Currently, I'm facing difficulties to perform dynamic analysis of masonry building using time history data (Time vs acceleration) [due to ground borne vibration] . The data received externally in .txt format. From my understanding, the data can be inserted the in Amplitude > tabular. However, I lack understanding regarding "Boundary Condition" of acceleration. What is the exact steps require for BC ?
Hi
A common approach to your problem is to apply the so called Large Mass approach. What you do is set up your model, find its weight, m, and then apply a mass below with a weight of, say 1000*m. The input acceleration you wish to apply, a, can then be turned into a force, F, using Newton's 2nd law, i.e. F = 1000*m*a.
There will be a small error but the error will be controlled by the factor you choose for your Large Mass.The approach works for time and spectral loads.
The Large Mass conncets to your model using MPCs (Multi Point Contraint equations, or rigid beams). Take care in applying these as theymay otherwise contrain vibration in your model in ways you do want. A simple modal analysis with/without BCs can show if anything is amiss when the MPCs are introduced.
Sincerely
Claes
Hai sir claes, Thanks a lot for your great explaination,
In my case, the model is made up as a large scale model of Multi story masonry building. Is it capable to perform the Large Mass approach ? or to obtain the overall building mass ?
Or is it another method can be done ? The sample of vibration data is shown in my previous thread :
https://www.researchgate.net/post/How_to_perform_Vibration_analysis_of_multi_story_masonry_building_with_TimevsAcceleration_ABAQUS
The vibration data is in the form of TIME HISTORY ( with total period of time 10s , increment of 0.01) , i need to apply these data at certain points on the model . Isit possible to perform this task ?
From my understanding, the data can be inputted in ABAQUS at Amplitude > Tabular , and set the amplitude at BC (correct me if im wrong as im a new user). However, im lack of understanding regarding on BC, what is the correct parameter of BC need to apply ?
Thank you sir, i appreciate your helps.
Hi
The Large Mass typically is applied at the building base to simulate motion from earthquakes. It is a single point input method. You can input per axis but that is it.
There are ways to introduce arbitrary synchronous signals to multiple location. However, in my opinion, this is fool's gold as you then also input constraints into your model that are very difficult to understand and/or predict.
A better approach is to shake your model where so is motivated and to check that response at other locations are in the right ballpark of what they should be.
For multiple load cases, you apply a Large Mass at one location at the time and combine data, as appropriate, using an energy approach or take the absolute values and sum over load cases.
If you need wave loads, i.e. waves impinging from a certain direction and propagating across your buidling, it is easier to switch code and use Code Aster which can make such analysis. http://www.code-aster.org/V2/spip.php?rubrique2
However, you can view your building as a point when it is smaller than 1/10th of the wavelength in the soil that is shaking the building.
Sincerely
Claes
Thanks a lot sir . such a great explaination !, it gives me some idea. thank you sir .
It looks as if you are modelling effects of a seismic event on your structure. If that is the case, it usually has an assumption that all the supported (restrained) nodes see the same input motion (i.e. acceleration vs time are all the same and in phase). I have seen models in ANSYS using a Large Mass approach, but in practice in such cases (using BAQUS for example) I see more often the acceleration time history is directly applied as a boundary condition to the boundary nodes.
If using the Large Mass approach, I would check if the motions at the Large Mass interacts with the structure are as expected / prescribed.
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