using truss elements as soil springs

Samira M. · Post by **Samira M.** » Fri Mar 06, 2009 6:52 am

I'm using truss elements to model soil springs taking advantage of "beam on winkler springs" concept. The superstructure is a 2D RC frame (of 3 stories and 2 bays). the problem is that the truss elements have to be fixed in their base otherwise the model won't run. I presume it is required to fix the foundation level nodes in the x direction and the end nodes of the truss elements in x and y directions. but the model requires the end nodes of the truss elements to be fixed in all 3 dofs. what's the problem?

Post by **fmk** » Tue Mar 10, 2009 10:45 am

i presume your problem is with having to fix rz at the end node .. the end of the truss for your model must be fixed in all 3 directions x,y and rz .. a truss member unlike a beam member has no rotational stifness .. a node with rotational dof which is only connected to by truss elements would thus have no stiffness term for the rotational dof .. the rotational dof at these nodes must therefore be fixed .. if you don't understand this i suggest a quick look at a book on matrix structural analysis.