Article No: 188
Tilt-up tackles time, tight-site challenges
By: Carole McMichael
Photography courtesy of Woodland Construction Company
The Boca Raton campus of Florida Atlantic University had already put one four-story tilt-up residence hall to good use. In great need of another like it, the school tapped 20-year tilt-up specialist Woodland Construction Company of Jupiter, Florida, to build an even larger, 600-bed student housing facility.
“Tilt-up is a fairly common construction system in this market,” says Clayton Fischer, founder and president of Woodland. “It is probably one of the methods of choice over masonry because it is fairly economical compared to it. You have a finished wall that you don’t have to stucco or plaster. It is a load-bearing wall, so you don’t need a perimeter structure. It has the durability to meet hurricane codes on wind loads, and it is a little less labor-intensive than masonry. Everything is basically done flat on the ground, so you don’t need scaffolding.”
When Fischer began to build tilt-up structures, he didn’t face any competition from other subcontractors and his business turned into quite a niche market. Now, a tremendous number of contractors have begun tilt-up construction. Woodland differentiates itself by doing the “Houdini jobs”—the more complex projects that call for casting on difficult sites, creating panels with different layers, and constructing many corners, circles, arcs, and varied openings. To that list, they add precise engineering to ensure every component fits correctly.
A matter of time
According to Fischer, time was the project’s greatest challenge—students planned to move into the rooms just before the fall semester began. If the building was incomplete, Fischer would have to pay to house 600 students in hotels and motels. Fischer devised a plan to keep construction moving forward: “The student housing facility was a hybrid building, with a structural steel frame,” he explains. “We built two- and three-story panels outside the building on temporary casting beds and attached them to the steel frame, so the walls were a closure rather than load-bearing—we did that for speed. As we were putting up the steel, we were building the wall panels. As soon as they got to the second story, we attached the panels to the frame and poured the floors. That way, the trades on the bottom floors could get started sooner than in a conventional building.” Woodland began construction in November and finished in July, on schedule.
The residence hall’s central lobby provides access to two identical seven-story towers, which accommodate 600 beds, or about 300 rooms. Rooms average 600 to 700 square feet. Units called “double doubles” have two bedrooms, closets, a common bath with a shower, and a small entry hallway. Guest apartments built with married couples in mind have a dining and living area and one bedroom. In the north tower, a staff apartment holds a living room, dining room, small kitchen and two bedrooms. In addition to a reception area, the lobby contains a computer lab, three study rooms, a classroom, a laundry room, a conference room and four offices.
Exterior walls are finished with colored stucco and a sand-textured finish. Fischer added applied moldings and cast a random pattern of reveals into the panels. To add more depth to the exterior walls, he applied horizontal molding at the tops of the first floor, the fifth floor and the roof parapet.
The building’s pleasant façade—the entry walkway is lined with towering Royal Palms—belies its strength. Every aspect is engineered to survive a Category 3 hurricane. Large windows incorporate a glazing impact system, on a par with bulletproof glass, designed to withstand 145-miles-per-hour winds without shutters. Foundation engineering, crucial to the project’s hurricane resistance, depended on the bearing capacity of the soil. Soil in the Boca Raton area is likely either coral rock or sand with a sugary texture that makes building difficult. Fortunately, the base at the campus was rock, providing better load-bearing capacity and enabling heavy equipment to drive onto the site. Fischer created large, isolated pad footings for each of the building’s columns, and then poured a concrete-slab floating floor.
A tremendous amount of engineering was also required to determine the structure’s wind loads, and tilt-up panels had to be engineered for in-place loads. More important, because they are cast on the ground, panels have to be reinforced before being lifted into place—they will never undergo more stress than during the erection process. Individual bars are tied in place before the pour, so the panels are reinforced to withstand their own weight.
“We also cast in special lifting hardware and connection plates,” says Fischer. “The lugs are placed to incur the minimum amount of stress on the panel. Their location is determined by a lifting engineer using a computer program so the panels come up plumb. We produce shop drawings for each individual panel, which has the dimensions for windows and doors as well.”
Fischer says that determining the layout of panels prior to casting is critical. “There are very small tolerances,” he says. His team makes chalk lines of the dimensions on temporary slabs and then attaches wooden forms. Then crew sprays the forms with a chemical bond-breaker before they pour the concrete, preventing the panels from sticking to the beds. The beds are broken down and recycled after use.
For the FAU housing project, Fischer cast the panels’ exterior face down. The reveals are “innies” and the moldings are “outies” that are attached later. The amount of steel added varied according to the panel, but because panels for this project were closure panels, they were not heavily reinforced. The crew placed No. 5 rebar every 16 inches vertically and No. 4 rebar every 12 inches horizontally. In the world of tilt-up construction, this is considered relatively light reinforcement.
“The panels were attached to the frame every 10 feet on-center, vertically, and every 5 feet on-center, horizontally, so the frame was taking the wind,” says Fischer. “At that stage, we tried to get 20 or 30 panels ready, and set up a big 6-inch pump to pour 500 or 600 yards of panels. The average panel is about 18 feet wide by 30 feet tall. The lobby, which is one story, was the only part of the project with load-bearing tilt-up panels.”
Meeting the deadline
To maximize use of manpower and equipment, Fischer ordered 500 to 600 yards of concrete each day. “The big trouble we had was getting as much concrete as we needed as fast as we needed it in a busy market,” he says. “They are so overbooked, we would request 100 yards an hour and were lucky to get 30. It made for some long days.”
Because of the summer rainy season, Fischer completed much of the casting at night when it wasn’t as likely to rain. The crew started work at two or three o’clock in the morning. The panels would cure enough to be walked on in seven hours, but didn’t have sufficient lifting strength for about three days. Fischer performed a compressive strength test, then a flexural strength test before beginning the construction process.
He used a 200-ton, 40-foot-long crawler crane with a 150-foot boom to reach to the top of the seven-story building. “We had eight lift points for each panel, so we ran the cable through rolling blocks to produce even pressure through all the lifting inserts,” he says. “To attach it to the steel frame, we set it in place, temporarily held it with 10 welds, then cut it loose and went to hook up to the next panel while the guys finished up the welding. On a typical tilt-up project, on a good day, we could probably erect 30 panels in eight hours.”
Building a workforce
According to Fischer, tilt-up construction is so common that finding workers in other trades who have experience with it is not a problem. However, because of a labor shortage in Florida, finding an experienced crew is a different matter. “Our company has a pretty intensive recruitment-training program,” he says. “Our workforce is predominantly Hispanic, from Central and South America. We get families—fathers, sons and cousins. They are smart guys who pick it up quickly and see that they can advance. I have one man from Guatemala who started as a laborer, but today he is one of my best superintendents. We train and promote from within. Some of the workers don’t have much construction experience when they come, but they have motivation. They are very loyal, and we take good care of them. Ten or 15 years ago, we could get kids here out of high school. But it is hot, tough work, so we hardly get any of them anymore.”
Woodland is known as one of the better tilt-up companies in this market, according to Fischer. He gains exposure predominantly through his work familiarizing architects and design engineers with tilt-up construction techniques. “They can call me any time and ask questions—sort of some free consulting on how to panelize,” he says. “I’m their go-to guy. Once they have confidence in you, they spread the word around. We also have a lot of repeat customers.”
Fischer sees a bright outlook for his trade: “In the future, for residential, you will see more tilt-up in Southern markets where you have hurricanes. Actual on-site residential is just starting to take hold.”
For more details on the tilt-up erection process, visit woodlandconstruction.com.