Shape Representation in IFC File Format

 

Abstract

The goal of this article is to introduce the shape representation types for Solid Model. It is useful when a user knows the theoretical background of shape representation in the IFC format and can imagine why the same entity with different representation is imported in a different way.

Introduction

One of the important things in the IFC file format is the shape representation. Each program has different requirements and demands.

A solid model is a complete representation of the nominal shape of a product with all points in the interior connected. Any point can be classified as being inside, outside, or on the boundary of the solid. There are several different types of solid model representations. [1]

Solid Model has 5 basic types:

  • SweptSolid
  • Brep
  • CSG
  • Clipping
  • AdvancedSweptSolid

And 3 additional types:

  • BoundingBox
  • Sectionedspine
  • MappedRepresentation

SweptSolid

The profile of an entity is defined parametrically or as a general polygon which is extruded along a line or curve. This representation provides geometric information about profile, begin and end point of the axis for a 1D member or height / thickness of a 2D member.

Brep

The entity is defines by points which are combined into lines and lines into a surface. As a result, there is no information about the geometry of a profile, length, height, thickness, axis of a 1D member or a middle plane of a 2D member.

CSG

A solid represented as a CSG model is defined by a collection of “primitive solids”, combined using regularized Boolean operations. The allowed operations are intersection, union, and difference. As a special case a CSG solid can also consist of a single CSG primitive. [1]

Clipping

The geometry of entity is based on another representation type, e.g. SweptSolid. The final entity is the extruded solid which is clipped / cut by a plane or other solids. It can be simply imagined as a member with defined structural shape in SCIA Engineer.

AdvancedSweptSolid

A swept disk solid is the solid created by sweeping of a circular disk along a three dimensional curve. During the sweeping operation the normal to the plane of the circular disk is in the direction of the tangent to the directrix curve and the center of the disk lies on the directrix. The circular disk may, optionally, have a central hole, in this case the resulting solid has a through hole, or, an internal void when the directrix forms a closed curve. [1]

BoundingBox

This type is for simplistic 3D representation of entities.

SectionedSpine

A sectioned spine is a representation of the shape of a three dimensional object composed of a spine curve and a number of planar cross sections. The shape is defined between the first element of the cross sections and the last element of this set. [1]

MappedRepresentation

A representation map is the identification of a representation and a representation item in that representation for the purpose of mapping. The representation item defines the origin of the mapping. The representation map is used as the source of a mapping by a mapped item. [1]

Conclusion

Each of shape representations is convenient for different kind of application. Architectural programs often use CSG and boundary representation whereas structural programs quite often require SweptSolid. The advantage of SweptSolid for SCIA Engineer is that it gives exactly the same information which is needed for creation of a SCIA Engineer native entity. Also MappedRepresentation is useful as it saves space in the file and provides information about repetition of the entity. SCIA Engineer uses the mapping for reinforcement bars i.e. stirrups on a member can be imported as one free bar with repetition instead of many stirrups.

References

[1] http://www.buildingsmart-tech.org/ifc/ifc2x3/tc1/html/index.htm

Marie Heřmanová

18/03/2013