A Dynamic Roof-Top Renovation
Like other commercial products, as they age, high-rise office buildings often face increasing competition to stay relevant. As more new, modern buildings are developed, tenant expectations increase and companies looking for office space must consider more than merely available square footage and lease rates. Many fully modern office properties are developed with integrated amenity packages that go way beyond the lobby coffee shop, to include restaurants, bars, fitness centers, and outdoor green spaces. To secure long-term leases with stable office tenants, building owners are forced to provide more than simply office space to keep up. At Chicago’s One Two Pru, the building owners recognized the need for a top-to-bottom rehabilitation of the 41-story structure built in 1955 to extend staying power with better tenant amenities.
The centerpiece of the transformation, executed in 2013, was the complete conversion of the 11th-floor roof landing as a 13,000 square foot amenities deck. Highlighted by a fully wired amphitheater, the reconceived rooftop green space perched directly above Chicago’s Millennium Park includes a bar, a fire pit, vast lounge seating, and a small lawn accompanied by a 12,000 square foot fitness center and a tenant clubhouse inside.
Molded polystyrene geofoam, from Atlas Molded Products, was a vital element in the successful reimagination of an inaccessible, unusable rooftop landing as a dynamic first-world lounge in the sky. Atlas geofoam is made from closed-cell polystyrene and boasts high-compressive resistance in a lightweight, and essentially portable, structural material. Easily cut and shaped, molded polystyrene geofoam allowed designers unbridled freedom in the depth and range of structural form they were able to add to the 11th-floor landing without overloading the 25-year-old roof and building beneath it.
A classic figure in Chicago’s skyline, One Two Pru rises elegantly above Millennium Park in two volumes. Pru One is the 41-story tower, build in 1955, as the first major building constructed in Chicago after World War II. Pru Two is the 11-story volume, which opened in 1990 and has won numerous awards for its distinctive architecture. While the floor-to-floor interior renovation took place largely in Pru One, the roof deck reconfiguration scope was exclusive to Pru Two.
The first thing designers at Wolff Landscape Architecture understood when looking at Pru Two’s roof conditions was that the existing roof’s 4 ½”-thick deck was going to present some design and construction challenges. In any roof-deck renovation, it is critical to be very mindful of the amount of weight being added to the building beneath it. In the case of Pru Two, designers were adding what is essentially a mini-park to a very thin structural surface. Furthermore, a building’s structural loading capacity is determined by the specific structural system and the design, and construction methodologies employed when the building was originally built. Each system and each building have a maximum loading capacity. Enhancing a building's structural loading capacity by adding reinforcements is rarely cost-worthy and never easy.
In addition to structural loading concerns, designers also had to consider the roof deck’s awkward accessibility. One Two Pru is sited along a collection of vaulted streets over an underground network of pedestrian tunnels connecting to local hotels, retail, city services, and the Chicago Transit Authority train system. Craning materials to the 11th floor from the street would be difficult and expensive, requiring special work permits and necessitating street closures and other complex safety provisions. The logistical alternative to craning was moving all the construction materials for the roof deck and interior clubhouse renovation through the building using the freight elevator.
The congested 11th-floor interior and exterior build-out situated in an intensely urban high-rise left the general contractor and subcontractors little room to maneuver inside or out. Material selection involved considerations on many fronts including product quality, cost, durability, weight, and deliverability.
Atlas Geofoam is a rigid, closed-cell plastic that is block molded into materials that are well-suited for roof deck and below-grade structural applications as well as Earth-facing building insulation around foundations or under slabs. At One Two Pru, two different types of geofoam were used to create a structural surface on top of the 41/2” roof deck slab. EPS 15 with a load-carrying capacity of over 500 pounds per square foot was used in areas that would be largely filled with plants and would have limited exposure to live-load, foot traffic. Chosen for its exceptionally high resistance, EPS 46 with a load-carrying capacity of over 2,500 pounds per square foot was used to create the structural platform across the design’s many grade changes and in live-load, weight-bearing areas.
Landscape architects generally appreciate the artistic freedom they are afforded using geofoam. Designers are easily able to create ADA-compliant sloped ramps, amphitheater risers, wavy undulating landforms, and even faux-rock formations when needed all from the same building material. In the case of One Two Pru, designers at Wolff Landscape Architecture especially appreciated the ability to largely see the shape of the assembled space in foam before covering it with soil, grass, plants, trees, and a wide array of building materials. This allowed them to make small adjustments to proportions, placement, and functionality that enhanced the design before the material was covered and rework becomes significantly more complicated.
Atlas Geofoam was also key to solving the roof deck’s accessibility challenges. Since all of the construction materials had to be brought to the 11th floor by freight elevator, Atlas worked with designers to pre-cut all the geofoam pieces to specific lengths so they could be loaded into the elevator by hand. Atlas devised a just-in-time delivery schedule to ensure a steady stream of materials in the required resistance as workers were ready to put them in place. This allowed the builders to maintain a limited staging area that was clear of excess material while ensuring construction progress on the rooftop remained unimpeded.
Lightweight, easy to use, and flexibly adapted to fit around any circumstance, Atlas Geofoam afforded designers unbridled freedom on the roof deck transformation at One Two Pru. Atlas’ willingness to organize and deliver acutely sequenced purchase orders is vital on many job sites with limited laydown areas and accessibility constraints. Atlas takes pride in our products, our performance, and our people. Working with clients as a team to understand and dissolve construction challenges, Atlas Molded Products outperforms the competition through a long-line of dynamic, versatile molded polystyrene foam building materials that can be structural, insulative, or both.