Challenges involved in modeling with surface elements in STAAD.Pro

The fact that one does not need to generate a finite element mesh when modeling with surface elements makes it an attractive modeling option for many engineers modeling their structures using STAAD.Pro. While there are advantages of using surface elements under some scenarios, before one uses surface elements for modeling, one should be aware of the challenges that come with it. As mentioned, an advantage of using surface element is that the minute details involved in the process of converting a physical object like a wall or slab into an analytical model consisting of a plate element mesh, is not something that the user has to bother about. However, in many situations, not knowing these details can lead to errors, some of which are impossible to detect because the underlying elements cannot be seen graphically.

For example in a structure, there may be beams elements and plate elements sharing common edges with the surface elements. For proper connectivity to be established in between these entities, each has to be divided or meshed such that the number of divisions match at the common edges or in other words these entities should share common nodes. Unless that happens, the connectivity would not be proper which would lead to incorrect load transfer between these entities. Although STAAD.Pro internally meshes the surface element into a plate mesh however the adjacent entities are not automatically split or meshed accordingly to establish proper connectivity and the onus is on the engineer to ensure that. For proper connectivity to be established, one needs to go in and set the number of divisions for each edge of a surface such that it matches with adjacent plate mesh or with segments of beams at the periphery. One would also need to ensure that columns adjacent to surfaces, usually modeled using beam elements, are also split to match with the surface divisions. Matching these divisions for regular rectangular geometry may not be that complicated but in a real life structure where slabs could be of any shape, have openings, walls may have cutouts and may have discontinuities, establishing proper connectivity could be a real challenge. So before modeling with surfaces just because it is simpler to model it that way, you need to consider the pros and cons.

In addition, although one can influence the edge divisions along an edge of a surface but one cannot control the internal meshing fully as irregularity in geometry may result in generation of a triangular mesh. In such cases it is difficult to estimate the number of elements that could be generated during the meshing process. Not knowing this beforehand can lead to unwanted consequences such as a massive increase in the size of the model, to a point where the program simply cannot handle such a massive volume of data. The following wiki deals with one such scenario

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/22310.error-total-jointelement-limits-exceeded-while-creating-surface

There is however one situation where the user has to use surface elements. That is when he/she wants to perform reinforced concrete design of a shearwall per the ACI, British or Indian codes. STAAD.Pro can perform a shearwall design if and only if that wall is modelled using the surface entity.

It may be worth mentioning here that modeling shear walls with plate elements is also very common. The following wiki explains how shear walls modeled with plates can be subsequently designed

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/23103.modeling-shear-walls-using-plates

So this is what it boils down to. When one needs to model a floor slab, roof or a wall that does not need to be designed as a shear wall, they are better off using plate elements. When a shear wall needs to be modeled and also designed as per one of the codes mentioned above, the user needs to make a choice as to whether to use plates or surfaces after weighing in all the pros and cons. In our view, the only drawback of using plates is that one needs several of them to model a wall or a slab and that increases the size of the input data. But even then one has a lot more control on the model size when using a plate mesh. In addition, fixing modeling errors are a lot easier for a model using a plate mesh.