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Simpson Co-generation Plant - Tacoma, Washington

Sitts & Hill Engineers, Inc. was hired as the Prime Consultant, Structural Engineer and Building Designer. Our scope of work included structural analysis and design of the new Cogeneration building, including building code requirements for exiting and fire ratings, as well as design of several auxiliary buildings and structures. Coordination with the owner and the electrical and mechanical engineers was critical to our success.

The main building consisted of two portions: the two story turbine hall (80' x 86'-8" x 76' tall) and the two story electrical room (51'-4" x 57'-4" x 54' tall), with concrete block (CMU) walls and steel framed floors and roofs with concrete slabs. The foundation consisted of concrete slabs spanning to grade beams and pilecaps supported by 18 inch diameter augercast piles. A separate structure inside the turbine hall was designed to support the turbine and generator. This turbine pedestal consisted of a concrete slab 4' thick by 46'-6" by 29' supported on six concrete columns 5' by 5' by 28' high. The pedestal was designed to be isolated from the main building structure due to vibration of the turbine and generator. Lateral deflections and vibration modes of the concrete structure were checked to avoid coincidence with those of the machines.

Other auxiliary structures included:

  • Foundations for power poles from the Cogen building to the substation consisting of pilecaps and augercast piles.
  • Foundation for the generator at the substation consisting of a concrete mat supported by auger cast piles.
  • Foundations for substation components consisting of concrete pilecaps supported by steel pin piles.
  • Foundations for power poles between the substation and the public power grid consisting of large diameter caissons.
  • Analysis of existing structures and design of repairs as required.

Notable facts include:

  • 65MW triple extraction, condensing, steam turbine generator
  • Boiler modifications for increased pressure and temperature
  • 220' circulator hog fuel stacker/reclaimer
  • Mill piping modifications to move steam demand to lower pressures
  • Conversion of boiler fans from steam turbine drives to electric motors
  • Approximately 20,000 lf of piping
  • Total of 141 piles driven to an average depth of 60 feet
  • Over 240 tons of rebar set
  • Over 3,500 yards of concrete placed
  • Over 51,000 square feet of masonry installed
  • Installation of the new condenser. The total weight of the condenser was over 170,000 lbs
  • Over 500 tons of structural steel fabricated and installed
  • 10,000 square feet of roofing installed
  • 3 roll up doors and 12 man doors installed