Tilt-Up Solution for Florida Electronics Company
By: Richard McPherson, P.E.
A new office building for Florida electronics company, Jabil, was constructed using precast concrete panels via Tilt-Up methods.
Due to panel width, off-site precast wall panels were not an option, so site-cast (Tilt-Up) was selected as a load-bearing component of the structural system. This presented numerous challenges since the proposed building was inside Jabil’s campus and was bounded by a lake and trees at the back, an existing building with active parking at each end. At the front, only a portion of an active parking lot was released for construction (Figure 1). But it contained the main trunk of a fiber-optic cable, which was close to the deadman location. Therefore, storage, job trailer space, parking, casting area, etc, were minimal and deadman location was critical. Traditional crane access along the back of the building wasn’t possible due to the lake and trees while the ends had limited space. Therefore, some of the end panels had to be cast when the crane had cleared the area.
Panel Casting
The decision was made to design as many of the panels as possible on the front and back exactly the same, which allowed the panels to be cast one on top of the other. A 51mm casting slab was placed inside the building, at each end after the crane exited, as well as at the front (Figure 2). Stacking panels has the potential for quality problems because the trowel finish the lower panel receives is generally not as good as the finish the casting bed receives. This leaves a wavy surface, which reflects in the exposed face of the stack panel and becomes apparent with certain angles of the sun. With this in mind, the panel was cast with maximum exposure to view (the front panel) down and stacked the back panel (which had a lake view). Special care was taken during finishing the bottom panel, resulting in a flat finish on the exterior face of the back panel with only minor imperfections.
Panel Erection
The erection sequence was started by coming in one end of the building and lifting first the back panel and then the opposite front panel, walking on and breaking up the casting bed while moving up to the next set of panels. This continued until the crane exited the opposite end of the building. While the remaining front panels were being set, the end and stair tower panels were cast on 51mm beds and the interior pile caps and elevator pits were installed. Next, the end and stair towers were set. With limited area and crane access, the panels had to be braced to the outside, which required a deadman for each brace (Figure 5). Consequently, the panel was erected and with the main crane still hooked to the panel, an all-terrain crane lifted the brace into place and a man lift with two operatives installed the six braces and bi-pods per panel (Figures 3–5).
The erection process was slow – completing only three panels per eight-hour day. The braces required bi-pods to reduce the slenderness ratio of the brace and increase their capacity. The deadmen were located so they could be left in place and were designed to resist the maximum brace load from a 70 mph hour wind. The combined vertical and horizontal loads with a factor of safety of 1.5 were used to design the deadman. Horizontal loads used passive earth resistance but only a portion of the friction resistance.
Panel Stability
Special challenges arose when the architect wanted to install spandrel glass in the panel legs on each side of the window opening, requiring a 95mm recess the full width of the leg (see SK-2 (Figure 6) and Figure 3). This design element made the panel very flexible both during erection and when they were in place and subjected to the constant coastal winds. During erection, calculations indicated the panel would crack at these 95mm recesses and then, when the panel was up (even if it didn’t crack), erection design winds would produce deflections at the top of the panel in the range of 317mm. These elements were obviously dangerous and not acceptable since panel collapse was possible. Therefore, strongbacks were required not only for erection, but also for in-place construction until floor beams were installed.
Standard strongbacks had only web connections, which allowed some rotation at the splice before resistance to bending became effective. This rotation was excessive for erection, but within tolerance when the panel was up. It was decided that during erection very stiff strongbacks would be necessary for minimal deflection and safety, so a pair of C12 × 20.7 were installed on each leg of the panels and in the section with the openings (Figure 7). The panels came up crack-free and once the panels were up and braced, the C12 × 20.7 strongbacks were removed from the panel with another all-terrain crane and man lift and replaced with a pair of vendor C 8 × 11.5 strongbacks (Figure 8). These remained in place until the floor beams were welded in place. The analysis was correct since the building was erected without mishap.
Conclusion
During my 46 years working with Tilt-Up, I have not encountered a more challenging project considering space limitations, exterior bracing, panel flexibility and safety. However, with thorough planning and careful execution, the project turned out well (Figures 9–11).
