# Getting Started with OpenSees -- Loads and Analysis
In OpenSees applying loads is a three-step process:
1. You must first define the loads in a load pattern 2. You must then
define the analysis and its features 3. The loads are then applied when
you execute the analysis
<h2>Load definition</h2> Loads are defined using the
OpenSees pattern Command.
Three types of patterns are currently available:
-
a. plain Pattern -- this pattern is used to define the
following:
-
i. nodal loads, such as gravity loads and lateral loads (or
load-controlled nodal displacements)
-
ii. single-point constraints, such as displacement control at a node
(typically used for a constant displacement at a node)
-
iii. element loads, such as distributed gravity loads along the element
(this is a new option, which still needs documentation).
-
b. UniformExcitation Pattern -- this type of pattern
imposes a user-defined acceleration record to all fixed nodes, in a
specified direction.
-
c. MultipleSupport Pattern -- this type of pattern
imposes a user-defined displacement record at specified nodes, in a
specified direction, or a ground-motion record.
<h2>Analysis definition and features</h2> The
analysis-definition part of OpenSees allows the user to define the
different linear and nonlinear analysis tools available. For each
analysis, the following items need to be defined, preferably in this
order:
- constraints : The OpenSees constraints Command is used to construct the
ConstraintHandler object. Constraints enforce a relationship between
degrees-of-freedom. The ConstraintHandler object determines how the
constraint equations are enforced in the analysis.
- numberer : The OpenSees numberer Command is used to construct the
DOF_Numberer object. The DOF_Numberer object determines the mapping
between equation numbers and degrees-of-freedom -- how
degrees-of-freedom are numbered.
- system : The OpenSees system Command is used to construct the LinearSOE
and LinearSolver objects to store and solve the system of equations in
the analysis.
- test : The OpenSees test
Command is used to construct a ConvergenceTest object. Certain
SolutionAlgorithm objects require a ConvergenceTest object to determine
if convergence has been achieved at the end of an iteration step.
- algorithm : The OpenSees algorithm Command is used to construct a
SolutionAlgorithm object, which determines the sequence of steps taken
to solve the non-linear equation.
- integrator : The OpenSees integrator Command is used to construct the
Integrator object. The Integrator object determines the meaning of the
terms in the system of equation object. The Integrator object is used
for the following:
::*determine the predictive step for time t+dt
::*specify the tangent matrix and residual vector at any
iteration
::*determine the corrective step based on the displacement increment
dU
- analysis : The OpenSees analysis Command is used to construct the Analysis
object. This analysis object is constructed with the component objects
previously created by the analyst. All currently-available analysis
objects employ incremental solution strategies. There are three types of
analysis currently available:
::*Static - for static analysis
::*Transient - for transient analysis with constant time step
::*VariableTransient - for transient analysis with variable time
step
<h2>Analysis execution</h2> The analysis is executed
using the OpenSees analyze
Command. This command moves the analysis forward by the specified
number of steps.
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