By Cheryl Piner, P.E.
From the March/April 2016 Issue
The North Carolina Department of Administration operates several efficient district heating and cooling systems to support a government complex in downtown Raleigh, NC. Several years back, as equipment in that city’s oldest chiller plant exceeded its useful life, the Department developed a vision to improve equipment service access, replace aging equipment, and solve equipment operation issues. The plan became a reality by converting this single level facility into a two level plant with pumps located on the first floor and two 1,100 ton chillers on the second floor.
Completed in December 2014, this renovation project serves as an example of how a historic utility plant can be improved to address significant elements and concerns. In this case, these included maintaining structural integrity, achieving code compliance, carrying out selective demolition, and balancing energy upgrades with maintaining historic structural elements.
Built in the early 1900s, the plant was constructed to house coal fired boilers. The facility is nestled in the middle of several historic buildings and was built to serve the surrounding government buildings and the State Capitol Building. In the mid-1990s, the plant was converted to a single level chiller plant. As constructed, it did not have grade level access for pumps or chillers. Its internal steel structure and piping made it challenging to crane equipment from the basement floor to grade.
Planning The Project
The project team met with staff members to discuss their needs and concerns so that all issues could be addressed in the new facility. This team included stakeholders from the state’s Department of Administration; RMF Engineering, responsible for MEP, civil, and structural design; BBH Design, the architect of record; FDH Velocitel engineering, the non-destructive testing (NDT) consultant; and Kirlin Carolinas, the construction manager.
The primary need identified was to have better access to the plant equipment. Using 3D modeling, the project team collaboratively developed several concepts that would enhance equipment access, replace aging equipment, and solve operational issues. The team was drawn to a multi-level plant concept. The 3D model allowed the team to “walk through” the plant during the design process, providing a clear visualization of the improved maintenance access of the new equipment and usability of the space. The proposed layout included a dedicated elevated chiller operating floor with pumps located on the lower level; this allowed for clear pump aisles and easy grade level chiller access on the upper floor.
An in-depth assessment and analysis was completed for the existing structure—a multiple-wythe brick structure with subsurface concrete trenches and complex foundations below the original boilers. In 1939, a building addition raised the roof and added a clearstory structure. The original boiler substructure was left abandoned below the existing slab after it was converted to a chiller plant.
Over time the addition of equipment and the entanglement of piping made the structure barely visible. Determining what needed to be removed or modified in the structure was crucial for detailing the new design so that accurate bid pricing could be obtained. Non-destructive testing and selective destructive testing were used to further investigate the concealed condition of the building.
Ground penetrating radar (GPR), which uses electromagnetic waves to identify variations below the slab, was performed over the existing floor slab to determine the amount of substructure that remained in place. This method also evaluated and located the lateral dimensions of the footings and foundations. Next, the footing thicknesses were estimated using wave dispersion techniques via holes drilled in the floor slab above the footings; this was verified by hand digging at several locations. Foundation bearing capacity was determined by evaluating in-situ soils using dynamic cone penetrometer (DCP) testing, which was later used in designing the footings for the new structure.
Additional items that were addressed included hazardous material identification and removal, penetration repair requirements, mortar repointing, and roof repairs.
Chiller Plant Up To Code
As standards and codes are constantly changing, ensuring historic buildings are in compliance with current codes is critical. By adding a new level in the plant, egress paths had to be reevaluated and improved to ensure occupant safety. Each level was reviewed, and egress paths were provided to allow access outside. Previously, the only access to the cooling towers on the roof was a 40′ ladder, which was challenging for maintenance staff. An exterior stair tower was added, solving roof top access and plant egress challenges. Also, plant doors were replaced with exterior swing doors, and proper exit hardware and window openings were filled to allow for adequate fire ratings on the exterior due to close proximity of surrounding buildings.
Due to the limited space between the existing roof structure and the floor slab, the new floor structure had to be restricted to a specific depth to allow for adequate height for equipment clearances and piping. The steel frame was laid out to maintain the depth needed while meeting the required strength to support the large 55,000 pound equipment and deflection requirements. The new chillers needed to travel across the elevated floor to their permanent position. A 300-psf live load was used over the area so equipment could be moved into and out of the plant as needed.
Vibration analysis also played a key role in the structural design to make sure the mechanical equipment vibrations did not disturb occupants or hinder the function of the equipment. Equipment pads, composite floor slab, and neoprene dampening pads were used to dissipate vibrations.
In short, with new technology and strategic planning, historic utility structures can be revived and retooled to provide full functionality and improved use and maintainability that will last for years to come.
Piner is a structural project manager with RMF Engineering, Inc., a firm specializing in district energy system planning, design, and commissioning. She is a registered engineer who specializes in the structural design of utility plant renovations and retrofit projects, and utility distribution systems.
Do you have a comment? Share your thoughts in the Comments section below. Or send an e-mail to the Editor at [email protected]