Carnton Mansion E.A. Johnson Center for Historic Preservation, Middle Tennessee State University, Murfreesboro, Tennessee, USA INTRODUCTION Efforts to describe and conserve historic buildings often require understanding complex spatial relationships; fortunately, these relationships can be easily visualized using three dimensional computer modelling. The Center for Historic Preservation recently developed a computer model of Carnton Mansion, a nineteenth century residential building located near Nashville, Tennessee; the building is associated with the American Civil War. This public service project involved determining the original exterior appearance of a missing wing once attached to Carnton Mansion. Today Carnton Mansion is operated as a historic house museum. The governing board of Carnton Mansion, which is considering the possible physical reconstruction of the missing wing to augment the museum's interpretive and operational programs, asked the Center for assistance in determining the missing wing's probable historic appearance. The Center, affiliated with Middle Tennessee State University, located in Murfreesboro, Tennessee, USA, is an instructional and public service research institute which complements the University's Historic Preservation Program and serves heritage-related organizations and agencies at all levels. This paper describes the Center's methodology involved in developing a computer model of the missing wing based on a variety of historical evidence. THE EVIDENCE FOR THE MISSING WING The surviving evidence for the missing wing consists of a variety of photographic, archaeological, and existing site information previously assembled by the author [1]. As shown on the south elevation drawing in Figure 1, the missing wing (drawn with dotted lines) once stood between two extant buildings: Carnton Mansion to the west (on the left) and a small smokehouse to the east (on the right). Two surviving historic photographs show only a small portion of the western (left in Figure 1) end of the wing, but these do firmly establish several aspects of the wing's appearance. Shuttered, double hung sash windows (nine over six panes) are clearly visible on both stories. The brick wall, brick
Carnton Mansion Overall configuration and chimney location based on evidence from walls of extant buildings and from archaeological remains on o 2 p 5' nt o CD O "-*} ac Windows visible in historic photograph Figure 1: Evidence for Missing Wing Window placement based on other similar buildings in Tennessee Bt OQ on
Structural Repair and Maintenance of Historical Buildings 645 cornice, and the (presumably wooden shingled) roof surface are also visible but obscure due to poor image quality. As shown on the east facade elevation drawing in Figure 2, a "ghost" of the wing's west gable wall remains visible on the extant east wall of the Mansion. This "ghost" consists of disturbances in the brick work as well as some surviving plaster and paint materials. It establishes the width of the wing (about nineteen feet) and such details of gable roof design as the roof angle and an exact cross-section of the brick cornice. With respect to the wing's conjectural east gable wall, the disturbed brick work and plaster remnants on the west wall of the smokehouse suggests that the existing smokehouse wall at one time was actually incorporated within the missing wing's wall (i.e., the wing and smokehouse shared a single common wall). There is another, less substantial "ghost" on the north side of the wing which probably represents a porch structure. Unfortunately, there is currently no other evidence to establish the nature of the wing's porch, so further consideration of the porch is omitted in this paper. Scattered stones at ground level indicate the probable location of the missing wing's wall foundations. As shown by the plan drawing in Figure 3, limited archaeological excavation conducted in 1987 by Steven Ruple [2] confirmed the location of a portion of the north wall foundation. The north wall appears to be approximately fifty three feet long. Unfortunately, the south wall foundation had been disturbed in subsequent trenching for utilities installation. This archaeological work also uncovered the foundation of a substantial chimney which probably originally contained a large fireplace to serve the cooking needs of Carnton Mansion. Obviously, further excavation of the site should be undertaken to uncover additional details regarding the missing wing's configuration and use. A particularly thorny problem lies in determining the number and location of doors and windows in the missing wing. It seems reasonable to infer that the north elevation of the wing contained exterior doors, given the porch "ghost" on this side and the nature of the ground elevation. However, it is necessary to postpone further speculation about the exact location of those openings as well as the configuration of the interior rooms, pending further archaeological exploration. Speculation about the south elevation of the wing seems more worthwhile, if we presume that this elevation contained additional windows similar to those in the historic photographs. Our conjectural version of the missing wing thus acquires four pairs of window on its south elevation, possibly located as shown in Figure 1. This window arrangement roughly parallels the configuration of surviving wings attached to several other early nineteenth century residential buildings in Middle Tennessee. Therefore, based on analysis of the available historical evidence, the probable original appearance of the missing wing begins to emerge. The wing's four-bay south facade appears in Figure 1. It's gable end wall elevation follows the shape of the "ghost" in Figure 2. The basic floor plan appears in Figure 3 (although interior room layout remains uncertain). However, these two dimensional drawings largely fail to provide the viewer with an accurate sense of the wing's architectural massing, especially as it relates to the associated Mansion and the smokehouse. To provide a better vehicle for
646 Structural Repair and Maintenance of Historical Buildings Wing "Ghost" Porch "Ghost" Figure 2: Wing "Ghost" on Mansion East Wall Conjectural locations of missing walls Excavated area with foundations of chimney and north wall Mansion Missing Wing Smokehouse Figure 3: Archaeological Excavation of Foundations
Structural Repair and Maintenance of Historical Buildings 647 presentation of these spatial relationships, the Center turned to three dimensional computer modelling. THE COMPUTER MODEL OF THE MISSING WING The Center's computer modelling relies on a three dimensional (3D) version of computer-aided-drafting (CAD) software running in a modest microcomputer environment [3]. Successive oblique, isometric, or perspective projections of a 3D computer model (with hidden lines removed and rendered surfaces) create the illusion of a scale model which can be manipulated at will by the viewer. Moreover, these projections can be reproduced on paper at any convenient scale for publication. The construction of a 3D computer model begins with the importation of two dimensional (2D) CAD elevation and plan drawings - such as those in Figures 1 through 3 - into the 3D CAD environment. The 3D model can then be rapidly constructed by combining selected features of the 2D drawings or by extruding 2D features into 3D shapes. (Alternatively, the model can be completely constructed within 3D CAD using measured dimensions.) Related features are drawn on different "layers" so that various parts of the building can be made invisible when desired. Once the model is complete, numerous projections can be generated at a rapid pace which would be extremely difficult using manual drafting methods. The Center constructed a simplified 3D model of Carnton Mansion designed to illustrate massing relationships among various components of the total complex. Figures 4 and 5 present isometric views of that model; Figure 4 illustrates the current appearance of Carnton Mansion while Figure 5 reconstructs the missing wing in its original relationship to the Mansion and the smokehouse. (Actually, Figure 5 represents the entire computer model; Figure 4 was obtained by simply hiding the layer containing the wing.) Perspective views are notoriously difficult to produce using manual methods; however, perspective projections can be quickly generated using 3D CAD. For example, Figure 6 presents a perspective view of the southeast corner of Carton Mansion and its reconstructed wing, while Figure 7 presents a perspective view of the southwest corner. These perspective projections accurately simulate views which would be experienced if the wing actually existed. The computer model of Carton Mansion omits many architectural details since they are not necessary to depict the overall massing of the building. For example, most of the bracket and moulding details in the cornices are omitted; likewise, much of the porch detail has been greatly simplified, as have the window treatments. As this approach indicates, the level of detail should be adjusted to suit the needs of the particular modelling project. Indeed, when necessary, it is possible to construct extremely detailed models which have a photo-realistic quality, including amazingly accurate light reflections. However, more extensive detail requires more production time and may also require more advanced hardware and software. Although this project involves only exterior features, it should be mentioned that the Center has also developed complex models which depict
8L &CD 2. r-k 3' 0> I O o" dd Figure 4: Extant Buildings Isometric View of Computer Model Figure 5: Reconstructed Wing Isometric View of Computer Model CTQ
Structural Repair and Maintenance of Historical Buildings 649 Figure 6: Southeast Perspective View Computer Model with Reconstructed Wing Figure 7: Southwest Perspective View Computer Model with Reconstructed Wing
650 Structural Repair and Maintenance of Historical Buildings interior space configurations. This approach can have great advantages: for example, spatial relationships between several rooms or between features with a single room can be more clearly portrayed than is commonly feasible using only two dimensional drawings. Computer models provide an alternative to the traditional use of photographs and physical scale models to depict spatial relationships. While it is true that photographs are more quickly produced and inherently record more information, they can only depict what actually exists - obviously, this limitation clearly applies to the problem of depicting the missing wing at Carnton Mansion. Physical scale models do provide excellent depictions of spatial relationships; however, they are very expensive and time-consuming to produce. In addition, physical models cannot be as readily distributed in mass to multiple viewers; on the other hand, photographs and 2D projections of 3D models are readily printable, and even the 3D computer model itself can be easily distributed by diskette (or other electronic transmission) for use on other computers. Therefore it appears that the computer model often proves to be the best approach for the depiction of spatial relationships, especially when it is necessary to depict a historical object which no longer exists in actuality. CONCLUSION AND IMPLICATIONS FOR FUTURE WORK The development of a 3D computer model of the missing wing of Carnton Mansion demonstrates the viability of computer modelling as an approach to depicting the spatial relationships often encountered in preservation work. The computer model is relatively easy to construct using modest microcomputer resources, and any desired projection of the model can be rapidly generated and easily distributed to appropriate viewers. Finally, mention should be made of the future potential of computer modelling of historic buildings. A series of related perspective projections similar to those in Figures 6 and 7 can be shown in rapid succession to provide a sense of walking around the building - this process, when refined, becomes animation. Although this process has just recently become available on the microcomputer level, future developments will probably provide real-time animation capabilities in a user-friendly and economical microcomputer environment. Combined with new types of computer interfaces, such models may ultimately be incorporated in the nascent "virtual reality" movement. REFERENCES 1. Johnson, E. A., The Missing Wing of Carnton Mansion: Conjectural Appearance and Suggestions for Additional Research, Center for Historic Preservation, Murfreesboro, Tennessee, USA, 1991. 2. Ruple, S. D., Archaeological Excavations of Two Outbuildings of Carnton, a Plantation in Williamson County, Tennessee, Nashville, Tennessee, USA, 1987. 3. Autodesk Generic CADD version 6.0 and Generic 3D Drafting installed on an IBM PS/2 Model 80 microcomputer based on the Intel 80386/20Mhz microprocessor, with output via a Hewlett-Packard LaserJet III laser printer.