7. Practical Examples and Case Studies

A 30-storey tower uses post-tensioned slabs. Improper accounting of creep led to:

• Excessive downward deflection at mid-spans
• Misalignment of cladding panels
• Post-construction retrofitting required

In this example, we explore the consequences of neglecting creep in the design of a high-rise building. Creep is a gradual deformation of concrete that occurs under a constant load over time. In a 30-storey tower utilizing post-tensioned slabs, failing to properly account for creep led to several structural issues. Specifically, the mid-spans of the slabs experienced excessive downward deflection, meaning that the slabs sagged more than anticipated. This sagging also resulted in the cladding panels— which are the outer envelopes of the building— becoming misaligned. As a result, post-construction retrofitting was necessary to address these issues, indicating the importance of considering creep during initial design.

Imagine building a tall stack of books. If you leave the stack under a heavy object for an extended period, the books might begin to bend or sag due to the weight. If you don’t measure this sagging during construction, it can lead to problems with alignment when adding decorations to the bookshelf. This analogy reflects the issues encountered in our high-rise example, where the unaccounted sagging caused misalignment and the need for remedial work.

Creep and shrinkage caused:

• Longitudinal shortening of segments
• Displacement of bearings and expansion joints
• Need for creep compensation measures during erection

In this case study, the impacts of creep and shrinkage are examined in the context of bridge segmental construction. Creep and shrinkage together can result in changes to the size and shape of concrete elements over time. Specifically, the longitudinal shortening of the bridge segments was noted, which means that the segments were physically becoming shorter than anticipated. This unintended consequence led to the displacement of bearings—structures that support and stabilize the bridge—and expansion joints that allow for movement due to thermal fluctuations. To mitigate these issues during construction, creep compensation measures were necessary to ensure that the bridge could be safely and effectively assembled. This highlights the significance of understanding how creep and shrinkage affect large structures like bridges.

Consider a rubber band that you stretch and hold for a long time. Over time, you may notice that the rubber band doesn’t return to its original length; it has permanently shortened. In a similar way, the concrete segments in our bridge example have 'shortened' under sustained load due to creep. Just as you would take extra care if you were depending on a stretched rubber band to hold something up, engineers must account for these changes to ensure the safety and integrity of the bridge.

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