aesthetically important; it is structurally critical when arcades are connected
by vaults to create the walkways—aisles and ambulatories—as well as the nave
ceilings of a church, which severely restricted the size and shape of the bays,
the structural units of a church. A pointed arch has an arbitrary shape that
can be adjusted for different spacing between columns while maintaining a
constant height, permitting the architect to create variable shapes, sizes, and
arrangement of supports.
Ribs are a second component of the Gothic system. Earlier, Romanesque
vaults, typically round in cross-section, were mostly smooth on their underside.
Flat bands (called douhleaux) were sometimes introduced above columns
to direct part of the weight of the vaults onto the columns and from them
to the outer walls of the churches. Because the vaults exerted strong forces
that pushed out the top of the columns and outer walls, these needed to be
thick and buttressed by other walls or they would be overturned. A pointed
arch and a pointed vault, because of their geometry, press almost straight
down on supporting walls or columns, reducing or eliminating the overturning
forces. If all these arches and the arched intersection of vaults (groins))
are connected with thin stone ribs, all the weight and other forces of the
building can be concentrated at points; that is, onto relatively thin columns
or piers. Thick columns (piers) and walls can be largely eliminated, making
possible the third innovation: point-support or skeleton construction, the
same sort of structural frame that made large buildings like skyscrapers possible.
A Gothic structure, such as that at St. Denis, resembles the skeleton of a
mammal. The ribs of the church carried forces exerted on them to a multitude
of columns much as the ribs of a mammal's chest disperse forces on them
to the spine and from there to the ground by way of the legs. Just as the tissue
between ribs are thin webs of muscle and skin, the vaults between the ribs
in a Gothic structure are thin panels (also called webs) of stone, and the walls
between the columns on the exterior of the building can be thin tissues of
glass. Structurally, a Gothic church dances lightly on its columns, whereas the
typical Romanesque church sits solidly on walls and thick piers. This difference
results in a dramatic reduction in the amount of stone needed to support
a Gothic building. Psychologically as well as physically, a Gothic church
built on this system gives the sensation of lightness.
In the chancel of St. Denis, its architect arranged seven chapels along lines
radiating from the center of the apse, effectively the high altar, and added two
more to make a transition from the rounded end to the straight sides of the
chancel. The new ribbed groin vaults covered these chapels as well as the
ambulatory, the passageway between them and the apse containing the high
altar. Views through the resulting layers of space from the chancel to the
outer walls, which are filled with some of the oldest stained-glass windows
in France, are dramatic and entrancing.