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The Sessions and Events schedule is now available.
H = Hilton Baltimore Inner Harbor; M = Baltimore Marriott Inner Harbor; and C = Baltimore Convention Center
Long-Term Slab Deflection: Practical Insights for Design Engineers
Wednesday, October 29, 2025 1:30 PM - 3:30 PM, H - Holiday 2
Structural engineers face growing pressure to minimize slab thickness for cost and carbon reduction while ensuring adequate deflection performance. Limited resources and minimal guidance from building codes on long-term deflection methods add to the challenge. This session, designed for practicing engineers but valuable to academics and contractors as well, provides insights from experienced professionals on best practices in long-term slab deflection analysis. Attendees will gain practical knowledge to enhance their design decisions.
Analysis, Design and Construction of Los Angeles County Museum of Art Presented By: Jacqueline Li Affiliation: Skidmore, Owings & Merrill Description: The architectural concept for the new Los Angeles County Museum of Art (LACMA) Building for the Permanent Collection developed by Atelier Peter Zumthor is an organically shaped building in architecturally exposed concrete. The exhibition level is supported on ten reinforced concrete service towers that extend from ground level to the roof. The organic plan geometry is structurally composed of a post-tensioned concrete framing system following a repetitive rhomboidal pattern. With cantilevers up to 85ft and a span of 165ft over Wilshire Boulevard, the LACMA structural system utilizes post-tensioned monostrand tendons in the slabs, the beams and within the walls. The tendon layout was calibrated for long term deflection to the allowable movement of a continuous glass facade around the exhibition floor plate. The building is monolithic without any joints and careful consideration for the construction sequence had to be placed to minimize cracking potential.
Three Reasons to Not Minimize Slab Thickness: More Prominent Radial and Spiderweb Cracking and Amplified Deflection Presented By: Terry Paret Affiliation: Wiss, Janney, Elstner Associates, Inc. Description: While ACI 318 sets forth design provisions that are intended to result in safe and serviceable two-way slabs, achievement of equivalent levels of safety and serviceability by a design that just satisfies the relevant code provisions is not a reasonable expectation because optimization of design strength is the enemy of serviceability. Similarly, minimization of slab thickness in pursuit of an optimized design is counterproductive with respect to serviceability, as can easily be shown by analysis and as evidenced by structures across the country that were designed under different editions of ACI 318. Even when a slab satisfies all relevant strength provisions, the thinner it is, the greater its deflection and the more noticeable radial and spiderweb cracking become. The presentation posits: that radial and spiderweb cracking derive from “tent-poling”, which is a flexural behavior; that a slab that is ACI 318 compliant might still exhibit radial and spiderweb cracking and deflection that are readily noticeable by users; and that reliance on calculated deflections rather than minimum thickness requirements to set slab thickness, and on the strip method of design, increase the propensity for potentially more objectionable radial and spiderweb cracking and deflection.
Mitigating Errors in Predicting Long-Term Deflections Presented By: Jared Brewe Affiliation: S. K. Ghosh Associates LLC Description: Measuring actual defections of a structure after some period of time is relatively easy; predicting those deflections in advance is inherently more complicated. This presentation will provide an overview of long-term slab deflection prediction methods and discuss ways to mitigate significant differences between predictions and performance. Project case studies will be used to relate theoretical approaches to practical methods.
Deflection Control of Slabs Based on Concrete Properties and Construction Sequence Presented By: Adam Lubell Affiliation: Read Jones Christoffersen Ltd. Description: The design of reinforced concrete slabs must satisfy both strength and serviceability requirements under all applicable loading stages, including different construction stages, final completion, and the long-term conditions accounting for sustained loading. For multi-story structures, the selected shoring / reshoring sequence to temporarily support the weight of new slabs by slabs constructed earlier can often be critical. Furthermore, the contractor selected concrete mixes compatible with the required strength evolution for this construction sequence as well as the other corresponding concrete properties such as modulus of elasticity, creep modulus and shrinkage can influence the deflection control outcome. Through analytical and field measurements, a typical evolution of concrete properties over the loading history are established and these properties are utilized towards optimized slab thicknesses for deflection control. Slab design examples are taken from common one-way and two-way framing systems to demonstrate the overall approach. Validation of the approach is demonstrated using in-field deflection measurements of real two-way flat slab systems with regular and irregular framing.