I have modeled a bulkhead wall with beam elements and assigned plastic moments. Tracking the bending moment during the seismic shaking shows that in some elements the bending moment reached the plastic moment capacity but the lateral deformation of the wall is not showing any sign of failure or excessive deformation. In other words, I am unsure if the plastic hinges are formed by exceeding the bending moment capacity. Is this a correct way of modeling an inelastic wall?

Are you using FLAC2D 9.0 or old FLAC?

I am using FLAC v8.0

Is any softening plastic hinge used? If only plastic moment is specified in the beam, once reaching the limit, the maximum moment will become constant, and will not set to zero. Another option is using liner element, inelastic behavior can be specified for liner elements by employing an elastic-plastic

material model.

I am not assigning softening plastic hinges because I do not know the location of the plastic hinges before running and I am running multiple scenarios. So, in this case, is the beam element with plastic moment assigned equal to an elastic beam, and plastic moment capacity is not affecting wall behavior?

it should affect the wall behavior the capacity is reached, you can check the example se_02 in the structural element manual, with the pmom value is specified, you can see the difference on the beam deformation.

The example se_01_02 in the FLAC 8.0 manual does not include the plastic moment but I found the discussion in the example se_01_05. it is still not clear that assigning the plastic moment, models the deformation of the post-failure of the beam when the plastic moment capacity has been reached if we do not pre-determine the location of the plastic hinges. In other words, I would like to know when the bending moment capacity exceeded what would be the total lateral deformation of my structural elements considering the post-failure behavior if that is possible.

You can clone state and create another branch, in the new branch, modify struct prop line, and specify a value for pmom, e.g.

struct prop 1001 e 2.0E11 area 0.0060 I 2.0E-4 pmom 10000

How is it possible to specify an elasto-plastic material for liner? From the manual, the only options are isotropic, orthotropic and anisotropic which are, I think, all linear elastic material. Isn’t it?

There is a failure envelop defined by a P-M diagram. Initially it’s linearly elastic, once the force and moment fall outside the failure envelope, those values will be adjusted to return the values to the failure surface.

So does this mean that the effect of plastic hinging is not modeled? I guess the concept of predefining the plastic hing locations is a bit confusing. If the moments and load exceeding the failure envelop then the effect of hinging should be accou ted in the model, correct?

I’m still not sure what happens if you specify pmom but do not specify a plastic hinge. It seems that FLAC will limit moment to the specified Pmom but will not compute any associated plastic rotations. The beam remains elastic, but it will also be out of equilibrium for the next loading step. This will result in additional rotations being computed on the next step, but using a limited set of mathematical steps that do not specifically incorporate plastic rotations.