Interconnected Silos Built with Post-Tensioning

The Seward Silo project involved the post-tensioning of three interconnected ash silos that are part of the Seward Re-Powering Project in Seward, Pennsylvania.

The General Contractor built three 187’-6” tall, interconnected, in-line silos: two 82’-4” diameter fly ash silos and one 64’-8” diameter bed ash silo. The silos were built using the slip-form method of construction and are believed to be the first interconnected silos built using post-tensioning as the primary circumferential reinforcement worldwide. STRUCTURAL TECHNOLOGIES/VSL’s scope included design assistance to the engineer of record, material supply, access equipment, technical support during the slip, and supervision and labor for strand installation, stressing, and injection grouting. The reinforcement included 366 horizontal post-tensioning tendons in the walls, using the VSL ES6-12 anchorage system and galvanized duct with 4 to 12 – 0.6” strands per tendon. Strand installation, stressing and grouting operations were completed at the four intersection wall locations and the eight external pilasters. The slip was completed in 9 days with STRUCTURAL TECHNOLOGIES/VSL technical support on-site continuously during the duct and anchorage placement. Once the slip was completed, access to work was achieved using swing-stages supported with steel beams placed at the roofs of the silos.

Significant snowfall and subzero temperatures made progress challenging, yet with a strong focus on safety, both cold related and otherwise, the job was completed with no incidents.  The job required close coordination between the various trades working in close proximity and constant communication between parties working above and below work locations to phase the work to avoid having personnel under an active work zone.

The strand installation, stressing and grouting operations were completed successfully, with cold-weather grouting made possible through a variety of heating methods.