> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/openbrimfea/properties-fea/strands-fea.md).

# Strands \[FEA]

A Strand defines a prestressing strand size — its diameter, cross-sectional area and weight per unit length. Like Rebars, it is a **property library** entry: it describes a strand, it does not place one. Strands are referenced by Tendon definitions and by section reinforcement layouts.

Strand entries feed **prestress force calculations and section analysis**. They do not create finite elements and add nothing directly to the global stiffness or mass matrices.

## Properties

**Diameter:** The nominal strand diameter.

*Engineering influence.* Diameter governs whether the strand pattern fits within the duct or the section, the minimum duct and deviator radii, and the spacing rules that apply. In a pre-tensioned member it also relates to transfer and development length, over which the prestress force builds up from zero at the member end.

As with rebars, diameter and area are entered independently, so nothing enforces consistency between them. Prestressing strand is stranded wire rather than solid bar, so its area is genuinely **less** than `πD²/4` — typically around 70–75% of it. That is expected and correct; do not "fix" the area to match the geometric circle.

**Area:** The cross-sectional area of one strand.

*Engineering influence.* This is the value that governs prestress. Together with the number of strands it gives the tendon's steel area, which converts jacking force into strand stress — and code limits on jacking are expressed as a fraction of ultimate **stress**, so an understated area makes the model report a stress that exceeds the limit for a force that is actually acceptable, or hides an over-stressed tendon behind an overstated area.

It also sets the axial stiffness the tendon contributes in a bonded section analysis, which affects the neutral-axis position and the cracked-section properties, and it drives the ultimate-strength contribution of the prestressing steel.

Increasing the area increases the force available at a given stress and increases the tendon's contribution to flexural capacity. A value of zero means the strand contributes nothing.

Use the published nominal area for the designation rather than a computed value.

**Weight:** Weight per unit length of the strand.

*Engineering influence.* Used for **quantity reporting** — take-offs and schedules — not for structural behaviour. Prestressing steel weight is normally already inside the concrete's unit weight, so this value does not add mass or load to the analysis.

## Verification

* Compare diameter and area against the published table for the designation. Expect area to be roughly 70–75% of `πD²/4` for stranded product.
* Open a Tendon that references the strand and confirm the reported steel area equals strand area × strand count.
* Check the stress at jacking: `force / (count × area)` should land within the code limit as a fraction of ultimate strength. A value far outside that range usually means the area or the count is wrong.
* Confirm the tendon force reported in **Prestressing Details…** is consistent with the stress you expect.
* Check that a change in strand count moves computed capacity proportionally.

## Common mistakes

* **"Correcting" the area to `πD²/4`.** Strand is stranded, not solid, and its true area is substantially lower. This inflates the steel area by around a third.
* **Entering the area of the whole tendon** rather than of one strand, then also setting the strand count — multiplying the prestressing steel by the count.
* **Confusing strand and rebar entries**, which have identical fields but very different material behaviour and code treatment.
* **Mixing unit systems** between diameter and area.
* **Expecting the Weight value to add load** to the model. It is for reporting only.


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