Error faces on limited dissolve

When using limited dissolve in blender its creating some bad faces like this

Original geo:

Error Face:

Here is the just the face in isolation and fixed face on the right.

The idea is to decimate the model and the frame shown here is part of a bigger model and just this is having issue when doing limited dissolve.

What are these faces called ?

What can I do apart from triangulation to fix it ?

Attaching the blender file here...

For converting triangles to quard faces use Alt+J (Tirds to quards).

For good result in your model, tweek Max Shape Angle:

The idea is to decimate the model and the frame shown here is part of a bigger model and just this is having issue when doing limited dissolve.

About decimating: there is no much difference between this 2 models:

Before limited dissolve:

after:

As you see tirs count stays same. That's because n-gons (that created by limited dissolve) based on same triangles, so there'is no any memory saving in doing this.

Well, the idea is not to triangulate but decimate. This is part of a larger model and the frame is giving issue

If you realy want to save some memory - use decimate modifier or (much better way) - instancing.

• Well, the idea is not to triangulate but decimate. This is part of a larger model and the frame is giving issue. hope its clear. – sreenivas Nov 9 '17 at 10:00
• As I said, it might not make sense to do it in this model. I have a perfect use case for it. Thanks for the link to the other question. Yes, its called concave face. That brought me to the solution. EditMode mesh > cleanup faces > split concave faces. Running this will eliminate the concave faces and triangulates them. Again that might look like triangulation in the current geo. It does fix the problem in my model. Thanks for you help. If you can just add the explanation of using split concave faces to your answer. I can accept it. Thanks! – sreenivas Nov 9 '17 at 13:05