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TR.31.9 Defining Starting Load

This command is used to directly specify the starting load vectors for use with load dependent Ritz vector method for the eigen solution.

General Format

DEFINE STARTING LOAD
MASS { X | Y | Z | XY | YZ | ZX } NGEN n

or 

REFERENCE LOAD Ri NGEN n
END

You may specify one of each direction or direction pair when using the MASS option. You may specify any number of reference loads.

where
n
=
The number of Ritz vectors to be extracted corresponding to each starting load.
Ri
=
The reference load number to be used as a starting load.

Program Versus User-Generated Start Vectors

Using the program generated method, the starting load vector is generated using the mass model of the structure. Assuming mass matrix only has translational components, the resulting load vector will have force components in the directions of all the translational degrees of freedom (DOFs) and will have zero values for all the rotational DOFs. This will guarantee a static deflection with components in many directions to start with the initial mode assumption. As long as the initial mode assumption has a component in the direction of interest, it will yield a correct set of Ritz vectors.

With the user-defined starting load, the load vector will have a force component in one translational degree of freedom in the direction of the dynamic load. This results in a static deflection mainly in the direction of interest. In some models where mass participation is predominant in one translational direction for the initial few modes this method can achieve 90% mass participation with only few modes.

Note: A single set of Ritz vectors can be derived from a single starting load. If all starting loads that participate in dynamic loads are defined, a single set of Ritz vectors can be extracted from all participating starting loads. The STAAD.Pro Advanced Analysis can extract a single set of Ritz vector only from one single starting load. Thus, the use of user-defined start vectors is best suited if the structural response is predominant in one translational direction. If responses in multiple translational DOF are predominant, it is recommended to use the program generated starting load vector.

Notes

Use of this command (and specifying the eigen solution in general) requires the Advanced Analysis.

This command requires that the SET EIGEN METHOD RITZ command also be used previously in the input file. Refer to TR.5 Set Command Specification for details.

Examples

The following uses starting loads in each direction, with 10 Ritz vectors in each except for the Y direction only, which will extract 5. 

DEFINE STARTING LOAD
* Load components in X direction only
MASS X NGEN 10
* Load components in Y direction only
MASS Y NGEN 5
* Load components in Z direction only
MASS Z NGEN 10
* Load components in X & Y directions
MASS XY NGEN 10
* Load components in Y & Z directions
MASS YZ NGEN 10
* Load components in Z & X directions
MASS ZX NGEN 10
END
The following example contains four reference loads:
  1. R1: all 3 translational degrees of freedom (X, Y, & Z)
  2. R2: in X d.o.f. only
  3. R3: in Y d.o.f. only
  4. R4: in Z d.o.f. only
Twenty five Ritz vectors will be generated for each of the starting loads. 
DEFINE STARTING LOAD
* Load components in all directions
REF LOAD R1 NGEN 25
* Load components in X direction only
REF LOAD R2 NGEN 25
* Load components in Y direction only
REF LOAD R3 NGEN 25
* Load components in Z direction only
REF LOAD R4 NGEN 25
END
Parent topic: TR.31 Definition of Load Systems
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