By Karen S. Hand, P.E., Needham and Associates Inc.

Successful Tilt-Up construction requires special attention to design and specialized care in construction to accommodate thermal expansion and contraction. In other words, the panels need to breathe. However, many Tilt-Up professionals fail to realize the importance of this principle.

This lack of understanding can be explained in part by the Tilt-Up industry’s tremendous growth over the past few years. Many engineers and contractors are now entering the industry without the appreciation for the concerns regarding expansion and contraction. Unfortunately, the buildings lacking adequate forethought in connection design may experience substantial problems within a few years of completion.

Even engineers and contractors who have worked for years in Tilt-Up construction are finding the industry is changing and presenting new challenges. As Tilt-Up gets taller, wider, thicker and heavier, connection design becomes even more critical.

EPA CENTER DESIGNED FOR EXPANSION

The EPA Science and Technology Center is a successful project that had to overcome a problem with thermal expansion during the construction process. Steel and concrete have different coefficients of thermal expansion. This fundamental, yet often overlooked, principle should be considered not only in the design but also in the location of connections.

In this example, a 6-inch-diameter pipe was to be added to the exterior of the building as an architectural feature. The design specified the tube to be supported with two rigid connections in the Tilt-Up panel at approximately 8 feet apart. The pair of supports would be spaced at around 45-foot intervals. There was a breakdown in communication regarding the placement of the expansion joints in the steel pipe that resulted in problems with the connections. When constructed, the long expanse of steel pipe extended over several pairs of panel supports before an expansion (relief) joint was made. The steel expands and contracts independently of the concrete, causing incompatibility of materials. The location of the rigid connection in relation to the expansion joint in the steel created high residual stresses at the rigid connection.

When a connection fails in this manner, many contractors initially blame poorly designed concrete. However, patching the concrete as a repair without searching for the root cause only provides a temporary solution. In the struggle between steel and concrete, the ductile steel always wins. The spalling of the concrete was a symptom of a bigger problem, a fundamental principle in material behavior. Once the steel expansion joints were properly located in relation to the panel supports, the concrete spalling was then patched, and the materials are now compatible and performing in accordance to their design.

Overcoming a Precast Mindset

The largest problem in the engineering community with the design of Tilt-Up connections is the precast panel mindset. Tilt-Up requires a specialty engineer and is most often procured by the Tilt-Up subcontractor well into the structure’s design phase.

Two scenarios can lead to problems. In one situation, the engineer-of- record may not be doing the design but has already specified connections and panels in the same way as the firm would specify precast panels.

In this case, the Tilt-Up engineer is not consulted until the steel shop drawings are returned, causing coordination issues with the steel design. In the second scenario, the engineer-of-record is a firm not typically involved in Tilt-Up design that decides to design the Tilt-Up without having the proper background. When such firms look for design guidance, they often turn first to precast panel design.

The problem with these scenarios is that Tilt-Up and precast are two very different applications. The goal for a Tilt-Up structure is not to create a rigid box but to create elements that adequately transfer the shear loads and the gravity loads to the foundation. If the panels can’t “breathe,” they will rip themselves apart trying to do so.

Engineers should not rely on precast and prestressed concrete technology for the basis of their connection design for Tilt-Up. Consider two major differences between precast concrete and Tilt-Up concrete: panel size and pretressed tendons. With limited panel size, the stresses due to thermal expansion are minimal compared to a Tilt-Up panel that may be on a magnitude of 10 times larger. Also, with prestressed tendons, the panel can handle more residual stress than a conventionally reinforced panel. Particular attention must be paid to base connections, edge of slab, edge of roof and corner connections. Connections that consist of continuous angles, chords and collector beam connections need special consideration.

With early involvement and better understanding of connection concerns by all engineers and contractors, your project can be successful. Just remember, let your panels breathe easy so you, too, may breathe easy.