Skip to content

Amplitude manager

An amplitude is a dimensionless function of time. Loads and boundary conditions reference an amplitude by name, and their magnitude at any moment is the specified value multiplied by the amplitude at that time. This separates how much is applied from when it is applied, so the same time history can drive several loads at once.

The amplitude manager replaces the former Amplitude branch of the data tree, which is no longer available. Open it from the numgeo tools toolbar or from numgeo → Manager → Amplitude.

How amplitudes are stored

Amplitudes are held in amplitude-database.tcl next to the GiD project. As with the other managers, only the amplitude name reaches the data tree, so the database file must accompany the project when it is copied by hand.

The plot

A live plot occupies the right-hand side of the window and redraws as parameters are typed. It is the fastest way to catch a wrong sign, a misplaced decimal point or a frequency that is an order of magnitude off, so it is worth watching while entering values.

The axis limits follow the data automatically, and can be set manually when a particular range needs inspecting.

Amplitude types

A new amplitude starts as a unit ramp from (0, 0) to (1, 1).

Ramp

A piecewise-linear transition from one value to another, held constant outside the transition:

\[ a(t) = \begin{cases} v_0 & t < t_0 \\[2pt] v_0 + (v_1 - v_0)\dfrac{t - t_0}{t_1 - t_0} & t_0 \le t \le t_1 \\[6pt] v_1 & t > t_1 \end{cases} \]
Parameter Meaning
t0, v0 Time and value at the start of the ramp
t1, v1 Time and value at the end of the ramp

This is the most common amplitude type. A ramp from (0, 0) to (t1, 1) is the usual way to apply a static load gradually instead of as a step, which avoids the numerical shock of instantaneous loading.

Rising sine / rising cosine

A harmonic signal multiplied by a linearly rising envelope:

\[ a(t) = \text{env}(t)\cdot\sin(\omega t) \qquad \text{env}(t) = \begin{cases} 0 & t \le 0 \\[2pt] \dfrac{A_1}{t_1}\,t & 0 < t \le t_1 \\[6pt] A_1 & t > t_1 \end{cases} \]

with cosine as the alternative carrier.

Parameter Unit Meaning
t1 – Duration of the rise phase
ω Hz Frequency of the carrier
A1 – Amplitude reached at the end of the rise

The envelope is what makes these useful for dynamic and seismic problems: the excitation grows from zero over the characteristic time t1 rather than switching on abruptly, which would excite the whole frequency content of the model at once.

t1 must be strictly positive.

Periodic

A truncated Fourier series, for signals that repeat but are not a single harmonic:

\[ a(t) = A_0 + \sum_{j=1}^{N}\Big[A_j\cos\big(j\,\omega\,(t-t_0)\big) + B_j\sin\big(j\,\omega\,(t-t_0)\big)\Big] \]

for t ≥ t0, and zero before.

Parameter Unit Meaning
N – Number of harmonics
A0 – Constant offset
t0 – Start time of the periodic phase
ω Hz Fundamental frequency

Below these, a table of A_i and B_i coefficients appears with one row per harmonic. The number of rows follows N automatically.

N must be a positive integer, ω must not be zero, and every harmonic needs its coefficient pair.

Tabular

Any piecewise-linear time history, entered as one time–value pair per line. numgeo interpolates linearly between the rows:

0.0   0.0
0.5   1.0
1.5   1.0
2.0   0.0

This is the type to use for measured signals — an earthquake record, a monitored water level, a load history from a test.

The parser is deliberately tolerant: spaces, commas, semicolons and tabs all work as separators, blank lines are ignored, and rows that do not contain two valid numbers are skipped rather than causing an error. Columns can therefore be pasted straight from a spreadsheet or CSV file without cleanup. A Paste from clipboard button is provided for exactly this.

Warning

Because malformed rows are skipped silently, a file with a header line or a stray text column will load without complaint but with fewer points than expected. Check the plot and the point count after pasting a long series.

Validation

Checked on Apply & Save:

  • The name must not be empty.
  • Ramp — t0, v0, t1, v1 must all be numeric.
  • Rising sine/cosine — t1, ω, A1 must be numeric, and t1 positive.
  • Periodic — N a positive integer, A0, t0, ω numeric, ω non-zero, and at least N coefficient rows present.
  • Tabular — at least one valid time–value pair.