Walls are meshed and represented with shell finite element for analysis. Currently, an option to include shell out-of-plane stiffness is provided to the user. If shell out-of-plane stiffness is not considered in analysis, walls are acted for only in their in-plane (membrane action).

The shell finite element is a quadrilateral shell element and it has a total of six degrees of freedom at its each node: three translational, two bending, and one drilling. The formulation for element stiffness matrix includes membrane stiffness and plate bending stiffness. These stiffness matrices are calculated separately and then combined to form element stiffness matrix for the shell element.

The membrane stiffness accommodates three degrees of freedom at each node, namely two in-plane translational and one drilling degrees of freedom. The formulation utilizes Allman type shape functions within Hughes-Brezzi variational formulation framework. It includes correction matrix to remove any existing membrane locking from element behavior. Also, another correction matrix is applied to calculated stiffness matrix in case of warped planes of shell (Ibrahimbegovic, at. al., 1990; Taylor, 1997; and Long at. al., 2004).

The plate bending stiffness is derived based on thin plate assumption and it is a typical Discrete Kirchoff Element (shear deformations over the thickness of the element ignored). The formulation includes three degrees of freedom at each node: remaining two rotational degrees of freedom and one translational degrees of freedom, perpendicular to the plane of the shell.

Unlike beam, brace, and column elements whose stiffness coefficients are integrated exactly, the shell finite element gives results that are "exact" or close to the theoretical solution only if a finer mesh is used and convergence achieved. For most practical purposes, the use of only a single wall element in a typical bay may significantly underestimate deflections. In some cases, it could also give wrong results. RAM Frame automatically meshes wall systems in a building. In general, a coarse mesh for walls lead overstiff wall systems while a fine mesh for walls gives more flexible results. Refer to Modeling Shear Walls for further information.

Since individual wall elements are basically two dimensional elements, singularity could occur during the solution process if shell out-of-plane stiffness is not included in analysis. RAM Frame provides a very small out-of-bending stiffness for walls to prevent such singularities if necessary.

RAM Frame assembles the stiffness coefficients of its elements in a 3-Dimensional fashion. Therefore, walls that intersect at an angle (and hence share common nodes) form a 3-D system and the 3-D behavior is captured by the analysis. However, member force outputs will be for the individual components of the wall, and not for the system.

RAM Frame assumes that wall finite elements at the lowest story are foundation nodes, i.e. they are fixed at the bottom. If both ends of the wall are foundation nodes, the internal nodes at the base created by the mesh are also considered foundation nodes. If only one end of the wall is a foundation node, the internal nodes are not considered foundation nodes. The reader is also referred to Walls at Foundation Level and Remarks on Meshing Walls for more information about how reactions at wall foundations level are calculated and how meshing walls impact foundation node configuration.

It should be known that a user can define two separate crack factors for concrete walls: one for membrane (in-plane) actions, and one for bending (out-of-plane) action. These crack factors are applied to the stiffness properties of concrete walls and hence, their stiffness for in-plane and out-of-plane are reduced with the crack factors.

Rigid end effects are ignored in walls.