pre-stressed pre/post-tensioned concrete bridge

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bernd
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Re: pre-stressed pre/post-tensioned concrete bridge

Postby bernd » Tue Aug 21, 2018 8:14 pm

HarryvL wrote:
Mon Aug 20, 2018 9:45 pm
Post tensioning ...


I am running out of time (and patience :twisted: ). I attach the file for those who want to try. This should work "as is". Actual post-tensioning would be the next step, but not before this simple load case converges.


CB_Post_Rev1.fcstdCB_Post_Rev1.fcstd
you are not the first one having problems in convergences ... topic https://forum.freecadweb.org/viewtopic.php?f=18&t=20276 and in contact topic this post https://forum.freecadweb.org/viewtopic. ... 90#p188716
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HarryvL
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Re: pre-stressed pre/post-tensioned concrete bridge

Postby HarryvL » Mon Sep 03, 2018 8:30 pm

HarryvL wrote:
Mon Aug 20, 2018 9:45 pm
Post tensioning ...


I am running out of time (and patience :twisted: ). I attach the file for those who want to try. This should work "as is". Actual post-tensioning would be the next step, but not before this simple load case converges.


CB_Post_Rev1.fcstdCB_Post_Rev1.fcstd
INTERMEZZO 2: https://forum.freecadweb.org/viewtopic.php?f=18&t=30697
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HarryvL
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Re: pre-stressed pre/post-tensioned concrete bridge

Postby HarryvL » Sun Sep 30, 2018 9:22 pm

Armed with the experience gained here: https://forum.freecadweb.org/viewtopic.php?f=18&t=30697 I returned to the challenge of modeling post-tensioning of a concrete bridge. I finally got it to work, but not after overcoming a few further challenges, i.e.:

1) I used symmetry to cut the model in half. Due to mesh refinement at the anchoring points, the model had simply become too big to be practical.
2) I had to introduce contact constraints at the vertical faces of the tendon to prevent it shooting off laterally
3) Convergence could only be achieved by a) reducing contact stiffness on the vertical faces (which is fine, because they only serve to keep the tendon in place) and b) reducing the temperature of the tendon (to mimic post tensioning) in 2 steps.

All-in-all it took me a full weekend to get it to work :shock: and I therefore do not consider this a practical approach for modeling post-tensioned concrete. A much more robust way to achieve this would be with simple, old-fashioned, small deformation interface elements ... which are unfortunately not available in CCX.

As mentioned, I use half a model with symmetry conditions applied:


CB_Post_Half_Bridge_Rev4_Model.png
CB_Post_Half_Bridge_Rev4_Model.png (280.09 KiB) Viewed 193 times


A 50x50mm tendon is placed in 51x51mm duct, which is cut out from the beam. The tendon is anchored to two steel anchor plates, which react against the beam.


CB_Post_Half_Bridge_Rev4_Model_CU.png
CB_Post_Half_Bridge_Rev4_Model_CU.png (326.99 KiB) Viewed 193 times


As before, the tendon is put under tension by dropping the temperature by 500 degrees. This process is achieved in 2 steps to ensure convergence. The deformation of the bridge under dead and live load before shortening the tendon is 171mm, which far exceeds design guidance (<L/300). However, after applying tension, the deformation reduces to 99mm, which just meets design guidance.

The result plots show that the actual tensile stress achieved in the tendon is approximately 1350 MPa (smaller than allowable) and that the required reinforcement ratio is 6%, which both agree well with the values obtained in the pre-tensioning example at the start of this topic.


CB_Post_Half_Bridge_Rev4_Result.png
CB_Post_Half_Bridge_Rev4_Result.png (86.58 KiB) Viewed 193 times


Time to move on to other challenges :D
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bernd
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Re: pre-stressed pre/post-tensioned concrete bridge

Postby bernd » Wed Oct 03, 2018 6:18 pm

Cool :-)

What does a small deformation interface element do? Hkw does this connect the cable and the concrete?
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HarryvL
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Re: pre-stressed pre/post-tensioned concrete bridge

Postby HarryvL » Thu Oct 04, 2018 5:05 am

A classical interface element is a zero-thickness element that connects the nodes on oposite faces of contacting solids. It describes coupled sliding and penetration/separation behavior of the contact through a stiffness matrix. The coupling of sliding and “separation” becomes evident in the description of slip surfaces in sand, rock or concrete, where sliding normally gives dilation. Let me see if I can program one in a macro to show how it works.