrought project,” Yoon says. “I don't think we got through a committee meeting without tears.”

Initially, the project had no budget, site, or program—only the public’s ideas and some important themes: Collier’s love of hiking, his patriotism and service, and his appreciation for MIT culture. “I was anxious that the memorial could become a collection of many things,” Yoon says.

So she sought to capture several concepts into one form. She presented five sketches to the committee, the final of which was an abstracted hand that drew from MIT’s motto, “Mens et Manus,” Latin for “mind and hand.” Five radial stone walls would form the hand’s fingers, connected by a palm, or vault; the open hand would represent Collier’s spirit of helpfulness, and the space under the vault represented his absence. His name and date of death would be inscribed on one face, while the solidness of the granite evoked the motto “Collier Strong,” a variation on “Boston Strong” used by family members who ran the 2014 Boston Marathon in his memory.

The memorial’s form—a single structure assembled from many pieces—resonated with the committee. “We’re all pieces of a puzzle,” Vossmer says. “We all fit into this community.”

Höweler + Yoon ArchitectureSite plan. The design retains a sightline to the spot where Officer Collier was killed in his parked police cruiser (indicated by the dotted line).

True Masonry

Yoon didn’t want to the structure to simply look like a vault. “I didn't want it to be concrete clad in stone,” she says. “I wanted this to work like a [masonry] vault, which means every stone is helping to support every other stone.” The memorial was built by bringing together a construction method that recalls the Middle Ages, but with contemporary design and fabrication tools.

Each spoke of the structure functions as a half-arch that buttresses the shallow vault at the center. The curved apertures carved into each radial wall create passageways through the heart of the structure as well specific sightlines, including a window from the campus to where Collier’s police cruiser was parked to the south.

Though the design follows basic principles of masonry construction, its complexity required a specialized team of engineers, consultants, installers, and contractors. Yoon enlisted structural engineering firm Knippers Helbig, in New York, and masonry-vault expert John Ochsendorf, a building technology and civil engineering professor at MIT and a partner at the structural engineering firm Ochsendorf DeJong & Block (ODB).

The beauty of an arch lies in its ability to support loads in pure compression, for which masonry is incredibly strong. Yoon wanted the memorial’s vault ceiling to be as flat as possible, which would generate intense lateral forces, or thrust, through the half-arches and, ultimately, the support walls. Ochsendorf and his team developed software that allowed Yoon to map the forces in the stones and see the effect of changing the walls’ shapes and configurations on the structure’s overall stability. The program also helped determine how to parse the granite walls into individual masonry blocks while still maintaining equilibrium in the five-spoke structure. As a result, joints between blocks are perpendicular to the lines of thrust to ensure stability.

Yoon and her team 3D-printed 1:50 scaled models of 30 different iterations of the structure to explore design ideas from the software analysis; ODB also used a few models to test how the structure behaved. Together, the teams found a configuration in which the entire memorial would stand in compression, which would theoretically eliminate tensile forces in the structure and thus the need for masonry reinforcement.

However, when Knippers Helbig tested how the structure would perform under seismic loads, temperature shifts, and foundation settlement, they recommended placing steel pins between several granite blocks. “The shallow arch geometry is very sensitive to any changes at its supports,” says associate Tom Reiner. The pins would keep the stones in the center of the vault aligned in case of movement and provide a ductile counterpoint to the brittle stone. “If the pins happen to become overloaded, they are designed to yield in a controlled manner before the stone fractures, allowing the structure to find a new equilibrium shape,” he says.

The addition of pins didn’t sit well with Ochsendorf. “My own desire for the project was that it would have the purity and simplicity of the stones only,” he says. In the end, however, his team agreed to use 58 stainless-steel pins, 1.5 to 2 inches in diameter and up to a foot long, throughout the structure. Grout also fills the ¼-inch joint between blocks.

Yoon chose domestic granite to signify Collier’s patriotism. The designers, along with some committee members, hand-picked 32 raw blocks from a quarry in Virginia to correspond to each stone in the computer model. The blocks were shipped to the Quarra Stone Co. in Madison, Wis., and fabricated with a combination of manual labor and robotic saws to their specific profile within a 0.5-millimeter tolerance. Some blocks required seven days of nonstop sawing to carve. The blocks were then trucked in groups to Cambridge.

"Live Long"
The keystone is typically the final piece of an arch or vault construction. But for the Collier Memorial, the keystone was positioned in place first, back in January. A crane suspended the 12,000-pound, 7-foot-by-6.5-foot block of granite 11 feet in the air, and seven pairs of hands guided it into place onto a scaffold.

By mid-February, the notched, five-sided keystone was ringed by five center stones on the scaffold. In this project, the center stones were the hardest to place, with up to eight faces to match, rather than two or three for the wall stones. “We developed this concept of coming from the inside and working our way out,” Reiner says.

Once workers lowered the scaffolding temporarily supporting the keystone, the masonry vault was allowed to find its own structural equilibrium. Ochsendorf and his team predicted that the deflection at the keystone would be between 5 millimeters—if fabrication and construction were nothing short of perfect—and 16 millimeters. The measured deflection was 6 millimeters.

Construction was slowed by Boston’s record-breaking snowfalls last winter. After each storm, workers had to clear snow from the walls' concrete foundation, which is supported by piles driven 30 feet below ground.

Presiding over all of it was Rob Rogers, the project manager for the general contractor, Suffolk Construction. Rogers had more than a professional tie to the project: Sean Collier was his stepbrother. During construction, he had to spend nearly every day in view of where his brother was murdered. “I know why I’m here, but I’m also treating it like a normal job,” he said, in February.

The true test of the structure came on March 31, when the scaffold supporting the center stones was lowered, millimeter by millimeter, allowing the structure to stand on its own. Afterward, the curvilinear stones at the base of the structure were set in place, giving the space under the vault its ovoid form. At a lecture preceding the April 29 dedication ceremony, Rogers said that among all his experiences in the past year, from running the Boston Marathon to meeting President Barack Obama, “setting that last stone and seeing everyone’s faces tops them all.”

Aside from the structure itself, the memorial contains many elements that honor Collier. To deter skateboarders from riding on the memorial’s smooth curves, 61 sets of 0.9-inch-diameter steel studs forming Collier’s badge number, 179, in Braille are embedded where the seven concave surfaces meet the plaza. Carved into one wall is a quote from Rogers’ eulogy for his brother, two years ago: “Live long like he would. Big hearts, big smiles, big service, all love.” Rogers had intended to say, “Live life like he would,” but Ferry, who suggested the engraving, thought the stumble evoked something even more poignant: a long life, cut short.

At night, in-ground lights mapped to the positions of stars on that tragic night in April 2013, illuminate the structure. It harks back to Yoon’s love of lighting and adds a temporal quality to the design. While the project is both expressive and emotional, it is the attention to these details that makes the Collier Memorial a part of MIT.


Project: Officer Sean Collier Memorial, Massachusetts Institute of Technology, Cambridge, Mass.
Client: Massachusetts Institute of Technology
Design Architect: J. Meejin Yoon, AIA
Architect of Record: Höweler + Yoon Architecture - Eric Höweler, Yoonhee Cho, Paul Cattaneo, Sungwoo Jang
Specialty Masonry Consultant: Ochsendorf DeJong and Block Consulting Engineers - John Ochsendorf, Philippe Block, Matthew DeJong
Structural Engineer: Knippers Helbig- Advanced Engineering - Thorsten Helbig, Matthias Oppe, Tom Reiner, Florian Meier, Hauke Jungjohann
Landscape Architect: Richard Burck Associates - Skip Burck, Robyn Reed
Civil Engineer: Nitsch Engineering - David Conway, Michelle DiBenedetto
Geotechnical Engineer: McPhail Associates - Ambrose J. Donovan, Brendan O’Neil
Lighting Designer: Horton Lees Brogden Lighting Design - Carrie Hawley, Barrett Newell
Electrical Engineer: AHA Consulting Engineers - Mikus Veners
Typographic Consultant: Francesca Bolognini
Research Assistants: William Plunkett, Corentin Fivet, Euipoom Estelle Yoon, Anna Kaertner, Cecile Lu, Marianna Gonzalez
Construction Manager: Suffolk Construction
Stone Fabricator: Quarra Stone Company
Granite Erector: Phoenix Bay State Construction Company
Granite Supplier (Quarry): Virginia Mist Group
Landscape Contractor: ValleyCrest Landscape Development
Granite and Field Surveyor: Feldman Land Surveyors
Geotechnical Construction Company: Hayward Baker
Concrete Placement: G&C Concrete Construction
Paver Supplier: Hanover Architectural Products
Lighting: Lumenpulse (moon lighting), Inter-Lux (recessed in-grade marker)
Masonry and Stone: RED Graniti - Steve Rousseau
Electrical Contractor: Gaston Electrical
Construction Labor: Liberty Construction
Site Development Contractor: James W. Flett.(excavation work)
Size in Square Feet: 6,500 square feet
Cost: Withheld