EXT Joint Load : Convert the beam to a FIXED-FIXED Beam. The loads applied as MEMBER LOAD are converted to fixed end actions. These are the negative of the support reactions you would get from all the loads applied using the MEMBER LOAD command on this fixed-fixed beam. These fixed end actions are the EXT Joint loads. See the attached screenshot where the fixed end beam reaction is shown .
The INT Joint Load is the amount (algebraic value) that needs to be added to this EXT Joint Load to keep it in equilibrium with the Reactions at that real support that is defined at that Joint. See below.
Following are the two examples for better explanation.
For the first case (NODAL LOAD model ) :
As mentioned above , Staad first restrains all the nodes (including the node on which the nodal load is applied ) .
As the nodal load in this case is also applied at fixed support node (the node on which load is applied has been constrained), so fixed end reaction (EXT) is “0 kip ” and then the compensating forces (INT) are -20 kip and -10 kip .
So, the net force 1 = EXT+INT = 0 -20 =-20 KIP
net force 2 = EXT+INT = 0 -10 =-10 KIP ( See the attached figure)
For the second case ( MEMBER LOAD ) :
In this case , the concentrated load is not applied at any joint ,so after restraining the available end nodes, program first calculates the
fixed end force (EXT) as - 22.22 Kip at the first node and - 7.78 Kip at second node.
And the compensating forces (INT) at first node 2.22 kip and -2.22 kip at second node.
the net force at first node = EXT+INT = - 22.22 + 2.22 =-20 KIP
net force at second node = EXT+INT = - 7.78 - 2.22 =-10 KIP ( See the attached figure)
In both the cases the net forces values as reported by Staad are same.
As you have defined the PRINT STATIC LOAD command , Staad reports the detailed breakup of the loads applied at the supports from which it derives the ultimate reaction values.
If you use the command PRINT SUPPORT REACTION ALL as shown below , you could see the net reaction value in the output file (20 kip and 10 kip are reported directly in the member load model , see the attached file).