This functions like re-bar in concrete or the roving in fibreglass layup. Basically this is a 3 mm hole, or holes, placed within your object, crosswise to the 3D layering, to tie all the layers together structurally, using a post print piece of ABS filament that is glued into the hole (s) using acetone or ABS slurry. The more "Lucius Gales" you use the more structurally sound your print. I have named this to honor a grandfather who was instrumental in putting in the second transcontinental railroad across the United States. I would appreciate these being called out after his name or at least LG's. A test I did after a print showed this form to be far stronger than a print without them and much more resistant to splitting at the layer.
Please inform me if this type of strengthening has been mentioned before and I will delete this post (if I can).
The .scad file is an example cube with eight Lucius Gale's in it. It is well documented as to what each part does. I hope these will be helpful with your structural parts.
Edit.1: I should add that it would appear you could just drill a 3mm hole for the same effect but that would only work in a solid piece with no infill.