Hello
I am performing an eigenvalue analysis on a 5 span bridge with Seismo Struct 5.2.2. My model only contains the superstructure, built with “elfrm” elements, which is held by “bl_sym” link elements (I want to do a pushover later) according to the stiffnesses of the piers and abutments. I perform the eigenvalue analysis with Seismo Struct and Statik 6 for the exact same model and get different results (I am looking here especially at the slowest longitudinal mode). Seismo Struct gives me a period of T=0.60s and Statik-6 T=0.64s. Because of the EA and the length of the bridge (337m), there is a significant longitudinal wave in the superstructure. So to investigate the problem I set the EA very high in both models and tried again. With the rigid superstructure I can calculate the right period by hand very easily and get T=0.524s. Statik-6 gives me exact this period with the rigid superstructure whereas Seismo Struct calculates T=0.488s. I then changed the “bl_sym” elements in Seismo Struct to “lin_sym” elements with the same stiffness and so I get also exact the T=0.524s as by hand and with Statik-6.
Is it possible that there is somewhere a problem using “bl_sym” elements in a system and performing an eigenvalue analysis? In my opinion there should be no difference in the result of the eigenvalue analysis using either “bl_sym” or “lin_sym” since I gave them exactly the same stiffness. And if there is a difference caused by the calculation of the program it should in my eyes be rather softer than stiffer with the bl springs.
Have you come accross a similar problem in the past?
Bets Regrds,
Pascal
Problems with eigenvalues using "bl_sym" link elem
Re: Problems with eigenvalues using "bl_sym" link elem
I am just wondering if maybe you included rotational stiffness about the transverse bridge axis in your link definition, Bridge87? Depending upon the actual boundary support conditions, a value of zero might be appropriate for this DOF.
This could artificially prevent a rotation which may actually be able to occur in the structure.
Just a thought. You've probably already thought of that, but it is the first thing that comes to mind.
Also, did you try the bl_kin element and get the same answer as with the bl_sym element? They behave slightly differently but should be identical for a simple eigenvalue analysis it would seem. Just another thought.
Best of luck.
This could artificially prevent a rotation which may actually be able to occur in the structure.
Just a thought. You've probably already thought of that, but it is the first thing that comes to mind.
Also, did you try the bl_kin element and get the same answer as with the bl_sym element? They behave slightly differently but should be identical for a simple eigenvalue analysis it would seem. Just another thought.
Best of luck.
Tim Huff
Re: Problems with eigenvalues using "bl_sym" link elem
Hello huffte
Thank you for your inputs. The rotational stiffness of all the links is set to zero for the transversal directions (horizontal and vertical) to the bridge. I only hold it with a rotation restraint in x direction (longitudinal to the superstructure) to prevent a rotation mode.
About the bl_kin elements I already thought yesterday evening, but I was too tired to try. I tried now and got the right result of T=0.524s. So it works with the lin_sym and the bl_kin links, but not with the bl_sym.
Best Regards
Thank you for your inputs. The rotational stiffness of all the links is set to zero for the transversal directions (horizontal and vertical) to the bridge. I only hold it with a rotation restraint in x direction (longitudinal to the superstructure) to prevent a rotation mode.
About the bl_kin elements I already thought yesterday evening, but I was too tired to try. I tried now and got the right result of T=0.524s. So it works with the lin_sym and the bl_kin links, but not with the bl_sym.
Best Regards