. Concrete bridges and culverts, for both railroads and highways . gesdesigned as arches have cantilever action also, orwhen the rise is small in proportion to the span,the stresses are chiefly the result of bending, andregardless of theory the span acts then more as abeam than as an arch. The uncertainty in refer-ence to cantilever or beam action of arches can beremoved by building an open vertical joint betweenthe arches over the piers, the presence of whichwill positively prevent any cantilever action. Whilesuch a joint removes a serious uncertainty of de-sign, it is very doubtful whether o
. Concrete bridges and culverts, for both railroads and highways . gesdesigned as arches have cantilever action also, orwhen the rise is small in proportion to the span,the stresses are chiefly the result of bending, andregardless of theory the span acts then more as abeam than as an arch. The uncertainty in refer-ence to cantilever or beam action of arches can beremoved by building an open vertical joint betweenthe arches over the piers, the presence of whichwill positively prevent any cantilever action. Whilesuch a joint removes a serious uncertainty of de-sign, it is very doubtful whether or not this expedi-ent is desirable, for the cantilever action freciuentlyadds as much strength to the bridge as does thearch and when properly designed and built to re-sist both sets of stresses, the presence of cantileveraction adds greatly to its strength and Walnut Lane bridge at Philadelphia, andthe Rocky River and Piney Creek bridges now un-der construction, illustrate to some extent the sa^ -ing which may be accomplished by the use of ribbed. _p-fl 144 CONCRETE BRIDGES AND CULVERTS. in place of slab arches, and yet all of these threebridges are only partially ribbed. They each con-sist of a pair of twin arch rings separated by adistance of from 10 to 20 feet, which space be-tween the rings is spanned by simple floor con-struction. The saving in the arch ring b}^ this ex-pedient is from 25% to 30% of the cost of the ring,which saving would be still further increased byusing entire ribbed designs. The Luxemburg stonearch bridge in Germany with a span of 275 feet,and completed in the year 1903, is of the same unusual example of ribbed arch design pre-pared by Mr. Turner of Minneapolis, is shown inFigure 27. It is one of several designs submittedfor the Grand Avenue viaduct in Milwaukee. Themain compression members are octagonal and arehooped. The use of ribs instead of slabs makes it possibleto place members of the proper strength where re-quired, as f
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