The objective of this tutorial is to build on the skills learned in Chapter 45 of the RAM Concept Manual, a flat plate tutorial for ACI 318, and show how post-tensioned (PT) slab design and engineering workflows can be improved using the PT Optimization feature. During the tutorial, a base design found using traditional manual optimization techniques is compared to the best design found by the Optimizer. The tutorial also outlines key modeling best practices that are important for successful optimizations.
Cost estimates reported by the program before and after the optimization are summarized in the table below:
This tutorial demonstrates the following:
This tutorial focuses on modeling tips and design workflows only and is not intended to be a step-by-step guide on how to use the PT Optimization feature. Please see the RAM Concept PT Optimization Feature Article for an outline of the basic steps required to run optimizations in the program.
The Chapter 45 tutorial model, which is installed in the Tutorial folder of the root RAM Concept program files directory, is useful for learning how to model manual and generated tendons, layout design strips, and review the results. While the PT layout in this model produces a valid design with no code failures, it is not intended to represent the most economical design for the slab. For example, the slab thickness and PT quantities could be decreased without producing design failures or sacrificing performance (deflection, vibration, etc).
We have modified the original tutorial model to produce a more economical design. The modified file can be downloaded here. Significant changes from the original tutorial model are summarized in the next section. The file has been prepared for optimization and is ready to be optimized without any modeling or criteria changes. This model represents the base design and used to judge the success of the Optimizer.
The significant changes from original tutorial model are listed below:
The revised model is saved with a manual design that was completed by Jonathan Hirsch, PE, the Development Manager of RAM Concept. He has 23 years of post-tensioned slab design experience, is an active member of the Post-Tensioning Institute (PTI), and currently chairs the PTI Education Committee.
During the manual design process, tendons were initially modeled with minimum precompression, and tendon quantities were gradually increased until a valid design was obtained. The total engineering time to produce the optimized manual design was about 1 hour.
Examples of traditional manual design methods that were used to select the base design include:
When reviewing the base design in the revised tutorial model, you will see that bottom reinforcement remains in many of the spans. Increasing PT force to reduce bottom service stress and eliminate this reinforcement in these spans either resulted in stress failures or increased stud rail reinforcement in other areas. This illustrates another important point with manual designs: often there is not a well-defined stopping point and the most efficient change is not intuitive. Changing tendon quantities in one area to reduce reinforcement cost can create a design issue that requires an increase in PT or reinforcement in another span, and so on. The Optimizer allows you to easily compare hundreds of possible designs and automatically eliminates the designs that do not satisfy code requirements. This eliminates the need for tedious manual iteration and saves hours of engineering time on a typical project.