With its all-precast hybrid moment frame system, a Sacramento mixed-use building combines form and function with seismic stability.
By Deborah R. Huso – Photos courtesy of Clark Pacific
In a city that has made history with its earthquakes, seismic stability is one of the most important features of new construction. When City of Sacramento officials initiated revitalization of a downtown infill lot that had been vacant for 15 years, they wanted a structure that would not only withstand earthquakes but would also be an architectural gem given its prominent location.
“It was a conspicuous vacant site in a key location adjacent to City Hall and a city park,” explains LPA Sacramento’s design architect Curtis Owyang. “The city wanted a project that would be a catalyst for redevelopment.” The result was the design of the 800 J Street Lofts, an eight-story, 340,000-sqaure-foot, mixed-use project with a parking garage, street-level retail space and residential apartments.
“The building was originally designed for a cast-in-place system,” explains Culp and Tanner project engineer Mick Wilson. “But when the client looked at the cost and versatility of precast, the architect reworked the plans.”
Owyang says the three key reasons for choosing precast were the speed of erection, lower cost, and the ability of the structure and architectural finish of the building to be one and the same. “We looked at every possible system from steel frame to wood,” he adds.
The $47 million J Street Lofts were not to be a conventional precast structure, however. They were, in fact, the first residential building in Sacramento to make use of an all-precast concrete hybrid moment frame system more typical of large commercial buildings.
A framing system that self-rights
The all-precast hybrid moment frame system manufactured by Clark Pacific for the J Street Lofts has a special post-tensioned, self-righting mechanism that protects the building’s structural frame from damage in the event of an earthquake. Wilson explains that the standard reinforcing steel and high-strength, post-tensioned cables in both the columns and beams allow more prestressing than with traditional precast. “The clamping action compresses the rebar when the joints open up,” he says.
In more traditional construction, beams, columns and floors provide the structural frame with an exterior façade attached. Since the hybrid moment frame is both frame and façade, the whole building system absorbs energy when the joints move.
“One of the main advantages of this system is that the building has beams and columns instead of walls,” Wilson adds. “The main structural system would be
intact in the event of an earthquake.”
Most structures designed to withstand earthquakes disperse energy by yielding to the seismic stress. While such a structure doesn’t collapse, it will often lean. “In cast-in-place, the building stops where it stops, which isn’t necessarily vertical,” says Don Clark, president of business development for Clark Pacific.
The hybrid moment frame of the J Street Lofts, however, has an elastic, unbonded post-tensioning system that lets the building yield but then come back to its original position. The system essentially acts like a rubber band, drawing the structure back into place in a seismic event and ideally allowing for immediate reoccupancy.
First developed in the late 1990s, the hybrid moment frame system has undergone repeated testing by the National Institute of Standards and Technology (NIST) and the National Science Foundation (NSF) and has exhibited the potential for zero residual drift. The J Street Lofts has been built to withstand the pressures associated with being in a level three seismic zone.
In the precast hybrid moment frame systems (also known as an architecturally clad structural precast system), structure and cladding are one and the same, saving substantially on construction time. Clark Pacific says the system allows a building to be erected with the same speed as steel framing while offering one of the strongest performing lateral resistance systems available. And there’s the added benefit of the weatherability of concrete.
It took three months to erect the J Street Lofts once Clark Pacific delivered the preset beams and columns. Once on site, they were locked together with steel cables, saving the client time and allowing the building to be leased sooner.
Form follows function
Another key feature of the all-precast hybrid moment frame system is its design flexibility. The structural columns and beams are part of the architecture of the building, serving as the building’s exterior finish system as well as its frame.
The framing system also allowed for extensive use of glass in the building’s residential units. The apartments have floor-to-ceiling windows, many of them consisting of up to 30 feet of glass, to highlight views and bring in natural light. The windows feature both translucent and tinted glass, mimicking the varied architectural features of the building’s façade.
“The glass was designed to work with the hybrid moment frame,” explains Owyang. The location of the glass varies with respect to the frame. In some places, the glass protrudes from the frame; in others, it is flush; and in some instances, the glass is behind the frame. This keeps the long window spans on the building from becoming
monotonous and helps the structure as a whole blend into adjacent city architecture.
“The façade has a lot of architectural play,” Owyang adds. “We also have different levels of sandblasting on the concrete to add detail and help the building relate to its surroundings.” The J Street Lofts’ architectural features also made for a complex job for the precaster. “Normally, one of the major advantages of working with precast is the repetition,” Wilson points out. In the case of the J Street Lofts, however, there were more than 1,000 different precast elements used out of nearly 1,600 panels. According to Wilson, the heaviest beams weighed more than 28,000 pounds, and the heaviest columns topped out at more than 50,000 pounds. Clark says it took about three months to manufacture the beams and columns in the factory. He estimates that it saved the client up to 10 percent on material cost plus weeks of construction time.
The dense columns and beams allow for thermal mass, which increases the structure’s energy efficiency. There is less heat penetration into the building in summer and greater retention of warmth within the structure in winter.
The double-tee floor system of the structure also allows for extensive open space in the residential units, creating a loft feel that mimics converted warehouse buildings. Wilson says that despite the varying shapes and sizes of the building’s beams and columns, the biggest challenge, in his opinion, was the floor support system, which consists of 820 double tees. “It was tough addressing the camber in the tees,” he explains, “and trying to keep them level enough for a residential building.” The poured concrete floors also serve as a sound and vibration barrier between floors and fire retardant between units.
The varying colors and textures of the J Street Lofts exterior finish aren’t all that’s eye-catching about the building. Owyang points out that a commissioned artist developed the handrails for the residential units’ balconies, where scrolling steel provides a whimsical accent to the structure. The artist also developed steel medallions to cover areas where post-tensioning occurs on the structure’s interior.
To date, the structure is nearly 100 percent occupied. “It’s a unique design,” Clark says, “and it was the first time an all-precast building had been built in Sacramento.”
Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.