Define the time-dependent
load sources
You need to define your time-dependent load sources in different ways
depending on their type as follows:
"Applied loads" are general purpose time-dependent loads
that you must apply to the structure as normal static node
loads. The time-dependent properties of the loads are defined
separately when you create your transient load cases as described
below.
"Initial displacements" are assumed to be present at time
zero and are immediately removed, causing the structure to react and
settle into a new state of equilibrium. They typically come from the
analysis results of a static or spectral load case. You simply
need to specify the load case that they come from as the "Source
case" when you create your transient load cases as described
below.
"Initial velocities" are assumed to be present at time zero
and are immediately removed, causing a structural response. Because
there is currently no dedicated datasheet for velocities, you must
input the initial velocities as node loads but with units of length/sec
and radians/sec instead of forces and moments, where "length"
is the length unit used
in the job. Note that if you change
the job length, force or moment units, the "velocity" node
loads may no longer be correct and it is up to you to convert them
manually.
"Base acceleration" applies time-dependent in-phase translational
or rotational accelerations to the base (restrained) nodes. They cause
all of the base nodes to vibrate in unison. You simply define them
in the transient loads datasheet when you create your transient load
cases as described below. For rotational accelerations you must also
specify the node that the rotations act about. The displacements,
velocities and accelerations obtained from a transient response analysis
using a base acceleration load source are relative
to the base (restrained) nodes, and so if you want to obtain
the net displacements, velocities or accelerations then you need to
superimpose the base displacements, velocities or accelerations on
the reported values, taking into account that they could be in opposite
directions.
"Harmonic loads" involve masses vibrating in a sinusoidal
motion at various points on the structure. The first step requires
you to calculate their dynamic force and input them as node loads. The
dynamic force of a vibrating mass is its mass multiplied by its maximum
acceleration. For example, a mass m
rotating about a circle of radius r
with a frequency of f has
a maximum acceleration of a = r(2pf)2
and a dynamic force of ma = mr(2pf)2.
This is also equal to the centrifugal force of the rotating mass.
Similarly, a mass m moving
linearly in harmonic motion (eg. like a piston) over a travel distance
of 2r with a frequency of
f also has a maximum acceleration
of a = r(2pf)2 and a
dynamic force of ma = mr(2pf)2.
Because harmonic loads oscillate from one side to the other there
is no need to input them as both positive and negative.
"Periodic loads" are similar to harmonic loads except that
they follow an arbitrary time-dependent repeating pattern rather than
being strictly sinusoidal. You should apply them as normal static
node loads. Their period and time-dependent properties are defined
separately when you create your transient load cases as described
below.
Note that for all load types, if
they are acting at some angle to the global axes then you must calculate
their global components and apply them in each of the global directions.
Create the transient
load cases
The next step involves setting
up your transient load cases. These can be input via the "Transient
Loads" datasheet. Each transient load case contains a source
load case (eg. a node load case that you have put your time-dependent
loads or velocities into or an analysed load case that contains initial
displacements), a problem type ("Applied loads", "Initial
displacements", "Initial velocities", "Base acceleration",
"Harmonic loads" or "Periodic loads"), a mass
case (ie. the mass case that was used in the dynamic frequency analysis
on which the transient analysis is based), the base accelerations
(if the "Base acceleration" problem type was selected),
a load factor (that can be used to factor up or down the effect of
the time-dependent loads), a frequency (for harmonic loads), a phase
(for harmonic or periodic loads) and a period (for periodic loads).
Each transient load case also contains a "Factor versus time table"
that defines the time-dependent properties of the applied load, base
acceleration and periodic load sources. If no table is defined then
the load source is just assumed to be suddenly applied at time zero
and held constant. Harmonic loads generally don't require a factor
versus time table, however a table can be defined to further vary
the time-dependent effect of a harmonic load source if required. If
both a load factor and a factor versus time table have been defined
then they are multiplied together and their product is applied to
the transient loads.
The source load case and mass case can be primary or combination load
cases.
If you want to combine load sources that have different problem types,
factors, frequencies, periods, phases or time-dependent properties
into a single transient load case then you can do it by simply adding
extra lines to the datasheet that have the same transient load case
number. Any lines with duplicate transient load case numbers can have
different source load cases, problem types, base acceleration, factors,
frequencies, periods, phases and factor versus time tables. The transient
analysis will simply combine the effects of the duplicate lines for
each transient load case.
If you want to combine transient analysis results with static analysis
results, refer to "Combining
transient and static results".
Run the transient
analysis
When running the
transient analysis you can select which dynamic modes to consider
(usually just leave the mode list blank to consider them all), the
damping type (none, modal or Rayleigh) and the number of time steps.
Each step is a snapshot in time at which the structural response is
calculated. You should choose the number of steps based on a good
compromise between analysis speed, the amount of data that gets generated,
and enough sampling points to get a good representation of the structure's
response. 1000 steps is usually a good starting point. If you set
it too low then you may miss some of the peaks and troughs in the
structure's response if they happen to occur between steps. If you
set it too high then the analysis will be slower and the amount of
data stored with the job will be larger. The number of steps also
has a direct affect on the smoothness of the animation and graphs
that you can get after the transient analysis.
After
the transient analysis
After the analysis, there are four main courses of action:
(a) Animate
the structure to see how it responds to the transient loads.
(b) Display
graphs of displacements, velocities, accelerations and phases.
(c) Convert the time steps to step
load cases.
(d) Generate
reports of the results.
These are explained in more detail as follows.
Animate the structure
In order to see how the structure responds to the transient loads you
can animate it via the "Show
animated dynamic response" button 
on the
side toolbar. You can choose any transient load case to animate and
specify the animation speed. An animation speed of "1x"
shows it in real-time.
Display graphs
Graphs of displacements, velocities, accelerations or phases versus
time can be displayed by right-clicking on any node and then selecting
"Harmonic/Transient
Response Graph" from the popup menu that appears. Once the
graph has appeared, you can change load cases via the load case selector
at the top of the graph and you can change the diagram type or axis
via the "Diagram Type" button.
You can also click on any other nodes while the graph is visible and
it will be updated for each node that you click.
Generate transient
step load cases
In order to be able to use the results from a transient analysis in
a practical way, you can convert any of the transient time steps to
load cases that contain displacements, forces, moments, stresses and
reactions, just like you would get from a static analysis. You can
do this by selecting "Generate
Dynamic Response Step Load Cases" from the "Loads"
menu, choose the transient load case that you want to create the load
cases from and then specify the starting load case number. You can
also use this tool to generate combination load cases that combine
the transient result load cases with other non-transient load cases
such as static, spectral or harmonic load cases.
You must first decide which steps you want to convert to load cases.
It is usually a good idea to only convert the steps that correspond
to peaks or troughs in the structure's response, otherwise you may
generate many load cases that are of no use. You can specify a list
of steps to be converted or you can request SPACE GASS to search for
the peaks and troughs by selecting "Steps at min/max values".
You can also limit the steps to a specific time range.
Multiple options can also be selected. For example, if you specify
a step list of 500-700, tick "Steps at min/max values" and
specify a time range of 4-6 seconds then SPACE GASS will choose the
steps between 500 and 700 that correspond to peaks and troughs within
the time range of 4 to 6 seconds. If you make your search too restrictive
then you may exclude all steps. Conversely, if you convert too many
steps to load cases then you may finish up with a huge number of unwanted
load cases.
Once the step load cases and combinations have been generated then
you can view them, get reports and use them in any of the design modules,
just like any other load cases.
Be careful!
If you re-run the transient response analysis then all the load cases
previously created from the transient steps will be deleted and you
will have to re-generate them!
If you
have a combination load case that combines static load cases with
step load cases from a transient response analysis then you must carefully
check that the transient step load cases actually exist before you
use the results of the combination. If the transient step load cases
have been deleted (due to a model, load or mass change or a re-analysis)
then the combination may just contain the results of the static load
cases. Because SPACE GASS allows combination load cases to include
non-existent load cases, it doesn't give any warnings or errors if
the transient step load cases are missing. It can't even detect that
they are missing because they have no input data associated with them.
One way for you to quickly check if combination load cases contain
transient analysis results is to look for "transient" in
the load case heading in analysis result reports (eg. "Load case
301 (LinComb+Transient)" or "Load case 302 (Transient)").
Generate reports
You can generate reports that
include "Transient Loads" (input data) and/or "Transient
Response" (output data). The "Transient Response" report
includes a list of the nodes and steps that correspond to the locations
of maximum translation, velocity, acceleration and rotation. These
are sometimes useful for finding the critical locations in your model.