ECONOMICS OF BRIDGE-BUILDING.
THE condition of Westminster Bridge naturally leads to a consideration of the other old bridges over the Thames in and near the Metropolis ; all of which were more or less faulty in their structure. Old London Bridge was cumbrous, unhandsome, and a perilous obstruction to navi- gation ; and the attempts to remedy the serious inconveniences it occa. sioned cost more than building the new one. Both Westminster and Blackfriars—the last of comparatively recent date—have required the expenditure of large sums to remedy the defects of their construction; and those at Putney and Battersea are dangerous and unsightly nui- sances, that can only be abated by rebuilding. To account for this state of things, we had recourse to the treatise of Professor HOSKIN% before referred to ; and there we found that it was occasioned by an imperfect acquaintance with the scientific principles of bridge-building. Nor, it would seem, have those principles been completely carried out by later engineer-architects. The aim of the bridge-builder should be to carry across the river a
road, as little elevated as may be above the shore, with the least possible obstruction to the course of the stream. To accomplish this object, he must solve the problem of producing a durable structure raised on the fewest and smallest points of support ; the arches being as wide in their span and flat in their curve as is consistent with strength. Suspension- bridges fulfil these conditions most completely ; the suspension-bridge at Hammersmith, for instance, being little if any obstruction to the river, and having the least elevation above the shore of any over the Thames. Standard bridges of iron admit of arches of wide span and slight elevation, being constructed on the principle of beams tied toge- ther : of this-the Southwark Bridge is an example, though its rise is unnecessarily steep. But neither kind of iron bridge is so solid and lasting as one of masonry ; nor so suitable for a great city where the traffic is heavy and incessant. Stone, especially granite, is the most du- rable, and therefore the fittest material for the bridges of a metropolis ; and was judiciously chosen by RENNIE for those of London and Waterloo.
The ancients in building a bridge made every pier massive enough to
serve as a buttress to resist the thrust of the arch it contributed to sup- port ; so that if one or more arches were broken down, the rest would stand as firm as ever. The principle of construction acted upon by modern architects is to consider the piers merely as supports to the arches ; the abutments at each end resisting the thrust of the whole series of arches. But they do not fully carry out this principle : their piers are wider and more massive than they need be for the arches to rest upon ; though not strong enough to act as buttresses. Thus the course of the stream is needlessly obstructed, to the detriment both of the navigation and of the structure itself. This new principle was first enunciated in 1716, by GAUTIER ; who lays it down as an axiom, that " the piers of bridges support only half the masonry of the two arches at their sides." PERRONET restated the principle, which he carried into effect in his bridge at Neuilly. It has been considered that the piers of a bridge should occupy a fifth part of the water-way : but they need not take up so much as a sixth or a seventh : those of Waterloo and Lon- don bridges are equal to one-sixth of the openings of the arches ; bit those of Staines bridge, by the same architect, are only equal to between one-eighth and one-ninth. PERRONET gives a different standard of proportion : he considers twice the length or depth of the keystones sufficient. The flatter the arch the severer is the thrust : those curves, therefore, should be sought for which have the effect of converting horizontal thrust into vertical pressure within the smallest space. Segmental arches exercise a mach severer thrust than arches of elliptical form with the same span and rise. PERRONET, however, suggests, that "it may be prudent in designing bridges for rivers of great width to introduce some strong piers, which in case of need may serve as abutments; placing them at distances of three or four arches apart." Mr. Hosxntc advocates reducing the piers in thickness, and extending the arches as wide as is consistent with a due regard to strength ; a thin pier in a water-way being exposed to less violent action from the water than a thick one, because it restrains the water less, and it is less liable to casual injuries. The abutments should be constructed to resist the thrust of the whole series of arches so strongly that their materials should be crushed before they yield to the pressure. By way of illustrating his views, he gives an improved version of the famous bridge of Santa Trinita over the Arno at Florence, composed of three elliptic arches of singular lightness and elegance : a design so perfect as almost to defy emendation ; yet, by simply lessening the width of the piers one-half, and adding the bulk taken from them to the abutments, the symmetry and beauty of the structure are very much enhanced. But while reducing the width of the piers, Mr. HOMING would extend their basis as widely as possible, to insure their stability.
The opening, or head-way of the arches above high-water, is a matter
of great importance, that appears to have been insufficiently considered: the neglect of it has caused the injury of many bridges, and even the destruction of some. A bridge erected over the Tyne at Hexham, by no less an engineer than the celebrated SMEATON, was washed away by a flood, owing to the want of sufficient head-way for the passage of the swollen current. The principle laid down by Mr. HOSKING, that the arches should spring from above the level of the highest tide, is one that ought in every case to be observed : how essential it is to the safety of the structure and the convenience of the navigation, is apparent from the fact, that in proportion as the aperture for the passage of the stream becomes narrowed by the rising of the tide above the piers, the scouring action of the water on the bed of the river becomes more violent. " While the water meets with no obstruction greater than that which the piers offer, no appreciable head should be formed nor should any severe action take place upon the bottom of the river; but if the springings of the arches be once immersed, the haunches and spandrils add, inch by inch and foot by foot, to the line of obstruction as the water attains a head ; which it does constantly with increasing effect : the bed of the river is acted upon, and the piers are undermined, to the utter destruction of the whole edifice, or if the bridge is not overturned bodily, it forms a thorough dam, and is productive of more mischief than would be occasioned by its fall, by holding the water up to flood the surrounding country." It is this "scouring action," we apprehend, that has undermined the piers of Westminster Bridge ; their erection by means of caissoons having increased their liability to be acted upon by the rush of the water at high tide. Even new London Bridge is not wholly free from this defect of construction ; though it exists in so slight a degree as to be objectionable rather on the score of propriety than of any injurious effect—the springings of the centre arch being just below the level of Trinity high-water. The improvement in the appearance of this noble bridge, produced by elevating the springing of the arch and increasing the extent of bead-way by a parabolic curve given to the ellipse, is very striking. Nor is the road-way raised by this means, the height of the arch remaining the same: the parapet is even lowered, and the road-way widened six feet, by means of a pro- jecting corbelled parapet. This last-mentioned desirable improvement, Mr. HOMING states, could be effected without interfering with the use of the bridge for a day ; as the foot-ways only would have to be dis- turbed, and these only one at a time. The ascent of the road-ways of Blackfriars and Westminster Bridges was originally one foot in fifteen ; that of Blackfriars has been recently reduced to one in twenty-four ; and it is intended to do the same with Westminster Bridge ; but a much greater reduction could be effected in the latter, by lessening the substance above the crowns of the arches. This forms a part of Mr. Hosxurn's plan for improving Westminster Bridge, and is one of the advantages attainable by his longitudinal central groining ; which would lessen the vibratory action of the traffic, and distribute it over so large a surface as to neutralize its effect. Not only would this groining enable the road-way to be lowered—increasing the strength and lessening the weight of the superstructure, and dimi- nishing the expense of its reconstruction—but it would have a strikingly picturesque effect when viewed from the river, and from the approaches to the bridge. The approaches, too, are so ingeniously planned as to afford a view of each face of the bridge to passengers going over it—which is not effected in any bridge with which we are acquainted ; while persons descending the steps to the water have a perspective view of the central groining, or longitudinal tunnelling, through the whole series of arches. The most important use of this ar- rangement, however, is to lessen the ascent and facilitate traffic, by dividing the road-way over the abutments into two branches, that make a detour ; the descent to the river being between them instead of at the sides, as usual. These improvements, together with the widening of the road-way by projecting corbelled parapets, have reference to the traffic over the bridge : narrowing the piers and ele- vating the spring of the arches above high-water, are important to the navigation of the river, by giving an increased extent of water-way to each arch, and preventing the current at high tide from making head above and scouring the channel beneath. It is not the absolute breadth of the river, but the width of unobstructed space for the flow of the tide and the navigation of craft, that is to be regarded : projecting abut- ments , like those of London Bridge, are rather beneficial than other- wise, inasmuch as by narrowing the stream the shore is cleared from mud. Such is the effect of the embankment of the New Houses of Par- liament; in a line with which, Mr. HOMING proposes to make his abutment on this side, and a corresponding one on the Surrey shore, by blocking up the end arches. In short, all Mr. HOSKING.8 suggestions for the improvement of Westminster Bridge are based upon scientific principles, and serve to exemplify not only the grand defects of the old, structure, but the main points to be aimed at in constructing new bridges.
There are several minor points worthy of consideration on the score of economy and stability ; but fur these we must be content to refer the reader to the work on the Architecture of Bridges, of which the treatise referred to forms a part.
Previous page
