Case Study – Glendale Library, SLC, Ballasted Solar System Required Creative Solution

The Problem

The new Glendale Library in Salt Lake City, completed in Spring 2015, offers 20,000 square feet of media and meeting space for a diverse populace in an older section of the city.  Designed with an EPDM membrane roof, and achieving LEED Certified Gold rating, Intermountain Wind & Solar was contracted to install solar panels.  However, with a 115 miles per hour wind speed rating, and a maximum allowable five pounds per square foot dead load on this building, they needed an unusually light ballasted solar mounting layout.  But the Class C exposure category of the building added additional complications.  The problem was apparent:  How do you stay under the 5 psf dead load requirement but provide enough ballast for the class C exposure?

solar racking for flat roof

The Solution

Intermountain Wind & Solar contacted TRA Snow and Sun, Inc., of American Fork, Utah, designers and manufacturers of solar mounting and racking systems.  TRA frequently engineers ballasted mounting systems, with arrays that are completely interconnected, one right after the other, commonly spaced out six feet apart with 100-200 lbs of ballast weight per tray.  However, this typical layout wasn’t going to work in the case of the Glendale Library.  The engineers at TRA and the folks at IW&S had to be creative to keep the weight of the system under 5 psf but provide enough weight for the wind speed and class C exposure.

The integrators originally considered spreading out the panels to lessen the impact/weight on each rafter/truss, so TRA Snow and Sun suggested a layout with all panels close together and the ballast trays linked as well, creating an interconnected solar mounting system.  However, since all of the ballast was tied together in the system, the weight per square foot was uniform over a small area and overloaded the localized rafters/trusses.

ballasted solar mounting

The solution to this loading issue was to separate the continuous, closely laid out array, and create smaller arrays. These smaller arrays were still interconnected in the north/south direction but with a  gap  created in the east/west direction.  Each smaller array only had one tray per 3 panels, therefore reducing the amount of ballast on the rafters/trusses drastically.  The new design allowed the load per tray to satisfy the required ballast for a 115 mph wind load and a class C Exposure.

According to Jake Owsley, TRA sales team member who worked on this design, “The result of our efforts created a uniform, functional system providing adequate maintenance space, while also satisfying the necessary requirements for the Glendale Library.”

ballast solar mounting system