# Getting Started with OpenSees -- Gravity Loads

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Gravity loads are independent of the type of lateral loading and here they are considered part of the structural model.

<h2>Nodal Forces & Moments Calculations</h2>

Because the beam is an elastic element, the vertical load distributed along the horizontal member can be represented by nodal forces and moments. The nodal forces are distributed equally to the two end nodes. The nodal bending moments are equal and opposite:

The nodal force is equal to one half of the superstructure weight:

Force = (4000kip)/2 = 2000kip

the distributed load is calculated by dividing the total load by the beam length:

DistributedLoad = (4000kip)/((42ft)*(12inch/ft))=7.94 kip/inch

The bending moment is then calculated from the distributed load:

Moment = (1/12)*(DistributedLoad)*(BeamLength)^2=(1/12)*(7.94 kip/inch)*(42*ft*12in/ft)^2 = 168074 kip-inch

<h2>Load-Pattern Definition</h2>

Like all loads in OpenSees, gravity loads require two steps. The first step defines the load into a load pattern, the second applies the load pattern and the associated gravity load. The plain pattern Command with a linear time series is used in the load definition:

''pattern Plain $patternTag (TimeSeriesType arguments) {

load $nodeTag (ndf $LoadValues)

}''

```Tcl pattern Plain 1 Linear { load 3 0.0 -2000 -168074 load 4 0.0 -2000 168074 } ```

<h2>Analysis Definition</h2>

```Tcl constraints Transformation ```

```Tcl numberer RCM ```

```Tcl system BandGeneral ```

test NormDispIncr $tol $maxNumIter <$printFlag>

```Tcl test NormDispIncr 1.0e-6 6 ```

```Tcl algorithm Newton ```

The type of integrator specified using the integrator Command is dependent on whether it is a static analysis or transient analysis. The gravity load is analyzed using the load-control integrator which takes the following form: integrator LoadControl $dLambda1 <$Jd $minLambda $maxLambda>

A load increment of 1/10 will be applied at each analysis step ```Tcl integrator LoadControl 0.1 ```

```Tcl analysis Static ```

<h2>Analysis Execution</h2>

The commands take the following formats:

analyze $numIncr <$dt> <$dtMin $dtMax $Jd> <br> loadConst <-time $pseudoTime>

```Tcl analyze 10 loadConst -time 0.0 ```


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