20 



SCIENCE. 



[N. S. ^OL.^VI. No. 392. 



road on tlie upper deck and a highway on 

 the lower deck. The other bridge has 

 arched trusses with parallel chords and two 

 hinges. It replaced the Niagara and Clif- 

 ton highway suspension bridge in 1898, 

 and as its span is 840 feet, it is the largest 

 arch of any type in the world. The naan- 

 ner in which this arch was erected fur- 

 nishes an illustration of the effort which 

 is made by engineers to conform the actual 

 conditions so far as possible to the theoretic 

 ones involved in the computation of the 

 stresses. Since the stresses in an arch hav- 

 ing less than three hinges are statically 

 indeterminate, stresses of considerable 

 magnitude may be introduced into the 

 trusses if the workmanship be imperfect, 

 the supports not located with sufficient 

 precision, and the arch closed without the 

 proper means and care. 



The Niagara and Clifton arch was first 

 closed as a three-hinged arch and then 

 transformed into a two-hinged arch by in- 

 serting the final member under the sum 

 of the computed stress due to the weight 

 of the truss, and that due to the difference 

 between the temperature at which the clos- 

 ure was made and that assumed as stand- 

 ard in the stress computations. This stress 

 was secured in the member by inserting it 

 when the hydraulic jack which forced 

 apart the ad.jacent ends of the shortened 

 chords registered the required amount of 

 pressure. The arch had been erected as a 

 pair of cantilevers from each side extend- 

 ing 420 feet out beyond the supports, and 

 when the closure was made the two arms 

 came together within a quarter of an inch 

 of the computed value. Such a result in- 

 volving the ' accuracy of the calculation 

 and design of the entire steel work, the 

 exactness with which the bearing shoes or 

 skewbacks were placed, and the perfection 

 of the shopwork ' has been truly charac- 

 terized as phenomenal.* In order to re- 

 duce secondary stresses to a minimum the 



members were bolted up during the canti- 

 lever erection and the bolts replaced by riv- 

 ets after the closure of the arch rib. 



The past decade witnessed the introduc- 

 tion and extensive development of arches 

 of concrete and of concrete-steel construc- 

 tion. In the latter kind a small amount of 

 steel is imbedded in the concrete in order 

 to resist any tensile stresses that may be 

 developed. During this period more than 

 150 concrete steel bridges have been built 

 in this country. In the same year in which 

 the largest metallic arch was completed, 

 the five concrete-steel arches of the bridge 

 at Topeka, Kansas, were finished. The 

 largest one has a span of 125 feet and still 

 remains the largest span of this type in 

 America, although it has been exceeded in 

 Europe. Considerably larger spans are to 

 be built this season, while others are in- 

 cluded in the accepted design for the pro- 

 posed Memorial bridge at Washington. 



It is the smaller steel structures which 

 are destined more and more to be replaced 

 by arches of this material. The steel 

 bridges require repainting at frequent in- 

 tervals, constant inspection, occasional re- 

 pairs and finally replacing by a new struc- 

 ture after a relatively short life, on account 

 of rust and wear, unless it is required even 

 sooner on account of a considerable in- 

 crease in the live load. The concrete arch 

 requires practically no attention except at 

 very long intervals. 



The safety of operating the traffic makes 

 it desirable to have as few breaks as pos- 

 sible in the regular track construction of a 

 railroad, and this constitutes an additional 

 reason why concrete or stone arches are 

 being substituted for the smaller openings. 

 The decreasing cost of concrete tends to 

 an extension of this practice to openings 

 of increasing size. Last year, however, a 

 bridge was completed which marks a de- 

 cided departure from previous practice. 



* Engineering Neics, August 4, 1898. 



