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Building description techniques for rationalized traditional construction
Building description techniques for rationalized traditional construction D S H Rosenthal
Techniques for constructing computer descriptions o f buildings viewed as assemblies o f 'standard details' o f in-situ construction are described and contrasted with those appropriate for buildings viewed as assemblies of standard components. These techniques have been successfully applied to the 'rationalized traditional' method o f housing construction, but certain problems impede their wider application.
fits if, on average, each entry i~ the Codex is shared by many instances of components in the building. Obviously, this condition will generally be satisfied in componentbased building systems. (This exposition of the OXSYS building description has been considerably simplified for the purposes of clariW.)
Systems capable of automatically generating a complete bill of quantities from a building description held in a computer have been implemented successfully in two fundamentally different contexts: component-based building systems, exemplified by the Oxford Method, and rat-trad (rationalized traditional) practice, exemplified by the SSHA (Scottish Special Housing Association). Superficially, the building description held in the computer in each case is similar, typically consisting of a set of paraxial cuboids representing 'components' and 'standard details'. However, the detailed techniques used to implement these descriptions are very different, being optimized for certain types of construction and inefficient for others. The difference is not simply that component-based systems, such as OXSYS I describe a building as components, and rat-trad systems, such as the SSHA's House Design 2, describe it as standard details. On the contrary, House Design's description includes many predefined components, such as windows and doors, and the OXSYS description is also capable of representing standard details. The difference is that component-based systems can operate efficiently using a description whose elements are discrete, whereas rattrad systems, in general, require nondiscrete elements.
As an illustration of how nondiscrete descriptions arise, consider applying discrete techniques to a typical problem of rat-trad practice. For simplicity, consider a twodimensional orthogonal description in which the elements are paraxial rectangles defined by a position, an X-extent and a Y-extent. An internal partition B is created, forming a junction with a pre-existing external wall A, which appears in plan as in Figure I. There will be constructional information associated with the junction of A and B, described by the standard detail for that type of junction. To refer to this detail a new component C, of zero thickness, can be created and placed at the junction, as Figure 2 shows. The inner surface of the external wall A has now been divided into three separate areas, one covered by C and the other two facing different rooms on either side of B. These two areas may well receive different surface treatments, and in any case will not be the same as the inner surface of another instance of the same external wall component. Thus additional zero thickness components are required (Figure 3).
DEVELOPMENT OF A NONDISCRETE DESCRIPTION
Room I
DISCRETE DESCR IPTIONS A discrete descriptive element is selfcontained as its attributes do not depend upon its relationship to other elements of the description. This implies that the quantity information associated with one instance c f a component is the same as that associated with any other instance of the same component. Hence, it may be 'factored out'. The ability of OXSYS to handle descriptions of many thousands of elements is based upon this technique; each instance of a component is described as an isolated box in space and a reference to the shared quantity information in the 'Codex'. Note that this technique only offers baneEdCAAD, Department of Architecture, Edinburgh University, Edinburgh, UK. Presented at 'Computer representation of shape' conference, Cambridge, UK, December 1978.
volume 11 number 6 november 1979
Room 2
Figure 1. Plan of existing wall A with internal partition B
0010-4485/79/060325-04 $02.00 © 1979 IPC Business Press
325
Since the other surfaces may also be subject to similar processes, they too must be covered in zero thickness components as shown in Figure 4. Note that these zero thickness components are different from real components in three ways, which are crucial for the design of the software:
/
F
• They must be generated automatically by the system as a consequence of the user's positioning of real components. • The system has to recognize of which, if any, standard detail they are an instance. • Their existence is dependent upon the existence of some real component. For example, if B is deleted,/, ] and K must also be deleted. Further, C, D and E must be coalesced back into a single surface L, since the basis for this division has been removed (Figure 5). To account for the effects of junctions between components, the description of each component has had to be divided into two parts, namely a static part, describing its interna~ attributes, and a set of surfaces, which dynamically change to reflect the relationship between each element and its neighbours. An individual surface cannot, in this descrip-
C
B
~ K
H Figure 4. Other surfaces must also be covered in zero thickness components
tion, be said to belong to a particular element; it is shared by all elements which it bounds. For example, surface C forms part of the description of both element A and element B. Similarly, surface D is shared by element A and room 1. It is in this sense that the descriptive elements can no longer be said to be discrete.
LESS RESTRICTED NONDISCRETE DESCRIPTIONS A
B
The use of descriptive elements composed of lower-level entities which, by being shared between related elements, contain information describing the interrelationship of the elements, has been fundamental to all the systems produced by EdCAAD for the SSHA. The preceding section has outlined the techniques employed by the House Design system 2 . Since then, similar techniques have been employed in two less restricted systems, Site Layout 3 and PIM (the Polyhedron Interrelationship Modeff).
Site Layout Figure 2. Component C refers to a standard detail
c
B
Figure 3. Components D and E refer to different surface treatments
326
In the Site Layout system, the site is modelled as a mosaic of polygons completely covering a ground height model. A nondiscrete description of this kind of mosaic, the DIME file s , is widely used in geographic mapping systems. It describes the mosaic as a set of nodes (each containing a position), and a set of links (each referring to a start and an end node and a left and right polygon). Although this representation is efficient for the quantification functions of Site Layout, it is difficult to input and manipulate interactively. This difficulty is particularly evident in the case of predefined elements, such as houses. The user would find it tedious to trace the boundary of each house entering the identities of each neighbouring element, and would make errors. Thus the polygons are input as discrete entities consisting of a ring of nonshared edges, many being generated by program from templates in a library. Before quantification, the site description is processed by the so-called Merge function 6p. This removes 'noise' from the digitizing process, computes the element interrelationships by identifying all shared edges and vertices, and reconstitutes the polygon description into a nondiscrete form, similar to the DIME file.
computer-aided design
Polyhedron Interrelationship Model In PIM, which has been developed largely as a research vehicle but is being used in a current project for the SSHA, the building is modelled as a set of general polyhedra, each formed of faces, edges and vertices. Traditionally, for example in GLIDE 8, the descriptions used to represent polyhedra have been derived from Baumgart's data structure 9. This regards edges as joining two vertices, separating two faces, and forming part of two rings of edges, one bounding each face. It gains considerable efficiency from these assumptions, since the structure requires only fixed-length records. However, it does incorporate the concept of discreteness, since in an assembly of polyhedra, a single edge may form part of the boundary of any number of pairs of faces. In PIM, unlike systems with discrete elements whose faces, edges and vertices belong to individual polyhedra, the faces, edges and vertices are each described only once, no matter how many polyhedra they may form a part of. Because each vertex is linked to all edges of which it is an end point, and each edge to all faces which it bounds, and each face to all polyhedra which it bounds, they contain information describing the interrelationship of the polyhedra in a form suitable for the rat-trad quantification process. The maintenance of this description as polyhedra are created, modified and destroyed, is analogous to, but more severe than, the Merge problem in two dimensions. It consists of four subproblems I°,4, the first three of which are three-dimensional equivalents of the parts of the Merge problem: • As new vertices are created, they must be compared (via a spatial index) with the existing vertices to ensure that they remain unique. • As new edges are created, they must be compared with the other edges linked to their start and end vertices to ensure that at most one edge links each pair of vertices. • As new edges and vertices are created, they must be compared with the edges and vertices in their neighbourhood, so that: o If an edge passes near an existing vertex it is split into two and linked to that vertex. o Ifa vertex is created near an edge, the edge is split into two and linked to the vertex. o If an edge passes near an existing edge, they are both split into two and a new vertex created at their 'intersection'. • As new faces are created, they must be compared with each of the existing coplanar faces. If their intersection with an existing face is nonempty, the shared part must be split off from each, and constituted as a separate face. PIM now includes algorithms for each of these operations, and their associated book-keeping. However, not all of the corresponding deletion operations have yet been implemented satisfactorily. It includes a polygon package operating in two dimensions on a nondiscrete polygon description, since this is required for the coplanar face comparisons.
PROBLEMS OF N O N D I S C R E T E DESCRIPTIONS Compared with discrete descriptions, the type of nondiscrete description outlined above suffers from two interrelated problems:
volume 11 number 6 november 1979
F
G
A
L
H Figure 5. Zero thickness components are dependent upon the existence o f some real component
• The amount of information required to describe an individual element is vastly greater, since it must represent not only the internal attributes of the element, but also its relationship to its neighbours. • This greater amount of information must either be maintained as part of the building description as the user inputs it (the approach taken by House Design and PIM) or generated en bloc after input is finished (the approach of Site Layout). In either case the demand for computation is severe. A further problem is that component-based systems are effectively told of which component an element is an instance, whereas in many cases rat-trad systems must recognize the appropriate standard detail for a junction. In contexts where the number of possible details is large, this can prove very difficult. These problems have so far limited the application of nondiscrete techniques to sparse descriptions, and this has severely restricted the applicability of CAAD in the rat-trad context.
CONCLUSIONS Nondiscrete descriptive techniques have been used succesfully in certain rat-trad contexts to enable computers to produce bills of quantities. However, their wider introduction is dependent upon the development of powerful factoring techniques, similar to those already available for discrete descriptions, to reduce the amounts of descriptive data and the computational load of maintaining them.
ACKNOWLEDGEMENTS I should like to thank my colleagues, especially David Stone, the staff of Applied Research of Cambridge Ltd, especially Paul Richens, and the CEDAR team at the PSA for the stimulating discussions which led to this paper. This work was supported by the PSA (Property Services Agency) of the Department of the Environment, and by the Science Research Council.
327
REFERENCES I Hoskins , E 'The OXSYS system' in Gero, J (Ed) Computer applications in architecture Applied Science (1977)
2 SSHA 'CAAD at the Scottish Special Housing Association' SSHA (Spring 1976) 3 Bijl, A and Shawcross, G 'Housing site layout system' Comput. Aided Des. Vol 7 No 1 (January 1975) pp 2---10 4 Rosenthal, D 'PIM: underlying algorithms' EdCAAD Res. Rep. 79/15 (June 1979) 5 Nagy, G and Wagle, S 'Geographic data processing' ACM Comput. Surv. Vol 11 No 2 (June 1979)
6 Holmes, C 'Plastic merge procedure tor mosaics of polygons' Comput. Aided Des. Vo{ 10 No [ (January 1978) pp 57 64 7 Liardet M, Holmes, C and Rosenthal, D 'Input to CAD systems: two practical examples' Proc. IFIP Working Conf. on AI & PR in CAD Grenoble (March 1978) 8 Eastman, C and Henrion, M ' G L I D E - A language for design information systems' Comput. Graph. (ACM/ SIGGRAPH) Vol 11 No 2 (Summer 1977) 9 Baumgart, B G 'A polyhedron representation for computer vision' Proc. NCC (1975) l e Holmes, C 'Analysis of contiguities between polyhedra' EdCAAD Res. Rep. 79/04 (January 1979)
the new internmional c o w r i n g the ical, and social of computerinformation sy .m=s Coverage includes: etechniques =legislation tinformation systems oworking group activities ecase studies =computer misuse oorganizational aspects tcurrent interest section Annual subscription £40.00 ($104.00) Further details from: Marilyn Fitchett, IPC Science and Technology Press Ltd., PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 5BH, England Telephone: (0483) 31261 Telex: 859556 Scitec G
328
computer-aided design
Techniques for constructing computer descriptions o f buildings viewed as assemblies o f 'standard details' o f in-situ construction are described and contrasted with those appropriate for buildings viewed as assemblies of standard components. These techniques have been successfully applied to the 'rationalized traditional' method o f housing construction, but certain problems impede their wider application.
fits if, on average, each entry i~ the Codex is shared by many instances of components in the building. Obviously, this condition will generally be satisfied in componentbased building systems. (This exposition of the OXSYS building description has been considerably simplified for the purposes of clariW.)
Systems capable of automatically generating a complete bill of quantities from a building description held in a computer have been implemented successfully in two fundamentally different contexts: component-based building systems, exemplified by the Oxford Method, and rat-trad (rationalized traditional) practice, exemplified by the SSHA (Scottish Special Housing Association). Superficially, the building description held in the computer in each case is similar, typically consisting of a set of paraxial cuboids representing 'components' and 'standard details'. However, the detailed techniques used to implement these descriptions are very different, being optimized for certain types of construction and inefficient for others. The difference is not simply that component-based systems, such as OXSYS I describe a building as components, and rat-trad systems, such as the SSHA's House Design 2, describe it as standard details. On the contrary, House Design's description includes many predefined components, such as windows and doors, and the OXSYS description is also capable of representing standard details. The difference is that component-based systems can operate efficiently using a description whose elements are discrete, whereas rattrad systems, in general, require nondiscrete elements.
As an illustration of how nondiscrete descriptions arise, consider applying discrete techniques to a typical problem of rat-trad practice. For simplicity, consider a twodimensional orthogonal description in which the elements are paraxial rectangles defined by a position, an X-extent and a Y-extent. An internal partition B is created, forming a junction with a pre-existing external wall A, which appears in plan as in Figure I. There will be constructional information associated with the junction of A and B, described by the standard detail for that type of junction. To refer to this detail a new component C, of zero thickness, can be created and placed at the junction, as Figure 2 shows. The inner surface of the external wall A has now been divided into three separate areas, one covered by C and the other two facing different rooms on either side of B. These two areas may well receive different surface treatments, and in any case will not be the same as the inner surface of another instance of the same external wall component. Thus additional zero thickness components are required (Figure 3).
DEVELOPMENT OF A NONDISCRETE DESCRIPTION
Room I
DISCRETE DESCR IPTIONS A discrete descriptive element is selfcontained as its attributes do not depend upon its relationship to other elements of the description. This implies that the quantity information associated with one instance c f a component is the same as that associated with any other instance of the same component. Hence, it may be 'factored out'. The ability of OXSYS to handle descriptions of many thousands of elements is based upon this technique; each instance of a component is described as an isolated box in space and a reference to the shared quantity information in the 'Codex'. Note that this technique only offers baneEdCAAD, Department of Architecture, Edinburgh University, Edinburgh, UK. Presented at 'Computer representation of shape' conference, Cambridge, UK, December 1978.
volume 11 number 6 november 1979
Room 2
Figure 1. Plan of existing wall A with internal partition B
0010-4485/79/060325-04 $02.00 © 1979 IPC Business Press
325
Since the other surfaces may also be subject to similar processes, they too must be covered in zero thickness components as shown in Figure 4. Note that these zero thickness components are different from real components in three ways, which are crucial for the design of the software:
/
F
• They must be generated automatically by the system as a consequence of the user's positioning of real components. • The system has to recognize of which, if any, standard detail they are an instance. • Their existence is dependent upon the existence of some real component. For example, if B is deleted,/, ] and K must also be deleted. Further, C, D and E must be coalesced back into a single surface L, since the basis for this division has been removed (Figure 5). To account for the effects of junctions between components, the description of each component has had to be divided into two parts, namely a static part, describing its interna~ attributes, and a set of surfaces, which dynamically change to reflect the relationship between each element and its neighbours. An individual surface cannot, in this descrip-
C
B
~ K
H Figure 4. Other surfaces must also be covered in zero thickness components
tion, be said to belong to a particular element; it is shared by all elements which it bounds. For example, surface C forms part of the description of both element A and element B. Similarly, surface D is shared by element A and room 1. It is in this sense that the descriptive elements can no longer be said to be discrete.
LESS RESTRICTED NONDISCRETE DESCRIPTIONS A
B
The use of descriptive elements composed of lower-level entities which, by being shared between related elements, contain information describing the interrelationship of the elements, has been fundamental to all the systems produced by EdCAAD for the SSHA. The preceding section has outlined the techniques employed by the House Design system 2 . Since then, similar techniques have been employed in two less restricted systems, Site Layout 3 and PIM (the Polyhedron Interrelationship Modeff).
Site Layout Figure 2. Component C refers to a standard detail
c
B
Figure 3. Components D and E refer to different surface treatments
326
In the Site Layout system, the site is modelled as a mosaic of polygons completely covering a ground height model. A nondiscrete description of this kind of mosaic, the DIME file s , is widely used in geographic mapping systems. It describes the mosaic as a set of nodes (each containing a position), and a set of links (each referring to a start and an end node and a left and right polygon). Although this representation is efficient for the quantification functions of Site Layout, it is difficult to input and manipulate interactively. This difficulty is particularly evident in the case of predefined elements, such as houses. The user would find it tedious to trace the boundary of each house entering the identities of each neighbouring element, and would make errors. Thus the polygons are input as discrete entities consisting of a ring of nonshared edges, many being generated by program from templates in a library. Before quantification, the site description is processed by the so-called Merge function 6p. This removes 'noise' from the digitizing process, computes the element interrelationships by identifying all shared edges and vertices, and reconstitutes the polygon description into a nondiscrete form, similar to the DIME file.
computer-aided design
Polyhedron Interrelationship Model In PIM, which has been developed largely as a research vehicle but is being used in a current project for the SSHA, the building is modelled as a set of general polyhedra, each formed of faces, edges and vertices. Traditionally, for example in GLIDE 8, the descriptions used to represent polyhedra have been derived from Baumgart's data structure 9. This regards edges as joining two vertices, separating two faces, and forming part of two rings of edges, one bounding each face. It gains considerable efficiency from these assumptions, since the structure requires only fixed-length records. However, it does incorporate the concept of discreteness, since in an assembly of polyhedra, a single edge may form part of the boundary of any number of pairs of faces. In PIM, unlike systems with discrete elements whose faces, edges and vertices belong to individual polyhedra, the faces, edges and vertices are each described only once, no matter how many polyhedra they may form a part of. Because each vertex is linked to all edges of which it is an end point, and each edge to all faces which it bounds, and each face to all polyhedra which it bounds, they contain information describing the interrelationship of the polyhedra in a form suitable for the rat-trad quantification process. The maintenance of this description as polyhedra are created, modified and destroyed, is analogous to, but more severe than, the Merge problem in two dimensions. It consists of four subproblems I°,4, the first three of which are three-dimensional equivalents of the parts of the Merge problem: • As new vertices are created, they must be compared (via a spatial index) with the existing vertices to ensure that they remain unique. • As new edges are created, they must be compared with the other edges linked to their start and end vertices to ensure that at most one edge links each pair of vertices. • As new edges and vertices are created, they must be compared with the edges and vertices in their neighbourhood, so that: o If an edge passes near an existing vertex it is split into two and linked to that vertex. o Ifa vertex is created near an edge, the edge is split into two and linked to the vertex. o If an edge passes near an existing edge, they are both split into two and a new vertex created at their 'intersection'. • As new faces are created, they must be compared with each of the existing coplanar faces. If their intersection with an existing face is nonempty, the shared part must be split off from each, and constituted as a separate face. PIM now includes algorithms for each of these operations, and their associated book-keeping. However, not all of the corresponding deletion operations have yet been implemented satisfactorily. It includes a polygon package operating in two dimensions on a nondiscrete polygon description, since this is required for the coplanar face comparisons.
PROBLEMS OF N O N D I S C R E T E DESCRIPTIONS Compared with discrete descriptions, the type of nondiscrete description outlined above suffers from two interrelated problems:
volume 11 number 6 november 1979
F
G
A
L
H Figure 5. Zero thickness components are dependent upon the existence o f some real component
• The amount of information required to describe an individual element is vastly greater, since it must represent not only the internal attributes of the element, but also its relationship to its neighbours. • This greater amount of information must either be maintained as part of the building description as the user inputs it (the approach taken by House Design and PIM) or generated en bloc after input is finished (the approach of Site Layout). In either case the demand for computation is severe. A further problem is that component-based systems are effectively told of which component an element is an instance, whereas in many cases rat-trad systems must recognize the appropriate standard detail for a junction. In contexts where the number of possible details is large, this can prove very difficult. These problems have so far limited the application of nondiscrete techniques to sparse descriptions, and this has severely restricted the applicability of CAAD in the rat-trad context.
CONCLUSIONS Nondiscrete descriptive techniques have been used succesfully in certain rat-trad contexts to enable computers to produce bills of quantities. However, their wider introduction is dependent upon the development of powerful factoring techniques, similar to those already available for discrete descriptions, to reduce the amounts of descriptive data and the computational load of maintaining them.
ACKNOWLEDGEMENTS I should like to thank my colleagues, especially David Stone, the staff of Applied Research of Cambridge Ltd, especially Paul Richens, and the CEDAR team at the PSA for the stimulating discussions which led to this paper. This work was supported by the PSA (Property Services Agency) of the Department of the Environment, and by the Science Research Council.
327
REFERENCES I Hoskins , E 'The OXSYS system' in Gero, J (Ed) Computer applications in architecture Applied Science (1977)
2 SSHA 'CAAD at the Scottish Special Housing Association' SSHA (Spring 1976) 3 Bijl, A and Shawcross, G 'Housing site layout system' Comput. Aided Des. Vol 7 No 1 (January 1975) pp 2---10 4 Rosenthal, D 'PIM: underlying algorithms' EdCAAD Res. Rep. 79/15 (June 1979) 5 Nagy, G and Wagle, S 'Geographic data processing' ACM Comput. Surv. Vol 11 No 2 (June 1979)
6 Holmes, C 'Plastic merge procedure tor mosaics of polygons' Comput. Aided Des. Vo{ 10 No [ (January 1978) pp 57 64 7 Liardet M, Holmes, C and Rosenthal, D 'Input to CAD systems: two practical examples' Proc. IFIP Working Conf. on AI & PR in CAD Grenoble (March 1978) 8 Eastman, C and Henrion, M ' G L I D E - A language for design information systems' Comput. Graph. (ACM/ SIGGRAPH) Vol 11 No 2 (Summer 1977) 9 Baumgart, B G 'A polyhedron representation for computer vision' Proc. NCC (1975) l e Holmes, C 'Analysis of contiguities between polyhedra' EdCAAD Res. Rep. 79/04 (January 1979)
the new internmional c o w r i n g the ical, and social of computerinformation sy .m=s Coverage includes: etechniques =legislation tinformation systems oworking group activities ecase studies =computer misuse oorganizational aspects tcurrent interest section Annual subscription £40.00 ($104.00) Further details from: Marilyn Fitchett, IPC Science and Technology Press Ltd., PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 5BH, England Telephone: (0483) 31261 Telex: 859556 Scitec G
328
computer-aided design