Description

If the load combination option is left out (i.e., neither ABS or SRSS is included), then the results from analyses will be combined algebraically.

LOAD COMBINATION 6 DL+LL+WL
1 0.75 2 0.75 3 1.33

If the ABS load combination method is included, then the absolute value of results from the analyses will be combined.

LOAD COMBINATION ABS 7 DL+LL+WL
1 0.85 2 0.65 3 2.12

If the SRSS load combination method is included, then the results from analyses may be combined both algebraically and using the SRSS (Square Root of Summation of Squares) method. The combination scheme may be mixed if required. For example, in the same load combination case, results from load cases may be combined in the SRSS manner and then combined algebraically with other load cases.

Notes

1. In the LOAD COMBINATION SRSS option, if the minus sign precedes any load case no., then that load case will be combined algebraically with the SRSS combination of the rest. 
2. The LOAD COMBINATION command should not be used to obtain the secondary effects of combined load cases through a P-Delta, Member/Spring Tension/Compression, Multilinear Spring, or Nonlinear Analysis. See the REPEAT LOAD command (TR.32.11 Repeat Load Specification) for details.
3. In a load combination specification, a value of 0 (zero) as a load factor is permitted.  In other words, a specification such as

   LOAD COMB 7
   1 1.35 2 0.0 3 1.2 4 0.0 5 1.7

   is permitted. This is the same as

   LOAD COMB 7
   1 1.35 3 1.2 5 1.7

4. All combination load cases must be provided immediately after the last primary load case.
5. The total number of primary and combination load cases combined cannot exceed the limit described in TR.2 Problem Initiation and Model Title.
6. A maximum of 550 load cases can be combined using a LOAD COMBINATION command.
7. Load combinations can refer to previously defined load combination numbers.

Generate Load Combinations

If the optional GENERATE command is included, this single set of absolute values is replaced by 64 sets of load combinations. Otherwise, the program will follow the usual method of ABS or SRSS load combination.

For a particular model, all 13 section forces for each of the members for the internally generated 64 load combination cases can be printed in an external text file. To do this, use the following SET command in the input file before joint incidences are specified.

SET CG TXT

This command will instruct the program to print the section forces for each member for 1 to 64 generated load combinations to a file named filename _CG.txt located in the same location as the STAAD input file.

Response Spectra in Load Combinations

The following steps are followed in order to derive member section forces for a response spectrum case.

1. Forces at start and end joints of a member are calculated for each mode.
2. Since masses are lumped at two member ends, it is assumed that the section forces due each of the modes are varying linearly between member ends. Thus at any section the section force is linearly interpolated for each mode.
3. The forces at each section for all modes are combined using modal combination method to arrive at the final result.

The section forces for each mode for any response spectrum load case can be printed in an external text file. To do this, use the following SET command in the input file before joint incidences are specified.

SET RS TXT

This command will instruct the program to print the section forces for each member to a file named filename _RESP.txt located in the same location as the STAAD input file.

Once the section forces are available it can be combined with other static load cases using SRSS/ABS and GENERATE options.

Design for Load Combinations per ASME NF

Steel design is performed for active load cases that may include primary loads and load combinations. If any load combination is an internal load generation case, instead of designing for one particular load combination case, the design is performed for all 64 load combinations. Thus, the steel design results may vary from what if it had been designed for only one combination case. If any load combination generation case becomes a critical load case for design, it is noted as such in the design output.

All section forces for the 64 generated load cases –whose parent load combination case becomes the critical design load– can be printed in an external text file. To do this, use the following SET command in the input file before joint incidences are specified.

SET NF TXT

This command will instruct the program to print the section forces for each member to a file named filename _RESP.txt located in the same location as the STAAD input file.