In a concrete structure with a post-tensioned slab, the slab experiences elastic shortening when the tensioning force is first applied and continues to shorten over time due to creep. If not accounted for in the expansion joint design, shortening of the slab on its own can lead to large enough movements to cause joint failures. An example of large movement at a post-tensioned slab is shown in Figure 11. Already large joint movement in prestressed slabs is often exacerbated by unique building characteristics. For example, the locations of shear walls or other sources of lateral stiffness in each building section influence movement at nearby expansion joints and can cause variable movement at adjacent joints. If a source of lateral stiffness is located near one end of a building section, movement will be greater at the joint located further away from the lateral stiffness. Similarly, in a terraced structure, terrace levels can have increased exposure to changes in temperature. If exposed portions of the building are not considered, thermal movements can be underestimated. When these two phenomena are not accurately accounted for by the structural engineer, it is impossible for the architect to make an appropriate selection of an expansion joint seal. Inaccurate predictions for building movements are compounded when value engineering alters the structural design or reduces the total number of expansion joints late in the design process. Changes in the quantity or location of expansion joints alter the expected movement across joints. In the event the structural engineer does not recalculate the anticipated movements and communicate them to the architect, or in the event the architect does not update their selection for the expansion joint seal, the joint could be sized incorrectly. After occupancy, as concrete creep gradually takes effect and the building undergoes cyclical temperature changes, errors made in the sizing of expansion joints will result in joint failures, which, in turn, will lead to water infiltration.
For expansion joints to perform correctly, the design team must be able to successfully match the expansion joint seal’s material capacity with anticipated movements. When predicting building movements, it is important for the structural engineer to fully consider all sources of volume change and any unique problems the building could face. For example, the structural engineer must consider the building’s stiffness against lateral displacement within each separate section of the building. Variable joint movement should be identified and accounted for in expansion joint design. Likewise, in a structure with terraces, the structural engineer should consider the level with maximum exposure as the controlling case for building movement and design all other levels considering that movement. Two references that are available to assist structural engineers in the prediction of movement at expansion joints are the Federal Construction Council Technical Report No. 65 by the Standing Committee on Structural Engineering of the Federal Construction Council (FCC) and the PCI Design Handbook: Precast and Prestressed Concrete by the Precast/Prestressed Concrete Institute (PCI). If value engineering is used, the project team should be cognizant changes in the number or location of expansion joints impact the anticipated movement across expansion joints. The architect should consult the structural engineer and update the expansion joint design before approving these types of changes.
Conclusion
Fortunately, the building failures discussed above are far from inevitable. Design team awareness and communication is the ultimate key to designing a structure that is equipped to handle volume changes of concrete and other building materials.
George R. Mulholland, SE, PE, is a principal at Raths, Raths & Johnson. He has 32 years of experience specializing in the evaluation, investigation, and repair design of distressed building façades and structures as well as providing expert testimony. Mulholland is the chair of the Structural Condition Assessment and Rehabilitation of Buildings, vice-president of the International Concrete Repair Institute’s (ICRI’s) Chicago chapter, and a committee member of the Precast/Prestressed Concrete Institute (PCI) Building Code Committee. He can be reached at grmulholland@rrj.com.
Demetria Boatwright, EIT, CDT, is a member of Raths, Raths & Johnson’s structural engineering practice. She has experience with a variety of projects involving condition assessment, field investigation, forensic research, and documentation of structural components and systems and distressed buildings. She is an active member of the Structural Engineers Association of Illinois (SEAOI), as a part of the Women in Structural Engineering (WiSE) committee, and serves on the communications committee of CSI’s Chicago chapter. Boatwright can be reached at deboatwright@rrj.com.
