MINI SESSION: Deflection of Structures - Advancements in Methods and Reflections in Application

Monday, November 4, 2024  3:00 PM - 4:00 PM, 401

Advancements in methods used to calculate the deflection of mild and prestressed reinforced floor framing systems including creep & shrinkage and cracking behavior of prestressed concrete will be presented. Also, built examples using methods in ACI 435's document will be presented. The combined topics will foster communication between researchers and practicing engineers.

Learning Objectives:
(1) Learn about key factors affecting deflection performance of slab and beam systems;
(2) Learn new methods of deflection calculations including creep, shrinkage, and cracking of prestressed members;
(3) Learn about real-world application of ACI 435's recommendations including how it compares to project surveyed results;
(4) Learn about how construction sequence such as shoring / reshoring can affect the deflection of slab and beam systems.

Coupled Pore Relative Humidity Model for Concrete Shrinkage and Creep

Presented By: Brock Hedegaard
Affiliation: University of Minnesota Duluth
Description: A new semi-empirical concrete shrinkage and creep model, called the CPRH Model, proposes a coupling between autogenous and drying shrinkage using a volume-average pore relative humidity and treats drying creep as an additional stress-dependent shrinkage, linking together all these phenomena. The proposed expressions are designed to facilitate traditional integral-type analysis, but also uniquely support rate-type calculations that can be leveraged by analysis software. Model calibration uses the Northwestern University (NU) database of creep and shrinkage tests to determine new model parameters. The proposed model uses minimal inputs that are often known or may be assumed by the design engineer. Comparison of the proposed model to historical time-dependent models indicates that the new model provides a superior fit over a wider range of inputs. This model will be incorporated into ongoing ACI Committee 209 documents, including reports on time-dependent models and structural analysis techniques.

Deflection Behavior of Beams Prestressed with Bonded FRP Tendons

Presented By: Wassim Nasreddine
Affiliation: Rutgers
Description: Prediction of immediate deflection for cracked prestressed concrete members using integration of curvature is evaluated and compared with deflection computed using a more direct approach assuming a uniform member stiffness based on an effective moment of inertia at the critical section where the moment is greatest. Integration of curvature takes account of changes in member stiffness and strand eccentricity along the member length. Results are evaluated using an extensive database for beams either fully or partially prestressed. The beams are simply supported under two-point loading and have a straight tendon profile. Several approaches are evaluated including a bilinear moment-deformation response, and those using an effective moment of inertia based on an effective prestress moment defined by an effective eccentricity of the prestress force or an offset in the cracked response.

Analysis + Construction: Experiences in the Application of ACI 435

Presented By: David Shook
Affiliation: Skidmore, Owings and Merrill
Description: Design engineers utilize a range of concrete framing systems for their designs to optimize for spans and cost effectiveness. They utilize a range of software tools which have a range of alignments with ACI 318 and ACI 435 provisions. Furthermore, design engineers typically do not know the actual deflection of the framing systems they design. The authors have collected a range of surveyed deflection data from built projects and compared them to design phase analysis models. Correlations and findings will be presented to aid designers in better aligning analysis methods/assumptions with actual floor framing deflection.

Analysis + Construction: Experiences in the Application of ACI 435

Presented By: Ricardo Henoch
Affiliation: Skidmore, Owings & Merrill
Description: NOTE TO REVIEWER: We would like this to be part of and precede our ACI 435 meeting that occurs 3-6pm. We would be ok giving the slot of 3-4p at the start of the meeting to do this so it wouldn't require an extra room. We are really just asking it be listed as a mini-session in the conference schedule/app.