For a 17-story, Class-A office building currently underway in Boston’s Seaport District, builder-developer Skanska USA​, along with Boston-based design firm CBT and a host of engineers, had to think outside of the box.

So, they looked to the ellipse.

The 400,000-square-foot building, which topped out in March, is shaped by an elliptical steel frame that bows out as it ascends, reducing lateral loads and allowing the interior to be relatively light on columns.

The project, 121 Seaport, is just one of several buildings to break ground in Boston’s burgeoning Seaport District in the last few years. The development run was sent into overdrive by the 2004 addition of new convention center in the neighborhood, with more than 7.6 million square feet of residential and commercial space built or on the way. And that’s just what’s being developed as part of developer WS’s Seaport Square master plan for the district’s core.

That massive undertaking has spurred other development in the area, including GE’s new $200 million headquarters, which broke ground there this week on the site of a former candy-making facility.

Using data to drive design

At 121 Seaport, the decision to use an elliptical frame came from two parallel charges. The first, according to Henry Celli, senior project architect at CBT, was Skanska’s goal to build the most energy-efficient structure the team could manage (the project is vying for LEED Platinum certification). The second was less quantifiable and came from the architects, who wanted to break from the typical steel-and-glass rectangular buildings cropping up nearby.

"As we were analyzing the Seaport District — it was our second project in the Seaport — we were noticing how the existing zoning was forcing a lot of projects into a regularized form, and we wanted to distinguish ourselves from that form in some other way," Celli said.

Zoning in the area encourages square or blocky construction that uses up most the site, Celli said, with a relatively low cap on project height due to the nearby Boston Logan International Airport. An added challenge was the MBTA Silver Line train that ran beneath one end of the site, making digging too close challenging and costly.

The design solution was two-fold. The heft of the building was angled on the site as much to mind the placement of the foundation relative to the subway tunnel as to offer a "strong gesture," Celli said, toward the adjacent Seaport Square Green park, both part of the Seaport Square master plan.

The project team derived the elliptical shape from building models generated to determine what form factors would allow them to reduce the risk associated with building near a subway line, limit solar heat gain on the façade, reduce the wind loads and, more generally, deliver a fresh, yet modest, take on office building design in a neighborhood filled with boxes.

Keep it glassy

The elliptical shell offers expansive views of the Boston Harbor and Financial District from the interior — which this reporter witnessed when visiting the site last week. Even with the interiors complete, 10-foot ceilings throughout the space and high-performance vision glass — which accounts for 80% of the façade — will largely maintain the sights.

Those high ceilings are possible in part because the project uses a chilled-beam mechanical system instead of typical HVAC, which Skanska first implemented at 101 Seaport — a similarly sized office project next door — and will use in 121, too. The system circulates water, instead of air, reducing energy consumption and lowering related costs. The project will also use a 40,000-gallon-tank rainwater reclamation and reuse system, which will cut water consumption throughout the project by 30%, Russ DeMartino, vice president of development for Skanska USA Commercial Development, told Construction Dive in an email.

"We used a lot of data to back up the design moves we were making," Celli told journalists during a presentation at Skanska’s Boston office last week. For example, for a 25,000-square-foot floor plate, CBT’s design requires 10% less cladding when the ellipse form factor is used compared to the rectangular one. The design choice also cuts energy usage by 14% thanks to a lower solar-heat gain coefficient, and the lighter load requires 15% less rebar in the building’s core.

Up-down, and up again

An "aggressive" construction timeline, according to DeMartino, required the team to consider alternative construction methods to speed up the build. The team settled on the still-uncommon "up/down" construction process to erect the structure. The approach, through which the substructure and the first part of the superstructure are built roughly in tandem, is particularly useful for large projects in urban areas where conventional foundation work can be challenging.

"If we designed each tower column foundation to support the entire 17 stories of building, each of them would have been 25 feet deeper and it would have increased the cost of the project by $6 million," DeMartino said, noting that using the method shaved six months off the job as compared to a typical bottom-up build.

Here’s how it worked: The project team designed a temporary foundation under each column that could support the dead load of the subterranean parking and the first six floors of the tower. A 7-foot-thick concrete slab was then constructed at the lowest level of excavation (three floors down) to connect with the building columns and spread the load of all 17 stories across the slab. From there, floors seven through 17 were constructed.

"Accordingly, the excavation crews were 'in a race' to install the bottom garage slab before the steel crews erected more than six floors of the tower," DeMartino said. Having Skanska as developer and builder helped manage that process, he added.

Even though the bulk of the building was angled to avoid it, the subway tunnel under part of the site presented a challenge to construction. Excavation walls can move between inches and a foot — which normally isn’t a problem, DeMartino said — except when building near an underground structure as massive and complex as a subway.

The project team used Autodesk’s BIM 360 project collaboration software to manage workflow and a combined Autodesk AutoCAD Civil 3D and Revit model to develop a plan for the site that used the permanent parking floors underground to brace the perimeter walls, nearly eliminating lateral movement of 121 Seaport’s permanent slurry wall.

"Ironically, this method is also much faster than a conventional building construction method, saving many months of schedule," DeMartino said.

BIM wasn’t only used to facilitate design. It also helped keep the project on track. Paul Pedini, vice president of operations for Skanska USA Civil, told journalists during the presentation that the model was also used to manage the schedule and even to see if equipment necessary for construction would fit where it needed to on-site. "[There are] so many uses for the model once you get it," he said.

Greening the area

In addition to the building’s atypical orientation and flared shell, the ground plan was also a critical environmental design factor, in this case by making the area more accessible to pedestrians. The project’s three-story podium is separated from the tower and largely fills the site. Its rounded edges match the form of the rising, rounded corona, while the building’s entry is pulled back from the street corner. A 70-foot-wide outdoor pedestrian promenade on the third level of the podium provides space for retail and greenery and visually separates the base from the tower.

For a part of the city that was once a place for visitors to drop their cars on the way into downtown, supporting the uptick in commercial and residential development with greenery and public space is important. "Ten or 20 years ago, [the area] was mostly parking lots," Celli said. "Essentially, where our building is it was a sea of parking lots. Most people would drive in, park in that area, then walk into downtown."

Looking out from the top of 121 on our tour, it was hard to imagine the sea of parking lots Celli mentions. Next door is 101 Seaport, also by Skanska and home to its Boston offices, and its other completed nearby project, Watermark Seaport Apartments, is also visible. For now, the view from the top of 121 is uninhibited, with wind whipping through the wall-less shell and the curve of the floor plate more apparent from the incomplete interior then it is likely to be for its eventual tenants. During our visit, crews were already working their way up to install the glass curtainwall, apply fire-proofing and begin to prepare the building for tenants.

Working on 121 Seaport with Skanska and CBT was McNamara Salvia as the structure engineers; Haley and Aldrich as the geotechnical engineers; Bala | TMP as the MEP engineers; Nitsch Engineering for the civil work; and Vidaris as façade consultant.