[ILLUSTRATION]
126. ROME. ROOF OF OLD ST. PETER'S RECORDED IN 1694 IN
CARLO FONTANA'S TEMPLUM VIATICANUM ET IPSIUS ORIGO. Detail of engraving same size as original
[after FONTANA, 1694, 99]
In publishing this design of what he refers to as "the trusses which sustained the
roof over the nave of Old St. Peter's" (LE INCAUALLATURE, CHE SOSTUEUANO LI
TETTI DELLA NAUE MAGGIORE . . . DEL ANTICA BASILICA VATICANA) Carlo Fontana
(1634-1714), disciple and collaborator of Lorenzo Bernini and architect in charge
of the Pontifical Office of Architects and Engineers, informs his readers that his
engraving was made after an "accurate drawing" (UN GIUSTO DISEGNO) tendered
him by an "informed person" (UNA PERSONA DILETTEUOLE); and that it was
because of the extraordinary constructional "sophistication" (INTELLIGENZA) as
well as the soundness of the timbers employed in these trusses that the roof of the
Constantinian basilica survived intact for so many centuries—to the extent that
when finally taken down, it was found to be in such good condition that its timbers
could be reassembled to sustain the roof of the Palazzo Farnese.
If timbers from the roof of Old St. Peter's were re-used in the Palazzo Farnese
(1546-1589) they must have come from the western half of the nave dismantled
by Bramante (1502) to make room for the construction of New St. Peter's. The
eastern half of the nave (closed off from the construction site by a provisional wall
under Paul III, 1534-1549) was demolished only in 1606 to make room for Carlo
Maderna's westward elongation of New St. Peter's. The roof timbers of this
portion were also re-used, this time for the Palazzo Borghese (1605-1621).
The names of the component members of the truss shown in Fontana's engraving
are enumerated on two scrolls which form part of the drawing. On these the tie
beams are referred to as CORDE MAGGIORI (B), the collar beams as CORDE
MINORI (C), the rafters as PARADOSSI (D), the center post suspended from the
apex of the truss as traue pendente adVso di monaco (E).
During the 12th to 13th centuries of its existence, the roof of the Constantinian
basilica of Old St. Peter's was, not surprisingly, in need of numerous repairs. A
complete account of them, including what in the sources is referred to as a "renovation"
by Pope Benedict XII (fecit fieri de novo tecta huius Basilicae sub anno
1341) is given by Michele Cerrati in Tiberii Alpharini, "De Basilica Vaticana
Antiquissima et Nova Structura" (STUDI E TESTI vol. 26 Rome, 1914, 13 note 2;
brought to my attention by my colleague Loren Partridge). There is no compelling
reason to presume that Benedict's renovation involved any basic changes in the
roof's design.
Fontana's rendering of the trusses of Old St. Peter's is in complete accord with
that which Vitruvius recommends for broad spans, except that all of the principal
members of the truss are doubled, and that the tie beams are fashioned in two
pieces, joined midway by an overlapping scarf joint. Owing to the extraordinary
width of the nave of Old St. Peter's (23.6 3m.), two-piece tie beams were necessary
since it would have been hard to find trees of sufficient height to yield single timbers
to span the whole space. Doubling all of the principal members was an extremely
wise constructional feature—probably the primary contribution to the longevity
of the Constantinian trusses—which evolved from the strategic function made of
the TRAVE PENDENTE. The scheme is a laminative one; a kind of truss-sandwich
is formed in which the structural components are assembled and joined in a function
that yields, in effect, a "pair" of trusses, but which is really a single homogeneous
creation of remarkable simplicity and purity of concept—revealing a mastery of static
mechanics that transcends Vitruvius and commands admiration today. Yet the design
does not seem to have found general acceptance. On the contrary, a medieval carpentry
truss, when it is impressive, gains our attention rather by its quaintness, its
intricacy of joinery and the complexity of its members.
The construction is ingenious. Transmitting the entire roof load to the two outer ends
of the tie beams, the principal rafters D-D are in compression and thus act as
columns as well as beams. Column action augmented by the deflection of beam
action is resisted by the horizontal strut C-C (collar beam) which functions in
compression. These minor chords support the rafter pairs midway in their span, a
construction that reduces the effective length of the rafters to approximately one-fourth
the nave span. Strut C-C is supported at mid-point by the vertical member
E-E (MONACO) which concurrently serves as a tension member to prevent sag in
the great lower chords. The scarf joint of these tie beams, tabled, locked, and
girdled by iron bands, prevents them from separating in the horizontal plane. In
contemplating the brilliance and simplicity of the design, remember that the wall-to-wall
span was above 84 feet—reflecting a state of theory of mechanics and
knowledge of structure existing in the 4th century!
