CASE STUDY 

Building Description 

The Rotunda, Exterior Lawn Side  The Rotunda (1826) at the Academical Village of the University of Virginia was designed by Thomas Jefferson to be the centerpiece of his celebrated campus plan. It was the library, the most important building at the university, the repository of knowledge. As such, it occupied the central location between the two long arcades composed of faculty pavilions and student rooms. Jefferson modeled his Rotunda after the Pantheon in Rome reducing its scale by one half. Unlike the Pantheon, it has two intermediate floors composed of oval rooms with the dome room at the top surrounded by niches for books. The 72 foot diameter of the space encloses approximately 4070 square feet.

Dome Room Interior, c. mid 19th Century  As the university grew so did the book collection resulting in an addition on the north side designed by Robert Mills in 1850. It burned in 1895, and Stanford White remodeled the Rotunda by taking out the intermediate floors to make it truer to its Pantheon precedent. For the United States bicentennial in 1976, the Rotunda was remodeled again with the intention of returning it to the original Jefferson scheme in its spatial configuration. However, mechanical systems and artificial lighting were discretely added. The building now accommodates a variety of uses; lectures, meetings, symposia, dinners and tours

Oculus in Rotunda Dome  Daylight is admitted to the Rotunda dome room through a central overhead oculus and 14 double hung windows around the perimeter. The conical skylight, 16 feet in diameter, admits daylight uniformly from all directions of the sky dome. The 7 foot by 4 foot clear glazed windows (85% transmission) create a perimeter zone of daylight under the circular gallery. However, the daylighting conditions in this perimeter zone are not uniform due to porticos at the north and south sides which shade the windows, fireplaces in lieu of windows on the east and west sides and trees of varied size and spacing which obstruct the daylight. The windows are placed on 18 degree radii and set within deep splayed recesses 0f 68% reflectance. The dome lined, with white metal acoustical panels, has a reflectance of 62% and the heart pine floor has a reflectance of 28%.

 

Measurements  

Rotunda Plan with Readings Grid  Footcandle readings were taken at 9:00am, noon and 3:00pm under both cloudy and sunny skies. Readings were taken at waist level on a radial grid as shown on the plans. Although the daylighting from the overcast, cloudy sky appeared visually to be unchanging, the illuminance meter indicated significant changing conditions throughout the time readings were taken The absolute amount of change was not great but at low light levels it was a factor of two or three.  The illuminance values in the dome room under a cloudy sky were quite uniform along circumferential points. There was a uniform gradient of 1:2 from the perimeter to the center. The levels in the reading areas under the gallery were quite variable.                    9:00 AM  12:00 Noon 3:00 PM Cloudy:  Sunny:

Rotunda Section  The absolute illuminance values in the dome room increased under a sunny sky but without reaching excessive levels. The approximately 1:2 gradient from perimeter to center along the radials still pertained. Values around the perimeter were always higher in one location due to the sun's position in the sky at different times of the day. The direct sun only reaches the dome room floor when it is above 56 degrees in the sky, the angle between the oculus edge and the column base. In Charlottesville, this only occurs at any time during the day from April 1 to September 12. For most of the year, therefore, there is no "hot spot" sun problem.

 

Analysis 

The design of the dome room is simple in concept but sophisticated in its realization creating a uniform daylighting environment. The oculus is balanced in terms of glare by the smooth curved vault leading up to it. All available light that enters the space bounces off of this ceiling and back toward the center thus distributing the light throughout The reading alcoves under the gallery provide enough room for a study desk at the window between the book shelves. The white wood trim and books reflect the light and diffuse it to reduce glare conditions. The deep window splays of white wood help to reduce the brightness contrast with the glazing to a comfortable visual level. 

Subjective impressions were recorded at the beginning and end of the day. 

Under a cloudy sky they were as follows: 
 

pleasant unpleasant light dark cheerful gloomy uniform varied comfortable uncomfortable

 

Subjective impressions under a sunny sky were as follows: 
 

pleasant unpleasant light dark cheerful gloomy uniform varied comfortable uncomfortable

 

The daylighting in the dome room of the Rotunda is a triumph of quality over quantity. In the central space, the illuminance values are within comfortable range for reading under sunny skies, but somewhat low for this purpose under cloudy skies. The perimeter zone is more problematic due to the large trees which were not there during Jefferson's time. There are no glare problems due to brightness contrasts except for the hot spot of the sun shining on the dome wall. However, it is the diffuse distribution of daylight that makes the strongest visual impression. Daylight suffuses the space under all sky conditions by lighting the great domed surface thus beautifully rendering the space. This effect allows visitors and users to appreciate the dome room as a single cohesive space which achieves the designer's intention of creating a symbolic yet functional centerpiece for the University of Virginia.  

 
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