> 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/analysis/analysis-step.md).

# Analysis Step

Says how many steps the analysis takes and how long each one is — the arguments of the OpenSees `analyze` command.

## 🔧 Grasshopper component

`Analysis Step (Alpaca4d)` — **Alpaca4d ▸ 07\_Analysis**

### Inputs

| Name    | Nick      | Type    | Default  | Description                                                                       |
| ------- | --------- | ------- | -------- | --------------------------------------------------------------------------------- |
| NumIncr | `NumIncr` | Integer | `1`      | Number of steps to run.                                                           |
| Dt      | `Dt`      | Number  | *(none)* | Time step, in `s`. Transient analysis only.                                       |
| DtMin   | `DtMin`   | Number  | *(none)* | Smallest step the solver may fall back to when a step fails.                      |
| DtMax   | `DtMax`   | Number  | *(none)* | Largest step it may grow to.                                                      |
| Jd      | `Jd`      | Integer | *(none)* | Iterations aimed for at each step, used to decide whether to grow or shrink `Dt`. |

### Outputs

| Name         | Nick           | Type         | Description                                                          |
| ------------ | -------------- | ------------ | -------------------------------------------------------------------- |
| AnalysisStep | `AnalysisStep` | AnalysisStep | For [Analysis Settings](/docs/basics/analysis/analysis-settings.md). |

## 📈 When to use it

**Use it when** you are running anything through [Run Analysis](/docs/basics/analysis/run-analysis.md). Every analysis needs one.

**Do not use it when** you only want modes → [Natural Vibration](/docs/basics/analysis/natural-vibration.md) runs its own eigenvalue problem and takes no steps.

## 💡 Choosing the values

**Static, linear.** `NumIncr = 1` and nothing else. One step applies the whole load.

**Static, non-linear.** Raise `NumIncr` and lower the [Integrator](/docs/basics/analysis/integrator.md)'s `Lambda` so their product reaches the load factor you want. Twenty to a hundred steps is normal for a pushover; more steps mean a smoother capacity curve and a better chance of converging.

**Transient.** `NumIncr × Dt` is the duration of the analysis. `Dt` must resolve both the excitation and the response:

* Small enough to sample the highest mode you care about — twenty points per period.
* Small enough to sample the input signal. A ground motion recorded at 0.005 s should not be run at 0.05 s.
* For [CentralDifference](/docs/basics/analysis/integrator.md), small enough to stay stable, which is usually much smaller than either of the above.

**Adaptive stepping.** Setting `DtMin`, `DtMax` and `Jd` lets the solver shrink the step when it struggles and grow it again afterwards. Worth it for long non-linear runs; leave them empty otherwise.

## 🔗 Relation to OpenSees

```tcl
set alpacaAnalyzeOk [analyze $numIncr $dt $dtMin $dtMax $jd]
```

The result is captured so that [Run Analysis](/docs/basics/analysis/run-analysis.md) can tell a non-convergence from a successful run — `analyze` returns a negative number on failure but does not raise a Tcl error.