It determines the extension of meshed area of the diaphragm. If Use Slab Edges for Exterior Boundary is chosen, the program uses slab edges as an exterior boundary of the meshed diaphragm (see Generating diaphragm mesh (Use Slab Edges for Exterior Boundary) ). It is important to note that the area between the most exterior beams and slab edges are also meshed. If Use Beams for Exterior Boundary is chosen, the program uses the most exterior beams as exterior boundary of the meshed diaphragm (see Generating diaphragm mesh (Use Beams for Exterior Boundary) ). Note that there should be a closed beam loop constructed with exterior beams. Otherwise, the program fails to mesh in the absence of a closed boundary if this option is chosen, and then it automatically switches to the other option.

These two options are provided because

• For some configurations, slab edge offset distance may be too small (only a few inches) and there are many beams running parallel to slab edges. If the first option is chosen, then it is very likely that very small distorted elements are created between exterior beams and slab edges. Thus, the user is advised to use the second option, which leads to a more balanced meshed configuration.
• For slabs with no beams (like concrete flat slabs) or no closed beam loop exists, then user needs to use the first option.