Tree Sculpture Engineering
Engineering a "Tree of Life"
On entering the new casino from the existing one, a guest first encounters the "tree of life," a massive column cover and suspended ceiling system. The overall impression is of a funnel of natural textures (wood, birch bark, and parchment) combined with shimmering, multicolored glass beads, spreading out across the ceiling.
The four distinct features of this tree are vertical wood poles encircling the base of the steel building column; eight wood branch assemblies that reach up and out; eight part-funnel frames that fit between branches, containing birch bark, parchment, and glass beads; and 18 hanging ceiling panels of varying shapes and orientations, also containing bark, parchment, and beads.
The "tree" is roughly 40 feet (12 meters) tall and casts its shadow over 4000 square feet (372 square meters) of floor area. It was built by MJM Studios of New York.
Because the tree of life occurs at a double column expansion joint, the major engineering challenge was to develop a lightweight system that could accommodate vertical roof deflections and lateral column expansion.
Preliminary designs proposed that the columns support the branches. However, we advised against roof supports and column supports in the same assembly in order to avoid differential deflection problems. Also, we wanted to avoid reinforcing the columns because the necessary coordination and construction would have delayed the installation schedule.
A thorough review of the design revealed that the roof framing could safely carry the branches and all panels. We specified that cables be added from which to hang the branches that were originally intended to be cantilevered. The branch and funnel assembly connects to steel frames that encircle the columns, forming hubs that each branch fastens to. The cables suspend these frames from the roof framing above.
We designed the hanging panels with tubular perimeter frames onto which the birch, parchment, or bead frames fasten. The tubes resist lateral thrust caused by bead-cable tension forces that are contained within the overall assembly. The tubes also allow reasonable spans between support cables. Nevertheless, enough cables were provided to ensure that the weight was distributed between all of the roof members above.
The tree of life leads into a retail area decked with a series of similar wood tree forms and overhead beaded canopies.
Another major architectural element is in the hotel lobby, where ten tree-like sculptures encircle a reflecting pool. Each steel tree has a 35-foot- (11-meter-) tall trunk tilted at 10 degrees from vertical with tapered, tubular steel branches at two levels. The trunk is clad with wood, and the branches are painted to look like wood.
Each branch level carries a draped canopy tensioned at each branch tip with a central opening for the trunk to pass through. The canopy is made from 3/8-inch- (9.5-millimeter-) diameter glass beads strung on 1/32-inch (0.8-millimeter) wire rope, spanning between 1/4-inch (6.3-millimeter) perimeter wire ropes.
The glass beads include a variety of colors arranged in patterns and were manufactured and assembled in India. When the 20 bead canopies and the larger, roof-supported canopy above them are backlit by the skylight and overhead lights, the effect is a layered, brilliant glow.
For this structure, the engineering challenge was to maintain the desired shape of the bead canopy with branches sized to satisfy specific aesthetic and lighting demands. The branches had to be sufficiently stiff to resist the upward, inward, and lateral pull of the canopy. Cutouts were needed in the tops of "branches" to accommodate the recessed lights.
We developed a structural analysis model using the software, STAAD/PRO from Research Engineers International. We modeled each tapered tube as a discrete rectangular tube section and determined that hollow sections were needed throughout the entire length to limit branch-tip deflections resulting from twist.
As a result of our analysis, supplementary longitudinal plates were installed below the top cutouts to provide hollow section behavior. All branches were fabricated from 1/8-inch (3.2-millimeter) and 3/16-inch (4.8-millimeter) plates welded together to form tubes. Each tube was bolted to a stub-out that had been shop-welded to the trunk.
The branch geometries were adjusted in detailing and fabrication so that they would deflect to theoretical locations in space when the canopy loads were applied.
We designed the bead strings with a safety factor of greater than six. During shop set-up of a bead canopy mock-up, tension measurements were taken at each branch connection location closely matched theoretical design values, thereby confirming the design. To further demonstrate the mock-up's safety, a worker walked on it without damaging it.
The trees' installation was made still more challenging by the curved soffits above, the lack of overhead rigging, and the limitations for handling equipment on the floor. The contractor, Themeing Solutions of North Las Vegas, Nevada, developed a winch-operated handling and lifting carriage to install each tree.
Further complicating matters was that the lower bead canopy, had to be installed after the tree was erected. A custom-made, trussed assembly was moved into place to lift and then to tension the bead canopy. One by one, each canopy corner was transferred to the branch tip connection. Finally the canopy rigging hardware was adjusted so that the canopy was correctly placed.
McLaren engaged in similar engineering processes for other themed architectural elements on the Mohegan Sun Casino. These included a variety of suspended ceiling panels, additional wood trees, large chandeliers, wall cladding and glazing systems, free-standing "torchieres," decorative guardrails, and other sculptures.
The expansion of the Mohegan Sun Casino is a billion dollar project designed by Kohn Pedersen Fox Associates that includes a new casino, retail areas, restaurants, showrooms, and a hotel tower. The project more than doubles the space available for gaming and improves the site as a tourist destination.
William B. Gorlin, P.E., S.E. is an engineer with M.G. McLaren, P.C., of West Nyack, New York.