By Rachel C. Palisin, PE, LEED AP and Amanda L. Miller, AIA
From the February 2020 Issue
Creating plazas and terraces over occupied spaces poses unique challenges. These outdoor areas need to address all performance aspects of a plaza on grade, yet also compose part of the building enclosure system. Considerations for elevated plazas and terraces generally fall into one of two categories: structural support and building enclosure performance.
Structural Capacity
Retrofitting plazas and terraces on existing roofs requires evaluation of the load capacity of the supporting structure. Whether a conversion to a “green roof” or an occupied terrace, or a renovation of an existing plaza space, rehabilitation may require augmentation of the existing structural framing to accommodate increased dead loads (plantings and fixtures) and/or live loads (occupants). For example, soil commonly weighs more than 100 pounds per cubic foot, which can quickly exceed the capacity of underlying framing. Also, with any new occupancies, egress may need to be supplemented, resulting in additional stairs, ramps, openings, guardrails, and/or parapets that must be coordinated with the existing structure. Determining these structural requirements early in the design process is a key component of managing project costs.
If increasing the structural capacity is cost-prohibitive or not feasible given the existing configuration, there are alternative options to achieve a new terrace space. Extensive plantings, which can flourish in shallower soil depths, or alternate materials, such as geofills, can reduce the load attributed to planting areas, while still providing desired green space. And, limiting access, such as by elevating some roof areas and restricting pedestrian traffic, can decrease the associated occupancy loading and minimize or eliminate the need to increase roof deck capacity.
Furthermore, dependent on a combination of site topography, geographic location, height above grade, and roof configuration, the finish materials alone may be insufficient to resist the lateral and uplift forces created by wind and pressure differentials. To prevent these elements from being dislodged and potentially becoming hazardous wind-borne debris, interlocking or lock-down paving systems that can resist these forces must be designed and specified.
Planting areas subject to high wind loads are vulnerable to scour (loss of plant and/or soil material), especially early in the establishment period before the plantings have produced full surface cover and a strong root system. Careful consideration of the types of plantings and soil/growth media at edges or corners of exposed elevated plazas is recommended; at a minimum, an erosion control material should be provided during plant establishment.
Energy Efficiency And Building Protection
As part of the envelope system, a terrace is an integral part of building performance. Whether part of a new amenity space or strictly for access by maintenance personnel, the plaza design will directly affect the ability of the roof to prevent water infiltration and balance energy code requirements for thermal performance and reduction of the heat island effect. The waterproofing system can range from a variety of cold- or hot-applied, single-ply, multi-ply, or fluid-applied membranes, each appropriate to the unique situations posed by variables such as existing substrate, anticipated use, and spatial configuration.
Additionally, insulation is generally necessary for compliance with ever-increasing energy code requirements. This may result in modifying roof accessories such as guardrails, parapets, thresholds, stairs, and ramps. In conjunction with insulation, ballast is usually required, often in the form of pavers, which can come in a variety of shapes, sizes, and materials. This surface material is subject to the material performance requirements discussed for plazas on grade, the aforementioned structural considerations related to wind, and also energy code requirements for surface reflectivity.
Despite challenges with waterproofing and structural integrity, plazas and terraces provide benefits for occupants and the surrounding environment.
Palisin, PE, LEED AP, is senior engineer with Hoffmann Architects, Inc., a firm that specializes in the rehabilitation of the building envelope. An experienced professional engineer with a Master of Structural Engineering degree from Washington University in St. Louis, she leads a team of technical staff in meeting the structural engineering challenges of plaza and building enclosure projects in the New York metropolitan area.
Miller, AIA, is senior staff architect with Hoffmann Architects in New York. After completing a master’s degree in Design Studies at the Harvard University Graduate School of Design, Miller organized the Harvard Black in Design Conference and was selected for the AIA New York Civic Leadership program. She develops design solutions for plazas that promote accessibility and enhance performance.
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