Innovative Post-Tensioning Work In San Jose
|Clark Pacific is using new pre-cast post-tensioning design in parking structure construction. (Photo: Courtesy of Clark Pacific)|
The new 1,840,000 square foot parking structure will allow some 5000+ vehicle stalls to support the major rental car companies at the airport (RAC Structure). The newly designed structure includes an attached structure housing all car service facilities (QTA Structure).
Clark Pacific is responsible for the pre-cast concrete parking structure including a state-of-the-art alternate post tensioning design — a $35,235,000 contract.
California Builder & Engineer asked Farid Ibrahim, P.E., director of preconstruction services, Clark Pacific, some questions about the concrete pre-cast elements of this construction project:
What exactly is this state-of-the-art alternate post tensioning? The "alternate post-tensioning" is an innovation that was designed specifically for this project. The pre-cast system (utilizing a post-tensioning technique at the collector beam elements) allows for a single pre-cast element to replace the work of two pre-cast elements and one cast-in-place element that would have been very complicated and expensive to construct. Up to 27 post-tensioning cables were at each of these beam elements. The elements represent an innovative solution that was only possible with pre-cast design.Describe the sequence of how columnsand/or beams are assembled. First, pre-cast columns are erected on the site, they are grouted into place, they are followed by pre-cast beams, and then by pre-cast double T floor elements which then receive a cast-in-place topping pour. The speed of pre-cast construction is a huge benefit to the whole project. Months are saved from the overall construction schedule using a total pre-cast design.Do you assemble the post tension components (including pouring concrete for them) on site, or assemble and cure the concrete off-site? All pre-cast elements are cast off-site and then brought to the site for just-in-time construction. Very little lay-down area is required. For an active airport, this is a considerable benefit. The pre-cast elements for the San Jose airports are being cast at Clark Pacific's West Sacramento plant (columns) as well as its recently opened Woodland plant (beams and double Ts).Is there a special mix of concrete you are using for the post tensioning vs. the foundation and floor of each level? Yes. Clark Pacific uses Self Consolidating Concrete (SCC) for its pre-cast elements. It makes it much easier to pour and imparts a nice form finish to structural elements.What are the major challenges to this project you are facing? Site constraints at an active airport facility.What are the seismic loads it is capable of withstanding? This pre-cast structure was designed to meet current code requirements for this region. San Jose is located in one of the most active seismic regions in the United States.Are there special kinds of equipment being used? We use a large crane to pick the pre-cast elements which are very heavy (some column elements weigh up to 92,000 pounds).Are there special construction techniques being used? Not really. Pre-cast is easy to assemble in the field and work progresses very quickly, and we don't need any special techniques to do so. That said, the post-tensioning (stressing) of the collector beam elements is a special technique more often seen in bridge construction.Who is supplying the concrete mix? Clark Pacific generally batches its own concrete at its plants, though a local Cemex plant is providing concrete for the Woodland plant (beams and double Ts).Crew size? Clark Pacific's crew size is around 20. Areas covered by the crew include engineering, crew supervision, layout, erecting, stressing, and grouting.