As the concrete industry chases lower carbon footprints and tighter budgets, structural engineers are under growing pressure to design thinner slabs without compromising constructability and serviceability. Yet, as a panel of experts noted during a session at the American Concrete Institute’s Fall 2025 convention, slenderness can be a double-edged sword. The panel warned that misjudging long-term deflection or cracking behavior can create aesthetic problems and lead to costly disputes, occupant complaints and, in extreme cases, performance failures.
The engineers who presented shared projects and research spanning art museums to high-rises. The moral of the double-sword story is that long-term deflection is as much a construction-sequence and material-property problem as it is a code calculation issue.
Sculptural concrete meets serviceability: The new LACMA
Jacqueline Li of Skidmore, Owings & Merrill offered a look at the structural design of the new Los Angeles County Museum of Art, an organically shaped, architecturally exposed concrete building that literally bridges Wilshire Boulevard. The continuous, joint-free structure spans nearly 800 feet, with cantilevers extending up to 85 feet.
For SOM, the challenge was twofold: Making the sculptural geometry stand up while ensuring the museum’s fully glazed facade could tolerate long-term slab movement. Li described a highly integrated design process in which the structure was the architecture, which notably features no ceilings or partitions, just exposed post-tensioned concrete.
To control differential movement between the roof and exhibition slabs, SOM modeled creep, shrinkage and cracking using LUSAS software and calibrated the layout of post-tensioning tendons to limit long-term deflection to within ¼ inch over 10 feet. The team issued drawings specifying predicted deflections at each stage of construction and occupancy. Li noted that was a somewhat rare decision that allowed the contractor and facade engineers to coordinate precisely when glass would be installed.
The entire building is isolated on triple friction pendulum bearings, allowing it to move together under seismic loads and to accommodate shrinkage without internal expansion joints, Li said.
“The structure is the architecture,” Li emphasized.
However, so is the deflection performance, monitored continuously through the building’s first five years.
About those “optimized” slabs…
Terry Paret, senior principal at Wiss, Janney, Elstner Associates, offered a cautionary counterpoint. In a presentation that blended forensic engineering and code commentary, Paret argued that minimizing slab thickness in pursuit of efficiency often produces unintended serviceability failures, notably radial and spiderweb cracking that alarm owners and occupants.
Paret showed images of cracked parking structures, podiums and office floors, many of which are fully ACI 318-compliant, to demonstrate that design strength optimization can be the enemy of serviceability without careful consideration.
He linked these crack patterns to what he called “tent-poling,” a flexural behavior that occurs when thin slabs deflect significantly between columns, causing circumferential stresses that show up as radial cracks. However, he made sure to note that it isn’t the same as punching shear.
“These are not hair-on-fire situations,” Paret said, implying flexural serviceability issues instead of pure strength failures.
Finite element analyses run by WJE showed that thinner, “marginally compliant” slabs produced dramatically higher deflections and wider cracks, even when shear stresses remained within limits.
Paret urged engineers to do four things:
- Treat minimum slab thickness as a safeguard, not an optional calculation bypass
- Document all value-engineering changes to thickness, as they often drive future disputes
- Recognize that ACI’s deflection provisions focus on strength and immediate loads, not total deflection as experienced by occupants
- Distinguish serviceability issues from structural safety, communicating that difference clearly to clients
“Don’t expect an ACI compliant design will provide assurance and service of behavior,” Paret said.
Improving long-term deflection models
Jared Brewe, vice president at S. K. Ghosh Associates and part of the International Code Council, then tackled the analytical side of the problem.
“Deflection is probably the easiest [structural performance metric] we can actually measure,” he noted. “And so if there’s one thing that we’re going to be judged on, it’s something we can all measure really easily, right?”
Brewe walked through the fundamentals of deflection prediction, from immediate WL⁴/8EI relationships to the λ multiplier in ACI 318 that approximates creep and shrinkage effects. But, as he demonstrated, the assumptions behind that multiplier can vary dramatically depending on real-world factors such as aggregate type, modulus of elasticity and construction sequence.
Using case studies, Brewe showed how measured long-term deflections often double those predicted by standard models. The discrepancies stem largely from uncertainty in sustained loading, early-age loading during construction and concrete stiffness variability.
A 5,000 psi mix might yield an elastic modulus anywhere from 3,200 to 4,800 ksi depending on aggregate quality and testing method, he said.
Brewe offered four tips for a more data-driven, materials-based approach to prediction:
- Calibrate models to local aggregates and mix designs rather than generic code equations
- Account for early-age loading and shoring/reshoring sequences explicitly
- Use realistic sustained load assumptions (often only 20–25% of nominal live load)
- Incorporate field monitoring to validate analytical predictions
In one case, measured deflections reached six inches where calculations had predicted three. Adjusting the modulus, cracking moment and long-term multiplier brought the analytical and field data into alignment, Brewe explained.
Field-validated models from Canada
Closing the session, Daniel Snodgrass of RJC Engineers presented a Canadian perspective, combining analytical modeling with field data from high-rise projects. Like Brewe, Snodgrass emphasized that construction sequence dominates long-term deflection behavior, particularly in markets where fast cycle times demand early form removal.
In Toronto, he explained, contractors routinely pour floors on three- to four-day cycles, stripping formwork before the concrete reaches full design strength. That means slabs often experience their highest loads at 70% of f′c, setting long-term creep and cracking behavior in motion early.
RJC has responded by specifying strength targets at 5, 6 and 9 days rather than the conventional 28-day design value. Using field-cored samples, lidar deflection surveys and time-history analysis, Snodgrass’s team validated that their predicted deflections generally matched measured results within about ±25%.
One project — a 70-story residential tower in Mississauga — used the early-age data to justify a 10% reduction in slab thickness, cutting embodied carbon by roughly 7% without exceeding serviceability limits. It doesn’t sound huge, Snodgrass said, but in a high-rise, that’s “massive” savings.
He also urged greater regional calibration of code equations for modulus and creep. In Western Canada, RJC found concrete up to 30% less stiff than ACI or CSA assumptions would suggest, while Toronto concretes performed stiffer.
“We have to get more to local benchmarks, local understanding, where [what graduates are] doing in the model code, they actually break it down by some limestone rocks, different types of aggregate,” he said.
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This is part 4 of SmartBrief’s recap of the American Concrete Institute’s 2025 fall convention. Click here for part 1, here for part 2 and here for part 3. For more insight into the latest news and trends affecting the concrete industry, subscribe today to ACI SmartBrief and SmartBrief for Civil Engineers.