Can I change the support conditions in my model and analyze different loads with different support conditions within a single model ?

Yes. One can have different support conditions specified within the same model, each with its own set of load cases followed by PERFORM ANALYSIS and CHANGE commands

Here are a few simple rules that one should follow for such definitions

1. Each set of support condition should be followed by corresponding load cases and PERFORM ANALYSIS and CHANGE commands.
2. Supports should be specified in the same order before and after the CHANGE.
3. The number of supports in subsequent specifications should not increase. If the later cases have more supports, it is required to specify dummy supports at such nodes for prior scenarios using FIXED BUT FX FY FZ MX MY MZ ( which means unrestrained ).

Note : The points 2 and 3 are specifically for users using basic solver. The Advanced solver ( which requires the advanced analysis engine license ) does not have these limitations.   

Here is an example file with multiple support specifications

STAAD PLANE EXAMPLE FOR MULTIPLE SUPPORTS
START JOB INFORMATION
ENGINEER DATE 28-Oct-14
END JOB INFORMATION
SET NL 2
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 0 10 0; 3 0 20 0; 4 15 20 0; 5 15 10 0; 6 15 0 0;
MEMBER INCIDENCES
1 1 2; 2 2 3; 3 3 4; 4 4 5; 5 5 6; 6 1 5; 7 2 6; 8 2 4; 9 3 5; 10 2 5;
MEMBER PROPERTY AMERICAN
1 TO 10 TABLE ST W12X26
UNIT INCHES KIP
DEFINE MATERIAL START
ISOTROPIC MATERIAL1
E 29000
POISSON 0.3
END DEFINE MATERIAL
CONSTANTS
MATERIAL MATERIAL1 ALL
SUPPORTS
1 PINNED
6 PINNED
2 TO 5 FIXED BUT FX FY FZ MX MY MZ
LOAD 1
JOINT LOAD
2 FX 15
3 FX 10
PERFORM ANALYSIS
PRINT SUPPORT REACTIONS
CHANGE
*
SUPPORTS
1 PINNED
6 PINNED
2 TO 5 FIXED BUT FY KFX 10000
LOAD 2
JOINT LOAD
4 FX -10
5 FX -15
PERFORM ANALYSIS
CHANGE
PRINT SUPPORT REACTIONS
*
FINISH

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