RAM Concept PT Optimization Tutorial

Objective

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:

• The Optimizer generates a more economical design when models are set up using best practices. For this model, the Optimizer found a design that was approximately 6% more economical than the design found by manual iteration.
• The Optimizer saves significant engineering time. For this model, the optimizer would have saved more than 1 hour of engineering time spent by an expert engineer. Larger time savings are expected in larger or more complex models. The time spent finding an economical design manually could have been devoted to other engineering tasks.
• The Optimizer investigates potential solutions that are not possible with manual designs due to time constraints. Manual designs tend to focus on reducing PT quantity only. Possible solutions that add PT but reduce other reinforcing costs are typically not considered due to time and budget constraints. The Optimizer does things like adjust tendon profiles to balance moments at joints and reduce punching shear demand, which would not be practical in a manual design. In this model, the best solution is one that increases PT cost but reduces reinforcement and stud rail costs, and in turn, the total model cost.
• The Optimizer automatically controls deflection. Deflection is not explicitly considered by the optimizer. However, it tends to find solutions that produce reasonable deflections. In this model, the most economical design had more PT, which resulted in a lower service deflection than the manual design.

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.

Tutorial Instructions

1. Review the Model Notes below.
2. Download the modified tutorial file.
3. Create a copy of the downloaded tutorial file.
4. Open the tutorial model and its copy in 2 separate RAM Concept windows. Note that opening 2 simultaneous sessions of the program on a single machine counts as only 1 license use.
5. Review the model using PT Optimization Tutorial - Best Modeling Practices as a guide.
6. Run the designs in one of the open models. Review the generated tendon design and the resulting cost data in the Estimate Report.
7. Run an optimization in the other open model. CONNECT sign-in and Bentley Cloud Services credits (ACUs) are required to run optimizations within the program. Please click here for more information on ACU credits. Click here for step-by-step instructions on how to run an optimization.
8. Load the best trial into the model after the optimization is finished. See Step 8 in the step-by-step instructions.
9. Review the results and cost data of the optimized design and compare to the base design in the original model using PT Optimization Tutorial - Results as a guide.

Model Notes

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:

1. The typical slab thickness was reduced to 8 inches.
2. The balcony slab thickness was reduced to 6 inches.
3. All drop caps were removed.
4. Tendon quantities and profiles were selected using a traditional manual iteration process.
5. All tendons are modeled on the tendon parameter layers since manual tendons are not optimized by the Optimizer.

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:

1. PT forces were increased until the top service stress near the supports at or near the code limit. The image below shows the allowable service tensile stress (blue diagram, 6*sqrt(f’c) per ACI 318-14 24.5.2.1) and the maximum service stress (red diagram) at one of the interior columns.
2. PT forces were increased in some spans to reduce the bottom service stress in positive moment regions below 2*sqrt(f’c) and eliminate bottom reinforcement (see ACI 318-14 8.6.2.3).

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.