The new federal courthouse in downtown San Diego comprises of two subterranean levels of concrete system and 17 levels of approximately 23,000 square feet each on a 70,000-square-foot footprint. The steel-frame building structure has an architectural concrete elevator core and architectural concrete staircase. Designs called for the concrete core to be placed ahead of the structural steel, which would trail the core a few levels below.
The elevator core is best described as a rectangular C shape, with the long side dimension of 45 feet and the two short returns each at 11 feet 3 inches. The 16 architectural lifts included several different pour heights, which required the formwork to be able to accommodate a 25-foot lift, followed by a 17-foot-6-inch lift, multiple 13-foot-9-inch lifts, numerous 21-foot-3-inch lifts, a 23-foot-9-inch lift, a 20-foot lift, and finish with a 15-foot lift. The panel on the architectural side along the 45-foot length was designed as a single panel to provide a seamless concrete finish. Weighing approximately 15,000 pounds, this panel required a spreader beam to accommodate the hoisting of its 47-foot width.
Because the exterior of the core is a highly visible architectural building element, exterior wall panels were set first in order to ensure proper elevation of the panels and proper alignment of reveals, plywood joints, tie locations, and panel joints prior to rebar installation. Four 14-foot-6-inch-wide work platforms, two on the long side and one on each short side, were mounted to architectural tie holes from the previous lift. They were designed to adequately brace and support the largest panels that would be required, and had three additional access platforms trailing them to remove reveal strips, tie hole cones, and retrieve the platform attachment hardware used to mount the platform to the previous lift. The overall height of the work platform assemblies reached 29 feet 9 inches. When the wall panels were set for the 25-foot-tall lift, the overall height of the formwork system stretched to nearly 55 feet.
A 43-foot-long, 7-foot-8-inch-wide truss platform was prefabricated in Atlas Construction Supply Inc.’s San Diego yard and delivered to the site to support the interior core wall panels. At the end of each side of the truss platform, flipper pales supported the platform via tapered void pockets cast into the previous wall lift. The hoisting operation of the truss platform from level to level simply required the lifting of the platform until the flipper pales clicked into place at the next level of void pockets. One of the important requirements of the truss platform was the provision to accommodate the installation of structural steel columns and beams below. The 7-foot-8-inch width of the platform allowed for the W27x94 horizontal steel beam placement below, which was centered 9 feet off the face of the long side of the inside of the core. The more challenging provision was the accommodation of the W16x36 columns located on the ends of the short return walls on the inside. The platform needed to stay above these steel columns so that the crew could travel back and forth between the outside platforms and inside platforms, which presented an interference problem with the installation of the steel columns. The solution was to make a section of the platform hingeable at both ends. These hingeable ends were each 6x6 feet. Most of the time, the hinged sections would be down for proper working access between inside and outside platforms.
When it came time to install the steel columns below, which occurred eight times throughout the course of the core construction, the safety pins of the hinging platforms were removed so they could swing up out of the way as the columns were placed.
The concrete core preceded the rest of the building and therefore needed its own access. A temporary stair system was placed inside the core under the truss platform. Ladders extended from the top stair landing to the lower truss platform, with another ladder leading to the upper truss platform. As the platforms were raised, additional sections of the temporary stair tower were added.
The panels were rated for 8-feet-per-hour concrete placement. A double laminate of 3/4-inch plywood with high-quality 5x10-foot sheets as the face laminate were used to achieve the architectural requirements. For the tie system, 1 1/4 to 1 inch taper ties were used, with architectural cones at primarily 5-feet-wide by 4-feet-6 inch-high spacings. The project architect predetermined the tie pattern and plywood pattern, which limited the flexibility to locate ties from perhaps more practical locations for the wall placements and platform attachment locations. At tie locations that would eventually be used to attach the exterior working platforms, a custom-made coil tie was used. It was implemented as a wet tie for the wall placement that would have the same finished look as the rest of the ties, but would allow for one-sided attachment of the mounting hardware for the platforms. The reason one-sided attachment was advantageous was that it didn’t require the inside truss platform also to reach down approximately 30 feet to secure a washer and nut on the backside of the wall.
As a result of the multitude of different lift heights, the exterior platform mounting locations varied widely relative to the construction joints. The distance from the bottom of the wall panels to the platform varied from as little as 6 inches to as much as 5 feet 3 inches. To accommodate such a large range, a system of sliding strongback channels for rough adjustment, combined with a screwjack assembly for fine adjustment, was used to effectively elevate and level the panels at the various conditions.
The top of the core had its own set of challenges. At 282 feet above ground level, the 11-foot-3-inch return walls extended to 20 feet long each. The return wall panels were swapped out with wider panels to accommodate the wider walls. The primary challenge was resolving the gravity load of the 8-foot-9-inch-wide wall section. The first lift of this extension included a 10-foot-tall section of suspended concrete that exists beyond the confines of the steel structure below by 3 feet 3 inches on both sides. To support the heavy cantilevered loads, a 60-foot-long 8-inch-wide shoring platform, comprising of two 36-inch-deep castellated beams, was installed on the steel frame one level below. The castellated beams were adequately stiff to provide negligible deflection of the approximately 15,000-pound gravity load of the suspended concrete wall above, cantilevered several feet beyond the steel frame below. One additional 15-foot lift was placed above this section, also with 20 feet return walls. This lift had the luxury of sitting above the 8-foot-9-inch-wide, 10-foot-high section of concrete, which both supported the gravity load of the next lift as well as provided an attachment location for a 5x5-foot bracket that would support the ends of the return wall panels. The 60x8-foot shoring platform could be unbolted at the center into two 30x8-foot sections, which then could individually be removed through the core itself.
The successful formwork application was a two-way effort, with both general contractor Hensel Phelps, Irvine, Calif., and Atlas Construction Supply. Both parties were heavily engaged with each other for many months prior to construction and all the way through completion.