Citadel Contractors sets record with 89-ft-wide panel.

By: Mitch Bloomquist | Tilt-Up Concrete Association

At 89 ft wide, the front entry panel for York Preparatory Academy High School in Rock Hill, South Carolina is the widest tilt-up panel on record.  Jason Swagert, Vice President of Citadel Contractors explains while the reasons for designing this wide of a panel are simple, the process for lifting the panel is slightly more complicated.

Citadel Contractors, Inc., of Apex, North Carolina, specializes in tilt-up concrete construction.  They own the previous record for widest panel as well at 82 ft 10 in.  That extremely wide panel was erected at Voyager Academy Charter School in Durham, North Carolina in 2011.  The new record was set on another school project.

The new high school in Rock Hill is not the first project Citadel has been asked to build for York Preparatory Academy.  “Our relationship with the general contractor has allowed us to do repeat business with them,” explained Swagert.  “Advantages such as expedited delivery, lower construction costs, design innovation and durability were considered to determine the use of tilt up construction for the project.”

The high school is one of four educational buildings of the charter school program in the Rock Hill area, three were built by Citadel Contractors in 2012 (Elementary, Middle and Student Union Buildings).  York Preparatory Academy was designed for a load of 973 occupants, with 16 classrooms, 5 laboratories, offices, and resource rooms to reach the fullest potential of the students.

Project Manager Alejandro Moreno outlined the aggressive schedule for the project.  “Construction of foundations and slab on grade started on February 11, 2013. Two weeks later we began construction of panels lasting approximately four weeks, followed by panel erection that lasted six days.”  The school is expected to open on August 1, 2013.  Swagert mentioned that a construction schedule of 18-24 months (start of construction to school in session) is typical in the area for public school jobs.  Considering that, a little less than seven months construction time for this project is quite an accomplishment.

There are a total of 64 concrete panels on the job averaging (excluding the record breaker) 25-ft in width and 35-ft-3-in in height. The 7 ¼-in-thick panels weigh approximately 60,000 lbs each.  The largest panel on the project, by far, is the record breaking 89-ft-wide, 28-ft-11½-in-tall, 7¼-in-thick panel weighing 115,780 lbs.

According to Ernie Garcia, Engineering Manager for Meadow Burke who did the lifting design, the key to the process from a lifting standpoint was the incredibly wide spreader bar.  Citadel employed a 46-ft-wide, 10-in diameter steel pipe, which enabled them to spread the rigging across the entire width of the panel.  Conventionally, a spreader bar between 20 and 25 ft wide is used.

Still, the placement of lifting inserts in a panel with a limited amount of surface area presented a major challenge.  “We were able to fit a two high by eight wide configuration in the narrow 4-ft-7-in high solid section at the top of the panel,” said Garcia.  “Due to the inserts being higher up on the panel, the lift required large amounts of vertical reinforcing.  However, most of this required reinforcing was already in place structurally and a minimal amount of additional reinforcement was necessary.”  In addition to the required lifting reinforcing, the design required a minimum modulus of rupture of 500 psi, minimum compressive strength of 3,500 psi at the time of lifting and two vertical double-stacked strongbacks.

The 16-point pick was made using the Meadow Burke Super Lift III System.  The lift required at least 200 ft of crane (Liebherr LR 1300 330 ton crawler) boom due to the total height of the rigging.

While large openings in the panel helped to reduce its weight, they added complexity to the structural and lifting design.  “The large openings reduce the amount of concrete working to resist bending,” said Garcia. “They also leave a only a very small section to resist moment.”

The desire for relatively thin legs between openings added to the decision to go so wide.  Had the façade been broken into smaller panels, each leg would have had to increase in width due to an increased reinforcing steel requirement and the coverage needed, or each individual panel would have had to be reinforced with strongbacks for lifting.  In either case, given a crane with the required capacity, going wider simplified the construction process.  In some cases, designers will stagger panels in this situation so that the full width legs of each panel are overlapping the legs of the adjacent panel, maintaining the slender appearance between openings.  Limiting the number of joints on the front of this building was also of importance to the designer.

Garcia was on site for the lift and stated that everything went according to plan.  “There was a small amount of flexing during the lift, but nothing unexpected from a panel this wide.  The entire process was very smooth.”  The enormous panel went from the slab to its braced position in the façade in twenty minutes.