1506

THEORY AND PRACTICE OF ENGINEERING.

Book II.

very thick abutments, but it is evident that very flattened arches tend principally not to over-throw their abutments, but to produce a horizontal disjunction in their piers, and causetheir upper parts to slide. It results from hence that the mass of the abutments situatedbelow the point where this disjunction would take place is scarcely of any utility tothe solidity of the arch, and only serves to support the upper part, which alone resists thethrust efficiently ; consequently means have been sought to avoid the construction of thismass, which occasions a useless expense. Among the plans proposed for this object arethe two following: in the first the abutment is only an ordinary wall, behind which thearch is prolonged, and abuts against a platform supported by inclined piers; in thesecond, there is substituted for the mass of the abutment, walls constructed in the prolongingof the heads, which support an arch, the summit of which is placed a little below the levelof the springing of the main arch; this latter method appears to unite all the advantageswhich can be obtained in such constructions.

It has also been the practice to incline the platform of the foundations, as well as thebeds of the masonry, to the side of the arch to be supported; and there is no doubt thatby means of this arrangement, the mass of the abutment is considerably diminished:it would also be very advantageous to distribute in the interior of the masonry roughstones placed upright, which should bind the beds together, and contribute effectually toprevent disjunctions.

In all that has preceded, we have supposed that the haunches of the arches were filledin with masonry, levelled even with the summit of the curve of the intrados, or followinga slight inclination from this summit; it does not appear necessary to fill the haunchesof an arch entirely in this manner, which augments the expense, as well as the loadit must support: if the haunches be filled with earth or gravel, besides the loadbeing very little less, the arch would be exposed to dangerous actions when thesematters should be penetrated and diluted by the wateV which would filter throughthem. To prevent these inconveniences, it is advantageous, conformable to a customgenerally practised, after having levelled the masonry of the haunches to an inclinationtowards the piers and the abutments, to a height sufficient to ensure the equilibrium of thearch, to raise on this masonry vertical walls parallel with the heads, to which from 18inches to 3 feet in thickness may be given, placing them 2 feet 3 inches apart: theintervals of these walls remain void, and are covered with landings of stone or smallarches, in order that they may have little thrust, and on which the pavement rests: careis moreover taken to contrive at the foot of the walls, or in the masonry which supportsthem, small openings, by means of which the water which would penetrate the haunchesmay be conducted to a single point, whence it runs away by a pipe placed across thevoussoirs. In adopting an arrangement of this kind, passages may be - reserved for theconvenience of viewing the construction, and making the necessary reparations, to preventthe changes which time might produce in the masonry of the arch: where, however, voidspaces exist in the haunches of the arches of a bridge, provision must be made for anaccident which might take place, by a total immersion of these arches in the water.Bridges constructed in mountainous countries, on rivers or torrents subject to great floods,or near the sea, where the tide rises to a great height, are exposed to the chance of thelevel of the water surmounting the summit of the bridge, or even rising above thepavement; in this case it might happen, that on account of the voids left in the haunches,the weight of the construction would become inferior to that of the volume of waterit would displace, and would then be exposed to be vertically raised and thrown downby the current, if the difference of the two weights were sufficiently great to surmountthe forces of adhesion which would be opposed to this movement.

The loss of weight undergone by masonry plunged in the water should always betaken into consideration; regard should above all be paid to it when the dimensions ofthe piers of a bridge are regulated, with the intention of rendering them capable of servingas abutments, and of resisting the thrust of the arches they support.

Thickness of Piers . — The piers of bridges may be considered under two principal pointsof view; as destined to carry the weight of the arches, or to serve as abutments to resisttheir thrtist. In ancient works they are always of a considerable thickness, and thebridges of Vicenza and Padua are almost the only exceptions from this practice.

Bridges of the middle ages, like the ancient, are constructed on piers thicker than wouldbe necessary for the uses of an abutment: when the engineers submitted the thrusts ofarches to calculation, and determined according to the hypothesis of Lahire the thicknessnecessary for the abutments, they thought that the piers should have the same thickness;and it was on this principle that the greater number of large bridges were constructedin the last century, whether in France or the other countries of Europe . The piersof the bridges of Blois, Saumur, Orleans, Moulins , Tours, Westminster, &c., were thickenough to serve as abutments, and the vaults were always centred one after the other;it would, without doubt, have been imprudent to expose bridges, composed of so great anumber of arches, to be entirely overthrown by the failing of one of the piers.