Problem:

In order to evaluate the total load from operating conditions and worst case scenario from 6 Static Earthquake load case. The following combinations were created in a model.

Static Earthquake load cases: 

E1           User                      0.6280   0.0000   0.0000

E2           User                      0.3830   0.0000   0.0000

E3           User                      0.0000   0.0000   0.6280

E4           User                      0.0000   0.0000   0.3830

E5           User                      0.0000   0.2160   0.0000

Create user combinations to evaluate all possible scenario of Static Earthquake cases:

Trans1 = E1+E5

Trans2 = -E1+E5

Trans3 = E1-E5

Trans4 = -E1-E5

Trans5 = E3+E5

Trans6 = -E3+E5

Trans7 = E3-E5

Trans8 = -E3-E5

Create user combination to find worst case (MAX) scenario from above cases

TRANS(POS)= sign Max  ( TRANS1… Trans4)

TRANS(NEG)= sign Max  ( TRANS5… Trans8)

TRANS = sign Max  [ TRANS(POS), TRANS(NEG)]

Create user combination to evaluate normal operating conditions

Func = Gr + T1 +P1

Create user combination to total the loads of Operating and Worst case Seismic load cases 

TRANSPORT = TRANS + FUNC

Is this listing of combination correct, why or why not?

Solution:

NO, this set of combinations is NOT correct.

The problem is because of load sequence, the results of Trans1 - Trans12 cannot be added into other user combinations to find the worst cases scenario.  

Help topic explains AutoPIPE generates combinations based on the non-linear load sequence only

Please see the following AutoPIPE help section: 
Help > Contents> Search Tab> enter "Add default Non-Code combinations" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information.

Non-linear analysis:

For non-linear static analysis results, the program will automatically generate combinations based on the load sequences only. For example, if GR is the initial state of T12, and T12 is the initial state of P12, and P12 is the initial state of W10, then the program will generate the combination, GT12P12W10 or GT1P1E10.

Combining 2 or more Seismic load cases in a combination is not valid.

The seismic cases have to be explicit in different +/- directions.\

For now using a non-linear analysis looks like for 12 'seismic' loads they will need 2 models where the seismic loads are defined e.g. E1 = trans1, E2 = trans 2, etc..

Otherwise they could run as linear analysis with their existing combinations which would be valid and evaluate TRANSPORT load combination for worse loads and contributing load cases then go back and run a non-linear analysis for this load sequence only

Then envelop the linear and non-linear results to capture final maximum. Some users do this for true seismic loading.

Based on the setting s above, recommend the following corrective procedure:

A. Update the defined Static Earthquake cases to match the intention of Trans1...Trans8, as followed:

E1           User                      0.6280   0.0000   0.0000

E2           User                      -0.6280   0.2160   0.0000

E3           User                      0.6280   -0.2160   0.0000

E4           User                      -0.6280   -0.2160   0.0000

etc..

B. Because the load case scenario will be evaluating, Gravity + Press +Temp + Seismic, change the default load sequence:

Load> Static Analysis Set> open analysis set to confirm all pertinent load cases are checked ON, "Gaps / Friction / Soil" setting is checked ON, press OK button, and change "Initial case for Occ. loads" from GR to OP1 (see online help for details on this setting), and press OK button to close dialog screen.

C. Run analysis

D. Select Tools> Combinations> 

i. Code Combination Tab:

After opening this tab, noticed that AutoPIPE had automatically created the following operating load cases GRTP4+E1, GRTP4-E1, etc.. In order to configure AutoPIPE to evaluate a group of load cases (10 max per combination) and determine which one is the worst case, create the following user combination:

Note: each of the load cases has M/S = S (stress), this combination would not be valid if one or more combination had M/S = M (moments). 

Create as many grouping combinations (ex. Max Stress 5-8) as necessary to evaluate all the combinations, max 10 at a time. The following is an example of the next group to be evaluated:

Finally, evaluate the actual group combinations as shown below:

ii. Non-Code Combination Tab:

After opening this tab, noticed that AutoPIPE had automatically created the following operating load cases GT4P4E1{4}, GT4P4E2{4}, etc (see online help to understand why GT4P4-E1(4) type comb. are not created for non-linear analysis). In order to configure AutoPIPE to evaluate a group of load cases (10 max per combination) and determine which one is the worst case, create the following user combination:

Note: the different Combination method used for non-code combinations, Res. Max, see online help for details on this option.

E. After all the correct combinations have been created, Press OK button to close the combination dialog screen

F. Review the combination results in the Results grids or Output report.

Note: a new enhancement has been logged, TFS-E239855 to help with generating this type of information. 

See Also

"Non-Code Comb." tab

Bentley AutoPIPE