A new 17-storey Boston office building is showing how technology can shape costs alongside the geotechnical and structural engineering.
It is easy to see why the location of the new 121 Seaport office in South Boston has been selected. The 17 storey tower – topped out in March – is part of a 9.3ha development in what is being touted as the city’s new innovation district, a stone’s throw away from the business district and nestled between stunning city and port-side views in each direction.
The scheme is one of Skanska Commercial Development’s earliest US investments and with the firm also acting as main contractor for the scheme, the priority is to create a high quality office space, with a speedy enough programme to ensure occupants are tempted away from competitors springing up around Seaport Square.
121 Seaport 1 rightwayup
The ambitious schedule – work started on site in July 2015 and aims to be finished by the first quarter of next year – is enough of a challenge, but the site has one other significant hurdle. Some 4m below ground level, running across the north west corner of the site, is the Silver Line metro tunnel.
While building tall above this obstacle was going to be complex but technically feasible, gaining permission from relevant authorities, not least the Massachusetts Bay Transportation Authority, would have added time and cost. A new plan was needed.
Engineers from the structural and geotechnical teams have collaborated through a suite of software to come up with an efficient geotechnical and structural design that is so sustainably-biased that the building has ambitions of obtaining LEED platinum certification.
Seaport view from 17th floor 2
The design team, including structural engineer McNamara Salvia and CBT Architects played around with a suite of software from Autodesk using Revit for the superstructure in combination with Autocad Civil 3D, collaborating between them and the construction team via the BIM360 construction management software.
The masterplan set out a concept for a building which was rectangular in plan. The team briefly entertained a new design that simply cut off the corner above the tunnel. But this idea meant reducing valuable floor space. The area’s proximity to the airport means it is subject to Federal Aviation Administration height restrictions so going taller to squeeze in more floor space was also not an option.
Instead, a reworking within Revit helped the design team create an efficient design which is now elliptical in plan. The building is uniform for the first three storeys above the tunnel which incorporate an atrium-style lobby space. Above this, the floor plates begin retreating from the site’s north west corner as it rises, pulling the building back from the area above the tunnel.
The elliptical design not only afforded the same floor space originally hoped for by the developer, but did so with reduced façade and less solar heat gain, along with virtually column-free floor plates to allow for flexible space and wide sweeping views.
“The way the building is shaped and oriented meant we were also able to reduce the wind load,” says CBT Architects project architect Henry Celli. “That gave more construction savings, including 15% less rebar.”
Canadian wind engineering specialist RWDI validated the designs in its wind tunnel testing facility, which put the results within 5% of the team’s models.
“The technology was kind of new to us and we wanted to make sure that [our models] were really correct,” adds Celli.
Below ground, efforts to save time and cost were also gathering pace. “We had to be ready for a marketplace that was rapidly approaching and we had to be first to market,” explains Skanska operations vice president Paul Pedini. “We had to come up with an idea that would get us out of the ground and built faster than the people, literally, across the street.”
The construction methodology was key, and the Skanska construction team opted to eschew a traditional basement excavation, which would have used temporary raker props.
Such props would have accounted for a third of the steel needed for the whole building. “It’s really ridiculous that there’s this such a lot of waste,” exclaims Pedini. “After you use these raker members you have to cut them up and pull them out because they’re all encased in the building; then you have to patch up those holes, which costs millions of dollars.
An “unconventional technique” called “up-down” construction emerged as a viable, value engineered alternative.
This involved installing a retaining wall beneath the site perimeter, with foundation piling beneath each interior column needed for the above ground structure. As excavations progressed for each of the below ground levels, site workers installed the parking garage floor slabs, which acted as bracing for the excavation. “The building is actually working as your temporary structure,” explains Pedini.
The approach also enabled foundations to be shallower, equating to savings of $6M (£4.6M).
Simultaneous steel and concrete work
Meanwhile, steel installation and concreting for the superstructure could be carried out simultaneously. “This way we could build up at the same time as down, and it saved about six months on the schedule, which was critical to the financial success,” he adds.
Showing the client “rudimentary drawings” to illustrate these iterations was “kind of underwhelming”, according to Pedini. So the team joined the Civil 3D model for the underground structure with Revit superstructure model, alongside the Navisworks project schedule, to create a single 3D planning tool.
This allowed nimble design and construction alterations as well as aiding the client side of Skanska to be brought up to speed with each change and what it meant for the programme. It even informed decisions about the types of construction equipment that would suit the job.
“The model is not just there for pretty pictures it’s a useful tool that lasts the entire cycle of the project,” says Pedini.
Skanska is hoping that 121 Seaport serves as something of a trailblazer in its use of technology. It is also working with Autodesk to beta-test BIM360 Project IQ, which analyses building information modelling data from the project to enable a “risk management” approach, creating so-called machine learning on the project. In other words, as more information and data is gathered and understood, the risks can be better predicted.
“The construction industry is probably the worst industry in the world for technology adoption,” says Skanska senior vice president Al Gogolin. “If you let us, we will go out there right now in hard hats, glasses and vests and we will pound nails until we’re blue in the face and it’ll take a job like this decades to be built.”
Given the new skills that projects like this dictate, Gogolin predicts that within five years, Skanska will have hired its first “data mining engineers” to aid this machine learning approach to engineering and risk management. Watch this space.