Common Details For Tilt-Up Construction
Words: James Baty, FACI
Connections are an essential part of any tilt-up concrete structure and play a critical role in how the structure ultimately functions. The success of any tilt-up project depends, in part, on well-designed and easily executed connections.
While tilt-up, as a construction method, is executed differently all over the globe, there are a few common details used throughout the tilt-up industry. The Tilt-Up Concrete Association (TCA) recently completed an update of its presentation of these details, which is now available for download at www.tilt-up.org/resources.
Cultivated from a large sampling of the current tilt-up market, primarily Canada and the United States, the details included are not intended to show every variety of connection, nor are they intended to show every possible combination of structure. They are intended to provide the user an opportunity to see the theory behind these typical connections. The mechanics of designing many of the connections employed to attach structural framing members to tilt-up concrete panels are similar to those used for other types of construction involving concrete.
There are essentially two basic types of bearing connections used in tilt-up structures. Both involve some form of steel embed plate with headed stud anchors. One is a standard bearing type connection, where the structural framing member either sits in a pocket cast into the panel or bears directly on top of the panel. Bearing connections are a positive type of connection and generally simpler to construct, thus they are usually more economical. They also minimize load eccentricity for panel reinforcing design. However, they are not as forgiving for misplacement.
The other common connection is the face-mounted type, where the structural framing sits on a seat attached to an embedded steel plate cast into the face of the panel. These generally involve more pieces to fabricate and install, thus they can be more expensive. They also result in a larger load eccentricity for panel reinforcing design. However, they are more tolerant of embedded plate misplacement. Face-mounted connections are compatible with continuous parapets.
Post-installed anchors are sometimes used to connect secondary framing to the panels. These connections allow accurate positioning of the connection since they are located on the panels after the concrete has cured. They are also used when a steel embed plate is forgotten or misplaced.
For some applications or conditions, specialty inserts such as coil inserts with coil bolts, slotted anchors and embedded structural shapes, such as stub beams, are used.
The TCA’s, library of common details for tilt-up construction contains examples of these connection types in each of the following categories:
- Roof Structure to Panel: These connections transmit gravity; wind uplift; out-of-plane wind and seismic, in-plane wind; and seismic forces.
- Supported Floor Structure to Panel: These connections transmit gravity, out-of-plane wind, seismic, thermal bowing, in-plane wind, and seismic forces.
- Panel to Slab on Grade: These connections transmit out-of-plane wind, seismic, thermal bowing, lateral earth pressure, in-plane wind and seismic forces.
- Panel to Footing: These connections transmit out-of-plane wind and seismic forces, in-plane wind and seismic forces, and uplift due to overturning.
- Panel to Panel: These connections transmit in-plane wind and seismic forces to resist overturning and differential bowing.
The design and detailing of tilt-up connections should accommodate standard industry tolerances. It is suggested that connections be designed to accommodate a misplacement of the embedded item of up to 1 in. in the plane of the panel. When possible, embedded items should use a symmetrical layout in at least one axis to simplify layout and placement in the panel.
Contractors will typically want to install as many connections as possible while the panels are still on the ground for safety and economy. The design and layout of the connection should take this into consideration.
To provide an in-depth look at the design of tilt-up structures, the TCA recently published, “Engineering Tilt-Up”. Authored by Timothy W. Mays, Ph.D., P.E. and Joseph J. Steinbicker, P.E., S.E., “Engineering Tilt-Up” contains basic engineering principles and code interpretation as well as in depth analysis of a tilt-up structure and its components. The thorough guide breaks down tilt-up design to educate beginners as well as experienced engineers.
In addition to the chapter material, “Engineering Tilt-Up” features three full-building design examples presenting simplified standard practice approaches to design on realistic yet complex situations and constraints that are considered on actual projects. “Adhering to the latest codes and standards, the book serves as a one-of-a-kind resource for tilt-up engineers looking for an excellent summary of standard practice in tilt-up and alternative ways to seek code compliance,” said Mays.
The Tilt-Up Detail Library, as well as “Engineering Tilt-Up,” are available at www.tilt-up.org/resources.