By Kristin Dispenza

When developers set out to build the BTV Crown Corporate Center – a speculative office building in Sacramento, Calif. – they had three challenges before them: their building had to be economical to build, meet California’s stringent seismic design requirements and be aesthetically pleasing in order to attract tenants in a competitive marketplace.

SEISMIC REQUIREMENTS

For project designers LPAS Architecture + Design, the biggest priority was to choose a structural system that could perform well under seismic loading and meet the region’s strict building code. All buildings designed for seismic loading have, first and foremost, a need for ductility. Ductility, as it pertains to seismic design, refers to the ability of structural members to maintain load carrying capacity despite large deformations. In this way, energy is dissipated and structural strength and integrity are maintained.

“In this area, tilt-up concrete would have been nearly an automatic choice for a one-, two- or three-story structure,” said project engineer Rory Rottschalk of Culp & Tanner Structural Engineers. “But because this building is four stories, steel framing, as well as other concrete construction methods, were considered.”

Mitch Bloomquist, Managing Director for the Tilt-Up Concrete Association, agreed with Rottschalk, stating, “While tilt-up concrete construction is certainly not new to California, it is not employed in multistory applications there as often as it is in other parts of the country.” California’s seismic design requirements are often cited as the reasoning for this; however, Bloomquist believes, “Projects like BTV will raise awareness of the building system’s potential and stellar performance in this region.”

Ultimately the team designed a system based upon a tilt-up ductile moment frame, which offered several advantages. The incorporation of moment frame code design provisions increased the R factor to nearly 50 percent more than that of pierced wall tilt-up construction (the larger the R factor the less the design force level is required to be). A by-product of using moment frame code requirements for beam and column proportioning was that large openings in the panels were required, which in turn allowed for large expanses of glass and increased the void to solid ratio, conferring an aesthetic benefit.

BEAUTY COUNTS IN A COMPETITIVE REAL ESTATE ENVIRONMENT

Improved aesthetics were particularly important for this project, because the team faced some preconceptions about tilt-up construction.

“There was a bit of a perception, regionally, that tilt-up construction results in a punched-opening box,” said Curtis Owyang, Design Principal, LPAS Architecture + Design. “But with the large openings in the moment frames, we were able to vary the window sizes and patterns, making some areas appear like glass curtain walls spanning multiple floors. This resulted in a higher-end look that sets the BTV building apart from other buildings in this market.”

The glass was also set back from the wall’s exterior surface, helping to reduce the visual flatness and create depth and shadow on the façade. Another advantage of having more window area was that it brought more light and views to the interior, improving the quality of space inside the building.

Using tilt-up construction allowed the architects to vary the massing of the structure, eliminating the ‘big-box’ effect and visually reducing the building’s scale. Panels offset from one another and treated with different window patterns create a layering of the façade, adding to the visual complexity of the design. Various textures and reveals, imparted using form liners, also add to the building’s visual appeal. Entries are two-story glass volumes that provide an inviting space while contrasting with the mass of the tilt-up panels.

THE RIGHT PRICE

According to Rottschalk, the high performance lateral system reduced the building’s lateral loads by 40 percent. The system also reduced the overturning loads, so foundations could be reduced in size and reinforcement level. This represents a considerable cost savings, making tilt-up construction a very economical choice overall.

An additional benefit of the tilt-up panels was that their mass and weight helped the acoustics of the building – an important consideration since the site is located at the intersection of two major freeways, I-5 and I-80.

A PRACTICAL PROCESS

The moment frame was tilted up using the same process as is used with any panel system and glass was attached in the same way as it would be with any other tilt-up. The overall pick weight was within normal range, even though the building was 4 stories tall. The frame’s columns were 14 inches x 36 inches wide and beams were 14 inches by 48 inches.

The fact that this building was the first of its kind is significant, pointed out Rottschalk, because there was a learning curve for the contractors. “Educating contractors on a new method can be time consuming and costly. But this project was design-build, which allowed the contractor to validate the design ideas from step one. Together, the team members decided such details as where to position lift points.” The design-build process also provided real-time cost information.

“We enjoy the collaboration,” said Owyang. “Because we are working with the actual builders of the design, they can help us achieve the desired look.”

Members of the construction team agreed. “From the contractor’s standpoint the design-build approach was a key component allowing us to pull together the subcontracting teams early in the process. The teaming and collaboration that developed during the design stage truly led to an increased efficiency, not only in cost and schedule, but also in constructability,” said Mark Cirksena of DPR Construction.

Cirksena continued, “We had the opportunity to show the design concepts to the builders and ask them, ‘How would you build this?’”

Design-build is commonly credited with streamlining construction, but Cirksena pointed out that another major benefit of the project delivery method was the ability to mitigate the safety exposures that are typically inherent in construction. Early collaboration allowed for forethought and planning, which facilitated risk management.

In the end, “for this project,” said Owyang, “the three crucial needs really came together perfectly: aesthetics, economy and seismic design.” This translated into a convergence of good outcomes, from protecting public safety to an improved experience for the building owner and tenants.