Elizabeth Heck, SE, PE 2023-02-03 09:06:57
A new county building in the Arizona desert turns to steel—and saguaros—to achieve energy efficiency and blend in with the surrounding landscape.
WHILE GOVERNMENT BUILDINGS are typically built to be efficient and functional, rarely do they incorporate biomimicry in their design elements.
But one government structure in the middle of the Arizona desert uses the former to achieve the latter.
The new Pinal County Attorney’s Office in Florence, Ariz., addressed the county’s constrained budget by condensing the building footprint and maximizing energy efficiency through biomimicry, specifically of the area’s famed saguaro cactus. The design translates the shifting shadows across the cactus into rigid self-shading fins on the building, which also includes a pop of desert color when the fins are extended for viewers to enjoy as they pass by.
The design process began with the design team meeting with the county attorney staff to develop a vision for the project as a “lighthouse of justice.” The scope of work involved designing a five-story, 56,000-sq.-ft building adjacent to the existing Pinal County Superior Courthouse. The main structural system is comprised of wide-flange steel beams and columns and interior concrete shear walls supporting metal roof deck and composite floor deck. Steel gravity framing was selected for its repetitive framing layout, quick installation, durability, and low maintenance—and steel’s aesthetic qualities made it the obvious choice for other key design features. In all, the project incorporates 266 tons of structural steel.
The north and south entrances and the fifth-level patio feature decorative steel canopies cantilevering from the exterior wall, visually indicating the location of the lobby. Steel was the obvious aesthetic choice for the materiality of the canopy as it was able to mimic the vertical steel on the façade of the building and bring the duality of form and function. The canopy consists of hollow structural sections (HSS) hung from the interior floor framing and then extending out to provide dappled shading at the entrances with no exterior columns. The fascia is detailed with exposed steel channels with the flange facing out, creating texture at the edges. In between the HSS8×8×¼ beams, smaller HSS purlins (HSS4×2×3∕16) were used as infill to create shade below. The canopy not only acts as a visual cue but also solves the challenge of the programmatically small lobby spaces by visually extending the lobby space from inside to the outdoors.
The building’s ribbed metal panel façade, designed to mimic a saguaro, is its feature design element, combining a modern, eye-catching design with a structure that reduces energy consumption. The galvanized steel panels are supported by square HSS framing and break the sunlight over shifting areas, allowing heat loads to redistribute until natural air convection can cool the exterior. Where windows were desired on the south façade, the wall system angles out from the building and becomes a self-shading device while maintaining the look of a continuous protective skin. The panels display a copper tone, a color native to the Arizona desert, and appear to change color throughout the day, creating a dynamic façade dependent on the sun’s position in the sky. Additionally, on the west side, ¼-in.-thick steel plates extend from the HSS framing and are painted yellow and orange to provide pops of color, similar to the colors of the blooming saguaro flower.
Designing the steel support for the building’s ribbed metal panel was a unique challenge for DLR Group, the project’s structural engineer. On the west side, the metal fins are attached to a frame that floats in front of the exterior glazing. On the south side, the metal fins are integral to the exterior wall. The design goal was to have the metal panels extend from the exterior wall adjacent to window openings, creating shade during the hottest portion of the day. Locating the shade fins adjacent to the windows meant reducing the potential location for structural support; creating a strut at the protruded end of the shade fin would obstruct the view from inside.
The solution was to cantilever the shade fins from two HSS jamb posts in-plane with the exterior wall’s metal framing. Several materials and configurations were analyzed via RAM Elements finite-element analysis (FEA) software to pick the design solution, comparing many different design parameters such as material compatibility, redundancy in support, and torsional resistance. In addition, the panels were designed to stringent performance criteria. The panels range from 14 ft to 19 ft tall and protrude 3 ft to 4 ft from the building, and the engineers aimed to keep deflections low and used higher gust wind loads during design.
In addition to the performance of the cantilevered fins, the structural steel jambs allowed for a consistent design and detailing for all the fins. Most of the assembly was fabricated in the shop, resulting in a modular design that the ironworkers were able to erect in the field quickly. The fins and jambs were fabricated separately, allowing for the jambs to be constructed on the south side at the same time that the surrounding cold-formed metal studs were being installed, which allowed for the exterior fins to be attached after the remaining façade materials were in place. This streamlined erection process allowed for better efficiency and quality control of the wall components, as the cantilever fins were not in the way. Once the entire wall was constructed, the erector was able to bolt on the cantilevered fin to the in-place steel jambs.
The design team for the new Pinal County Attorney’s Office was able to provide a project with 52% external glazing while reducing energy usage and solar glare. This is a high percentage considering the harsh climate of the surrounding desert. How did they successfully achieve it? Biomimicry.
Biomimicry is not the same as bio-inspired design, which makes a building look like an element of nature, and it is not simply adding a green roof or a plant wall—rather, it studies and then replicates nature’s forms, processes, and ecosystems to create designs that affect the way a building functions. In other words, the design mimics nature to create conditions conducive to the resilience of a building.
The vertical fins of a saguaro cactus provide continuous self-shading and redistribution of heat. This ability to self-shade breaks sunlight up into smaller areas that shift continually, preventing any one area of the cactus skin from overheating. This makes the saguaro viable while giving it a beautiful and distinct character. By creating a 3D SketchUp model of a saguaro cactus and using a daylighting simulation model, the design team confirmed that no part of the plant received more than 15-to-20 minutes of direct sun at any one time, avoiding sunburn.
Using the lessons learned from the saguaro, the exterior façade of the building is built with rigid self-shading fins. Additionally, the ribbed metal panel skin allows the head load to shift until natural air convection cools the fins. On the south façade, which is the exterior face with the most sun exposure throughout the day, the fins are carefully placed to shade the windows while also maintaining the look of the continuous skin.
Although the metal panel skin absorbs heat, the “breathable” facade system is mounted a few feet off the building to allow air to circulate and dissipate heat. In addition, the heat-reflecting, high-performance glazing aids solar control, and the low-E coating manages the light spectrum, balancing visible light transmittance and reducing glare. This high performance created opportunities for downsized mechanical equipment, leading to reduced long-term energy costs.
The bolted connection between the jambs and cantilever fin was a custom configuration. The connection consists of an HSS rectangular member cut at the angle required for the fin. The HSS assembly has a fixed-end plate with a pair of welded threaded studs to allow for the fin assembly to bolt directly to the HSS support. This connection helps distribute the force couple and torsion from the cantilevered fin to the HSS jamb supports while preventing thin-wall buckling of the jamb members.
Additionally, having the entire fin assembly consist of structural steel meant that the fin subcontractor only had to manage one source for material and also facilitated a close working relationship between the subcontractor and the fabricator to ensure the design intent was properly implemented.
Although Pinal County had a specific site in mind for the project, the proposed location provided many design challenges. Therefore, DLR Group’s High- Performance Design Team researched four options to find the most efficient building and site solutions while delivering a positive design impact on the community. The predesign process ensured that the location, shape, and size met functional planning requirements and used life-cycle analysis to determine the full cost savings in utilities.
Using annual daylighting and energy analysis for different building iterations, the design team calculated the impact of building location and size on energy use. The results were used to find opportunities to bring more daylight inside the building and with improved views for the occupants. Four options were studied, two each on the east and west sides of the existing building. Massing models were developed with building areas varying from 60,000 sq. ft to 70,000 sq. ft over five floors with 15-ft floor-to-floor heights and a 40% wall-to-window ratio.
The different iterations were modeled with light-reflective colored surfaces, and the interior offices brought in daylight by eliminating high-partition cubicles. Sliding glass barn doors not only created a transparent connection to the outdoors but also eliminated door swings, which allowed for a narrower building. The final data-driven solution reduced the footprint to 56,000 sq. ft reducing energy use while maximizing natural daylighting.
In addition to the data-driven design processes used in predesign, this project was analyzed using Tally, a Revit plug-in that completes a life cycle used and provides comprehensive embodied carbon reporting. Pinal County Attorney’s Office was one of the founding projects reported as part of DLR Group’s pledge to the SE 2050 Commitment, which states: “All structural engineers shall understand, reduce, and ultimately eliminate embodied carbon in their projects by 2050.” Analyzing the embodied carbon of the building allowed the design team to establish a baseline and use this data to design lower embodied carbon-intensive projects in the future.
Owner
Pinal County, Arizona
General Contractor
FCI Constructors, Inc., Glendale, Ariz.
Architect and Structural Engineer
DLR Group, Phoenix
Steel Fabricator
Alstate Steel, Inc. Albuquerque
Elizabeth Heck (eheck@dlrgroup.com) is a senior associate and structural engineer with DLR Group in Phoenix and was the lead designer for the Pinal County Attorney’s Offi ce project.
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