Arch Bands and Vault Structural Strength
The role of groin and transverse arch bands is often misunderstood, attributing them a purely decorative function (Willemsen 1984, 12 and 14).
This perception is reinforced in cases where the groin arches have accidentally fallen off over time, leaving the web intact, which is the case for some vaults at Castel del Monte, figure 20.
Figure 20. Room at Castel del Monte with the groin and transverse rib arches fallen off.
The groin and transverse stone arch bands have a primary role in the erection process, providing a foundation and a guide for the web formation. After the web erection is completed and the construction timberwork is removed, they take on the more limited role of armatures, structural reinforcements; they also serve decoratively to cover over the joints between web sections.
Because of the cusped shell construction, the web sections above the groin arches become ultimately much stronger structural elements than the groin arch bands. Fitchen discusses in depth the superior strength achieved by shell structures with two-dimensional curvatures. (Fitchen 1961, chap. 3.)
Leaving aside complex physical explanations, the strength of shells structures can be experienced in a simple experiment with a business card. Holding the card from two opposite edges between the fingers of one hand, the card flexes with no effort when the fingers squeeze on the card. Next, squeeze gently the card from the two free sides using the fingers of the other hand, putting a slight curvature on the card. Now, squeezing the card as done before finds much more resistance. The card shows extremely high buckling strength when curved in the direction perpendicular to the sides being squeezed. Two dimensional curvatures give even more strength to shell structures, such as in the case of dome vaults.
More strength is added to a cross vault by the cusps formed above the groins arch bands by the joint between two adjacent web surfaces.
As noticeable in figure 20, the builders took great care in precisely dove-tailing the courses from adjacent web sections, a measure that binds the two shell structures, giving much more strength to the finished cross vault webs. The web merge line above the transverse arch has instead a wandering and shoddy look. These are clear indications that medieval builders had a good appreciation of these structural functions by the web shells versus the arch bands.
Noticeable also is the fact that there is no masonry feature along the groin arches that indicates the thought of merging the web masonry with the groin arch bands, which would increase the resulting strength of the finished vault. The web masonry at the groin edge is shaved as a perfect smooth surface to mate with the extrados surface of the groin arch bands. This is consistent with the outlined construction approach where the groin arch stone bands were intended by medieval builders as construction forms on which the webs were erected.
The primary role of the arch bands in the erection process is missed when studying these structures, centuries after they were built and the technology itself is lost in time. These armature bands can be removed together with the temporary support timberwork after the vaulting is complete. The structural impairment to the web is seemingly insignificant; they keep on the role of armatures that stabilizes the web, especially for the transverse arch bands.
Indeed the vault is less stable at the transverse arch crown, compared to the cross vault center where the crown is better stabilized horizontally by the two arches intersecting at right angle. A closing wall at the location of the transverse arch, a spandrel wall above the transverse arch band, or fill masonry above the web are other techniques used to stabilize and stiffen the vault web above the transverse arches.
These arch bands, both the diagonal and transverse arches, become part of the decorations after the construction is complete.