Fig. 575. State Street Subway while Work on Bridge is Progressing. 12 in. beam and hanging it so that the bottom of the beam is even with the bottom of the girder, a stiffer and thinner floor is obtained without increase in the amount of metal used. An elevation of the Garfield Boulevard bridge is shown in Fig. 572. It is essentially three standard spans with cresting and false arches to relieve the severe plainness of the unornamented structure. At 61st, 63rd and State streets, which are crossed by the many tracks of the Lake Shore Englewood yard, an entirely different type of bride is used. The girder type was abandoned an account of its obstruction of the spaces between cars used by switchmen, the danger to trainmen and the impossibility of putting in switches or cross-overs within the limits of the bridge. A deck bridge was adopted supported by lines of posts at the curbs and at the centers of the streets. On these posts are longitudinal box girders which support the ends of 15 in."I" beams. Centering on the tracks, and at right angles to the "I" beams, are 4 in. "Z" bars that act as ties distributing the load on the rail to the adjoining "I" beams. The whole is covered by a floor plate on which are the rail plates and guard angles and to which the rail is fastened by clips and bolts. To allow for expansion and contraction these bridges are Fig. 576. 63d. Street Subway, Showing Bridge and Masonry. built in panels of six tracks each, which are connected with each other by plates loosely bolted through slotted holes. Fig. 573 gives the characteristic features of the deck bridges. Fig. 575 presents a view of the State street subway at the time the work on the bridge was in progress. Fig. 576 shows bridge and masonry work on 63d street subway. Figs. 577 and 578 show the arrangement and number of tracks crossing 63rd and State: street. These two streets forming, as they do, a continuous subway nearly two thousand feet in length, and crossed by sixty-four tracks, constitute, I believe, by far the most extensive and most expensive subway yet constructed in Chicago. Discussion of Track Elevation in Chicago-November 16, 1898. Mr. E. H. Lee:* I wish to bring up a couple of questions that have occurred to me and which I am sure the gentlemen here present could discuss very entertainingly to the society. In the first place *E. H. Lee, Principal Assistant Engineer. Joint Elevation of Tracks, 16th and Clark streets. I notice that some of the engineers have used concrete, and they not only have adopted it for their retaining walls, but also have used it for abutments and for coping. All their masonry work has been composed of that material. On the other hand I note by observation that some here use rubble masonry for retaining walls. I should be glad to learn the reason for the use of concrete; whether the reason or reasons are entirely from motives of economy or whether there is some other motive behind it. Another question which has occurred to me is as to the relative economy and the relative value from an engineering standpoint of the different girder floors that have been adopted. Various devices. have been used, but broadly the two distinguishing features seem to be the I-beam floor covered by flat plates, with some such protection as asphaltum or tar and the trough or buckle plate floor carrying ballast and ties. I think it would be especially valuable to the society to have the relative cost of these floors and their relative advantages. One thing some of us have learned, is the elasticity of gradient and alignment which may be used. The topography of the country here is of such a nature that engineers have not been called upon to go to extremes in these respects. Perhaps track elevation has developed a few extreme examples of stiff curvature and grades. For instance, I know of curves as high as 22 degrees that have been used successfully day after day with all kinds of traffic, and I know of at least one crossing grade that was used for a time successfully with as high as a 7% grade and a 15 degree curve on it. If some of our members would give us the details of difficulties that have been met, it might lead to an interesting discussion, and lead other engineers besides themselves to appreciate some of the difficulties which have been met in dealing with track elevation here. Mr.L.H.Evans (C.& N.W.): Mr. President, I can start that, I think, by giving a few of the reasons for using dimension stone for the abutments, and concrete for the foundations on the Northwestern work. At the present time we are completing, or have nearly finished all the preliminary work for the track to be elevated next spring, part of which is about five miles of retaining wall. We have been able to put in concrete cheaper than rubble; it is an entirely different class of labor, so we have made the foundation for the retaining wall, which is practically the same area as the rubble wall above it, of concrete. That allows us to work one set of men who are common laborers entirely at that class of work, and the masons, high-priced labor, fill out with the rubble. In that way we were able to advance the work very rapidly, completing as much as four hundred feet of wall a day. The area of the wall would be close to a cubic yard of concrete in the foundation and a cubic yard of rubble above the foundation. I think, too, that the concrete footings will last better than Lemont rubble would at the ground line. |