• Dhiaa
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Topic: Surface pressure and crack patterns

Hello,

I am using ATENA 3D Engr and I have the following two questions:

1) I am applying a uniform pressure on a steel plate with is fully attached to a concrete block. I want to monitor the applied force through the load-deflection curve. From previous posts and the user manual, it said that I can sum the loads on all the surface nodes from the output text. However, that text gives me the 4 convergence criteria and the load and deflection at that node only. I am specifying 'value at a node' in 'monitoring points.' How can find the force in all the nodes at that surface in order to add them?

2) At the Run window, the load-deflection curve and the text output showing the load in MN unit even I have a pressure in MN/m2 in input. can you explain that?

3) From ATENA documents (theory and user manual), it says that the cracks are averaged per element and showing at the middle of elements. Also, in previous posts, it mentioned that I can specify the crack spacing at the input. Moreover, I get different crack pattern when I change the mesh size. It seems that I am forcing the program to form cracks in the pattern that I want. Can you clarify that point because I can get many cracks located at different locations based on number of elements, for example?

Quote Post 1

  • dpryl
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Re: Surface pressure and crack patterns

Hello,

1. Please see ATENA Troubleshooting, 2.1.27 How can I monitor a surface (or line) load?

2. You are monitoring a point force/reaction (or many of them). The point value is not recalculated to any surface.

3. If your crack pattern is strongly mesh-dependent, it typically means your mesh is problematic or there is some other issue with your model. Please look into the ATENA Troubleshooting Manual for possible problem sources.

dpryl's Website

Quote Post 2

  • Dhiaa
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Re: Surface pressure and crack patterns

Hello Dpryl,

Thank you for your fast replies.
1) I check it and I can get the external force at each node in the surface where I applied load. However, some of the loads are negative. Even I use the absolute value, they don't add to the total load specially they are in MN and should divide them by the area. Is there any step missing here or it means that there is another issue?
2) I didn't understand what you mean by 'The point value is not recalculated to any surface.'

Quote Post 3

  • dpryl
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Re: Surface pressure and crack patterns

Hello,
ad 1: if you are using quadratic elements, it is normal that some of the nodes have negative contributions. If you are interested in details, please see the Theory manual.

Of course nodal forces are in MN. If you like to calculate something else, I am afraid you need to do so outside of ATENA (e.g., in a spreadsheet). Background: the ATENA solution core works with nodal displacements and forces internally. Simply said, all loads and other boundary conditions are converted into that during the analysis.

Ad 2: please do not forget you are simply monitoring nodal forces. ATENA does NOT convert the values from the monitors in any way (e.g., it does NOT calculate pressure) - you need to do that by other means.

dpryl's Website

Quote Post 4

  • Dhiaa
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Re: Surface pressure and crack patterns

Hello Dobromil,

Thank you for your previous comments and emails, they helped me. Now, I have some related questions:

1) When I want to sum the nodal forces, I sum them based on their tributary element area. Does ATENA converts surface pressure based on tributary area?

2) When I sum the nodal forces as in 1, I get a total pressure equal to the applied one if its value is about 10 MN/m2. However, if I increase the pressure to 20, 30, 40, 50, 60, I get a summed pressure of 18.8, 26.25, 33.12, 38.92, 44.03, respectively. I tried different number of load steps, but the differences were in fractions. I am using the Standard Arc Length method. Do you have any idea why is that happening instead of getting the same value of pressure?

3) I tried to apply a pressure on a surface, a point load on a single node, and a prescribed deformation on a single node with different number of steps and element size, but the failure load is about 45% higher than the experimental results. Is there any possibility that I can send my model to you since the distributor hasn't got their key for v5 yet?

Regards,

Dhiaa

Quote Post 5

  • dpryl
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Re: Surface pressure and crack patterns

Hello,
ad 1. I must admit I don't understand your question - if my previous answer has not explained it enough, please make sure not to mix nodal forces with pressures, and explain what conversion you like to have (or not to have).

Ad 2. The Arc Length method scales the load. See ATENA Troubleshooting 2.2.6.1 Solution methods or the more detailed explanation in ATENA Theory.

Ad 3. If the AL load scaling (see 2. above) does not explain this difference, please see ATENA Troubleshooting, 2.1.22 My analysis results do not match the experiment/expectations. How can I improve my model? If you still can not identify the source of the difference, send us your model (see ATENA Troubleshooting, 2.1.1 I have a problem not listed here).

dpryl's Website

Quote Post 6