BY STAAD PRO FORUM

What is a surface element? When should I use Surface elements?

Answer :

In STAAD, a surface entity is an object that represents a collection of elements. When the program goes through the analysis phase, it subdivides the surface into a number of elements by itself. This process is called meshing or mesh generation. But during the modeling phase, the user does not see any of those elements which keeps the model small and easy to manage. An entire wall is hence represented by just a few "Surface" entities, instead of hundreds of plate elements.

The advantage of using surfaces is that the minute details involved in the process of converting a physical object like a wall or slab into the mathematical model which consists of plate elements is something that the user doesn't have to bother with. 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. Also, not knowing how many elements will be generated during the meshing process 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. Consequently, we recommend that surfaces be not used as much as possible.

There is however one situation where the user has no choice but to use surfaces. That is when he/she wants to perform reinforced concrete design of a shearwall per the ACI, British or Indian codes. STAAD can perform a shearwall design if and only if that wall is modelled using STAAD's SURFACE entity.

So when you wish to model a floor slab, roof or a wall that does not need to be designed as a shear wall, we recommend that you use plate elements. In our view, the only drawback of using plates is that you need several of them to model a wall or a slab, and that increases the size of the input data. But at least you can detect and remove every error in the model, and know exactly how many elements and nodes there are in that model

Question :

It is my understanding that plates can only be 3 or 4-noded elements, is that true? If I have more than 4 nodes for my wall or slab, should i use surfaces??

Answer :

Not really. STAAD has the ability to mesh a region which has more than 4 corners. Please have a look at the information at the following link for details

Question :

Is there any disadvantage in using surfaces?

Answer :

When you use surfaces in your model, the resulting mesh of finite elements is not easily apparent. In fact, the only place in the program where you get a good view of the meshed state of the surface is in the post-processing mode in the node displacement page.

The inability to view the meshed state of the surface makes it very difficult to see the flaws in the modelling. In your structure for example, there may be members which are sharing space with the surface. Unless those members get meshed automatically along with the surface, and more importantly, unless the surface and member meshing is done in such a manner than all the resulting pieces are connected to each other properly, there is no assurance that the members and surface will be able to transfer loads to each other. The inability to see their meshed condition makes it difficult to see if the meshing is done properly.

Replacing the surface with quadrilateral elements, and meshing those element is a way to get around the problem

Question :

I am comparing two models. One has surfaces for the slabs. In the other, I have used plates. They do not give the same results. Why?

Answer :

The plate element mesh in the two cases may not be the same. Go to the Node-displacement page of the post processing mode. The element mesh that has been obtained from the surfaces will be displayed in that page. You can make a visual comparison of this mesh with the plate element model and inspect if any significant differences are apparent

thank you so much