Base connection options are varied for tilt-up panels, but most use a slab connection through continuous reinforcement with lap splices.

Although tilt-up is well established in the architectural, engineering, and construction (AEC) communities, in many instances professionals are unaware the building method is a type of precast concrete. The American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318-05) Section 2.2 defines precast as any structural concrete element that is cast in one position and moved to a different, final position. The commentary in Chapter 16 of ACI 318 also names tilt-up concrete as precast. Specifically, tilt-up construction is site-cast precast concrete. However, since the term precast in the AEC industry has a very well-defined connotation, use of the word often takes engineers by surprise. What most engineers and architects would refer to as precast is shop-cast precast concrete.

Designing site-cast panels
Because it is precast concrete, a number of benefits to structural engineers are embedded in the definition of tilt-up concrete. One of the benefits of tilt-up construction is the increased level of inspection by the engineer and special inspector. Since most forms are broad and shallow, they provide the maximum access for optimal quality control. Because of this, ACI 318 7.7.3 allows the designer to use a smaller cover due to "plant conditions"—which is a phrase that the commentary would include site-cast conditions when quality control provisions are met. The result is a larger design depth, which provides improved strength and serviceability performance.

Tilt-up concrete buildings typically use tilt-up wall panels as load-bearing structural walls in single-story or multi-story applications. The size of panels has increased significantly with the improvement in crane and erection technology. Today, panels can reach nearly 100 feet in height, according to the Tilt-Up Concrete Association. When smaller cranes are used or there is limited casting area, technology is also available to stack multi-story tilt-up panels on top of each other. For instance, a nine story building may be constructed from a five-story tilt-up panel supporting a four-story tilt-up panel.

Lateral performance can also be enhanced by improved design coefficients and factors for Seismic Force Resisting Systems. The load-bearing tilt-up panels meet the criteria of "bearing wall systems" as defined in the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-05). Therefore, it is possible to classify tilt-up shear wall panels as ordinary precast shear walls, intermediate precast shear walls, and special reinforced concrete shear walls. To design an ordinary precast shear wall, Chapter 21 of ACI 318 requires that all requirements of Chapters 1-18 be met. To meet the requirements of the intermediate concrete shear walls, refer to the requirements in Chapters 1-18 and Section 21.13; to upgrade further to the special reinforced concrete shear wall classification, the requirements of Sections 21.2 and 21.17 will also be necessary.

Detailing base support of single-story tilt-up panels
As individual elements, tilt-up concrete panels possess beneficial strength and serviceability characteristics for both gravity and lateral applications. However, the greatest debate concerning single-story tilt-up concrete panels centers around the connections of the panels under lateral loading conditions, particularly connections at the base of the panels. One of the most significant points of discussion centers on the requirements of Section 7.13. Section 7.13.1 states, "In the detailing of reinforcement and connections, members of a structure shall be effectively tied together to improve integrity of the overall structure." Section 7.13.3 adds, "For precast concrete construction, tension ties shall be provided in the transverse, longitudinal, and vertical directions and around the perimeter of the structure to effectively tie elements together. The provisions of 16.5 shall apply."

Above grade, the design and detailing of this requirement is similar to those requirements for shop-cast precast and cast-in-place structures. These requirements can be satisfied by considering the support of the diaphragm and the longitudinal edge elements connecting it to the wall at each framing level. When additional capacity is required, the single-story panels may be tied together with continuous weldable reinforcement block-outs, welded stitch plates, et cetera. It should be noted that any edge-of-panel embeds used for these purposes should be properly reinforced to prevent cracking from thermal forces, shrinkage, et cetera.

The debate of the panel connections becomes more significant when discussing those connections at or below grade. When the overturning analysis is performed for shear walls in a single-story tilt-up concrete building, in a significant number of cases there is no net uplift from overturning at the base of the panels. For those instances when there is net uplift on the base of the panel, engineering mechanics dictate a positive connection to the foundation. The detailing of these connections should meet the requirements of Section 16.3.2.2 of ACI 318, which states, "Where tension forces occur, a continuous path of steel or steel reinforcement shall be provided." However, when no uplift is present, how can an engineer designing tilt-up panels meet the requirements of Section 7.13.3 and how should the attachment for uplift be made?

The Tilt-Up Concrete Association has authored two position papers on the subject. TCA-P-001 Connecting Tilt-Up Panels to Footings and TCA-P-002 Code Guidance for Seismic Design of Tilt-Up Buildings provide direction regarding the support of concrete panels. Typically in tilt-up design, reinforcing bars project from the base of the panels and extend into a temporary slab leave-out called a pour back strip. This reinforcement is lapped with reinforcement that projects from the main building slab and is able to provide a positive connection for in-plane and out-of-plane horizontal loads the panel is designed to resist. ASCE 7-05 provides a minimum out-of-plane force for structural walls and their anchorage in sections 12.11.1 and 12.11.2. In cases where there is no net uplift from overturning, tilt-up engineers use the connection with the slab to provide additional dead load for resistance to an overturning uplift that is already zero. Section 16.5.1.3 (c) states, "When design forces result in no tension at the base, the ties required by 16.5.1.3(b) shall be permitted to be anchored into an appropriately reinforced slab on grade."

The rationale above provides a clear load path of how slabs and single-story panels can be detailed to create a building envelope with heightened structural integrity when no net uplift on the base of the panel is present. ACI 551R-05 advises in Section 2.8.6 that panels may be interconnected or connected to foundations for in-plane and out-of-plane loads, but does not require the implementation of such a connection. Rather, that section is intended for cases where a connection to the slab to resist those forces is either not present or is not adequate without added resistance provided by additional connection to the foundation.

Given that it is possible for an appropriately reinforced slab-on-grade to stabilize the base of a panel, is it possible to detail a tilt-up panel to a foundation without a connection aside from a non-shrink grout bed? The ambiguity created between Section 7.13.3 and Section 16.5.1.3 (c) regarding panel/footing connections continues to be a point of discussion. Any interpretation will likely be based on the derived meaning of the phrase "tie elements together." Are the elements only precast elements or are they all structural elements?

Conclusion
TCA-P-002 concludes, "Connecting Tilt-Up wall panels to the footings is certainly one method to satisfy the building code requirements for transfer of forces, but it is not the only way. To arbitrarily require a particular connection/load path is not logical and may not produce the desired results. Whatever load path is used to transfer forces, all elements must be designed accordingly for those forces." Much has been written concerning the design of building shells for strength, ductility, integrity, and redundancy. In contrast, little has been written regarding those requirements when keeping buildings on the foundations in seismic events or unexpected movements in the subgrade.

It is not recommended to omit or limit any connection of the base of a tilt-up panel to a foundation—or between any structural elements—required by engineering mechanics, the building codes, or prescriptive measures. When connections of tilt-up panels below grade to a foundation are required, engineers should appropriately protect them against corrosion. The Tilt-Up Construction & Engineering Manual, 6th Edition also notes that connections made on the exterior face of the panel are subject to accelerated corrosion. It should be noted that applying a covering of concrete alone is not adequate for waterproofing. Different regions and connection details may require different waterproofing applications, therefore engineers should consult a local water-proofing consultant.

 

D. Eric Smith, P.E., is a project manager with Pruitt Eberly Stone, Inc., in Atlanta, as well as a lecturer at Southern Polytechnic State University in Marietta, Ga., and a member of the Tilt-Up Concrete Association’s Education and Meetings Committee. He can be reached for questions at 770-457-5923 or esmith@pesengineers.com.