Point load on shell element

[JasonGeng]

I modelled floor slab as shell elements in SAP. If I have a point load vertically applied on some shell element, what is the best way to mesh this shell element? Do I need to mesh it manually so that the point where load is applied is one of the joints of the shell elements?

See the attachment for a clear picture for what I am asking.

Thanks.

 

[masomenos]

I didn't see an attachment.. but yes, you need to mesh your shell element such that there is a joint where you want to assign the joint load. Under Edit menu you will find several options to mesh/divide your shell elements. I find that adding gridlines in order to mesh by gridline is particularly useful, but there are several other mesh options

 

[JasonGeng]

Thanks for your answer. But I am wondering why SAP and Etabs are different when dealing with this problem. In Etabs, there is no need to mesh the shell elements so that the points where the loads will be applied happen to the joints of the meshed shell elements.

Any special reason?

 

[masomenos]

I'm not as familiar with ETABS, but I'm pretty sure that ETABS will not automesh shells automatically in order to handle a joint load added without manual meshing. SAFE is the only CSI program to do this automatically. What might be happening in ETABS is that the automatic line constraint by default is ON.. whereas in SAP, the edge constraint by default is OFF. Using this constraint, ETABS is probably trying to do a displacement interpolation in order to connect that joint load onto the closest surrounding joints.. a procedure which can be inaccurate if the closest surrounding joints are far apart from the joint load.

As you probably know, this problem does not occur with frame elements. You can add joint loads or intersect frames with other beams or bracing and both SAP and ETABS will automesh the frames to account for this.

 

[JasonGeng]

I have modelled a simple structure with 4 columns supporting one piece of slab with shell element. I did not mesh the the slab, when I applied a point load on the slab, I still get a reasonable reaction after analysis. It may be because the line constraint in Slab is alway ON. See attached model.