The Mechanically Separated Wall

The MVLEM element (and its SFI-MVLEM and E-SFI-MVLEM cousins) has a curious 3D implementation in OpenSees. In plane, the element is a two node link with a fiber section and some rigid beams while out of plane the element is a linear-elastic plate.

The mechanics of the in-plane response are separate from the mechanics of the out-of-plane response. In a way, the MVLEM elements are like processed food, more chicken nuggets than whole chicken.

The rationale behind this paste of mechanically separated chicken is logical. In a building, some other system will resist lateral loads in the out-of-plane direction. And stiffness attracts load, so the wall element won’t see much out-of-plane loading.

But if the wall element has been damaged due to in-plane loading, the out-of-plane response would be affected too. And vice versa, out-of-plane nonlinearity will affect the in-plane response.

To demonstrate the point, take a single MVLEM 3D element with Concrete01 and Steel01 fibers and first load and unload the element in-plane, then load the element out-of-plane. The details of the wall (reinforcing pattern, flexure or shear dominated, etc.) are not important for this demonstration.

The in-plane load-displacement response and subsequent out-of-plane load-displacement response are shown below. The MVLEM element yields in-plane but responds linear-elastically out-of-plane.

Now if we use a twoNodeLinkSection element with a fiber section discretized in two directions (instead of only one direction like the MVLEM element), a more mechanically unified wall model, we see the out-of-plane response is affected by prior yielding in the in-plane direction.

The MVLEM element (and SFI-MVLEM and E-SFI-MVLEM) using an elastic plate in the out-of-plane direction is an unrealistic backstop that could lead to unconservative estimates of the lateral load carrying capacity of a building model.

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After Jamie Oliver shows how the nuggets are made, the kids still want to eat.