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TR.35 Load Combination Specification

This command may be used to combine the results of the analysis. The structure will not be analyzed for the combined loading. The combination of results may be algebraic, SRSS, a combination of both algebraic and SRSS, or Absolute.

Note: The LOAD COMBINATION specification is not appropriate for obtaining secondary effects for combined loads. Refer to notes below.

General Format

LOAD COMBINATION ( { SRSS | ABS } ) ( GENERATE ) i a1
i1 f1 i2 f2 … (fSRSS)

Where:

ParameterDescription
i Load combination number. This can be any integer smaller than 100,000 that is not the same as any previously defined primary load case or load combination number.
a1 Any title for the load combination.
i1, i2, … The load case or load combination numbers which are to be combined. For the SRSS option, a minus sign may be used to indicate that this load case should be combined algebraically rather than as an SRSS case.
f1, f2, … The corresponding factors to be applied to loadings. Values greater than or equal to 0.0 are allowed. 
fSRSS An optional factor to be applied as a multiplying factor on the combined result of the SRSS load combination (see examples below).
GENERATE Optional command for SRSS or ABS load combinations which will generate load combinations representing all possible positive or negative actions in each load degree of freedom. When used, a single set of absolute value analysis results is replaced by these 64 load combination analysis results.

A hyphen may be used at the end of a line to continue this command onto the next line.

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.

Note: If GEN is included this single set of absolute values is replaced by 64 sets of load combinations.
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.

Note: The case factor is not squared.  
Note: If GEN is included this single set of absolute values is replaced by 64 sets of load combinations

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.

CAUTION: For models with significantly larger number of members or load combination cases generated, this file can be come extremely large. This output option should only be used when the individual cases require review and not used in normal circumstance.

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.

CAUTION: For models with significantly larger number of members or load combination cases generated, this file can be come extremely large. This output option should only be used when the individual cases require review and not used in normal circumstance.

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.

CAUTION: For models with significantly larger number of members or load combination cases generated, this file can be come extremely large. This output option should only be used when the individual cases require review and not used in normal circumstance.

Example of Simple SRSS Combination

Several combination examples are provided to illustrate the possible combination schemes

LOAD COMBINATION SRSS 8 DL+SEISMIC
1 1.0 2 0.4 3 0.4

This (LOAD COMBINATION SRSS 8) illustrates a pure SRSS load combination with a default SRSS factor of one. The following combination scheme will be used:

v = 1.0⋅(1⋅L12 + 0.4⋅L22 + 0.4⋅L32)1/2

where
v
=
the combined value
L1, L2, and L3
=
values from load cases 1, 2 and 3.

Since an SRSS factor is not provided, the default value of 1.0 is being used.

Examples of Algebraic & SRSS Combination in the Same Load Combination Case

Example 1

LOAD COMBINATION SRSS 9
-1 0.75 2 1.3 3 2.42 0.75

The combination formula will be as follows:

v=0.75×L1+0.75×1.3(L2)2+2.42(L3)2
where
v
=
the combined value
L2 and L3
=
values from load cases 2 and 3

In the above specification, a minus sign precedes load case 1. Thus, Load 1 is combined algebraically with the result obtained from combining load cases 2 and 3 in the SRSS manner. The SRSS factor of 0.75 is applied on the SRSS combination of 2 and 3.

Example 2

LOAD COMBINATION SRSS 10
-1 0.75 -2 0.572 3 1.2 4 1.7 0.63

Here, both load cases 1 and 2 are combined algebraically with the SRSS combination of load cases 3 and 4. Note the SRSS factor of 0.63. The combination formula will be as follows:

v=0.75×L1++0.572×L2+0.63×1.2(L3)2+1.7(L4)2