> For the complete documentation index, see [llms.txt](https://alpaca4d.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alpaca4d.gitbook.io/docs/basics/results/modal-analysis-report.md).

# Modal Analysis Report

Reads the modal properties OpenSees writes after an eigenvalue run: masses, centre of mass, participation factors and participation mass ratios.

## 🔧 Grasshopper component

`Modal Analysis Report (Alpaca4d)` — **Alpaca4d ▸ 08\_NumericalOutput**

### Inputs

| Name        | Nick          | Type  | Default | Description                                                                                                          |
| ----------- | ------------- | ----- | ------- | -------------------------------------------------------------------------------------------------------------------- |
| AlpacaModel | `AlpacaModel` | Model | —       | A model from [Natural Vibration](/docs/basics/analysis/natural-vibration.md). Any other model has no report to read. |

### Outputs

Each output is the corresponding block of the report, as lines of text.

| Name                                         | Nick                        | Description                                                         |
| -------------------------------------------- | --------------------------- | ------------------------------------------------------------------- |
| EigenValueAnalysis                           | `EigenValueAnalysis`        | Eigenvalue, frequency, period per mode.                             |
| TotalMassOfStructure                         | `TotalMassOfStructure`      | Total mass, all six directions.                                     |
| TotalFreeMass                                | `TotalFreeMass`             | Mass on unrestrained DOFs — the mass that can actually participate. |
| CenterOfMass                                 | `CenterOfMass`              | Coordinates of the centre of mass.                                  |
| ModalParticipationFactors                    | `ModalParticipationFactors` | Participation factor per mode and direction.                        |
| ModalParticipationMasses                     | `ModalParticipationMasses`  | Participating mass per mode and direction.                          |
| ModalParticipationMasses\_Cumulative         | `..._Cumulative`            | Running total down the modes.                                       |
| ModalParticipationMassesRatio(%)             | `...Ratio(%)`               | Participating mass as a percentage of the total.                    |
| ModalParticipationMassesRatio(%)\_Cumulative | `...Ratio(%)_Cumulative`    | Running total of the percentages.                                   |

## 📈 When to use it

**Use it when**

* You need to show that enough modes have been included. Most codes ask for **90 %** of the mass in each direction — read it off **ModalParticipationMassesRatio(%)\_Cumulative** and raise the mode count until you get there.
* You are checking the total mass of the model against a hand calculation. This is the quickest way to catch a missing density or a mis-scaled [Mass Point](/docs/basics/loads/mass-point.md).
* You need the centre of mass, for instance to place a [Rigid Diaphragm](/docs/basics/constraints/diaphragm.md) master node or to estimate torsional eccentricity.

**Do not use it when**

* You only want periods and frequencies → they come straight out of [Natural Vibration](/docs/basics/analysis/natural-vibration.md).

{% hint style="info" %}
The outputs are **text**, one line per row of the report — they are the report as OpenSees wrote it, not parsed numbers. Plug them into a Panel to read, or split them yourself if you need the values.

The report is read from `ModalReport.txt`, written next to the Grasshopper document. If the file is missing, the model did not come from [Natural Vibration](/docs/basics/analysis/natural-vibration.md), or the document was never saved.
{% endhint %}