# Great Neapolitan Earthquake of 1857/Part I. Ch. XIII

CHAPTER XIII.
EFFECTS OF SHOCK UPON BUILDINGS—THE 4TH MODIFYING CONDITION CONTINUED—RELATIONS OF ROOFING—MODES OF FALL.
As respects roofing of the ordinary class, of heavy timber framing, with or without common rafters superposed, or with heavy lathing only, to carry the ponderous ridge and furrow tiling already described; all that has been stated of the modes of giving way of flooring applies also, in like cases of wave transit, to it; with this addition, that it frequently happens, owing to the low pitch of Italian roofs, and the enormous weight of the tiling, added to the rudeness of workmanship in the framing, and the want of sufficient iron work in fastenings (iron being a dear commodity, and none made in the kingdom but by the old Catalan process, and all imported heavily loaded with duty); from all this, the roof frames often give way at the tye beams, and the principal rafters then thrust out the side walls, as the roof falls by the shock, if either emergent steeply or vertical. This is almost always the case when the roof is of considerable span, as over churches. In such roofs, "common rafters," are laid upon longitudinal "purlins," or beams stretching across the principals; and when the direction of wave transit is anything nearly horizontal, and in or near the line of the ridge of the roof the ​whole roof rocks endways upon its bearings at the level of the caves or "wall plates," and these purlins act as "battering rams" upon the gable walls, which they almost invariably carry away wholly or in part. A good illustration of this is afforded by Photog. No. 80, of the west end of the church at Picerno, where the common rafters are nearly all gone as well as the tiling, from the right hand side of the roof, and the front gable nearly all thrown down by the E. and W. rocking of the roof, and the inertia of the gables themselves. When the wave is nearly normal, and transverse to the ridge of one of those large heavy timbered roofs, with the ends of the principals resting directly upon the masonry, of the top of the side walls, or occasionally perhaps upon a heavy wall plate of ill-squared timber, the inertia of the whole is so enormous, that in almost every instance, the wall first reached by the wave was thrown outwards, by the shove from the roof and its own inertia together, and the whole roof then dropped nearly plumb down upon, the area of the building, crushing everything before it.
From this tiled roofing, however, whether carried away completely, or only disturbed upon the walls, very few seismometric observations of value, can be made. The chief of these consist, in the occasional indications afforded, by the amount of draw-out of timbers from their sockets in the walls, either longitudinal or transverse. The tiling itself, is so loose, the interstices between the overlaps so great, that it is very seldom partially disturbed, never probably in situ, unless by nearly vertical emergence of wave, it is either carried away altogether, or presents no signs of movements that can be distinguished, from the ordinary ​irregularities, looseness, and twistings, of the lines of tiles.
There are two other forms of roofing, viz. arched or groined, and dome roofs, which frequently occur in the better order of churches, and which become highly instructive. Upon these some observations will be necessary. The Neapolitan churches, which are nearly all either Lombardic, Roman, or Palladian, in style of architecture, are very commonly roofed with solid brick (or more rarely stone) arching. The monasteries and public civil buildings, are also very frequently arched, both in floors and roofs, and many of the better built and more modern villas and palazzi, are so likewise, more especially in the upper stories and corridors. Semi-cylindric arching, and hemispherical domes, are the more common forms of church covering of this character, built in brick and mortar, with timbering and tiling over that, or with solid stone sloping pavement, for the outer skin of the roof. Groining of intersecting cylindric arches, is not unusual, in the monasteries and civil buildings, and hemispherical doming, intersecting and connected by cylindric arch-bands, is often met with in the more recent churches. The arching in villa architecture is of comparatively late date, and executed in hollow pottery laid in mortar.
The inertia of all these forms of roofing is very great. Intersecting arched groining universally splits along the crowns of the arches, whenever the direction of wave transit is transverse or oblique, to the line of the axis or springing, and with moderate amount of emergence. If the wave be very abnormal, and the structure pretty large, transverse fissures at right angles or more or less oblique ​to the former also form. When the emergence is steep, three orders of fissures, if not more, are produced in large arched vaults, one along the crown, and two others parallel to it, and distant from 40° to 50° at either side of it. Where the lateral movement amounts to even a very few inches, the detached masses descend, between those standing at each abutment, enough to destroy equilibrium and either they fall through, and more or less, from both abutments follows, or the whole comes down together. When the emergence is steep, the two lateral fissures, are further removed from that at the crown, and a very moderate vertical shock, suffices to send outwards, both side walls or abutments, and the whole vault drops between.
The gable ends or semi-tambours of such cylindric vaults, consisting (essentially) of a semicircular plate of masonry resting upon its diameter, level with the vault springing, give out at top at both ends of such vaulted roofing, when the direction of wave transit is along the line of the axis or near it, forming a large fissure transversely, at the junction of the gable and vault ring (or near it), which is usually, most open at he end first reached by the wave, from reasons obvious from what has preceded; the difference is greater as the wave is more subnormal, as gravity conspires with inertia then, to bring out the first reached gable, but acts against inertia, in the second.
When the wave is subnormal and transverse to the axis of a cylindric vault, its greatest overthrowing power is exercised, when the angle of emergence is such, that the line of transit passing through the centre of gravity of the vault, (or of its transverse section), also passes through the joint (either at one side or the other of the crown), that is ​removed 45° from it, which, as the centre of gravity is about 0·64 $r$, the mean radius of the arch ring being $r$, gives an angle of emergence of about 15°. Hemispheric domes, are also most readily, fissured or overthrown, by a wave of small emergence. Where the emergence is steep, the fissures run in curved lines, transverse to the line of transit, and cross each other more or less, when viewed in the line of the vertical axis, and few or none of them pass through the crown. When apertures are formed (such as lights) in the dome, and especially if near its springing the fissures are directed from their upper angles, and great sector-shaped masses are dislocated. An almost vertical or completely vertical shock does not seem to affect domes at all as much, as horizontal or subnormal ones.
The directions of the fissures in any case, depend not merely upon the direction of the wave transit, but also upon the planes of the curving joints, of the structure and upon its details of construction, to such an extent, that general principles can only to a very limited extent be made available for deductive observation from domes.
The Photogs. Nos. 81 (82 Coll. Roy. Sot.), illustrate the general character of the fall of curvilinear roofing. No. 81 is of Tito Cathedral, where the emergence was steep, and the roofing a combination of cylindric vaulting and domes; it shows (see p. 99) the form of a very formidable double fissure in the crown of cylindric arching or vaulting, and the intermediate fragments given down by inertia; beyond this fragments of the lateral domes are visible, and more fully seen in No. 82 (Coll. Roy. Soc.). In No. 168 (see p. 296), the fall of the cylindric vaulting of the nave and chancel at Polla is seen, and its effects in fracturing and ​sending outwards the side walls at its fall. No. 84 (Coll. Roy. Soc.) is an exterior view of the apse of the church of St. Maria Maggiore at Vignola, where the wave was sub-abnormal, with moderate obliquity and emergence, and where the fragments of the tower and of parts of its conical roof, were projected on to the roof of the church.
Conical or prismatic roofs of this sort over towers, being generally of timber were not frequently disturbed; when fractured, however, they are so in ways extremely capricious and perplexing. The partial fall of the tiled roof of the apse at Vignola, was due to the drawing away of the heads of the principal rafters from their support by the movement of the curved walls consequent on the large fissures visible in them.
It may be remarked that generally no feature of architectural construction is more characterized by its destructive effects upon the remainder of the edifice when shaken than are those vaulted and domed roofs. Their inertia is enormous, the centre of gravity is high above the walls, and they are deficient in tenacity and flexibility. They therefore not only are dislocated and fractured separately, but their rocking to and fro as a whole on the tops of the walls leads to the destruction of the latter. Upon examining the gigantic ruins at Rome, of the Imperial Baths of Titus, Caracalla and Nero, &c., an eye that has become conversant with seismic observation, at once perceives that the destruction of these enormous edifices, was but little due to the feeble hand of the barbarian, and was mainly produced by the earthquakes that desolated the city between the fifth and ninth centuries, acting thus upon their massive vaults and domes of brick-work.