314 



ENGINEERING. 



bility of a central pier, it was decided to have 

 but one span. The conditions favor this in 

 some respects, for the banks are precipitous, 

 and each city has an upper and a lower sec- 

 tion. Therefore, two roadways were called for, 

 one of them 160 feet higher than the other. 

 A glance at the illustration will show how in- 

 terdependent are the three parts, namely, the 

 upper platform, the main arch, and the lower 

 platform. The outward thrust of such an enor- 

 mous arch would be tremendous at the piers 

 upon which it rests, but the lower platform, 

 like the taut string of a bow, receives part of 

 this strain, and could probably sustain the 



proached one another. The arch rests upon 

 the masonry through steel rollers placed in 

 steel coussinets, which in turn rest upon cast- 

 iron beds that distribute the pressure over the 

 masonry. The lower platform is divided into 

 five bays by suspension-rods. It is composed 

 of simple lattice-girders 29^ feet apart and 10 

 feet between chords. The floor-beams are 10 

 feet apart, and five rows of longitudinals re- 

 ceive the plates of the roadway. A novel 

 method was employed in mounting the arch. 

 The piers and approaches to the arch being in 

 place, shears were erected over each pier on 

 the upper platform, and two cables, presumably 



FIG. 1. BRIDGE OP Luiz I, OPORTO, LISBON. 



whole of it in case of need. The great height 

 of the arch 240 feet above low water offers 

 a dangerous leverage for high winds, and 

 against this the upper platform with its lattice- 

 girders affords ample provision. Taken in con- 

 nection with the spread of the arch-trusses it 

 renders the structure remarkably strong. The 

 span of the main arch is the greatest in exist- 

 ence 566 feet. The thickness of the arch at 

 the key is 26 feet, about ^ of the span. The 

 thickness increases downward until it reaches 

 55 feet at the piers. This reverses the plan 

 followed in the great arch at Maria Pia, where 

 the main girders approach one another and 

 are united at the springers. The engineer, Mr. 

 Seyrig, was led to make this change in view ot 

 the different service expected of the bridge. 

 The Maria Pia is distinctively a railway-bridge 

 where the load moves en masse, and the moment 

 of strain at the key of the arch is of the first 

 importance. With the " Luiz I Bridge," as it 

 is called, at Oporto, the case is different, and it 

 became possible to diminish the central thick- 

 ness of the arch in view of the character of the 

 expected load. Again, Mr. Seyrig assumed, 

 though there may be two opinions in this par- 

 ticular, that the architectural effect would be 

 better, with wider supports at the springers. 

 Another and more obvious reason was that a 

 roadway 26 feet wide had to be provided at the 

 springers, and this would have seriously inter- 

 fered with the proper distribution of cross- 

 braces at that point, had the main arches ap- 



of steel wire, were stretched across the river, 

 passing over the shears and anchored securely 

 on the shoreward side. These shears and 

 cables are shown, still in position, in the draw- 

 ing of the completed bridge (Fig. 1). They 

 are also shown more in detail in Fig. 2. On 

 the cables were traveling-pulleys, which could 

 be run out to any point over the river. Through 

 the pulleys the hoisting-tackle ran and was 

 worked by windlasses at the base of the shears. 

 Gas motors were used to drive the windlasses, 

 and gave perfect satisfaction. The successive 

 pieces of the arch were floated out on boats 

 and hoisted thence by means of the tackle. 

 The workmen soon learned to handle pieces 

 of 6,500 pounds, raising them 195 feet, into 

 position, where they were temporarily held by 

 guys running upward and backward to an- 

 chorages on the piers. When the two demi- 

 arches approached one another, measurements 

 were taken, and one was found to be 3 inches 

 lower than the other. It was therefore raised 

 to the proper level. When everything was 

 ready to insert the key-pieces, it was found that 

 a difference in level of 2 inches existed, and 

 that the space was too small by about Hinch. 

 The engineers had observed that the degree of 

 separation was greatest in the morning ; ac- 

 cordingly, everything was arranged to mount 

 the last two pieces at an early hour on Aug. 11, 

 1885. One of them fitted accurately, while 

 the other was T 2 7 of an inch too large, but this 

 error was soon corrected, and the most difficult 





