Great Neapolitan Earthquake of 1857/Part I. Ch. X

by Robert Mallet

CHAPTER X.

EFFECTS DUE TO FLEXIBILITY AND ELASTICITY OF THE MATERIALS IN BUILDINGS—FLEXIBILITY OF BRICKWORK.

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All the preceding observations of course have taken no account as yet of the reactions produced on the walls by roofs and floors: they refer to the walls considered as standing alone. The actual extent of elastic flexibility of stone and brick masonry, especially of the former; is not commonly considerable; and unfortunately, as yet, no precise measures of these exist for any class of masonry. Were it not for this property, however, no building would stand, even a very moderate shock; and were the velocity of the wave confined within the limits of the velocity of the centre of oscillation of the structure, considered as an elastic compound pendulum, whose time of vibration is due to the length of a simple pendulum equal to the height of that centre above the base, and were the amplitude of the shock within the limit of elastic displacement of the masonry, &c., at that centre, no building would be thrown down.

A well-constructed brick and mortar wall, of 30 or 40 years' induration, and 40 feet in height, unsupported, of two bricks, or 1·60 feet in thickness, has been observed by myself, to vibrate nearly 2 feet transversely at the top, before it fell, in a storm of wind; and that not until after many such oscillations had disintegrated many of the horizontal joints, and produced several vertical fractures. The point of greatest flexion traversed along the length of the wall, as each oblique gust of wind impinged upon it, like the waves of a rope suspended from one end, and jerked transversely at the other.

An octagonal brick chimney stalk, with a heavy granite capping 160 feet in height above the ground, and 15 feet diameter at the base, was observed by me, instrumentally, to vibrate in a moderate gale of wind, when a few months built, nearly 5 inches at the top.

These are illustrations of the extent of flexibility in good brickwork, which possesses it in a far higher degree than stone masonry, the bond of the mortar being better, the flexibility greater, both in the brick and thick mortar joints, these very numerous, and the elasticity more nearly alike in both, than in stone masonry. When the joints are much fewer in proportion, the stone relatively to the mortar, highly elastic and rigid, and the bond, so far as adhesion of the mortar is concerned, small, (indeed, in the case of many hard, siliceous stones, such as granite, almost nil,) the result of this difference is, that a well-built and indurated brick wall, when fractured, breaks indifferently nearly, through joints and bricks; but in stone walls, the line of fracture is confined to the mortar joints, with rare exceptions, the rigidity of the several blocks, transferring the whole of the compressions and extensions due to the strains to the mortar alone. From this cause, it was observed very uniformly throughout this earthquake region, that when brick construction was superimposed upon stone

Photo Pl. 63

Vincent Brooks, lith. London

At Polla

work, as not unusual in churches, the brick-work, although of so much less density, fell as one mass, with fractures of severance along the lines of junction of the two; and vice versâ, when the brick-work, as in a few cases, was beneath, and stone-work above, and when the latter was thrown, if it did not push the brick-work over in its fall, the latter remained comparatively unharmed.

The limit of flexibility of stone masonry exposed to earthquake shocks depends, to an immense extent, upon the flatness and superficial area of the beds of the individual stones, and the completeness with which "breaking joint" and "thorough bonding" are preserved in the setting.

When the masonry consisted of rounded, lumpy, quadrated ovoïds, of soft limestone, as already mentioned in the general description of the poorer and older towns, and of which the Photog. No. 63, of a part of Polla is an example, the whole dislocation occurred through the enormously thick, ill-filled mortar joints; and almost all buildings thus formed, fell together at the first movement, in indistinguishable ruin. In the Photog. No. 64 (Coll. Roy. Soc.) of Pertosa, a poor, but more modern town, the class of masonry was a little better, and, as may be remarked, the ruin less complete.

Where, as in a few examples observed, the masonry was of the best class (and such as would be so recognized in England), the buildings thus constructed, stood absolutely uninjured in the midst of chaotic ruin. Some examples of this will be found in the second part—none more striking than that of the Campanile of Átena, a square tower of about 90 feet in height, and 22 feet square at the base, in which there was not even a fissure, while all around nearly was prostrate. This tower was, however, also aided by iron chain bars, built in at each story. The great viaduct carrying the military road at Campostrina is another example. Indeed, it was evident, that had the towns generally been substantially and well built, or, rather, the materials scientifically put together, very few buildings would have been actually shaken down, even in those localities where the shocks were most violent, and their directions the most destructive. Thus the frightful loss of life and limb, were as much to be attributed to the ignorance and imperfection displayed, in the domestic architecture of the people, as to the unhappy natural condition of their country, as respects earthquake.

In a wall of parallelopipedal blocks, properly overlapping and breaking joint, the aggregate tenacity, of a vertical serrated transverse line of joints, may be represented, as Professor Rankine has shown, by an equation of the form

$\mathrm {T} =n{\frac {n+1}{2}}f,\delta$ x $bht$

the last letters being the dimensions of the wall at the serrated section: $n$, the number of courses; $f$, the co-efficient of friction, which may equally be taken as the co-efficient of adherence of the mortar, irrespective of its own coherence, and $d$, the specific gravity of the stone or brick. Unfortunately, we still need better experimental data as to the adhesion of mortar, in directions both parallel, and transverse, to the bed surfaces, to enable us to apply the numerical results to earthquake-applied strains producing vertical or inclined serrated fissures. The like difficulties do not arise with horizontal transverse fractures.

The strain is here applied almost with the rapidity of a blow. Almost the whole stress falls instantly upon the less elastic mortar joints, at their surfaces of contact with the stone, when exposed to direct pull, and the mortar joint parts off from the stone with a resistance of only one-half that due to its statical adherence, or to its statical coherence. This fact is rendered familiar to the senses, by the facility with which two bricks from an indurated building, that would require a slowly applied load of perhaps half a ton to tear them directly asunder, may be caused to part and drop asunder, by a slight blow from a hammer upon one of the bricks while the other is held in the hand. As applied to our subject, this sufficiently indicates, that the portion of the total force of shock required to produce fissure, or horizontal fracture of the base, of the severed masses, to permit overthrow, is, when these are large, relatively very small; so much so, where the masses, are large in relation to the surfaces of fracture, and the co-efficient $f$, of adherence very low as in the case of the Neapolitan provincial mortar, that it may be frequently neglected in calculations of seismic statistity. At p. 139, et seq., will be found the method of calculating the velocity due to fracture of the horizontal mortar joints, at the base of walls overthrown, which is the most important and frequent case of fracture that occurs, in seismometric observation.