2019 version - Harmonic case not foundable in manual combination

[Anonymous]

Hello there !

I tried to verifiy a bridge according to a document we can find in my country. 

To go through all the process, I created a modale case, a FRF case and a harmonic case (plus the usual load cases). Then I need to create a manual combination that include the usual load cases and the Harmonic case. But I cannot find that harmonic case in the input datas. Whereas I have vidéo on the 2017 version of ROBOT that show that it is possible. 

Please Help Me 😃

Sincerely yours. 

Benjamin. 

[Krzysztof_Wasik]

Hi @Anonymous 

Harmonic load case can be combined with other (static) load case. It should be visible in load case list in both, manual and automatic combination module regardless used Robot version. Share your model for verification.

[Anonymous]

Hi @Krzysztof_Wasik ,

Thank you for your answer. 

You can find the model here : http   s://we.tl/t-hUnoZ157yM 

The website wouldn't accept the file extension : 

• "The attachment's passerelle centrale seule-analyse setra.rtd content type (application/octet-stream) does not match its file extension and has been removed."

Thank you for your time. 

[Krzysztof_Wasik · Accepted solution]

Hi @Anonymous 

Switch off damping in harmonic analysis. Then harmonic case will be visible in combinations dialog. From 2019 harmonic analysis with damping is calculated as a compound load case (Harmonic, Harmonic+, Harmonic-) and cannot be added to manual combinations.

See What is new in 2019 

[Krzysztof_Wasik]

Hi @Anonymous 

Robot file when saved as RTD without results and ZIP-ed can be attached to post on forum

[Anonymous]

Thank you very much @Krzysztof_Wasik  !

It does work indeed and it looks like I can go through the process to justify the project. 

Thank you for your time and reactivity. It realy is very appreciable. 

Have a good day !

[baccawendy]

Hello,

For RSA versions greater than 2019, it seems that the only way to combine a harmonic case with a static case is to neglect the damping. However, neglecting the damping does not seem to me to be an appropriate solution to determine the dynamic loads. Indeed, damping participates largely in the dynamic response of the structure. In addition, for a steel structure walkway as in the example of the French 15th webinar, it is recommended to use a constant damping of 0.4% for the SLS analysis and to change to 2% at the ULS for a case of vandalism or manifestation which authorizes a greater damping: these effects are completely erased if the damping is removed in the case of harmonic analysis. Moreover, the entry into resonance of a structure without damping can lead to a total divergence of the solution which will tend towards infinity. I found on the structure of a metal footbridge which presented in the harmonic analysis with damping of 0.4% a maximum deflection of 5 cm and which passes to several tens of meters if I neglect the damping. It is then impossible to derive anything from the corresponding ELS manual combination.

What are your recommendations to determine the combined effects of a static case and a harmonic case with versions of RSA greater than 2019 please? (for example when implementing the recommendations of the French SETRA guide for pedestrian bridges).

It is always possible to determine the maximum deflections and stresses in the static cases and in the harmonic analysis separately and then to sum manually, but there is a risk of adding maximum stresses which do not occur at the same place of the structure. This solution goes in the sense of security but is not very strict, unless you combine the results from node by node in an array, but this is a lot of extra work when it was done automatically on the older versions of Robot.

Than you very much for suggestions and / or clarifications on this subject.

[Krzysztof_Wasik]

HI @baccawendy 

Harmonic load case component (when damping in harmonic analysis) can be extracted from Harmonic case as single load case with "Select component" option. Then such a case is visible for combination generation.

Refer to video

Please keep in mind that results for combinations generated in that way are vaild til next structure calculation (which can change harmonic case results).

[baccawendy]

Thank you very much @Krzysztof_Wasik for this very quick and helpful response. The video is particularly useful for understanding how to extract the sub-case with the greatest forces from the harmonic case.