Essentia Health’s Vision Northland project, located on the shores of Lake Superior in Duluth, Minnesota, includes approximately 928,000 square feet of new construction and roughly 120,000 square feet of renovation to existing facilities and is scheduled to be completed by the end of 2023. The project has created a civic asset capable of being both the standard bearer of healthcare in the region and a physical celebration of the community it serves. This formidable project is a healthcare destination for Northern Minnesota. While receiving the benefits of world-class healthcare, facility occupants can enjoy year-round expansive views of Lake Superior.
New construction adds a nine-story Out-Patient building, a 17-story In-Patient tower to Essentia’s existing Duluth clinics, and a two-story over-build on existing Duluth Clinic DC-1. The In-Patient tower reaches a height of 290 feet, topped out with a mechanical penthouse and helipad. The project had to overcome many complicated region-specific issues, including wind loads from Lake Superior, heavy snow loads with drifting, a frigid winter climate necessitating close attention to thermal continuity through the exterior building envelope, and the steep slopes of existing grade. The complexity of many of these issues was compounded by the new project’s location being constructed adjacent and attached to an existing operating hospital. In addition, design and construction occurred during a global pandemic. The design team also had to work closely with the construction team to deliver early structural packages to meet the demanding construction schedule. Construction documents for foundations and early steel mill order packages were delivered to the construction team four months and eight months before the overall architectural design was finalized.
The project is built into the hillside of Duluth, Minnesota, on the shore of Lake Superior. The facility climbs the hillside starting from the shoreline of the lake at Superior Street, rising 100 feet in grade change up to 2nd Street. The footprint covers approximately 600 feet, spanning an alley and frequently traveled 1st Street. The street crossings create several long-span transfer conditions. In addition, the new facility is also built integral with many existing buildings across multiple city blocks. See Figure 1 for a building cross section.
Interconnecting the new building with existing buildings on the campus was complex due to the varying ages of the existing construction and the differing floor-to-floor heights. The new building is laid out to align in elevation at the north end with 2nd Street (Level 6 in the new building) and at the south end along Superior Street at Level 1. The site elevation change and the requirement to align with existing building floor levels results in the varying floor-to-floor heights in the new structure of up to 24 feet.
Site-Specific Geological Challenges
From the very beginning of the project, the primary goal was to provide a healthcare destination for the region. Achieving this goal required careful site selection for the new structure to integrate the new building into the existing facility. Siting for the building required the demolition of several existing buildings and the consolidating of some existing facility services.
The project is located within what is known as the iron range of Northern Minnesota, which means this area is known for hard rock formations. A detailed geotechnical engineering evaluation was performed to evaluate existing site conditions and develop criteria for the new foundations. Generally, the soils present at the site comprise fill-over glacial deposits, weathered bedrock, and bedrock. With maximum column loads of approximately 8,100 kips, it was necessary to construct all new foundations on existing bedrock. The in-situ rock was determined to have an allowable bearing capacity of 50 tons per square foot. Foundations for the new structures consist of spread footings bearing on the rock. Rock-anchored mat foundations are utilized at the braced frame locations to resist uplift and overturning forces.
Depth to competent bedrock varied, and reaching bedrock for foundation bearing and removal for the new building program required extensive excavation, drilling, and blasting of the existing bedrock. Approximately 79,500 cubic yards of rock removal was required for the project, including approximately 53,000 cubic yards of the rock requiring blasting. With an open and operating hospital on an active construction site, rock blasting was limited to specific times of day to limit the impact of sound and vibration on existing hospital functionality. Excavation work and rock removal took four months for the Outpatient building and ten months for the Inpatient building.
Building Superstructure Overview
Due to location and labor constraints (need to describe the constraints), structural steel was selected as the material of choice for the main structural frame. The structural system for the project is comprised of conventionally framed composite metal deck floor slabs. Steel framing is designed with headed studs to take advantage of the composite behavior and help reduce steel tonnage. The total project steel tonnage was approximately 12,000 tons).
The building utilizes steel-braced frames as the lateral force-resisting system. Since the project is located in a region of very low seismicity, the wind loads from the open lake surface and surrounding terrain controlled the lateral force-resisting system design.
Most of the steel for the project was typically ASTM A992 Grade 50. However, all truss members, plate girders, braced frame members, and large column sections utilize ASTM A913 Grade 65 steel to reduce steel sizes and keep the steel as economical as possible. At the time of design, grade 65 steel was only being rolled in column sections.
The Out-Patient building steel trusses span over 50 feet and support five stories above, allowing vehicle clearances in the alley. Similarly, the inpatient building spans over highly trafficked 1st Street and an open ambulance bay area using story-height steel trusses (18´-0˝ deep) spanning up to 95 feet and supporting up to three stories above. Two pedestrian bridges also took advantage of long-span steel trusses over 80 feet in length to connect new and existing parking garages across 4th Avenue.
Development of Project Specific Loading
Architectural programming combined with the site located in northern Minnesota created unique snow and wind loading conditions. The project was designed in accordance with the governing Minnesota state building code, which references IBC 2012 along with subcode ASCE 7-10. Code-prescribed wind loads for this site were developed based on an exposure category D due to the proximity to Lake Superior, even though the site is in an urban area. Despite the code provisions, the site terrain made the design team question how much wind loading this building would experience. To optimize the structural design, a wind tunnel study, and a snow and ice evaluation were performed by Novus Environmental Inc. (now SLR). A wind tunnel study was performed for a few reasons: to refine or reduce code-required wind loads and to develop components and cladding loads.
Wind tunnel results yielded wind loading significantly lower than those required by code. Per ASCE 7 chapter 31.4.3 Limitations on Loads,
“Loads for the main wind force resisting system determined by wind tunnel testing shall be limited such that the overall principal loads in the x and y directions are not less than 80 percent of those that would be obtained from Part 1 of Chapter 27 or Part 1 of Chapter 28.”
The code describes a few conditions that could allow for further reduction; however, the site did not meet these criteria; therefore, the reduction in wind loading was limited to a twenty percent reduction from the code-prescribed loads.
Limiting structural vibration was also a primary focus in the design of the new structure. Expansive surgical and imaging floors with stringent limitations on allowable vibration were of particular concern in bays with long-span conditions and surgical suites over transfer trusses. An initial analysis by EwingCole’s design team verified that all structural steel met the criteria for sensitive equipment per American Institute of Steel Construction (AISC) Design Guide 11 – “Floor Vibrations Due to Human Activity.”
EwingCole’s design team collaborated closely with consulting firm Novus Environmental to further refine the analysis. Vibration limits were set per the Facility Guidelines Institute (FGI) 2018 Hospital guidelines reference. For example, Patient Care areas were designed for a Footfall Vibration RMS Velocity (micro-inches per second) of 6,000, general surgery at 4,000 or less, and imaging at 2,000 or less. Finite Element analysis was performed to develop a full understanding of framing vibration accelerations. With such stringent criteria, an iterative process was used to refine the framing design based on actual room layout and excitation. The framing was analyzed based on walking excitation within the rooms and outside of the room in the corridor, as well as the impact of mechanical equipment in close proximity to sensitive areas. See Figure 2 for a mapping of floor vibration performance at one of the typical patient floors.
Meeting Architectural Goals
The landmark nature of the project led to many monumental architectural features and open interior spaces. The outpatient building boasts a six-story atrium made possible using horizontal trusses on levels two through four to brace the exterior curtainwall and transfer wind loading at a multistory atrium space. Clearstory curtainwall spanning up to 35 feet without intermediate support was constructed from Levels 4 to 6.
The north In-Patient Tower atrium also has a tall expanse of curtain wall. The north curtain wall is hung from the Level 8 steel to minimize vertical mullion sizes and maintain the clear without horizontal support.
The majority of the Out-patient Building’s southeast corner is supported by what is known as a “V” column at the Lake Superior entrance. This column supports nine levels of the Out-Patient building through transfers onto plated wide flange members, all fabricated to meet architecturally exposed structural steel (AESS Category 3) requirements. This category of AESS steel required a visual mock-up of the welds and painted finish for architectural approval. All welds were ground smooth, and the paint was finished with a smooth texture since the columns were visible from a close distance.
Along with the curtainwall, the building façade features many elements, including metal panels, Porcelanosa stone, sunshades, and projecting brickwork to blend the new with existing buildings. HSS tube hangers, braces, and horizontals support a complex brick façade. The exterior brick mimics the brickwork of neighboring 19th and early 20th-century buildings with projections. It reveals detailed hung and cantilevered steel, all supported by spandrel beams offset 13 feet from the closest columns. Detailed calculations were required to meet the brick live load deflection limits.
Steel plate girders were used on Level 6 of the Out-Patient building so that columns supporting floors 7 through nine could be pulled toward the interior of the building. This allowed ample outdoor balcony space on the sixth floor with no columns.
HSS tubes create multiple extended wing walls with curtainwall façade. These tubes featured end plate connections with thermal isolation material separating the external wing wall feature and interior conditioned space. Thermal break material was integrated into the cantilever connections to maintain envelope continuity.
Designing for the Future
This project used overbuild capacity in the existing 1st Street Duluth clinic design to add two stories. Additionally, structural provisions were made for an expanded future roof garden for women and children. The Out-Patient building was also designed to support a future expansion of two stories atop the building.
Novel projects require and promote innovative project solutions. Essentia’s Vision Northland presented abundant and unique challenges based on architectural goals, existing conditions, underlying geology, site topography, and a harsh northern climate. The structural design was optimized for long horizontal and vertical spans in the presence of refined yet substantial wind and snow loads. On-going hospital operations in existing buildings required a construction schedule to minimize the effects of blast vibrations. In addition, the new structure was designed for stringent occupant footfall vibrations. Growth of the new structure was considered with future overbuild loads in the current iteration of the design.
On July 20, 2023, Essentia Health held the Grand Opening for the hospital to welcome patients and caregivers with all the accolades one can imagine for such a formidable facility. The exhaustive efforts of the design team, contractor, and administration led to a successfully designed, detailed, fabricated, and erected facility to provide Duluth, Minnesota, with a state-of-the-art healthcare facility ready for this generation and the next.■
Project Team
Owner: Essentia Health
Architect: EwingCole
Structural Engineer: EwingCole
Acoustic/ Vibration Consultant: Novus Environmental Inc.
Construction Manager/Concrete Contractor: McGough Construction
Steel Fabricator: LeJuene Steel Co.
Steel Erector: Danny’s Construction Co.