【求助】meshing not work

bobo

I use IDEAS8 to create a geometry in Master Modeler which is a frame system(small girders go through the webs of big girders). I use join to have all girders belong to the same part.  
Then I went to "Meshing" and first create the mesh on small girders and then began to create the mesh on big girers, HOWEVER, when I wish to define mesh by using the "mapped" button, it says  "Selected face cannot be mapped meshed" , I then change to use "free", but I only can create nodes instead of elements. I was totally confused by that.  
So please fill me some ideas  about how this happens and how to solve that.  
Thank you very much!

CAEngineer

change the element density. sometimes it will work! It depends on the shape of surface or volume you used.

bobo

I try to change the desity of the mesh for the web of big girders, but it still does not work,  
If there is someother method? Thanks

CAEngineer

give me your model, maybe I can help you!