The new office complex that will soon be the corporate home of Office Depot posed quite a few challenges as a tilt-up project for concrete contractor Woodland Construction Co., Jupiter, Fla. Not only were the three towers five stories tall, the architect also wanted a stepped look on the exterior with a flush interior. Additionally, the elevated concrete decks had to meet strict flatness factors due to interior track wall systems.
Numerous architectural details occur throughout the tiltup panels. The 2-foot overhang at the parapet runs all along the front side of each building and was cast as an integral part of the upper panels.
Credit: Woodland Construction Co.
“We got involved very early working with the construction manager,” says Woodland president Gary Fischer. “We worked with the design team to make sure it was constructible.”
That spirit of cooperation is very evident in the solution to one architectural challenge. Woodland formed the four-story panels of varying thicknesses with overlaps to achieve the stepped-back look the architect wanted. But to get the proper offset for the fifth floor required casting that wall as separate panels, then placing them onto the lower panels. Additionally, a 2-foot eyebrow parapet on the top floor of the main towers was placed as an integral part of the top panels.
Going to a two-part exterior wall introduced an additional bracing challenge on the 90-foot-tall buildings. The solution was in the construction sequencing. After erecting the four-story panels, crews worked around the bracing to install the interior steel framing.
The elevated floors in the buildings posed an interesting challenge. The owner wanted them to be very flat, which ordinarily could be handled by designing them for composite action and using loose shores to control deflection during placement. However, in this case the floor slabs were being built from the top down. With no lower floors from which to shore, that approach was unfeasible.
Instead, Eldon Tipping of Richardson, Texas-based Structural Services Inc. was brought in to work with the engineering team to develop a design and placement plan for the composite metal deck floors. Again, early planning helped.
“We worked together to develop a cambering strategy,” Tipping says. “It turned out to be very close to the behavior we got in the field.” Observing how the steel framework and metal deck deflected as the concrete was placed allowed Tipping to guide the contractor's crew on where to make minor adjustments in the slab thickness.
Stacking the fifth floor wall panels atop the four-story panels already in place provided one of the bigger challenges on this project.
“We also implemented a number of different quality measures,” Tipping says. One consisted of the contractor placing a horizontal chalk mark on panels and columns 12 inches above the finished floor grade so the finishers could use a template to confirm that the adjacent floor was at the proper grade. Crews also did preplacement surveys and post-placement surveys. “We worked with Woodland to produce floors that were level after they deflected,” notes Tipping. And the plan worked.
Things were a bit tense on the first placement, but that anxiety soon dissipated. “Following the engineer's design, the floor settled in just like it was supposed to,” says Gary Fischer.
To minimize shrinkage cracking, Tipping suggested using a combination of steel and synthetic fibers instead of wire mesh. Propex's Novomesh 850—formerly known as Stealth e3—was selected and used at the standard application rate. The reported performance to date has been good.
Based on the structural framing plan, as well as an as-built preplacement survey, each deck pour was
planned to accommodate settlement during concrete placement and still end up very level.
After the fifth floor deck was placed, the wall units were lifted and set atop the lower tilt-up panels, then braced to the concrete deck. After those panels were in place, the roof structure could be erected and the rest of the floors installed.
“We converted nearly everything to tilt-up,” says Woodland CEO Clay Fisher. “There were six shear walls at the buildings' cores that were originally designed to be cast-in-place. We worked with the structural engineer to have those redesigned as three-piece tilt-up components.” The crews left rebar projecting from each piece, which was tied together as the pieces were stacked and then filled in with concrete “pour backs.”
They also converted the concrete elements in the two promenades, which connect the three towers, to tilt-up panels. That helped both with scheduling and in keeping the project on budget.
“The whole team, including the architect and the structural engineer, really embraced our constructability requirements,” says Clay Fischer. “We accommodated them, too, in many cases, but they both compromised to keep the buildings constructible.”
Another advantage that came into play on the project was suggested by concrete producer Maschmeyer Concrete, Lake Park, Fla. “They suggested we allow them to set up a mobile batch plant on the site to supply all the concrete,” says Clay Fischer. Woodland's contract included all the concrete work on the project, so there was plenty of demand.
An onsite batch plant alleviated any traffic delays. Concrete always was available and close at hand.
Having an onsite plant meant concrete availability and traffic delays were never problems. The company simply used five or six ready-mix trucks at the jobsite to deliver the concrete. Some days were slow, but there also were a number of 500 to 600 yard placements. “Although it didn't save us money on the material, it improved our efficiency,” Clay Fischer says. “We never had anybody standing around waiting for concrete.”
It also lessened the effect of a congested site where not all the panels could be laid out at once. Throughout the project one set of panels had to be lifted out of the way and put into place to allow another set of panels to be formed up and placed.
“Even though we had previously worked with the developer, we didn't automatically get this job,” says Clay Fischer. But in the end, Woodland's expertise in tilt-up construction made it a win-win project. The facility's shell is expected to be completed by the fall of 2008.
- Project: Office Depot Global Headquarters
- General contractor: Balfour Beatty, Ft. Lauderdale, Fla.
- Concrete contractor: Woodland Construction Co., Jupiter, Fla.
- Concrete producer: Maschmeyer Concrete, Lake Park, Fla.
- Statistics: Close to 700,000 square feet of prime office and support space