Learning Forum

[JamesUnder]

I am simulating a seismic earthquake load on a structural frame which supports two heavy objects (1500kg & 200kg), and want to know if/how I combine the Static Structural stress results with the Response Spectrum stress results.

In a Response Spectrum Analysis it is my understanding that the response deflection and stresses computed are only resulting from the applied excitation. In very simple terms (to avoid long post), the RSA finds the loadings on the structure at the modal frequencies, scales these loads (via partic. factors, mode coeff. etc.), then applies each in a linear elastic structural solution, then combines those results using mode combination methods to get a single result. In other words it finds the loadings on the structure that result from the forces (earthquake) which are accelerating/displacing the MASS of the structure at a particular frequency. So another point of view could be imagining the RSA in a gravity-less environment. If a structural frame, fixed to a base platform, was floating in space and an excitation was subjected to the base, the structure is still going to experience loading throughout because its mass is still being moved about.

Because of this, my thoughts are if you want to determine how the structure handles a seismic excitation, WHILE the structure is also experiencing the additional structural stresses due to gravitational force acting down on the two heavy objects which it supports, then the static structural stress results need to be added to the RSA stress results. Feeding a static structural solution into the pre-requisite Modal Analysis setup only serves to apply pre-stress / update the stiffness matrix of the modal equations, so doing this won't add any structural stresses (a separate analysis has shown including pre-stress has negligible effects on my structures stiffness, so I don't include pre-stress in my setup anyway).

So assuming my thought process is correct, how do I combine these deformations and stress values? I have read you can use 'Solution Combination', or convert them to linearized stresses then add, or extract the results as a data file then manipulate and add. There are some forums mentioning these methods but no solid guide, and I am struggling to gain enough info from the Ansys manuals to apply to my system. I have attempted Solution Combination and used SRSS combination to account for negative stresses. However the Solution Combination only lets me solve for Normal Stress and Shear Stress - which are in different regions - but I want to know the Equivalent (von-Mises) Stress.

[dlooman]

Your observations are correct. Perhaps you can do an SRSS of the static and RSA SEQV values using an expression that operates on SEQV created through User Defined Results.

[lfrausto]

Hi James, and Dlooman,

I have a similar situation to James. I'm not sure how to combine the static results of my strucutre to the response spectrum results. For example, If I wanted to know the reaction forces of my boundary conditions that take into consideration the seismic response and static loads on my structure, how can I do this in Anysys? Would it be as simple as adding the results of the static response and spectral response? In other words, take my reaction force values from the RSA case and add them to the static case?

[ORO]

I am in the exact same situation. Thank you JamesUnder for explaining much better that I would have been able to. As far as I know it is not possible to directly use the RS results directly in a results combination. If someone has more informations on this subjects that would be interesting !