Remembering the dead and embracing the living are the twin forces driving the architecture of the National September 11 Memorial & Museum. Although designed by different teams and created for different purposes, the Memorial and the Museum overlap physically and metaphorically. For many people visiting Ground Zero, the two projects will fuse together as a single experience ' a continuum of outdoor and enclosed spaces that elicit a range of emotions and interpretations.

The memorial, designed by Michael Arad and Peter Walker, opens this month on the tenth anniversary of the attacks on the World Trade Center. Based on Arad's entry to the 2003 international competition that drew 5,201 submissions, it forms an 8-acre plaza comprising outdoor rooms shaped by granite, bronze, water, and trees. Arad called his entry 'Reflecting Absence' because it preserves the footprints of the Twin Towers as square holes where water cascades into pools that reflect the Lower Manhattan skyline. 'I imagined a pair of voids cut into the surface of the Hudson River,' says the architect of his original idea. 'Instead of an object, I designed a plaza where people could gather.' He recalls going to Washington Square around 2:00 a.m. a couple of days after the attacks and sharing the park silently with others who had come there. 'I realized the important role that public places play in our civic life,' he says. 'They're the glue that binds us together as a society.'

After the competition jury (which included designer Maya Lin, architect Enrique Norten, landscape architect Michael Van Valkenburgh, and artist Martin Puryear) put Arad's design on a short list of eight finalists, it recommended he team up with a landscape architect. So Arad brought in Walker to collaborate on the project. Responding to criticism that the original scheme was too austere, Arad and Walker integrated more greenery into the plan and used trees to reinforce its geometry. 'We envisioned the trees as points on an abacus,' explains Arad. 'When approached from the east or west, you see the trees in rows. But from the north and south, they appear to be placed randomly, as in a forest.' Walker notes, 'Our challenge was to create a park here yet maintain the strength of the plane.'

Daniel Libeskind's master plan for Ground Zero placed the memorial 30 feet below the streets, so parts of the massive slurry walls surrounding the site could be integrated in the design. Arad, however, brought his memorial plaza to street level, wanting to connect it with the rest of the city. Underneath the plaza, though, he inserted galleries that would look through the cascading water into the voids of the missing Twin Towers and display the names of the 2,982 people who lost their lives in the WTC attacks of 2001 and 1993.

Even after the jury selected 'Reflecting Absence' as the winning design in January 2004, Arad and Walker continued to make changes in response to comments from many different groups. The process wasn't always pretty and often involved heated debate, but Arad says he's proud of the result and feels it retains the integrity of his original design.

The biggest change was eliminating the underground galleries, which he says was painful at first but brought the plaques with the names of the victims up to the plaza level. 'Now we have a more seamless sequence of sidewalk, plaza, names, water, and voids,' says Arad. Other changes came in response to various interest groups, such as the disabled, who said people in wheelchairs would have trouble seeing the voids beyond the bronze panels displaying the names. So Arad chamfered the corners of the panels wrapping the voids and cantilevered them above the walkways so wheelchairs could roll underneath. 'These changes made the design better,' states Arad.

Finding the right trees for the plaza proved to be a complex task, because they needed to grow in a tough urban environment in just 6 feet of soil and create a uniform leaf canopy. The designers ended up selecting white oaks, growing them in New Jersey, then transferring them to the memorial plaza.

Just as the memorial navigated a tortuous process of design and redesign, so did the September 11 Museum. Begun as a cultural facility with two mismatched institutions, the Drawing Center and the International Freedom Center, as tenants, the project morphed in concept and design as those organizations dropped out for different reasons. After winning the competition to design the cultural center in 2005, the Norwegian firm Sn'hetta had to shift gears several times as the program and size of the project changed (and shrank) radically. When Arad was forced to abandon his scheme with galleries tucked around voids, the Lower Manhattan Development Corporation needed to find a new way of bringing visitors underground. So Sn'hetta partner Craig Dykers suggested using his building as an entry pavilion to an underground museum that Davis Brody Bond would design. Although it will occupy some of the space that Arad's galleries would have, the museum will not look into the voids, display the names of the dead, nor have the same connection to the memorial.

While the museum isn't scheduled to open until September 11, 2012, the entry pavilion's exterior is mostly done and provides a sense of scale to the memorial. To help emphasize the horizontal nature of the memorial, Dykers and his team tilted their building up to the east so the plaza seems to slide underneath it. Visitors will enter on the east where the building is widest, go through security, get tickets, and then move downstairs to the museum or upstairs to a small auditorium. A private room on the second floor for family members of 9/11 victims will provide views of the memorial and space for contemplation.

Dykers had originally wanted to clad the building with glass prisms, but that strategy proved too expensive. So his team developed a system of stainless steel panels in which some are perforated and some are opaque. Bead-blasted and scratched finishes help catch the changing daylight while providing blurred reflections of people visiting the site. The architects designed the steel-frame pavilion with angled supports that respond to the different structural demands of the varied infrastructure below it. 'The memorial looks to the past and the skyscrapers to the future,' says Dykers. 'We wanted our building to be about the present, the everyday.'

Design Architect:
Handel Architects — Michael Arad and Gary Handel, partners; Amanda Sachs, David Margolis, Robert Jamieson, Cristóbal Canas, and Garrett Brignoli, project team

Architect of Record:
Davis Brody Bond — Steven Davis, Carl Krebs, David Williams, Joseph Grant, Richard Franklin, project team

Landscape Architect:
PWP Landscape Architecture — Peter Walker, Douglas Findlay, David Walker, Matthew Donham, project team

Architect:
Snøhetta — Craig Dykers, partner in charge; Anne Lewison, project manager; Aaron Dorf, project architect

Architect of Record:
Adamson Associates International

Architect:
Davis Brody Bond — Steven Davis, Carl Krebs, David Williams, Mark Wagner, Oliver Sippl