Editor's note: The first part of this two-part series ran in the April 2007 issue of CONCRETE CONSTRUCTION magazine. To read the first part again, click here
After diligent planning of the project—accounting for wall assembly, cranes, choosing the casting surface, panelization of the walls, panel dimensions and weights, and layout and erection sequencing—it's time to take a good look at the final details and successfully complete the project.
Finalizing the design
Panel detailing: Corner details for solid panels include simple butt joints, smooth-edge miters, sharp-edge miters, and modified butt joints. It's important to remember to consider these corner details when planning the erecting sequence.
Structural connections: The structural connections that need to be carefully detailed are found at beam joists, decks, interior walls, and foundations. The beam joist connection can either be a pocket or an embedded plate. Erection is faster with pockets, and they provide less eccentricity. Pocket placement is less forgiving than embedded plates, if done right, pockets are less costly.
For the deck connection, an angle needs to be fastened to the panel before the panel is tilted into place. Put a mark on the drill bit or use a short bit to avoid poking holes in the face of the panel. Also check the length of the anchors. For interior wall connections, use embedded plates in the top of the panel. Top slopes should follow roof drains. If the wall is intended to support joists, use open-top pockets. If a fire rating is required, then usually only one mat of rebar is needed. Seismic design may result in more stringent requirements.
Structural design of the panels: Panels have to resist loads from many forces. Live loads come from wind, snow, rain, and seismic forces. The structure itself imparts dead loads, including the weight of the panels. If applicable, panels must resist live loads from the floors. The additional weight of the architectural layer of concrete in a sandwich panel can be a factor in taller panels.
Rebar placement is an important consideration in panel design. In a single-layer of reinforcement, the vertical bars are positioned in the middle of the structural thickness of the panel, with the horizontal bars on the outside of the verticals. In a double-layer of rebar, be sure to maintain cover for the vertical bars. Pay attention to reveals and their depth over reinforcement. In these situations the horizontal bars are usually between the two layers of vertical bars. Although many may consider two layers of reinforcing as a lot of extra work, depending on the panel size and loading, it often is more cost-effective and results in a much stronger panel at a lower cost.
Indicate chair sizes on the panel drawings so the Tilt-Up Concrete Association (TCA) Certified Supervisor can inspect them prior to concrete placement. Make sure the superintendent checks the rebar placement prior to placing the concrete. Indicate the number of vertical bars on the panel drawing, not the spacing. Use plastic chairs specifically designed for tilt-up applications. Discuss chairs with the design engineer and rebar subcontractor and make sure the site superintendent can adequately inspect chair placement.