The Portland (OR) Aerial Tram connects the upper facilities of Oregon Health & Science University Hospital (OSHU), located at the top of a canyon hillside, with the lower facilities of OSHU at north Macadam on the bank of the Willamette River, just south of downtown Portland. Since there is virtually no more ability to expand the hillside facility, some room in the North Macadam district and the Tram offered a viable, economical transportation link. The overall goal is a series of connections to link disparate communities through urban design, neighborhood restoration, transportation infrastructure, and landscape design. The Tram began operating December 14, 2006.
Figure 1: Courtesy of Julie Akers
The tram is the centerpiece of the connections strategy. It is designed as a minimal intervention, with light and open structures that dematerialize transportation infrastructure.
The Aerial tram project consists of three structures: upper and lower stations and a central support tower.
The Upper Station is an open steel structure, a covered platform on braced legs balanced on a steep site wedged amongst hospital buildings. It is faced with expanded aluminum cladding, providing transparency between the hospital complex behind it to the open landscape of the city beyond. A tram entry provides a connection from the OHSU campus to the Upper Station, passing through the newly constructed Patient Care Facility.
Two 79- passenger cars move passengers between the Patient Care Facility and the Lower Station at North Macadam. The tram car is a curvilinear form of glass and painted steel intended to virtually disappear against the sky.
The Central Tower is a steel structure whose geometric form is the result of the forces acting upon it. The inclination of the tower aligns with the angle of the resultant forces acting on the tower. The forces under consideration are the self weight of the structure and the horizontal in-operation tram cable tension loads.
The Lower Station is the public center of a newly developing urban neighborhood. Like the Upper Station, it is an open network of exposed steel frame construction and expanded aluminum cladding.
Figure 2: Upper station – south elevation
Structural Description
The cars will operate between the two stations, each suspended on its own track ropes and will be pulled by a third haul rope connected to a drive engine at the Lower Station. The 3,300-foot track ropes will be fully tensioned at all times exerting substantial loads on all three structures. In addition to the tension load on the tram cables, forces due to wind and temperature variations and weight of the loaded tram cars, lateral loads exerted on to the platform level at the Upper Station, will vary from 500,000 to 800,000 pounds. Considering that this load will be permanent, continually vary in magnitude (millions of times during its service life) and applied at the 9th level, approximately 150 feet above the top of the pile cap which is built on a narrow hill slope surrounded by existing buildings, the design team faced a complex design challenge. Moreover, stringent lateral displacement and torsional rotation limits had to be met to keep the track cables in alignment and the aerial tram in operation.
The 200-foot tall Upper Station has a dual structural stability system. Lateral and vertical loads are shared by a concrete core wall, which also serves as an elevator shaft and stairwell, and four diagonal steel legs. The steel legs are made of 1-inch thick plates and form a parallelogram in shape measuring 6.0 x 4.0 feet. The overall concrete core is approximately 10 feet wide by 20 feet long. The steel legs resemble two compasses and provide stiffness in all directions. This design provides substantial lateral and torsional stiffness, is highly redundant and creates a unique architectural form.
The Upper Station structure was modeled and analyzed in 3D using SAP2000. Required to meet stringent displacement limits, the whole structure including drilled pier caps and drilled piers were incorporated into the model. Continuous sustained loading due to tram cables which varies depending on the location of the tram car on the cable, creep effects on concrete and fatigue of structural steel members were considered. Special attention was given to the long term deflections due to creep of concrete.
Connection details between the steel legs and between the steel legs and the platform level girders were documented in 3D, in addition to the typical 2D structural drawings. This made the presentation of the complex connection designs much clearer for the construction team. Constructability issues such as the connections between platform level framing members as deep as 7 feet to the 15-inch thick concrete core wall required special detailing by the design team. This required steel members being embedded in the concrete walls at the 9th level to facilitate the connections due to the high loading.
The Central Tower provides the intermediate support for the aerial tram. The tower measures 196.5 feet from the drilled pier cap to its highest point and slightly leans toward the Lower Station. Its steel-plated construction forms a striking sculptural shape and provides the necessary structural stability. The trapezoidal cross-section varies in width along the tower’s height, with the narrowest section occurring at approximately two thirds of the tower height. This variation of cross-sectional dimensions with height reduced the risks associated with vortex shedding. At its base, the trapezoidal cross section measures 22 feet wide and 20 feet long, at the neck region it narrows to 8 feet wide and 8 feet long. At the top, the cross section measures 8 feet wide and 32 feet long.
Some sections of the steel plating were subjected to high compressive stresses under service loading. Analysis of the tower using a 3D SAP2000 model indicated that design level seismic and wind loads, in combination with certain tram load cases, created substantial stress demands on the steel plates. In contrast, thermal analysis showed that stresses due to temperature variation across the steel plates were not significant. To alleviate the high compressive stresses, the designers opted to utilize the benefits of composite construction by placing concrete infill at the corners of the tower where compressive stresses were substantial. Utilizing 10,000 psi self-consolidating concrete allowed the designers to increase the efficiency of the structural system. Steel plate thicknesses were limited to 5/8 inches, except at the neck region where 1-inch thick plates were used. Furthermore, local buckling analysis was performed in SAP2000. Vertical stiffeners and diaphragm plates were designed and placed along the tower height accordingly.
The Lower Station has a simpler structural system. A reinforced concrete basement contains the mechanical equipment to operate the aerial tram in addition to the bollards that serve as the fixed ends of the tram cables. The station is covered with a 45 foot tall steel canopy. The structure is subjected to substantial uplift due to potential high water levels and lateral forces due to the tram loads.
Foundations
The foundation system consists of re-inforced concrete drilled piers which are 24 inches in diameter. The Upper Station steel legs and concrete shear walls are supported on a 10-foot thick reinforced concrete mat with 76 drilled piers that are embedded 35 to 50 feet into the underlying basalt formation.
The Central Tower is on a drilled pier cap, five-feet thick, which is supported on 35 drilled piers ranging in length from 35 to 55 feet below the pile cap.
The Lower Station structure and basement is supported on a reinforced concrete mat that varies from 2.5 to 4 feet thick. The mat is then supported on 39 reinforced drilled piers extending to 20 feet down.
All drilled piers of the Lower station and at the Central Tower are also 24 inchesin diameter.
Figure 4: Lower Station. Courtesy agps architecture
Construction
Construction of the Stations and Central Tower started with the foundation work in mid-2005. Structural steel work on the Upper Station began with the erection of the steel legs. Steel leg segments and joints were fabricated at the Thompson Metal Fab Inc. steel plant and the segments were welded together at the construction site. Special care was taken during the erection process. The 3D Sap2000 model created for the analysis of the structure was used by the contractor for erection sequence analysis, and the position of the nodes of the steel legs was closely monitored. The Central Tower was fabricated in three pieces by Thompson Metal Fab and transported to the construction site with barges. The 90-foot base piece, weighing 112,000 pounds, was installed on June 12, 2006. Construction crews later installed the 60-foot second tier and the 45-foot third tier of the tower on June 21 and July 5, respectively. Welding of the one-inch thick steel legs of the Upper Station and pieces of the Central Tower was done in several stages, and with utmost care to avoid deformation of the welded plates.
The extended group effort between the City of Portland Engineers, agps architects, Arup Structural Engineers, Contractor, Steel Fabricator, Steel Detailers and Erector resulted in a construction performance that dramatically exceeded everyone’s expectations and made possible a successful project.▪
Figure 5: Upper station – East Elevation
Atila Zekioglu, P.E., S.E, is a Principal of Arup and leads the firm’s Buildings business in the Los Angeles office. He served as the Structural Engineer of Record for this project.
Steve Carter is a Principal of Arup and the Los Angeles office Staff Group Leader. He served as the Project Director.
Edwin Shlemon, P.E, S.E, an Associate Principal in Arup’s Los Angeles office, and served as the Lead Structural Engineer for this project.
Murat Melek, Ph.D., is Structural Engineer who served as Project Engineer for this project and specializes in three-dimensional analysis and design.
Project Statistics
Horizontal Travel Distance, Dock to Dock 3387.6 feet
Vertical Travel Distance 476 feet
Travel Time 180 seconds
Maximum Number of Trips per Hour 16
Weight of Steel Upper Station Support Legs 1,070,000 lbs. of fabricated plates for legs 668,000 lbs. at platform framing
Central Tower 560,000 lbs. of Fabricated steel plate. 900,000 lbs. of 10,000 psi concrete in tower
Lower Station 193,000 lbs. of roof framing and lateral bracing
Figure 6: Central Tower. Courtesy of the City of Portland