EYE IN THE SKY
Engineer uses Algor FEA to Overcome Alaskan Wilderness in Tramway Terminal Design

The Mt. Roberts Aerial Tramway (bottom right) above Juneau, Alaska, is the first North American tramway built and installed by Poma of America, Inc., Grand Junction, Colorado. The upper terminal tower was designed and analyzed by Les Okreglak of Pol-X West, Inc., Carson City, Nevada with FEA software from Pittsburgh-based Algor, Inc. Rock anchors (top right) support the weight of the tower. Tramway photo courtesy of Rick Spear, Poma of America. Anchor photo and model courtesy of Les Okreglak, Pol-X West.

September 4, 1998, Pittsburgh, Pennsylvania - Two thousand feet above Juneau, Alaska, from the Mt. Roberts Aerial Tramway upper terminal, visitors can view pine covered mountains from the enclosed platform, browse through boutiques and dine in café-style restaurants. The 170-foot terminal tower and 3,000-square foot platform are connected by a covered walkway, which leads to a loading deck where passengers board and exit large tramway cabins carrying 60 people at a time.

Open for public operation since August 1996, the Mt. Roberts Aerial Tramway is the first tramway in North America manufactured and installed by Poma of America, Inc., Grand Junction, Colorado. To ensure that the upper terminal platform and tower would withstand the unpredictable Alaskan environment, Poma of America enlisted the finite element analysis (FEA) expertise of Les Okreglak, P.E., principal engineer and president of Pol-X West, Inc., an engineering consulting firm in Carson City, Nevada.

Mr. Okreglak used FEA software from Pittsburgh-based, engineering software producer Algor, Inc. to analyze the massive support tower, cables, platform and covered walkway. The task proved to be no small feat for Mr. Okreglak, who had to consider 120 mph winds, snow and ice as well as the loads imposed by the operation of the tramway. He also had to design individual members of the tower structure in such a way that each could be transported by helicopter since no road access was available to the construction site of the tower.

Okreglak built the 3-D tower model in CADKEY and transferred it to Superdraw III, Algor's precision finite element model-building tool. From Superdraw III, Mr. Okreglak used Algor's Beam Design Editor to apply loading and boundary conditions to the tower geometry.

"Algor's beam modeling capabilities enable me to easily make adjustments to the model as specifications and design needs change," said Mr. Okreglak. During one instance in the design phase, the specified snow loading value was increased substantially, according to Mr. Okreglak. "I was able to easily modify the design to accommodate the larger load."

Once the design was optimized, 3-D CAD models were used to make detailed drawings of the steel leg assemblies that had to be transported separately and constructed on Mt. Roberts. The completed Algor beam model contained over 1500 beam elements, which Okreglak analyzed in six different load cases using linear static stress analysis.

Besides wind, snow and operational loads, Mr. Okreglak's main concerns included deflection that could result from the weight of the 250-ton steel tower and axial forces in a series of support cables. These cables connect to 36 rock anchors in the mountainside and provide the sole source of support for the tower. Mr. Okreglak used 3-D topographical maps to determine the placement of the anchors and tower foundations. He applied fully constrained boundary conditions to these points in the FEA model.

Over the course of six months, Mr. Okreglak worked with Poma engineers to modify the design by adding and changing support members to meet changing design specifications. The model needed to be perfected since no prototype testing was possible on such a large structure. According to Mr. Okreglak, the final analysis results were extremely accurate.

"Once I completed the analysis results, another analysis firm double-checked my calculations. They matched Algor's results almost exactly." Mr. Okreglak said. "Actual deflections of the tower were between two and five inches. This is very small when you consider the size and mass of the whole assembly."

According to Mr. Okreglak, the tramway tower project was challenging because of the environmental factors he had to account for in his design. "Designing the tower members for transport by helicopter was very difficult because the members had to be a very specific weight and length," Mr. Okreglak said. "I could not have completed this design without Algor's beam modeling."