Client: City of Tacoma Public Assembly Facilities Department
Architects: McGranahan, Messenger Associates; Wendell Rossman – Rossman, Schneider & Gadberry – Ensphere dome concept
Contractors: Merit Construction – Jim Zarrelli
Glulam suppliers: Western Wood Structures, Inc.
Structural engineers: Hine, Wessel & Associates, Tacoma

With a span of 161.5m and a height of 48m, this is one of the world’s largest timber structures. Designed for 23,000 seated spectators, the Dome is home to American football, rodeo, ice hockey, basketball, tennis and motocross. This ‘ensphere dome’ is a practical modification of the geodesic domes pioneered by Buckminster Fuller (1895 - 1983). A pure geodesic dome is achieved by projecting an icosahedron (a figure of 20 sides, each surface comprising equilateral triangles) onto a sphere. Unless it is hemispherical, there are irregular lengths amongst the edge members at the base, which is inconvenient in forming entries to the building and in making connections to the ring beam. Also, when a sports dome of more that 100m diameter is required, it would produce too high a structure, both practically and for the economy of the materials. The ensphere system, developed by Wendell Rossmann, solves these problems.

The system involves a combination of hexagonal and triangular shapes. The outermost, bottom ring of the network is purely triangular, with the appearance from the inside of a circular Warren truss.

Higher up, the network looks like a series of diamonds. At the nodes there are six incoming members, and the hexagonal pattern is emphasised when looking up at the crown. The overall resolution of the geometry is thus aesthetically satisfying, as well as practical.

To interpret these geometrical concepts in timber, ribs are necessary, and these are generally formed with glulam, as is the case with the Tacoma Dome, where the primary members are laminated Douglas fir. These ribs measure from 170mm to 220mm in width, and 750mm in depth. Typically, the principal members are 15m long. For erection, units were connected on the ground to create modules that were light enough for craning, but stable enough to gradually add to the increasing ‘giant igloo’. Only two months were required to complete this erection process.

Layers of 50mm tongued and grooved Douglas fir, selected, dried and endjointed using finger joints, is used for the structural sheathing. The external membrane is an advanced polyurethane material, self-coloured to display the geometry. At the base of the dome, a pre-stressed concrete ring beam acts to resist the structure’s outward thrust.