# 8.8.3.37 IfcShellBasedSurfaceModel

## 8.8.3.37.1 Semantic definition

An IfcShellBasedSurfaceModel represents the shape by a set of open or closed shells. The connected faces within the shell have a dimensionality 2 and are placed in a coordinate space of dimensionality 3.

Informal Propositions:

1. The dimensionality of the shell based surface model is 2.
2. The shells shall not overlap or intersect except at common faces, edges or vertices.

## 8.8.3.37.4 Formal representation

ENTITY IfcShellBasedSurfaceModel
SUBTYPE OF (IfcGeometricRepresentationItem);
SbsmBoundary : SET [1:?] OF IfcShell;
DERIVE
Dim : IfcDimensionCount := 3;
END_ENTITY;