Unique precast concrete moment framing provides design flexibility and strength.
By Deborah Huso
View a detailed drawing of the precast concrete frame system used in this project (PDF)
Trendy urban dwellers have long been attracted to the idea of living in loft apartments or condos created out of old warehouse buildings. This breed of homeowner appreciates the exposed structural elements as design features as well as the openness of large rooms unobstructed by walls and suffuse in natural light from large windows. With this in mind, Los Angeles developer John Laing Homes decided to re-create that warehouse-style, loft-living experience in a brand-new building in Marina del Rey, Calif., with the construction of Element Lofts
What makes Element Lofts so unique is not that it resembles a re-created warehouse fit for luxury urban living, but that it’s one of the first multifamily structures in California to use precast concrete moment framing. While Element’s signature visible feature is its precast double-T construction – a common feature of parking garages – here that structure creates an exposed concrete form that is at once a beautiful design feature and part of the secret of making this building strong and capable of supporting long spans of unsupported space.
Design flexibility and strength
Director of Urban Development for John Laing Homes, Robert Megna, says one of the main reasons the company selected precast concrete for building Element Lofts was because of the emphasis it places on structure. “This is an industrial area with a new residential component,” he explains. The five-story Element Lofts with its sleek concrete form and visible framing fits its surroundings perfectly. “It’s very rare to build precast residential,” Megna adds, and this is actually the first precast structure on which he has worked.
What Megna likes most about Element’s moment frame structure, however, is its strength. At Element, the concrete columns are joined to precast double-T concrete floor beams via a proprietary connection feature developed by manufacturer Dwydag and project engineers Englekirk Partners Inc. “This allows for long spans between party walls,” notes Megna. “Most units have no walls or columns apart from what’s used to set off bathrooms.” There are uninterrupted spans in Element of as much as 50 to 60 feet.
Lawrence Ho, project engineer for Englekirk Partners and principal in charge of the Element construction, says the Dwydag connectors make for an exceptionally strong system. The connectors are essentially high-strength bars bolting the frame together to absorb the energy from seismic events. Ed Wopschall, president of Hanson Structural Precast, which provided the concrete for Element, says eight structures in California have used this moment frame structure in the last four years to resist earthquake stress. “It’s relatively inexpensive because it eliminates big concrete shear walls,” he explains.
“The system has a lot of ductility, and it’s flexible and tough,” Ho adds. He explains the ductile moment frame of Element, which eliminates shear walls, means the diagonal stress often caused by earthquakes will have no impact on Element. Instead the beams and columns absorb seismic stress. “That’s a big concern in earthquake-prone California,” says Wopschall, because Element has the ability to withstand a large seismic event.
“I designed the first building using ductile connectors in ’94,” says Ho. “It’s a system that’s been well-tested and has performed well in earthquakes before.” Element is, however, the first precast concrete residential structure on which he has worked.
From a design angle, the moment frame structure allows for long, clear spans between exterior beams with no need for load-bearing walls in between. This allows loft owners lots of open floor space and the flexibility to put interior walls where they want them. And as for the double-T floor and ceiling beams being 2 feet thick, they created taller ceilings in the loft, making them 10 feet instead of 8 and establishing an eye-catching architectural look on the interior with coffered ceilings.
The long spans of open space made possible by the moment frame structure also provide for large window openings that reach floor to ceiling, drawing in substantial natural light that is then reflected off the coffered ceilings and polished concrete floors. That means loft owners can take advantage of “daylighting” and cut down on electric usage. “Concrete is a natural insulator, too,” points out Megna, meaning it will help maintain indoor air temperatures and the coolness provided by the units’ exposed ductwork. And with many units built to have window openings on both sides, residents can open them to create cross breezes on mild days.
Another special advantage of the precast concrete construction is the fact that it significantly minimizes sound transmission the way conventional wood framing and sheetrock would. Concrete actually helps reflect sound between units, meaning that residents won’t hear sounds from neighbors above and below. The soundproofing has been enhanced even further with the addition of Enkasonic sound-control matting between floors, which breaks up noise transmission even further.
Precast concrete has advantages with regard to fire control as well, being naturally resistant to flame penetration. This can lower insurance premiums for owners. And as Megna points out, the walls between units will be drywalled, which fulfills code-required ratings for fire-resistant buildings as well as provides a substrate to which loft owners can apply paneling or other decorative features. Wopschall says there is a two-hour fire rating between Element units.
Saving time and materials
Any developer and contractor will appreciate the money that can be saved by speeding up the construction process and minimizing material waste, something John Laing Homes certainly took into consideration when planning Element. “Because the precast concrete is manufactured off site and craned in, it saved us three months,” Megna explains. This also meant Element’s foundation could be prepared simultaneously with the construction of the concrete forms in the factory.
Ho notes that precast concrete construction was ideal for the Element Lofts building site, which lacked a substantial staging area for conventional construction, as it is surrounded by other buildings on three sides. At 70 feet wide and 500 feet long, precast construction allowed crews to begin erecting columns and beams at one end and move toward the other end.
There were no delays resulting from having to wait for good weather to pour concrete since most of the concrete components of Element were made in the factory. Furthermore, the steel reinforcement rods in the concrete will not be as susceptible to corrosion in years to come, because most were installed and sealed in a factory setting.
Megna says from the time John Laing Homes ordered materials – enough precast concrete for a 123,000-square-foot structure – until they were delivered to the site, it took four months. It took an additional two months to install the precast concrete forms, mainly because crews had to pour mat slabs as they went along. Otherwise, the installation time would have been even shorter. Shorter construction time can often also mean shorter financing periods for the developer, cutting significantly into interest costs on construction loans. Wopschall says the concrete frame components were delivered to the building site using standard transport trucking. “Some loads were over 60,000 pounds,” he notes, “which meant we needed special permits.”
Cost savings don’t end with the time and materials saved in construction, however. Wopschall points out that in the long run, owners will save on maintenance costs also, as very little maintenance is required on the building’s concrete façade.
Megna says John Laing Homes did make some voluntary upgrades to Element. On the patios, crews waterproofed joints at transitions between the concrete and in-fill walls and masked the operation with a stucco coating that is the same color and texture as the concrete structure. President and CEO of Hanson Building Products North America Richard Manning says precast is a “new and expanding market in Southern California,” and he expects that market to continue to grow. “Having produced 100,000 square feet of Hanson product for and precast components for other residential and commercial buildings, Hanson is more than prepared to service this growing segment of the precast market.” Hanson Structural Precast has long been providing precast concrete for commercial buildings, sporting complexes, parking facilities and bridges, but this movement into residential is new and, as Element seems to suggest, very promising.
Why element sells
Even though Element Lofts won’t be ready for move-in till next spring, Megna says he has already sold most of the 50 units, which range in size from 900 to 1,600 square feet. Smaller units can be purchased for $630,000, while the penthouses sell for as much as $1.5 million. Megna says he’s been surprised by the diverse demographic of buyers, which ranges from young professionals in their 20s and 30s to empty nesters.
Megna thinks part of the attraction lies in Element’s unique structure. While there are other condos in the neighborhood, they’re all conventionally wood framed. “It’s beautiful, unique and timeless,” he says. “Element is so different that we’ve managed, even in a down market, to sell all but nine units.” Wopschall feels part of the draw is that Element promises low maintenance and lots of space. “Large areas of the units are very much open,” he explains. “It gives the impression of being less confined.”
Ho agrees: “People really like the look of it. It’s a product that stands out by itself.”