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Award of Excellence: Industrial Category

Structural Modifications to the Piti Power Plant Stack Foundation Pedestal
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Piti, Guam

Submitted by Structural Technologies, Inc.

Piti Power PlantIn early 1998 construction began on Guam’s first privately funded and operated power-generating facility. Piti Power Plant Units 8 and 9 were constructed of structural steel with precast concrete perimeter wall panels. The slow-rotation diesel generator was exhausted via a 150 foot (46 m) tall structural steel stack, consisting of two inner oval-shaped stacks, surrounded by a larger circular-shaped stack.

Shortly after the construction of the concrete foundation pedestal for the stack, the contractor undertook a series of pullout tests of the large diameter anchor bolts. All tested anchor bolts failed at pullout loads of less than 10% of that load required to be resisted. It was revealed that the 220 smooth (unthreaded) anchor bolts, without an end-bearing plate, were installed with the shafts of all anchor bolts being painted. It was also discovered that the embedment length of the anchor bolts was insufficient to resist the accumulated pullout loads from a group of anchor bolts for the outer stack. The mode of failure in this instance was a cone-failure mechanism due to group load effects.

To reestablish the anchor bolt pullout capacity, each bolt was removed, sandblasted, and the hole from which it was removed was acid etched. The anchor bolts were replaced and epoxy-resin was pressure-injected into the small annular space. To increase the global pullout capacity of the pedestal due to the inadequate anchor bolt embedment length, additional #10 vertical reinforcing steel was epoxy-doweled into 6 foot (2 m) deep concrete core holes. The size and depth of the reinforcing steel enabled the potential concrete cone-failure surface to be strengthened. To verify that all anchor bolts were able to withstand the intended loads, all 220 anchor bolts were proof loaded to their intended force for a period of one minute. The work was carried out in 21 days.

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