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The design of Frederick-Firestone Fire Station No. 5 in Frederick, CO, exemplifies cutting-edge structural design. Built atop an abandoned coal mine, the design team faced extraordinary challenges, including mitigating subsidence risk by crafting an innovative foundation and superstructure. More than just a building, the new fire station No. 5 is a testament to advanced engineering solutions and seamless teamwork. From its design as three separate buildings to its unique foundation that floats on the soil to the combination of cold-formed steel (CFS) infill wall framing with CMU veneer for durability and easy repairability, every aspect of this structure has been meticulously crafted to withstand the unpredictable forces of nature.
Coal Mining and Subsidence Risk
Beginning in the 1850s, Northern Colorado developed coal mining alongside precious metal mining. In addition to providing the fuel needed to transport gold and silver ore out of Colorado, coal warmed the growing number of homes in Denver and other burgeoning towns across the Front Range, making coal mining a key economic driver of the area.
Frederick, 39 miles north of Denver, was the home of the second-largest coal mine that Colorado Fuel & Iron, a company foundational to the industrialization of the American West, ever operated. Opened in 1907, it employed around 500 workers and produced an estimated 1,500-2,000 tons of bituminous coal per day at its peak in 1927. The mine closed in 1960.
Coal mines were constructed using the “room and pillar” mining technique, which involves creating a network of tunnels and rooms supported by coal pillars and allows for controlled collapse. Miners tunneled into a coal seam and dug rooms off the side between pillars of coal. Beginning at the tunnel's far end, they shaved coal off the pillars until the room was deemed unsafe. Then it was left abandoned to cave in. Some rooms caved immediately, and some are just now doing so decades after a mine's closure.
Geologists refer to the event of room collapse as a “subsidence,” which can take the shape of a bell-shaped pit, hole, trough, or sag at the earth’s surface. Tension cracks can also form around the perimeter of the subsided ground. The closer these rooms and shafts are to the surface, the more impact the subsidence has on structures built above them.
The Frederick-Firestone Fire District (FFFD), which currently serves the Towns of Frederick and Firestone and areas of unincorporated Weld County, considered several sites for the new station No. 5. The fire district chose the final site location due to the clear need for additional fire protection services in the area which was identified in FFFD’s 2021-2026 Strategic Plan.
Population growth within the district was driving this need, as it was projected to continue up to a staggering 7% per year. To accommodate this growth, the Towns of Frederick and Firestone needed to continue to annex unincorporated areas and increase commercial and residential development, leading to a rapid increase in calls for service and expansion of their service area, which now includes 36,000 residents across 36 square miles.
In situations where response times are measured in seconds, new fire station locations are carefully chosen according to how quickly apparatus can arrive at the scene after a call for service is received. According to its website, FFFD’s stated goal is to get lifesaving equipment and rescuers to an emergency scene within 5.5 minutes of receiving notification from the dispatch center. FFFD chose the final site for the new fire station No. 5 according to its ability to best meet this goal upon completion of a capital infrastructure needs assessment.
The location came with significant subsidence risk, estimated at 0.45 feet over the building’s length. To minimize the potential risk, the Colorado Geological Survey report recommended the station design consist of several smaller buildings, each no longer than 78 feet in length, instead of one large facility, which would reduce subsidence to 0.23 feet, nearly half the original estimate. With these guidelines, the building structure would not be subject to subsidence-induced surface strains capable of producing greater than slight damage in a worst-case event.
Design for Resilience, Flexibility, and Sustainability
In accordance with the geological report recommendation, the station design consisted of three separate buildings with complete isolation joints through the foundation and superstructure. The design team considered three foundation options:
- Deep Foundation System: Deep foundation systems comprised of straight-shaft drilled piers founded in claystone bedrock are a common foundation type along Colorado’s Front Range. Large foundation loads and/or the foundation’s ability to resist the effects of expansive soils necessitate the use of piers. Shale bedrock typically has high bearing capacities and limits foundation movement over time. However, the geological report recommended a flexible structure with foundation segments less than 78 feet in length to allow the subsidence to occur. With the inherent rigidity of this foundation system, the design team was concerned that focused areas of severe damage would occur within the structure, and this approach was eliminated as a viable solution.
- Shallow Foundation System: A shallow foundation system using concrete spread footings is also common for buildings in the area and was also considered for the project. Although independent footings supporting the building columns would allow for differential settlements across the site, the induced stresses and strains within the superstructure could be considerable, as well as unpredictable, depending on the subsidence profile. These factors eliminated this foundation type.
- Shallow Raft Foundation System: The selected foundation system uses shallow raft foundations broken up into three monolithic pieces that could float on the soil below while providing the necessary shear and bending capacity to resist the additional forces induced by a subsidence event. By limiting the differential deflections within each raft, the induced stresses and strains in the superstructure resulting from the soil subsidence are more manageable and predictable.
A finite element foundation analysis was performed and included nodal displacements in a parabolic profile to emulate a variety of subsidence events, resulting in additional concrete thickness and reinforcing steel in the final design while maintaining safe subgrade bearing pressures. In addition, the displaced foundation profiles were analyzed to resist the applied gravity, wind, and seismic loading to ensure the long-term performance of the structure should subsidence occur during the structure’s life.
The resulting raft foundations are 18 inches thick and reinforced with #6 reinforcing steel top and bottom, spaced at 12 inches on center with 8-foot square, 30-inch-thick integral foundations at the column locations. A reinforced turned-down edge detail at the perimeter of each raft foundation was used to extend the foundations 36 inches below grade to protect against frost heave.
Like the foundations, the primary superstructure system needed careful consideration to evaluate performance and flexibility requirements. Fire stations are typically designed with highly durable surfaces, including intermediate or special-reinforced concrete masonry (CMU) walls, due to CMU’s ability to be thoroughly washed down to disperse contaminants from apparatus and firefighter gear. Unfortunately, structural CMU walls lack the necessary flexibility to survive building settlement without requiring substantial repairs, which would not be an acceptable solution for an emergency response facility.
To create the necessary flexibility above the foundations, the primary superstructure consisted of fixed-base ductile steel moment frames encompassing each of the three buildings. Although the project site was found to be Seismic Design Category A, the design incorporated a response modification factor, R of 3.0. The design team incorporated the increased moments and displacements due to the settlement that would occur during subsidence and then analyzed and designed the steel superstructure to resist the applied gravity, wind, and seismic forces. In several locations, over 50% of the bending moment reaction at the foundation was caused by this assumed coal mine subsidence, going from 55 kip-ft due to soil settlement to 106 kip-ft after adding the vertical and horizontal loads. A 2-inch isolation joint through the foundations and a 3-inch isolation joint between the walls and roof structures were maintained to allow for the predicted tilt of the structure during a subsidence event, plus horizontal drift under lateral loading and P-delta effects.
The moment frame column locations along the interface between individual buildings were offset to one another to allow for the constructability of the beam-to-column moment connection joints. Without this, the steel erector would not be able to complete the required bolting and welding for the connections, as the adjacent framing would block the far side of the joints.
All the design disciplines were carefully coordinated to ensure the structure could be efficiently constructed and provide long-term performance for the Owner. The station design specified flexible utility connections across the expansion joints for electrical, mechanical, data, and plumbing lines to allow for differential settlement across the three buildings over time. The expansion joints were located at physical transitions between the different building surfaces wherever possible at both the interior and exterior of the buildings, for example, along the interface between roofs and exterior walls, at wall corners, along separating parapets, etc.
The station’s design also addressed the challenge of maintaining structural integrity after a subsidence event for both the secondary structural elements and the architectural finishes. As mentioned previously, fire station design typically employs concrete masonry CMU walls.
However, CMU walls were deemed not ductile enough for this project due to the settlement concerns. Therefore, in lieu of traditional CMU walls, the interior containment perimeter and the exterior envelope design specified CFS infill wall framing with CMU veneer. This pairing combines a lighter, ductile, and more sustainable material choice in CFS with the necessary durability of CMU. Although masonry veneer with CFS wall framing backup is a common system for building exterior walls, this was employed throughout the apparatus bays and containment areas within the fire station as well.
By designing and detailing the CFS wall framing to allow for the predicted deflection and movement of the building structure, including careful coordination of the veneer control joint locations, the intent of this approach is to limit potential damage after a subsidence event to drywall and non-structural veneer repairs, preserving the superstructure's performance and operation. After careful evaluation, the design team felt this system and approach was the best selection to balance functionality for the Fire District, while also achieving the geological report goal of incurring only slight damage levels and maintaining the structural integrity of the superstructure and CFS wall framing after a subsidence event.
Modern Design Achieved Despite Significant Challenges
The subsidence and estimated movement impacted all design disciplines. The team worked together and creatively, embracing the challenge of designing a structure required to provide emergency response 24 hours a day, 365 days a year despite the site's significant constraints. The station design included material choices that address the hazardous contaminants inherent in the operation of a fire station while providing a modern design that respects the community context and includes all the necessary amenities for firefighters’ health, wellness, and workplace efficiency. ■
About the Authors
Russ Leffler, PE, SE, P.Eng, MLSE , is a Principal at Salas O'Brien, with technical expertise in many areas, including industrial structures, public works projects, CFS engineering design, and blast-resistant design.
Bryan Peters, PE, PMP, graduated with a civil engineering degree from the University of Nebraska-Lincoln and is a Senior Project Engineer at Salas O'Brien.