The diagrams of transverse strength and stiffness are to be used to determine the depth of the beam; the diagram which gives the lowest load being followed. Stiffness is calculated on the assumption that the deflection should not exceed inch per foot of span. The diagram for weight is intended to show how much the load is increased by the weight of the beam itself. The diagrams are accompanied by four tables, containing values of the modulus of rupture, of the modulus of elasticity, the products of their moduli with the co-efficients of transverse strength and of stiffness, and the weight of timber of different kinds. D. K. C. Proportions of Eye-bar Heads and Pins, as determined by (Transactions of the American Society of Civil Engineers, vol. vi., pp. 263–268.) 66 In experimenting on this subject during the years extending from 1857 to 1866 the Author ascertained as an absolute, unchangeable fact," (1) that a pin the diameter of which was twothirds of the width of the bar was the least size which would develop the full strength of the bar; (2) that the required sectional area across the eye, for any given width of bar, increased with the diameter of the pin; (3) that the best shape of the eye-bar head depended upon the material and mode of manufacture; "weldless" eyes being circular in outline, and "hammered" eyes Gothic, or, in other words, having 50 per cent. greater sectional area behind the pin than on either side of it. The Author directs attention to the fact that the diameter of pins in some instances will be governed by the bending stress, and sums up his general conclusions in a table shewing diameter of pin, &c., expressed in terms of the width of bar, as follows: : Examples.-If a bar 4 inches by 1 inch be attached to a 3-inch pin, the sectional area across the eye (table, under "No. 2, Weldless Eyes") should be 4x1.56 square inches if weldless, and 4x1.33=5.33 inches if hammered, and the maximum thickness of bar which could be used without imposing too great a bending stress upon the pin would be 4x25=1 inch. The sectional area behind the pin would be 3 square inches in the first instance, and 2.66×1.50=4 square inches in the other, as previously explained.1 B. B. Pneumatic Caisson Foundations of the Prague-Smichow Bridge. By J. REITER. (Mittheilungen des Architekten- und Ingenieur-Vereines im Königreiche Böhmen, vol. xi., pp. 7–9, 13–16, vol. xii., pp. 20–24.) Before describing the above-bridge foundations, the Author gives a short outline of the history of pneumatic pier foundations, and enumerates some of the principal instances where the method has been applied. He then describes successively, the staging for the erecting and sinking of the caisson of the Prague-Smichow bridge, the caisson itself, the sinking apparatus, and the practical results obtained. The staging was composed of two double rows of piles, running parallel with the line of the river and embracing between them the space in which the caisson was to be sunk. The inclosure of the site of the pier was completed by a few transverse piles, at the down-stream end at right angles to the longitudinal rows, and at the up-stream end, in the form of a wedge. All these piles were strongly connected by ties, stays, and braces. A platform was established at about 8 feet above mean water-level and extended over the whole area of the staging, but the central part, within the inner piles, was made removable. Here the caisson was put together and, when ready, suspended to the staging by ten strong screws with chain attachments. After slightly lifting it, to allow the clearing of the removable portion of floor underneath, it was let down into the river. The bearings of the screws were supported by the inner piles, which were of greater height than the outer ones, inclined struts joining their ends and serving to stiffen the whole structure. There was besides at the top level a travelling crane, two narrow longitudinal platforms, and on one side, at one 1 Compare with Mr. Berkley's results (Proc. Inst. C.E., vol. xxx.). The proportions recommended and adopted by him after many experiments were: Diameter of pin, 0.75; metal section across the eye, 1.25; metal section behind the pin, 1.00. Thus the 4-inch by 1-inch bar would have a 3-inch diameter pin, and a sectional area of 5 square inches across, and 4 square inches behind the eye.-B. B. PNEUMATIC CAISSON FOUNDATIONS OF PRAGUE-SMICHOW BRIDGE. 289 end, a small overhanging platform where another crane unloaded the material from boats alongside the staging. The Author states that the cost of the staging amounted to £4 88. per square mètre, or 88. 2d. per square foot of area covered by the caisson. A comparison between numerous stagings of this kind led him to conclude that this is a fair mode of expressing the cost, and that, as a rule, it varies between £4 and £6 per square mètre, or 78. 4d. and 11s. per square foot of caisson area. The caisson was entirely made of wrought iron, and had in plan the shape of a rectangle, to which were added, against the short sides, a semicircle and an ogive. Its clear width was 22 feet, and its total length 64 feet. A series of transverse girders, 3 feet 4 inches apart and 2 feet high, formed the floor upon which the masonry of the pier rested. Against their bottom flanges were fixed the plates of the ceiling of the caisson chamber, there being for the latter a clear height of 7 feet 2 inches above the cutting edge of the caisson. There was therefore a fair margin allowed to make up for the amount of which the caisson edge penetrated the ground below the level in the chamber, without interfering with the excavation. The floor girders were fixed on to the sides of the caisson, and also supported on triangular brackets projecting from these sides. Before commencing the sinking, the spaces between the brackets were lined with masonry. The transverse stiffness, due to the flooring, was increased by an arched plate rib in the centre of the chamber, and by four horizontal cross stays. The shafts, two in number, had an elliptic cross section (diameter 5 feet 3 inches and 3 feet 4 inches), and their vertical axis was made to coincide with the centre of gravity of each half of the chamber, so as to reduce to a minimum, the transport of soil inside. These shafts could be lengthened according to the depth to which the caisson had to be sunk. This: was also the case with the prolongation of the external plating of the caisson above the floor, formed of a series of horizontal rings. In the present instance there were four such horizontal rings, each 3 feet 5 inches high and inch thick. The total weight of wrought iron in one caisson amounted to 52 tons. The comparison of the weights of ten similar caissons has led the Author to conclude that the weight of such caissons may be accurately expressed in function of the area covered by them; it amounts, with very little variance, to 450 kilogrammes per square mètre, or 92 lbs. per square foot. In the PragueSmichow bridge the weight was 445 kilogrammes per square mètre. The erection and caulking of the caisson on the staging took about twenty-one days. After having been slowly let down into the river, its weight with that of the masonry lining caused it to sink some 10 inches into the coarse sand of the river bed. The spaces between the floor girders were then filled with concrete, and the shafts lengthened. The air-locks were fixed on the top [1877-78. N.S.] U of the latter and put in communication with the blowing machine. This was placed, with the accompanying engines and boilers, on barges moored to the staging. It had been proposed to extend the foundations to a depth of 19 feet 8 inches below ordinary mean water-level; but on account of a depression in the strata underneath the bed of the river, the sinking had to be carried down to a depth of 30 feet 2 inches. When 18 feet 9 inches were reached—which had required the additional weight of four courses of granite facing, each 1 foot 7 inches high, filled internally with rubble masonry-the work had to be stopped to lengthen the air shafts. During this operation, the compressed air escaping from the caisson chamber, a further sinking of 10 inches took place. Upon introducing again compressed air, the caisson rose, and it was found that when the internal pressure in the chamber exceeded by 7 lbs. per square inch the atmospheric pressure, the caisson could be maintained in suspension. This observation gave an easy means to determine approximately the amount of friction developed along the sides of the caisson; for the total weight in the caisson was balanced by the upward pressure of the air in the chamber added to the friction in the ground. The total weight, including all masonry and ironwork, was 916 tons; the pressure exerted by the compressed air, taken at 7 lbs. per square inch, 638 tons; the difference, 278 tons, represents the part of the load taken up by the friction. This friction was developed in a layer of 13 feet thickness, composed of 9 feet 9 inches of sand and 3 feet 3 inches of coarse gravel irregularly stratified. The surface of the caisson bearing against the ground was 183.2 square mètres ; therefore the friction per square mètre was 1·5 ton, or about 2.8 cwt per square foot. This figure agrees very well with those obtained by Gerber and Bolzano in their experiments during the construction of the foundations of the Simbach and Lech bridges. They had to deal with a stratum of gravel, and found the friction there to vary between 1.6 ton and 1.8 ton per square mètre, or 3 cwt. and 3.35 cwt. per square foot. Two plates accompany this Paper, and fully show the details of the caisson and its staging. A. O. B. Bridge over the Danube to connect the Buda and Pesth Railways. By J. SEEFEHLNER. (Zeitschrift des Arch.-u Ing. Vereines Hannover, vol. xxiii., cols. 29-50, 3 pl.) This bridge was necessitated by the continually increasing importance of the Hungarian capital. A project was accordingly prepared in the Public Works Department, which comprised the extension of the existing station at Steinbruch on the Pesth side of the river, and its connection with the Franz-stadt station and the station at Uj-Buda, also branch lines to the pig farm at Steinbruch, the public slaughter-house and cattle market, the Pesth Custom House, the goods station, and the new terminus in Pesth; at the site selected the breadth of the river at low water is 1,150 feet. By the terms of the specification, the masonry was to be of “Neustift" stone, a hard and very compact limestone. For coursed rubble work, the area of the face of any stone was not to be less than 12 square foot, and in ashlar work the stones had to be at least 1 foot thick. As regards cast iron, its ultimate resistance to crushing was to be 45.75 tons per square inch, and in the case of wrought and rolled iron 22 tons. The main girders were required to bear a tensile strain of 4 tons; the cross and longitudinal girders 4 tons per square inch; and all parts of cast steel a strain of 10 tons per square inch. The tenders of MM. Filleul-Brohy having been accepted, the work was commenced in August 1873. The foundations of the river and land piers were laid in caissons, those for the latter being rectangular, while the former had semicircular ends. The various parts of the caisson were brought riveted to the yard, the plates being bent and punched, so that only the fitting together was done at the site of the bridge. A scaffolding or staging was built over the site of the land piers; on this was laid a platform to support the winches which lowered the caissons. The staging piles were driven by a 4-HP. steam rammer; the weight of ram was 24 cwt., and the fall about 20 feet. The cost of driving was about 1s. a yard, through a soil consisting of coarse sand and rubble. The construction and setting-up of the caissons took twenty-seven to thirty days on an average, and their aggregate weight for all the bridge piers was, in round numbers, 38 tons. The caissons for the foundation of the river piers were sunk by the plenum or compressed air process in the usual way, but there were special mechanical contrivances for emptying and filling the buckets which expedited the work considerably. The capacity of the buckets was 1 35 cubic foot, and they could be filled and emptied eleven times in ten minutes, so that the average daily excavation amounted to 32.7 cubic yards. While the preparation of the foundations was in progress, a heavy flood occurred which destroyed the staging, and filled the caissons with water. This was pumped out, at the rate of 68 tons per hour, by Letestu's steam pumps, the peculiarity of which consists in the form of the piston. The materials and workmen were taken from the building-yard to the piers in large boats towed by a 4-HP. steamer, and the material was lifted out of the boats by steam cranes and winches of improved design. The steam cranes (Chrétien's patent) were capable of lifting 4 tons, and consumed about 5 cwt. of coal daily. |