Hello everyone!
I'm modelling a shear wall subjected to a monotonic test (to calibrate the model so then I can model the cyclic test) and I have the following problem regarding the boundary conditions:
If I model the top boundary condition as fixed with no rotation (this is with the model prescribed node value and relative Uy so it will be fixed at the displacement corresponding to the one generated by the vertical load) the output force-displacement curve is really stiff and the peak load is achieved at a very small lateral displacement. While if I fixed-fixed both ends of the wall the failure is just flexural on the mortar joint and no shear failure is observed and thus the curve does not reach the expected capacity. Is there any way to get a medium response?
Here is my *SCD file (I don't know why I cannot attach it). when running the analysis, the monitor of the F-D curve also plots in the background the experimental curve I want to calibrate.
https://drive.google.com/file/d/1oyc1Ol ... sp=sharing
Please let me know if you can successfully access to it.
Thank you for your help.
Boundary condition Shear wall
Re: Boundary condition Shear wall
Could you also share some info (article?) of the experimental setup? probably your boundary conditions are not correct.
You have to zip it.Here is my *SCD file (I don't know why I cannot attach it). when running the analysis, the monitor of the F-D curve also plots in the background the experimental curve I want to calibrate.
Re: Boundary condition Shear wall
The papers are currently under review. The experimental test I want to model is the one I did on masonry walls retrofitted with TRM. Currently, I want to calibrate the URM masonry wall. I can tell you how the loads were applied. It was intended a double fixed BC but in the laboratory this is not always 100% achievable.
During construction, the samples were laid on a metallic C-profile filled with concrete. In addition, the base was constrained at both ends by two T-shape devices. Both the base and the end devices were fixed to the strong floor of the laboratory by means of post-tensioned steel bars. On top of the wall a reinforced concrete (RC) beam was placed. On top of the RC beam laid a stiff metallic H prole stiffened with ribs, where the vertical load was applied with two jacks of 1000 kN capacity each. The jacks reacted against a stiff frame anchored to the strong floor. Between the RC beam and metallic prole, a 3 mm thick Teflon sheet and a 3 mm thick PVC sheet was inserted to provide a smooth horizontal surface to reduce the friction transfer of the horizontal load to the RC beam. Between the RC beam and the PVC sheet a layer of cement-based mortar, with thickness of 5 to 10 mm, was inserted in order to level the end surface of the beam and guarantee the vertical load transfer avoiding stress concentration due to irregularities.
The shear compression tests were performed in two steps. Firstly, the vertical force was gradually applied under force control. The valves of the jacks were closed once the designed compression stress was reached (Sigma=0.3 MPa).
Secondly, the horizontal shear force was applied with a hydraulic actuator anchored to a reaction wall. With the valves of the vertical actuators closed, no displacement or rotation of the top of the wall was possible at this stage and applying horizontal load induced a double bending condition. As a consequence of the testing mode, the vertical load increased together with the horizontal load.
See attached a picture of the experimetnal set-up
Is there any way I can enhance the bc I'm currently using in my model to represent the behavior previously described?
Thank you!!
During construction, the samples were laid on a metallic C-profile filled with concrete. In addition, the base was constrained at both ends by two T-shape devices. Both the base and the end devices were fixed to the strong floor of the laboratory by means of post-tensioned steel bars. On top of the wall a reinforced concrete (RC) beam was placed. On top of the RC beam laid a stiff metallic H prole stiffened with ribs, where the vertical load was applied with two jacks of 1000 kN capacity each. The jacks reacted against a stiff frame anchored to the strong floor. Between the RC beam and metallic prole, a 3 mm thick Teflon sheet and a 3 mm thick PVC sheet was inserted to provide a smooth horizontal surface to reduce the friction transfer of the horizontal load to the RC beam. Between the RC beam and the PVC sheet a layer of cement-based mortar, with thickness of 5 to 10 mm, was inserted in order to level the end surface of the beam and guarantee the vertical load transfer avoiding stress concentration due to irregularities.
The shear compression tests were performed in two steps. Firstly, the vertical force was gradually applied under force control. The valves of the jacks were closed once the designed compression stress was reached (Sigma=0.3 MPa).
Secondly, the horizontal shear force was applied with a hydraulic actuator anchored to a reaction wall. With the valves of the vertical actuators closed, no displacement or rotation of the top of the wall was possible at this stage and applying horizontal load induced a double bending condition. As a consequence of the testing mode, the vertical load increased together with the horizontal load.
See attached a picture of the experimetnal set-up
Is there any way I can enhance the bc I'm currently using in my model to represent the behavior previously described?
Thank you!!
- Attachments
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- FSETUP.png (3.79 MiB) Viewed 4361 times
Re: Boundary condition Shear wall
Ok, so you may want to play with the m1 parameter.
It controls how much dilatant your material will be. A value of 0 gives the maximum dilatancy, while a value of 1 gives the minimum.
Since your wall is fixed, and the experimental test showed an increasing vertical reaction during the application of horizontal load, it means that you have some dilatancy.
Unfortunately, it should be calibrated with appropriate shear tests on a mortar joint.
I cannot access your SCD file, I need premissions.
Could you attach it here (ZIP it first)?
It controls how much dilatant your material will be. A value of 0 gives the maximum dilatancy, while a value of 1 gives the minimum.
Since your wall is fixed, and the experimental test showed an increasing vertical reaction during the application of horizontal load, it means that you have some dilatancy.
Unfortunately, it should be calibrated with appropriate shear tests on a mortar joint.
I cannot access your SCD file, I need premissions.
Could you attach it here (ZIP it first)?
Re: Boundary condition Shear wall
Thank you for your reply.
I've have done what you said, I've have changed the parameter of dilatancy m1, without any success in modifying the stiff behavior.
Please find attached the zip file.
I've have done what you said, I've have changed the parameter of dilatancy m1, without any success in modifying the stiff behavior.
Please find attached the zip file.
- Attachments
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- URMMonotonic.zip
- (502.41 KiB) Downloaded 244 times
Re: Boundary condition Shear wall
I had a look at your model, and even the elastic range is very different. I don't think it's a problem of boundary conditions... they seem to be applied as you described.
Are you sure about the material parameters? Starting from the elastic constants?
Do you have any experimental data on the material used in the experimental compaign?
Are you sure about the material parameters? Starting from the elastic constants?
Do you have any experimental data on the material used in the experimental compaign?
Re: Boundary condition Shear wall
Hello!
Yes I have. The elastic modulus and compressive strength of both mortar and bricks were experimentally obtained. The tensile strength was computed indirectly, but they are in agreement with the literature.
Regarding the compressive fracture energy of the brick I faced a problem. The one I experimentally obtained is too low for the software. If I input the corresponding data I get the error regarding fracture energy being too small or the length of the element too large, so I increased the parameter until I didn't get the error. Do you think this has sth to do with the error?
More weird is that I drastically reduce the elastic parameters of the masonry and even then the response was stiff. This is why I thought I had sth wrong with the boundary conditions.
Yes I have. The elastic modulus and compressive strength of both mortar and bricks were experimentally obtained. The tensile strength was computed indirectly, but they are in agreement with the literature.
Regarding the compressive fracture energy of the brick I faced a problem. The one I experimentally obtained is too low for the software. If I input the corresponding data I get the error regarding fracture energy being too small or the length of the element too large, so I increased the parameter until I didn't get the error. Do you think this has sth to do with the error?
More weird is that I drastically reduce the elastic parameters of the masonry and even then the response was stiff. This is why I thought I had sth wrong with the boundary conditions.
Re: Boundary condition Shear wall
In fact, this is what I noticed.More weird is that I drastically reduce the elastic parameters of the masonry and even then the response was stiff. This is why I thought I had sth wrong with the boundary conditions.
Now do not focus on non-linear parameters. You first have to understand why the elastic regime is soo different.
If the boundary conditions are how you described them, it looks correct.
Now there are some possibilities:
- Are you sure about how the reference curve has been obtained? I mean, is it the total base force (accounting for the whole wall thickness of 310 mm) vs the total top displacement?
- Are you sure that there is no slip in the experimental test in the X direction?
- The reference curve comes from a monotonic test? or is it the envelope of a cyclic test? In this case it can make sense because of cyclic damage
Re: Boundary condition Shear wall
- Are you sure about how the reference curve has been obtained? I mean, is it the total base force (accounting for the whole wall thickness of 310 mm) vs the total top displacement? Yes
- Are you sure that there is no slip in the experimental test in the X direction? From the experimental data (LVDT readings and pictures) it doesn't seem likely that there was a slip in the X-direction. In another tests we had this problem and it was registered by the sensors
- The reference curve comes from a monotonic test? or is it the envelope of a cyclic test? In this case it can make sense because of cyclic damage. As you said, the background curve is in fact the envelope curve of the cyclic test. However, even when the monotonic response should be a bit stiffer it should not differ that much.
Thank you!
Re: Boundary condition Shear wall
It really depends on how much the first cycles induced tensile damage.However, even when the monotonic response should be a bit stiffer it should not differ that much.
No, if the elastic response is not captured correctly, it makes no sense to change the nonlinear parameters. First we have to understand why there is such a huge difference.What do you suggest I do now? change the elastic parameters until I reduce the difference between both curves in the elastic range? Is there another way besides changing E, ft and fc?
Also lowering E to match the experimental response does not seem reasonable, because I've seen that you have to lower them by a factor of 10 to match the experimental response, and it is way to much!
Could you share some data?
- Experimental results from which you obtained the elastic parameters of the mortar and units
- Cyclic imposed displacement history
- Cyclic response