For any grillage modeling, there are two fundamental aspects to be taken into account: Proportions & Arrangements

• Proportions: The element's lengths of the grillage should be similar, i.e., the longitudinal and transversal girders' distances should be on the same order of magnitude: A ~ B;
• Arrangements: The transverse elements should be orthogonal to the longitudinal elements as this is a better solution when compared to skew transverse girders, as shown in the 2nd picture below. 

 

More information can be found in the respective Training example, with documentation and files:

communities.bentley.com/.../3d-composite-grillage