Prestressing tendons in slabs: from optimization to documentation

The design of a prestressed slab has two stages: selecting the prestressing and preparing the construction documentation. The Igent Suite platform handles both. In IgentForm you select and optimize the prestressing tendons, and in IgentDraw the drawings, the 3D model and the data needed on site are created.

The screenshots come from videos recorded in the French and English versions of the programs. The video above shows stage 2 (documentation in IgentDraw); the videos for stage 1 and for beams are at the end of this tutorial.

Stage 1. Selection and optimization of tendons — IgentForm

The tendon profile is created automatically

When tendons are defined, the program detects the supports by itself and generates the high and low points of the profile. The tendon also has anchorages and a duct assigned, chosen from a database.

A prestressing tendon in a slab supported on columns. On the left, the tendon profile with high and low points.

Punching shear first

The optimization starts with a check of the punching shear conditions. The calculation note shows the course of the check for every column.

The note with the punching shear check for successive columns.

For columns that do not meet the punching shear conditions you define slab thickenings.

Slab thickenings defined at the columns that did not meet the punching shear conditions.

Optimization on sections

Tendon optimization is carried out on sections through the slab, which the user has to define. In the optimization parameters you indicate the section for which the program is to look for a solution, and you set:

• the number of tendons — the maximum number per meter and the spacing,
• the stress level in the tendons — the range of the tensioning force and the step with which the program is to change it.

The program looks for a solution that meets all the conditions and gives the result: the number of tendons and the tensioning force.

Optimization parameters for the selected section and the result: the number of tendons and the tensioning force.

Finally, the slab thickness

In the last step you can optimize the slab thickness by giving a range and a step. The tendon eccentricities are calculated automatically for every variant.

What the program takes into account in the calculations

The calculations take into account the loads from the tendons, the prestressing losses and the code conditions. The calculation note shows the course of the check of successive conditions.

Stage 2. Documentation — IgentDraw

What comes from IgentForm and what IgentDraw calculates

This is the most important rule of this stage:

From IgentForm come the calculated tendon profiles. On their basis IgentDraw itself calculates the duct profiles and the coordinates of the tendon supports (chairs).

So the designer does not redraw the profiles from the calculations and does not determine the positions of the supports manually — the documentation is created from the same data on which the calculations were made.

The theoretical position of the tendons can also be loaded from a DXF file. IgentDraw then recovers the slab geometry and creates the final tendon profiles with ducts.

A slab with prestressing tendons in IgentDraw.

Duct and anchorage databases

The program contains databases of tendon ducts and anchorages. You choose from them the elements used in the project.

Tendons with ducts at the edge of the slab.

What you get

Result	Format	Contents
Drawing	DXF	Drawing of the prestressing tendons
3D model	IFC	Tendons with ducts and safety zones
Tendon data	TXT	Tendon schedule and data of the tendon supports

The drawing of the tendon profiles saved in DXF format.

The IFC model opened in a viewer: tendons, anchorages and reinforcing bars in one file.

The tendon data file opened in a spreadsheet: length, number of strands, mass, tensioning force and theoretical elongation of each tendon.

Tendons and standard reinforcement in one project

Prestressing tendons and standard reinforcement can be kept in one IgentDraw project.

Prestressed beams — IgentBeam

Multi-span beams are handled by IgentBeam — a multi-span beam calculator that calculates the beam using the finite element method after every parameter change. The program optimizes the tendon profile, automatically generates the loads from prestressing and lets you export them to Autodesk Robot Structural Analysis. IgentBeam can also work together with IgentForm on tendon optimization.

IgentBeam: beam geometry, load cases, tendon profile and internal force diagrams.

Summary

1. IgentForm — the tendon profile with automatically detected supports, the punching shear check, optimization of the number of tendons and the tensioning force on the sections you define, and finally optimization of the slab thickness.
2. IgentDraw — on the basis of the calculated tendon profiles the program calculates the duct profiles and the coordinates of the tendon supports, and then creates the DXF drawing, the IFC model and the tendon data file.
3. IgentBeam — calculations and optimization of the tendon profile for multi-span beams.

What next

• Prepare the standard reinforcement of the slab according to the tutorial Slab reinforcement drawing from a DXF background.
• Would you like to check tendon optimization on your own slab? Download the demo version — it works with full functionality for 30 days.