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Deflection Calculation of Concrete Floors

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Instructor information: Bijan O. Aalami, Ph.D., M.ASCE

Course Length: 1.5 Hours

This webinar was co-sponsored by ASCE's Structural Engineering Institute and ASCE Continuing Education.

Purpose and Background

A reliable assessment of immediate and long-term deflections of concrete floors is the primary consideration for in-service design of concrete floors. Prediction of probable deflections is a routine challenge for structural concrete design engineers. This webinar provides the knowledge and numerical examples necessary for estimating a concrete floor’s deflection in the environment of an everyday consulting office. The webinar covers the recommended code limits and their application to practical conditions. Starting with a review of the simplest method of estimating deflections, the discussion moves progressively to more rigorous schemes, and concludes with the practical application of the Finite Element Method. It covers the consideration of crack formation and its impact on a concrete floor’s deflection, a critical consideration for conventionally reinforced slabs. Crack width calculation, where applicable, is also discussed. Detailed numerical examples illustrate the application of each of the methods presented. A comparative evaluation of different methods illustrates the merits of each method. The webinar also covers the application of post-tensioning to concrete floors.

The presentation is targeted to practicing concrete design engineers, with the objective to provide them with practical knowledge and the tools needed to estimate floor deflections. The webinar starts with the background in allowable values, the code permissible thresholds, and the application of code stipulated values to practical conditions. It proceeds to the review of the current methods of deflection calculation, starting with the simplest and more approximate, and progressively leads to the more rigorous, but practical design-office tool for estimating deflections. The discussion includes the application of the Finite Element Methods for floor design with due allowance for cracking. Each method is followed with a numerical example and concluded with an evaluation of its merits and the level of its approximation. Different options for the estimate of long-term deflections due to shrinkage, creep and time of application of load are discussed in detail. The discussion is followed by numerical examples for long-term deflections. The webinar concludes with an estimate of location and width of probable cracks and the means to control them.

Learning Outcomes

  • Selection of allowable deflections for specific applications
  • Application of different methods for estimating deflection values and the differences among them
  • Selection of a suitable method of calculation for a reliable estimate of probable deflections
  • Impact of cracking on deflection of concrete floors
  • Application of finite elements in estimating deflection of floor systems
  • Estimate of crack widths and their control
  • Significance and computation of long-term deflections

Webinar Benefits

  • Become familiar with the interpretation and application of ACI-318 allowable deflection values for concrete floors; flat slabs, and other construction schemes
  • Learn the background and application of different methods of calculating deflection of concrete floors
  • Find out the differences between the different calculation schemes; how they compare and which is the suitable method for your project
  • Become familiar with the use of the Finite Element Method in estimating the deflection of concrete floors, with or without allowance for crack formation
  • Attain a full understanding of the impact of creep, shrinkage and time of application of loads on the long-term deflection of concrete floors
  • Learn how to estimate the long-term deflection of a concrete floor and its code compliance
  • Explore the application of post-tensioning in deflection control and cracking of concrete floors
  • Find out how probable crack widths are estimated and their control in concrete floors

Intended Audience

Structural engineers, building officials, plan checkers, and students engaged in the design of conventionally reinforced or post-tensioned floor systems, who are seeking to learn about practical procedures for estimating the immediate and long-term deflections of concrete floors, as well as the state-of-the art tools available.

Webinar Outline

  • Acceptable deflection values
  • Code permissible deflection limits; interpretation and evaluation
  • Review and presentation of methods of deflection calculation
    – Simplified ACI-318 equivalent moment of inertia Ie
    – Improved ACI-318 based on Ie
    – Recommended dimensional values
    – Closed form formulas
    – Finite Element Methods, with allowance for cracking
  • Long-term deflections; shrinkage, creep, impact of amount and location of reinforcement
  • Credible estimates of long-term and immediate deflections
  • Estimate of crack width and its control