step controls in transient structural analysis sittings

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  • Last Post 18 February 2018
Rana Nasser posted this 07 February 2018

Hi everyone!

I have some questions about the step controls in transient structural analysis sittings,

First, what is the difference between "time and substep" in the auto time stepping definition? and how this affect the solution of my  model?

second, the transient load in my model is an acceleration time history produced from a passing train and measured by an accelorometer installed on the structure which take a reading every .002 seconds, so how good is the step controls that I used in the attached model? 

*** The accelerometer that I'm taking about is ch(1) in the drawing pdf. The experiment was on a water structure (regulator), but the attached archive represent a 1 meter strip of the abutment.  

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peteroznewman posted this 07 February 2018

Hi Rana,

I have used accelerometer data in a Transient Structural analysis. I opened your archive and pulled the data out of the acceleration load. There are a issues with your data.

Issue 1) There is a huge 10 second gap in the data.

Delete the first two lines and subtract 10 from the Time values unless you must specifically simulate the first 10 seconds, the acceleration history should begin at zero with no gaps.

Issue 2) The mean acceleration is 0.05 m/s^2 for 200 ms.

When I have acceleration data, it usually starts and ends at zero, and has some oscillation in between. Your data has a mean acceleration of 0.05 m/s^2. Are you trying to impart an impulse force on your structure or just vibrate your structure?  If the latter, then subtract 0.05 from all the acceleration values in the Z column and you will be left with just the oscillations.

Issue 3) Your last data point has zero acceleration.  If you were trying to impart an impulse force, then you want time to observe the response after the impulse ends at 0.2 seconds, so you will want zero acceleration for 0.8 seconds after that for a total simulation time of 1.0 seconds (or more!).

 

 

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peteroznewman posted this 07 February 2018

One more comment about acceleration. Ground motion tends to have displacement and return. That means when a ground impulse wave travels by, it should have negative, positive and negative acceleration, and at the end, the total displacement is zero. 

 To answer the question about Time Step controls, here is the appropriate time steps for such an event:

ANSYS will interpolate between the values in the acceleration table.

Regards,

Peter

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Rana Nasser posted this 08 February 2018

issue no. 1: I'm not trying to impart an impulse force on the structure it's just a vibration occurred due to passing trains which were close to the structure. the train takes about 30 seconds to pass and the available accelerometers were 500 Hz frequency, so I had more than 10000 reading for each acelerometer. To impart this acceleration to the model I decided to take a part of the readings which has the high peaks, so I inserted the readings from the second 10 to 10.2 as a trial, but now I after reading you comment I think I must subtract 10 from the part of readings that I have chosen to avoid changing the main function of the imparted data.

issue no. 2: I have already filtered the data coming from the accelerometer and subtracted the mean from it, I have done this for the whole 10000 point. I don't know if I have to do this again for the 100 points that I decided to impart to the model or that will affect my simulation negatively( make me far away from the real data)? 

issue no 3: I think I need more explanation for you idea!

about the time step control:

1- The fist 2 rows of the acceleration data are always program controlled, I can't change or delete them!.. by the way what dose N/A in the tabular data steps cells mean? 

2- If you wont think that I'm greedy, I want to know what's under the black box when I'm changing the step controls data?! 

...And as usual I can't find words enough to thank you peter for your kindness and your continues help. I'm really looking forward to meet you some day I think you are really a good friend!!      

Rana Nasser posted this 08 February 2018

one more question! what is the program that you used to visualize the acceleration - time history data in the first comment?   

peteroznewman posted this 08 February 2018

Hi Rana,

Please put your 30 second accelerometer data in a zip file and attach it to your post so I can see the whole record.  Also, please describe where the accelerometer was placed.

The program I plotted your data with is JMP, but a very old copy, Version 5.

Consider two curves of acceleration data: red and green curves in the illustration below.

The green curve is the full record of the acceleration data. The red curve is cutting out a piece of the full record. The problem with applying the red data to a transient simulation is that the gradual ramp up to the peak value on the green curve is missing. The red curve is like a sudden impact and it excites higher frequencies in the structure that would not have been excited by the green curve. If you want to use just a portion of the data, you could multiply the data by a Hann window function to ramp up and down the amplitude.

The End Time in your Step Control has to be equal to or greater than the data in the Time column in your acceleration load. Any times greater than the End Time in your acceleration load is labeled N/A.  Once your time starts at zero for the acceleration load, you won't have this problem.

Best regards,

Peter

Rana Nasser posted this 13 February 2018

Hi peter, 

did you fined the attached files which contains the accelerometer's reading & layout of the acceleorometer's distribution? 

peteroznewman posted this 13 February 2018

Sorry Rana, I only found them now because you added the post above, which gave this thread a new date.

When you edit an old post, or attach files to an old post, the thread doesn't get a new date so I don't know that there was a change. It's better to add a post to attach files so that I will know something changed.

I had a quick look and have attached a zip file for you.  I will study this some more later. I'm a little concerned that the spreadsheet showed the units for the channel as µm/m.  That is not a unit of acceleration, that is a unit of strain.  Are you sure these sensors are accelerometers or are they strain gauges?  This question is more important to get right than the choice of which section of time history to analyze.

I also didn't fully understand the pdf showing the location of the sensors relative to the structure. Please expand on the description of where Ch1 is located.  Is it on the structure?

Best regards,
Peter

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Rana Nasser posted this 15 February 2018

sorry for being late, the sensors that we used can read different parameters and we use a compatible software on a laptop in the site to choose the parameter which we want it to read and adjust the units manually, but at the experiment time we set the program to take the acceleration readings and forgot to edit the units! so the reading are acceleration (m/sec^2).

the field test was held on a regulator ( a water head structure that is used to control the water flow in rivers) all the 6 sensors were distributed on the structural elements of the first bay of the regulator -except the raft-. So, yes accelerometer no. 1 was set on the abutment of the regulator vertically.

 I have reviewed the attached files and I have taken  your notes into consideration and there is no N/A cells any more, but could you please explain how did you extracted c1a file?, this is very useful for me by the way because that's my first field test too and if you don't mined I want to learn more from you, thank you peter for your dedication!!

** the attached file contains my trials to extract the time history sample according to your notes, but it was different from that in ca1 file. 

 

Attached Files

peteroznewman posted this 16 February 2018

I'm glad to know that the data is as you said.

The only difference between your extraction and mine is that you picked your zero one time step away from mine. After deleting one cell, and shifting all cells up one, the two curves are aligned, and only off vertically by a constant of 9e-6 so I think you have done a good job isolating a section of data to use.

Location of Accelerometer on Cart

A cart with wheels rolls over bumps in the floor, which creates a transient vibration in the cart structure. The cart consists of a base where the wheels are attached, and a column with a component on top. One accelerometer is on the base and another accelerometer is under the component where it bolts to the column. The acceleration-time history is recorded as the wheels roll over bumps in the floor.

Take the wheels off the base and bolt the base to a servo-controlled shaker table. Play back the recorded acceleration-time history from the base accelerometer into the shaker table controller. The base, column and component will reproduce a similar transient vibration on the shaker table that was observed when the wheels rolled over the bumps in the floor.

Make an ANSYS model of the base, column and component structure but without wheels, and add a fixed support on the base where the accelerometer was mounted. Now use the base accelerometer data as an acceleration load on that model.  The base, column and component will show a transient vibration like it had when the cart was on wheels.

But play back the acceleration-time history from the component accelerometer into the shaker table controller. It will not reproduce the transient vibrations that were observed in the base, column and component. Similarly, insert the acceleration data from the component accelerometer into the ANSYS model with the fixed support on the base, a similar transient will not be seen as was observed when the cart had wheels.

Now unbolt the component from the top of the column where the component accelerometer was mounted, and bolt that component onto the shaker table. Play back the acceleration-time history recorded by the component accelerometer and the component would experience a similar transient vibration that it saw when the cart rolled over the bumps in the floor.

But play back the acceleration-time history from the base accelerometer into to the shaker table and the component will not experience a similar transient as it saw when the wheels were rolling over bumps in the floor.

Make an ANSYS model of the component and add a fixed support to the component where it bolts to the column. Use the component accelerometer as the acceleration load for the ANSYS model of the component. The component model will show a similar transient that was obsvered as when the wheels rolled over the bumps.


Do you see how the location of the accelerometer determines what can be in the ANSYS model if you want to use that accelerometer time-history as an acceleration load to the model?

It seems to me that you want an accelerometer mounted to a stake in the ground to record the ground motion in order to use that data as an acceleration load to a structure that is mounted to the ground. The accelerometer mounted to the top of the structure is seeing the response of the struture to the ground motion, so it not ideal to use as an acceleration load for the ground motion.

A Seismograph instrument may be more appropriate than an accelerometer, since you are trying to record the seismic vibrations in the ground and use that to simulate vibrations in a structure that is built on the ground.

 

Rana Nasser posted this 17 February 2018

thank you so much for this illustration, I have discussed this issue before with my professor (the discussion was after the filed test so I couldn't do the 2 scenarios) and his opinion was that we are interested in the load which affect the structure directly so we will take the readings of the accelorometer no. 1 (ch1) as an input and verify the model by comparing the acceleration on the structural elements at the same positions of the acceleromrters in the field test with the data comes from the test, so he thought that the dissipation of the load due to the soil between the railway and the regulator will not affect my model because we are taking the readings which represent the real vibration that hit the structure directly  

peteroznewman posted this 17 February 2018

I don't have a clear picture of where ch1 accelerometer was mounted. Do you have any photographs from the field test?

Rana Nasser posted this 18 February 2018

The attached file contains pictures for the ch1 accelerometer location, I hope they can illustrate the location clearly!  

Attached Files

peteroznewman posted this 18 February 2018

Thank you Rana, that is a great help.

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