Harvesting Green Energy From a Gray Garage

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Renewable energies can provide a great boost to sustainability and energy independence by harnessing the sun, wind, and water that is around us. Each of these sources needs different conditions for optimal harvesting. Solar energy requires large surface areas with direct sun exposure to place photovoltaic (PV) arrays. In the built world, parking lots and parking garages provide a prime opportunity to expand solar energy production. Parking garages offer raised solar opportunities in urban and suburban environments, and in turn, a solar canopy can provide weather protection at the top level to increase a garage’s service life and reduce maintenance. These benefits have led to an increasing number of municipalities requiring solar canopies over new parking garages and exploring options to add canopies to existing garages.

Adding solar array canopies to new or existing parking garages requires specific design considerations and customizations for each application.

Choosing the right type of array and framing system for a solar array canopy involves answering several questions:

Is covering the entire garage feasible? What is the desired electrical output? Will the orientation of the garage affect the sun exposure? Is the owner hoping to achieve a certain aesthetic? How will the canopy manage precipitation runoff?

While covering the entire garage is often ideal, it can be impractical for post-installed canopies on existing garages. Elevator or stair headhouses located in the corners can block the structural members that would be the connection points for new canopies. End bays of garages are often longer than the middle bays, so heavier framing is needed to span these areas. Unusual geometry such as an L-shaped garage may also limit coverage, but even partial coverage can be a worthwhile investment to an owner looking to save on both energy costs and maintenance. Where only partial coverage is achievable, the owner may review the layout with an electrical consultant to determine whether the proposed canopy’s output can meet (or exceed) the site demand.

One of most critical factors for a solar canopy’s effectiveness is the orientation and exposure of the garage itself. In the northern hemisphere, a solar array is most effective when it is oriented towards the south. A solar array should be tilted to face the sun as directly as possible, so for a garage with a predominantly east-west layout, a monoslope or stepped array will maximize its effectiveness (Figs. 1-2). Monoslope canopies are typically the most efficient framing methods, while a stepped array will allow more sunlight onto the roof deck.

Garages with north-south orientations offer flexibility for array layouts, since the preferred south-facing layout is impractical for the garage spans. An inverted V-shaped canopy can assist with water management to direct water away from the sides and towards an internal drainage system. This system may be more desirable in northern climates, so snowfall can be better retained; however, this may require the designer to account for higher loads from increased snow loading. When only minor efficiencies are to be gained between systems, the framing type can simply come down to the garage owner’s aesthetic preference.

Canopy water runoff can be managed with either a gasketed or non-gasketed system. A gasketed system involves supporting the solar panels directly on the purlin framing, with EPDM (Ethylene Propylene Diene Monomer) gaskets between each panel. This system allows easier panel access from the underside of the array, with the ability to replace individual panels relatively easily (Fig. 3).

A non-gasketed system typically requires a metal roof deck under the array, which eliminates the need for gaskets to create a watertight surface, as precipitation is collected by the deck (Fig. 4). While this system reduces the need for maintenance (there are no gaskets to maintain or replace), it makes access to the panels more difficult. Any malfunctioning panels must be replaced from the top side. For this reason, non-gasketed systems often require aisles between solar panel areas, which reduces the overall output of the system. Both water management systems provide benefits to the garage maintenance and the user experience. Vehicles on the top level are shaded in warmer climates and shielded from precipitation including snow in colder climates. Omitting a water management system will concentrate water falling on the concrete surface which may prematurely deteriorate sealant joints and the concrete surface. In colder climates water falling from solar joints may create ice buildup on the parking deck as the surface is less likely to melt in the shade.

Another type of solar panel array is a column tree system, where individual columns each support a panel array, but the system does not form a full canopy over the parking surface, but rather covers just the parking stalls. This type of system is popular for on-grade parking (Fig. 5), as each tree is supported by its own footing. This system lacks the economy of a full canopy frame and is particularly difficult to install on existing garages. The cantilever column lateral system for the tree style array imposes a large moment force at the base. Garage columns may not be able to support these moment loads, as discussed later in this article.

Regardless of the system chosen or any potential layout variations, precast concrete garages typically do not require strengthening or supplemental reinforcement to support a new steel-framed solar canopy. The only additional gravity load is the dead weight of the steel frame and canopy itself because snow load will have been accounted for on the garage roof deck. The relative lightness of the canopy compared with a multi-level garage also means that added seismic loads are relatively negligible. Because the canopy is an open structure, the added lateral wind load is relatively minor when compared with the base garage loads. Wind uplift must be considered at the canopy base connections but is not likely to affect the base structure. Although a garage will rarely require foundation or superstructure strengthening to support the addition of a solar array canopy, confirming the existing structure and foundations can handle the additional weight is required.

The typical framing layout of a steel canopy consists of structural steel columns supported by steel girders along the garage perimeter and in the long direction middle, with jack beams running over the top of the girders and purlins over the jack beams (Fig. 6). The jack beams run perpendicular to the drive aisle. A typical parking bay is roughly 60 feet wide comprised of two 18-foot-long parking spaces with a 24-foot-wide drive aisle running between them. For a two-bay garage, the jack beams need to span approximately 120 feet with an intermediate support. Splices are typically placed at the third-points of these beams, so each piece is approximately 40 feet long to assist with shipping and erection. Member lengths will vary depending on the number of 60-foot-wide trays a garage contains, but the basic framing system is generally consistent for the various array shapes described above. The canopy columns typically land on the garage column or wall elements above the top level. In cases where the column or wall elements do not rise above the top level, the new steel columns may require openings in the roof deck to connect to the columns below.

The canopy system is typically very light with a shallow profile, so neither wind nor seismic loads generate significant forces on the canopy relative to the base garage; however, they must be addressed. Designers can select from a few options for the canopy lateral force resisting system. Because parking garages are relatively rigid structures, the designer can analyze the canopy framing performance with minimal increase in the garage drift.

A moment-framed system for a garage canopy is the most common for the 60-foot span over the parking trays as braces would impede vehicular access. The steel connections are relatively simple; however, the overall framing is heavier, as the members must be stiffer to limit story drifts. A variety of column base connection options are available and discussed here.

In the long direction and along the perimeter of the garage, the designer may employ tension-only cable X-braces parallel to the drive aisle. These braces are an efficient method of limiting drift in the long direction of the garage, though they require more detailing from the designer and increased maintenance as they are more prone to corrosion and deterioration. X-braces are also typically used for horizontal bracing of the canopy diaphragm.

For new garages, designers can coordinate canopy connections, but in an existing garage the construction type and geometry can drive the canopy design for a post-installed condition.
Parking garages may be constructed of precast concrete, cast in place concrete, steel framing, or a combination of these systems. While connection details will vary based on the construction type, the principles of how the connections work are consistent.

Whether the base connection is pinned or fixed is a primary consideration in the canopy design. The canopy manufacturer typically prefers a fixed moment base connection to increase the moment frame efficiency and reduce overall steel tonnage. Providing a fully fixed connection, however, is contingent on the capacity of the base garage framing below and the access to install the fixed connection.

Garages where the columns rise above the top parking level (typical in precast garages) are well-suited for fixed moment connections. Exposed steel columns allow the canopy column to connect directly to the existing column and an exposed concrete column allows the new canopy to connect with the column to transfer moment into the column. For concrete columns, a top connection with vertical, post-installed dowels is typically insufficient or impractical to engage enough development length, the common approach for a fixed base connection is a “saddle” connection anchored to the sides of the column, as shown in Figure 7. By engaging the sides of the column, the connection can “grab” the column lower and engage more vertical column reinforcement while mitigating the anchor breakout forces from the applied moment loads.

While manufacturers typically prefer fixed base connections, these connections can sometimes be impractical due to accessibility or limited capacity of the base structure. In these cases, a simple pinned connection may be used, which requires stiffer framing above to limit canopy deflections but is an easier base connection to install. Pinned connections may be preferred for ease of installation, limited capacity of smaller cast-in-place columns, or conditions where the column does not penetrate the top level. The pinned connection is similar to a standard building column base, with four vertical anchor bolts through a steel baseplate (Fig. 8).

The lateral system in a precast concrete garage often consists of intermediate shear walls in both directions. Shear walls (e.g. litewalls) are particularly useful parallel to the drive aisles, as they can support offset ramps between levels and do not require that the parking decks on either side of the wall be at the same elevation. These walls, however, are typically lightly reinforced and may not support the moment demands from a fixed-base connection. Furthermore, the shear walls panel joints often align with the column grid, so the new steel column base connection must bridge the wall panel joint. Because of these limitations, the designer often uses pinned interior base connections, even if the perimeter columns use a fixed base connection.

A new parking garage offers more flexibility to incorporate solar array canopies into the base design. Including allowances for a canopy can provide a more economical design and an expedited schedule for canopy construction. If the canopy erection is integrated into the garage erection, the crane does not have to reach over the existing garage, and the erection is not limited by equipment on the roof level of the exiting garage. This can offer initial cost savings to an owner.

If the canopy is installed after the garage, the owner may need to consider several additional factors. Site logistics and construction loading may require smaller steel framing segments if a crane cannot reach the roof level and smaller erection equipment is used on the roof level of the garage.

The design phase is the ideal time to include moment-framing capacities in the columns. Showing the assumed solar design loads on the drawings will facilitate the base connection design for the array. Designers can provide extra reinforcement or extend columns to make the moment connections easier to complete. Furthermore, in concrete columns, the designer may even include anchor bolts for future canopy attachment, as shown in Figure 9. These bolts will allow for a clean finished aesthetic but may limit the future solar array anchor design options as the canopy designer must coordinate baseplates with the pre-installed anchor bolts.

Parking garages are particularly well-suited for the installation of steel framed solar array canopies. The system is customizable to work with both new and existing garages through a variety of lateral systems and base connections. An engineer well versed in the different conditions described herein can provide value to an owner trying to maximize their investment and support renewable green energy from their gray garage. ■

About the Authors

Jake Phelan, PE, is a Consulting Engineer in the Waltham office of Simpson Gumpertz & Heger Inc. (SGH). He has designed multiple precast parking garages and designed or peer reviewed solar canopy attachments for garages designed both by himself and by others.

Mike J. Bolduc, PE, is an Associate Principal in the Waltham office of Simpson Gumpertz & Heger Inc. (SGH). He has 25 years of structural design and renovation experience including commercial, healthcare, and industrial buildings along with a specialty in precast concrete parking garage and solar canopy designs.