18 



SCIENCE. 



[N. S. Vol. XVI. No. 392. 



the result of efforts to eliminate ambiguity 

 in the stresses of the trusses, to reduce the 

 effect of secondary stresses, and to secure 

 increased stiifness as well as strength in 

 the structure. Double systems of webbing 

 have been practically abandoned so far as 

 new construction is concerned. The sim- 

 plicity of truss action thus secured permits 

 the stresses to be computed with greater 

 accuracy and thereby tends to economy. 

 Before the last decade very few through 

 bridges and those only of large span were 

 designed with end floor beams in order to 

 make the superstructure as complete as 

 possible in itself and independent of the 

 masonry supports. Now this improved 

 feature is being extended to bridges of 

 small spans. Similarly dropping the ends 

 of all floor beams in through bridges so as 

 to clear the lower chord and to enable the 

 loAver lateral system to be connected with- 

 out producing an excessive bending move- 

 ment in the posts has likewise been ex- 

 tended to the smaller spans of pin bridges 

 and is now the standard practice. The 

 expansion bearings have been made more 

 effective by the use of larger rollers, and 

 of bed plates so designed as to properly 

 distribute the large loads upon the masonry. 

 In the large spans of through bridges the 

 top chord is curved more uniformly, there- 

 by improving the esthetic appearance. 

 These chords are also given full pin bear- 

 ings, thus reducing the secondary stresses. 

 The stiffness of truss bridges has been 

 secured by adopting stiff bracing in the 

 lateral systems and sway bracing, instead 

 of the light adjustable rods formerly used. 

 At the same time adjustable counter ties 

 in the trusses are being replaced in recent 

 years by stiff ones, while in some cases the 

 counters are omitted and the main diago- 

 nals designed to take both tension and 

 compression. 



Some of the same influences referred to 

 above have led to much simpler designs for 



the portal bracing by using a few members 

 of adequate strength and stiffness similar 

 in general character to those of the trusses. 

 Such steady progress in the design and 

 construction of railroad bridges of moder- 

 ate span has, unfortunately, no adequate 

 counterpart in highway bridges. The con- 

 ditions under which highway bridges are 

 purchased by township and county com- 

 missioners are decidedly unfavorable to 

 material improvements in the character of 

 their details. It is a comparatively rare 

 occurrence that the commissioners employ 

 a bridge engineer to look after the inter- 

 ests of the taxpayers by providing suitable 

 specifications, making the design, inspect- 

 ing the material, and examining the con- 

 struction of the bridge to see that it con- 

 forms to all the imposed requirements. 

 These provisions are only made in some of 

 the cities, and accordingly one must examine 

 the new bridges in cities to learn what pro- 

 gress is making in highway bridge building. 

 The lack of proper supervision in the 

 rural districts and many of the smaller 

 cities results in the continued use of short 

 trusses with slender members built up of 

 thin plates and shapes, whose comparative- 

 ly light weight causes excessive vibration 

 and consequent wear, as well as deteriora- 

 tion from rust. Under better administra- 

 tion plate girders would be substituted for 

 such light trusses, making both a stiffer 

 structure and one more easily protected 

 by paint. The general lack of inspection 

 and the consequent failure to protect high- 

 way bridges by regular repainting will 

 materially shorten their life and thereby 

 increase the financial burden to replace 

 them by new structures. Some progress 

 has been made in adopting riveted trusses 

 for the shorter spans for which pin-con- 

 nected trusses were formerly used, but liie 

 extent of this change is by no means as 

 extensive as it should be, nor equal to the 

 corresponding advance in railroad bridges. 



