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How to Enhance Mechanical Connections

Choosing the right connection for your project hinges on many factors – from function and ease of production to constructability and surface aesthetics.

By Evan Gurley

The Baha’i Temple in Wilmette, Ill., took over three decades to build and was the first architectural precast structure built in the U.S.  The specified stainless steel connections were costly, but have proven their worth over time by protecting the temple’s magnificent surface aesthetics, as seen in this dramatic nighttime photo.  Photo courtesy Chris Smith Photography (Chris@outofchicago.com)The behavior of connections used in precast concrete products greatly influences the structural integrity of the entire structure. Whether analyzing large precast concrete wall and floor panel junctions or the load transfer mechanisms used in precast concrete pavement slabs, the design and construction of the joints and connections are crucial components that ensure the stability and robustness of the structure. The overall integrity of the precast concrete structure can be substantially enhanced by minor changes in the amount, location and detailing of connections and connection hardware.

Example: exotic precast temple

The Baha’i Temple in Wilmette, Ill., is considered to be the first architectural precast structure constructed in the United States. The precaster on the project, John Joseph Earley of Earley Studio, used innovative mix designs and manufacturing methods to create the exterior shell of this exotically beautiful structure. Construction on the temple began in 1919 and the doors subsequently opened in 1953. It was constructed using architectural precast concrete panels bolted to a steel and concrete skeleton using expensive stainless steel connections that eliminated the possibility of rust.

Even at the start of this temple’s early 20th century construction, connections played a vital role. For example, proper connections ensured that the precast panels were correctly positioned and held in place, they resisted the vertical and horizontal loading conditions, and they prevented the architectural components from rust and other damage to architectural aesthetics.

Connection performance criteria

As defined in ACI 318 (ACI 318, “Building Code Requirements for Structural Concrete and Commentary”), a connection is a region that joins two or more members. The primary purpose of a connection is to transfer the load to the supporting structure, restrain movement, and/or to provide stability, depending on the structure or component. Precast concrete manufacturers use numerous sizes, shapes and types of connections.

It is common for precasters to develop their own connection details to best fit their production and erection needs. If this is the case, the precast manufacturer is typically included early in the design stages, as it may be able to provide additional insight and address any questions that may arise.

In order to ensure that the connections in a precast concrete structure or component meet the minimum design and performance criteria, let’s consider the following factors:

    • Building Code and industry standards requirements
    • Economy of erection
    • Constructability of connections
    • Simplicity of connections
    • Connection hardware

Close-up photo of the Baha’i Temple reveals the amazing intricacy and exacting detailing of the structure’s celebrated aesthetics.  Photo courtesy © Thomas Barrat | Dreamstime.comCodes and standards. Connections used by precasters must comply with the applicable governing codes and standards, which outline the materials, design and construction of structural precast concrete products in buildings and other structures. Codes and standards provide a means of establishing minimum requirements for acceptance of design and construction.

When reviewing the ACI 318 code, we see numerous sections highlighting the minimum standards for connections in precast concrete building elements. These requirements touch on all types of connections used by precasters when dealing with building system elements.

Connection issues arise on projects, and each must be solved on its own merits following the limitations set by the building codes.

Economy of erection. If the connection makes the precast component difficult to handle, the speed of erection slows. This reduces the efficiency of installation and increases the overall cost. With proper up-front planning by the engineer and precaster, connection issues can be significantly reduced or eliminated altogether to increase the speed, efficiency and economy of erection.

When precast concrete wall panels make up the exterior envelope of a building, the economy of erection dictates that the connections be designed to allow the crane to set the panels in place with temporary anchorage/bracing, permitting the crane to advance to the next piece. Alignment and permanent anchorage can then be accomplished without the use of a crane. This is just one example of how a proper connection detail allows the installation process to be more efficient.

Constructability. Constructability can ultimately reduce the fabrication period. It includes the following criteria:

    • Avoidance of congestion
    • Simplicity of connection design
    • Repetition of standard connection designs and types
    • Integrity of tolerances and clearance for connections and connection materials/hardware
    • Allowance for field adjustments
    • Accessibility of connections
    • Avoidance of form penetrations
    • Elimination of unnecessary work (post-stripping)
    • Shortest crane hook-up and erection time

Simplicity. Connections should be designed to be as simple and practical as possible. Simplicity should be one of the chief goals in designing all connections, as each complication adds to the expense of installation. In some cases, local practice may suggest one type of connection over another, such as the use of bolts rather than welds. All connections need to be accessible and allow for tolerances and adjustments.

The precast concrete manufacturing process has a significant advantage over alternate construction processes, as precast products are manufactured in a quality-controlled environment that ensures the precise placement of connections to match exacting specifications and tolerance restrictions.

Connection Hardware. Hardware designed for connections should take into account the tolerances for both the precast concrete components and the structure. These may require clip angles and plates with slots or oversized holes to compensate for dimensional variations in elevation. Sufficient minimum clearance between precast units and other structural components should be provided to allow for product, interface and erection tolerances. Hardware should be designed to compensate for additional stress at maximum anticipated clearance.

Other important factors to consider include strength, ductility, volume change accommodations, durability and fire resistance.

Types of architectural connections

There are numerous types of connections for all precast concrete products, but the most common connections for architectural precast concrete components are:

    • Bolted connections
    • Welded connections
    • Dowel/anchor bolt connections

Bolted connections. Bolted connections simplify and speed up the erection operation, because the connection is immediately strong. Final alignment and adjustments can be made later without tying up a crane. Staggered bolt layout allows easier access for tightening with a pneumatic wrench for bolted connections. High-strength bolts may be snug-tightened (bearing connections) or slip-critical (clamp-like connections).

Welded connections. Welded connections are most commonly used in the erection of precast concrete. Welded connections are structurally efficient and adjust easily to varying field conditions.

Dowel/anchor bolt connections. The strength of dowels in tension or shear depends on dowel diameter, embedded length, bond development and loading. Recommended practice is to provide sufficient embedment to develop full dowel strength. Typically, dowel work is performed by a subcontractor.

For more information on architectural connections and connection details, visit the NPCA website at precast.org to view the Architectural Connections Guide (see Sources).

Conclusion

Precast concrete systems enable fast and effective completion of many types of buildings and other structures. Effective design and construction are achieved through the use of suitable connections to cater to all service, environmental and ultimate load conditions.

Connecting the elements is not just a question of fixing one piece to another; it is to ensure the structural integrity of the whole structure. To achieve structural integrity, the designer must appreciate the critical importance of connections and understand how connections influence the flow of forces through a structure under shear, vertical and/or horizontal loads.

Evan Gurley is a technical services engineer with NPCA.

Sources

 


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