Precast Connections
Precast Connections: The "glue" that holds it all together
In the erection of a precast structure, connections are the “glue” that keeps it all together. As such, there is a vast array of connections available to meet the individual requirements of the project. The transfer of loads, restraint of movement and creation of stability is the role of connections.
Connection Design Considerations
Strength
A connection must have the strength to avoid failure during its lifetime.
Ductility
The ability of a connection to undergo relatively large deformations without failure.
Volume
Change Accommodation Restraint of movement due to creep, shrinkage, and temperature change can cause large stresses in precast concrete components and their connections. It is better to design the connection to allow some movement, which will relieve the build-up of these stresses.
Durability
When the connection is exposed to weather or used in a corrosive environment, steel elements should be adequately covered by concrete, painted, epoxy-coated, or galvanized. Stainless steel may also be used, however, the added cost should be considered carefully.
Constructability
The following reflects only some of the items that should be considered when designing connections:
Standardize connection types
Avoid reinforcement and hardware congestion
Avoid penetration of forms
Reduce post-stripping work
Consider clearances and tolerances of connection materials
Avoid non-standard product and erection tolerances
Plan for the shortest-possible crane hook-up time
Provide for field adjustments
Provide accessibility
Determine if special inspection is required per the applicable code for the material and the welding process
Provide as direct a load path as possible for the transfer of the load
Fire Resistance
Connections, which could jeopardize the structure's stability if weakened by high temperatures from a fire, should be protected to the same degree as the components that they connect.
Aesthetics
For connections that are exposed to view in the final structure, the designer should incorporate a visually pleasing final product.
Seismic Requirements
Structures and/or components that must be designed for seismic loads may require special consideration. Consultation with a structural engineer with experience in seismic design is recommended.
Tolerances
The designer must realize that normal allowable fabrication, erection, and interfacing tolerances preclude the possibility of a perfect fit in the field
Connection Materials
Headed Concrete Anchors (studs) are round bars with an integral head. These are typically welded to plates to provide anchorage to the plate.
Steel Shapes including wide flanges, structural tubes, channels, plates, and angles.
Reinforcing Bars are typically welded to steel sections to provide anchorage to the steel.
Reinforcing Bar Couplers are typically proprietary devices for connecting reinforcing bars at a joint. Precast producers of these services can provide technical information.
Deformed Bar Anchors are similar in configuration to deformed reinforcing bars and are welded to steel shapes to provide anchorage similar to headed concrete anchors.
Bolts and Threaded Connectors are used in many precast concrete connections. Use of ASTM A36 or A307 bolts is typical. Use of high-strength ASTM A325 and A490 is usually not required.
Specialty Inserts are available from many precast producers of these devices. They include standard threaded inserts, coil threaded inserts, and slotted inserts that provide for tolerances and field adjustment.
Bearing Pads are used predominantly for structural applications to support beams, double tees, and similar components. Use of random fiber oriented bearing pads (ROF) is recommended.
Shims can be hard plastic or steel and are often used to provide adjustment to align a precast concrete component for elevation or horizontal alignment.
Looking to learn more about connections? Try the PCI resources below!
Offered as a free downloadable resource is PCI’s Designer’s Notebook: Connections (DN-32), which provides an overview of design concepts with regard to connections and architectural enclosure systems.
A “must have” resource for practicing engineers is PCI Connections Manual for Precast and Prestressed Concrete Construction which provides design examples.
Types of Connections
Click on any of the connection type graphics below to get a better look!
Structural Connection Types
Cladding Connection Types
The structural connections are generally used in structures such as parking garages, office buildings and warehouses where panels sit on a footing whereas, the cladding type of connections are used to attach the outside cladding to the underlying structural frame that could be made of various materials ( ie. steel)
Connection Examples
Pic #1 is a Double Tee to Corbel
Pic #2 is a Spandrel Beam to Column
Pic #3 is a Panel to Panel Connection
The role connections play in the erection of a precast concrete structure are key to its strength and durability. Producers and engineers work diligently to employ connections that best fit with the project needs to meet and/or exceed all applicable codes.
For further information please refer to www.pci.org.