Design considerations for building movements caused by volume change

by sadia_badhon | November 26, 2020 9:49 am

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by George R. Mulholland, SE, PE, and Demetria E Boatwright, EIT, CDT

As a building material, concrete is particularly susceptible to volume changes, which can be caused by thermal effects, creep, and shrinkage. When materials like concrete are constrained to prevent volume changes, large internal forces and stresses develop. Therefore, it is important for architects and engineers to work together in order to avoid inappropriately constraining building materials; ensure the internal forces and stresses caused by volume changes are accounted for in design; and coordinate the location, configuration, and design of building expansion joints. Failing to consider or properly accommodate building movements caused by volume change can result in structural problems and water infiltration.

Sources of volume change in concrete structures

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Changes in temperature over time cause all building materials, including concrete, to expand as the temperature rises and contract as it falls. Expansion and contraction due to temperature changes occur in all directions and repeat cyclically throughout the life of a structure. The magnitude of thermal expansion and contraction is relatively straightforward to calculate and is dependent on the material’s coefficient of thermal expansion, the temperature change it experiences, and its dimensions.

Shrinkage and creep are two further sources of volume change impacting concrete structures. The American Concrete Institute (ACI) defines shrinkage as a “decrease in either length or volume of a material resulting from changes in moisture content or chemical changes,” (referenced from the American Concrete Institute [ACI] CT-13, ACI Concrete Terminology). Creep is defined as “time-dependent deformation due to sustained load.” Shrinkage and creep are closely related and difficult to predict. Shrinkage occurs gradually in all directions as concrete cures and dries. Shrinkage-compensating concrete, or adjusted water-cement ratios and aggregate size can be used to limit the impact of shrinkage on structural movements. As indicated in ACI’s definition, creep is also time-dependent. It occurs in the direction of a sustained load and increases over time, continuing for several years after construction is complete. Since creep and shrinkage are closely related, prediction models for concrete structures often combine them together as time-dependent deformations (consult ACI R318-14 Commentary Section R24.2.4.1). The final magnitude of time-dependent deformations can be roughly two times that of elastic deformations (those occurring instantaneously when loads are first applied to the structure). The exact proportion depends on the concrete mix design, the curing conditions such as moisture and humidity, the concrete component’s type and dimensions, the age of the concrete when loads are first applied, and the direction and type of loading.

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In prestressed structures, the pre- or post-tensioning force causes additional creep effects, which manifest in the gradual shortening of members along the axis of the prestress force. The magnitude of shortening caused by pre- or post-tensioning is dependent on the length of the concrete member along the axis of the prestress force, the time at which the prestress force was applied, the average prestress force applied to the concrete, and the modulus of elasticity of the concrete and the prestressing tendons.

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Problem #1: Inappropriately constrained building materials and a lack of consideration for internal forces and stresses induced by volume change

When a structural engineer inappropriately constrains concrete structural members, preventing building movements associated with volume change, connection distress is an inevitable result.

At a parking garage in a northern climate, the exterior wall was made up of multiple precast concrete shear wall panels. The structural engineer did not consider volume change of the individual panels or the building as a whole when they selected rigid panel-to-panel connections. Even though they designed each wall panel to act separately when resisting lateral loads, linking the panels together with rigid panel-to-panel connections created one continuous wall. The connections consisted of embed angles cast flush with the edge of two adjacent panels and a steel plate welded across the vertical panel joint to the embed angle in each panel. The embed angles were recessed into the panel face, creating a pocket, which was grouted after panel installation. See Figure 1 for the original embed angle detail. Shortly after construction, volume changes caused by a combination of creep, shrinkage, and thermal effects led to a buildup of stress at the connections. This culminated in prying at the embed angle connections, delamination and spalling of the surrounding concrete, and tension/shear failure of the embeds (tension/shear failures of embed angles similar to the ones discussed in this article were studied by H.S. Lew, et. al. and reported on in their article “Performance of precast concrete moment frames subject to column removal: Part 1, experimental study”). Typical connection distress is shown in Figure 2. A more appropriate connection design (and the eventual repair design) would consist of horizontal slotted bolt connections, which allow for volume change movements across the vertical joint after construction and prevent stress concentration at the connections. The repaired connection types are shown in Figure 3.

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For best practice, when designing a concrete structure, it is imperative the structural engineer consider and calculate expected movements due to temperature change, creep, and shrinkage. It is their responsibility to communicate the expected magnitude to the rest of the design team. The expected movement should be accommodated by appropriately designed expansion joints and connections. If a concrete member must be constrained in a way that inhibits volumetric expansion and/or contraction, the member and its connections should be designed to resist anticipated induced stresses.

Problem #2: Lack of design team coordination for building expansion joint locations

Building expansion joints can effectively alleviate forces and stresses caused by volume change, but it is important the design team coordinates structural and architectural expansion joint locations.

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Poor coordination of expansion joint location became particularly problematic at a building in a northern climate. The first floor plan of the structure was larger than the remaining stories. Above the first floor, concrete framing and exterior masonry walls were inset to create terraces at the second floor. At one terrace, aluminum railings located on top of a masonry knee wall spanned across a building expansion joint. Figure 4 depicts gaps and damage at the railing connections caused by movement at the expansion joint, pulling the railing apart. Further, Figure 5 shows masonry knee walls that were installed with expansion joints either missing or offset from the expansion joint in the terrace concrete slab. Different expansion joint seal types, the lack of appropriate seal transitions and movement at offset joints eventually lead to water infiltration into the space below.

Damage to the railings and masonry knee walls and water infiltration into the space below could have been avoided if the design team had properly coordinated expansion joint locations and worked with the expansion joint seal manufacturers to provide proper expansion joint transitions—instead, their lack of communication and poor detailing led to an expensive problem.

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Problem #3: Improper building expansion joint configuration

Not to be overlooked, expansion joint configuration is equally important to accommodating building movements.

Expansion joint seals are typically designed to work in tension across the joint width. As shown in Figure 6, oddly shaped expansion joints may include many right angles to fit the joint around columns and walls. When an expansion joint includes right angles, both tension and shear forces are applied to the expansion joint seal. Shear forces can result in failures at transitions and shear locations, leaving the joint vulnerable to water infiltration. At a similar project, the design team intended to include right angles in their expansion joint to accommodate a column penetration. However, they did not adequately plan for incorporation of the expansion joint seal within the substrate materials along the vertical column face parallel to the anticipated movement. There was no room for continuation of the expansion joint seal, and it was terminated at the face of the column. Instead of the planned expansion joint seal, a urethane sealant joint, which could not accommodate the anticipated shear movement, was installed along the vertical column face. One potential repair for either condition listed above would be to raise the shear portion of the expansion joint above the drainage plane and cover the joint with counterflashing, as shown in Figure 7.

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In addition to poorly designed joint shape, other problems involving joint configuration occur at slab-to-wall and wall-to-roof transitions. Figure 8 depicts a roof/parapet wall interface where the roof expansion joint is not continued up the interior face of the parapet. Roofing patches and sealant at the metal coping joints are present, indicating failures at these locations. At another structure, an exterior masonry wall was located above an exterior concrete slab and ran parallel to an expansion joint in the slab. The masonry veneer was designed to be supported on one side of the expansion joint while tied back to the structure on the other side of the joint, as shown in Figure 9 (page 27). If installed in this way, movement at the expansion joint would have pulled the masonry veneer away from the backup. The lack of cut sections in the architectural drawing set meant that this issue was not discovered until construction. Ultimately, the masonry veneer was supported by shelf angles directly over the expansion joint and exterior exposed slab. The as-built gap provided between the slab topside and the underside of the shelf angle was approximately 25 mm (1 in.), which did not allow for installation of the expansion joint seal. Visible in Figure 10 (page 27), the waterproofing solution consisted of flexible flashing and face-sealed cover plates over the expansion joint, under the supported veneer brick and extending out onto the exposed exterior concrete slab. This solution eventually failed, leaving masonry wall removal and expansion joint reconfiguration as the next step in remediation of the joint.

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For successful expansion joint configuration, the design team should avoid oddly shaped expansion joints and check transition details carefully. Expansion joint seals should not be installed to resist shear movements unless the seal material is selected specifically to accommodate these movements. In exposed conditions, where possible, the expansion joint seal should be installed above the drainage level, for example, within a curb. The designers should also discuss the feasibility of their transition details with the contractor regarding both installation and future maintenance.

Problem #4: Lack of design team coordination for building expansion joint seal material capacity versus actual movements

As mentioned before, the structural engineer is responsible for calculating expected movement at expansion joints and sharing their predictions with the rest of the design team. The architect uses this information to select an expansion joint seal with sufficient material capacity which can accommodate the anticipated movement. If the structural engineer fails to predict movement accurately, or the lines of communication breakdown, the seal selected by the architect could have insufficient capacity.

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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.

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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.

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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.

[13]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[14].

[15]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[16].

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