Great Neapolitan Earthquake of 1857/Part I. Ch. XIV
CHAPTER XIV.
EFFECT OF SHOCK ON BUILDINGS—FIFTH MODIFYING CONDITION—EFFECTS OF APERTURES ETC. IN WALLS.
We at length come to the fifth and last head of constructive modification, as affecting and affected by, the shock, viz., the effects of wall apertures, windows, doors, &c. A few words of illustration will be sufficient for this. Assuming the simplest case, as in Fig. 86, of a normal
wave; whenever the wall is pierced by one or several rectangular or other apertures, it may be viewed as divided into different segments such as that , , , each having a separate moment of stress of its own, and giving rise to a separate dynamic couple, the extremity of one arm being in the centre of gravity, , the other at the base and junction with the similar adjoining segments, the tendency of the oscillation being, to alter the angular position of each separate mass, and to produce separation from the others in directions parallel to , to , and to .
The fissures tend to form, as in a solid wall, perpendicular to the line of transit of the wave; but as in a 'solid wall (all of horizontal courses) these must also follow the joints, so must they here where the joints above all such rectangular apertures are those of arch voussoirs at various inclinations. Hence, when the wave is normal the fissures form through the nearest vertical arch joint generally, and through to the next aperture above or below, as in Fig. 87; but when subnormal the fractures are through
the joints of the voussoir nearest square to the line of transit, as in Fig. 88; and very generally in such a case the fissures run, from the angles of the apertures. Hence the angle made by the joint, of the plate-band arch or low segment arch, above a window or doorway, through which the fractures run, with the vertical, forms an approximate measure of the angle of emergence. The precise position of the fracture is of course varied by innumerable minuter conditions, such as variation of thickness in the wall, difference of strength at different points otherwise similar, and many others, which must be looked out for, when the phenomena are found perplexed.
Abnormal and subabnormal waves, produce like effects here, to normal and subnormal, except that they are by the former produced in two planes of walls, meeting at an angle usually right, and accompanied by disturbances transverse to the plane of each wall. When the wave is of very steep emergence or vertical, then diagonal fissures are produced in two directions crossing each other, and often accompanied by vertical fissures also, from causes obvious on reference to the statements already made as to the fissures in solid walls due to such waves.
A remarkable example of fissures of this sort will be found in a subsequent page, occurring at Polla. Their general character is that of Fig. 89.
These figures apply to wall apertures of the usual moderate size of windows and doors. When very much larger and wider, and covered at top with plate-band arches or stone lintels, two or more fissures often take place, by the opposite movements of the first and second semivibration of the wave, and large fragments fall out.
The usual character of fracture in arches of considerable curvature may be illustrated by the Photog. No. 90 (Coll. Roy. Soc.); but many others will occur in the succeeding pages. When the 'width of arch aperture is very considerable-eighteen, to twenty or thirty feet for example—and a large mass of wall overhead, fractures transverse to the plane of the wall, usually occur not only at or near the crown of the arch, but (as in the case of roof vaults) 40° 50° even more, at either side of it, and the mass above probably descends more or less, and then secondary diagonal fissures are produced by its descent, which have no relation to the angle of emergence of the wave, and must not be confounded. as indicatory of it, with fissures. previously produced. An example of this will be recorded in the city of Naples.
Very steep emergence with large semicircular arches, usually produces two sets of fractures also, as in Fig. 91.
Some examples will be given in the narrative also of other singular secondary effects upon the stones and voussoirs of arches by continued oscillation produced by normal or subnormal waves.
When the angle of emergence becomes extremely steep it may occasionally be observed that little or no trace of obliquity of fissure is to be found. The wave, in fact, emerges so nearly vertical that its horizontal elements are those mainly effective in dislocation, and produce only fissures by resolution, where there are piers and window apertures horizontal ones, as in Fig. 92.
Stone staircases, the steps of which are bedded into the walls, often produce most complicated effects, both by primary and secondary fracture, and, as objects for deductive information, should usually be avoided.
The choice of buildings best suited for observation in an earthquake region, will have been discerned from what has been stated, though much must always depend upon the observational power and sharpness of the observer, and something upon prior experience (acquirable, however, in a very few days' work). Buildings of the simplest character, large, well-built, not too much injured, and cardinal, are the most important points.
There are many detached objects the observation of which can afford valuable results, in reference to the path of the wave and the direction of its transit, besides the dislocations of the shell of buildings; such as the swing of lamps or candelabra, of hung pictures, &c., and of twisted objects, such as vases, obelisks, &c., which, however, do not demand detailed consideration in this place. Several examples occur in Part II.