T.1 Creating Load Case 3
In this procedure, you will add a load combination of the first two primary load cases.
If you started your model in STAAD.Pro Physical Modeler, first select the Loading page in the Analytical Modeling page control bar. The Load & Definition dialog opens on the right side of the program window.
Here you will see that the two load groups you created inSTAAD.Pro Physical Modeler have each been added as primary load cases to the analytical model. The loads you added to the physical model are also transferred as load items in the respective load cases.

On the
Loading ribbon tab, select the
Combination Load Case
tool in the
Loading Specifications group.
The Create New Load Combinations dialog opens.  Type 75 Percent of [DL+LL+WL] in the Name field.

Set the
Type option to
Normal.
This combines the loads algebraically.
The other combination types available are called SRSS (square root of sum of squares) and ABS (Absolute). The SRSS type offers the flexibility of part SRSS and part Algebraic. That is, some load cases are combined using the square root of sum of squares approach, and the result is combined with other cases algebraically, as in:
$A+\sqrt{{B}^{2}+{C}^{2}}$where A, B, and C
=  the individual primary load cases

Type
0.75 in the
Default ɑ1 field.
This is used as the default factor for normal load combinations.

Click
[>>].
Both primary load cases are added to the Load Case Definition list using the default factor.
This indicates that the results of the two load cases will be multiplied by 0.75 and then summed to produce load combination results. Note that such a load combination method should only be used for linear analysis.
 Click Add. The load combination is added to the Load Cases Details section in the Load & Definition dialog as load 3.
 Click Close.