bracing, thus leaving a longitudinal passageway for wagons at the center of the structure.

The saving in weight of metal per lineal foot of four-track structure on tangents by adopting braced towers instead of solitary columns was found to be about 140 lbs., or nearly 9 per cent of the total weight.

Plate Girders versus Open-Webbed Girders

In designing these elevated railroads for Chicago, many estimates were made for both plate-girders and open-webbed girders, which demonstrate that there is practically no difference in the weight when both girders are properly designed. As the open-webbed girders cost a trifle more per pound to manufacture, there is no economy in their use; nevertheless they were adopted for all structures running longitudinally in the streets, in order to comply with certain city ordinances. The observance of these ordinances was sometimes carried to extremes, producing ridiculous combinations of solid and open web in the same girder, and an evident waste of material and labor. For this the engineers are not to blame, because they did not frame the ordinances.

Best Sections for Columns

Investigations concerning strength, capacity to resist impact, facility of erection, economy of metal, etc., determined that the section for columns located in the street should be two 15-in. rolled channels with the flanges turned inward and a 15-in. rolled I-beam riveted between to act as a central web or diaphragm, the flanges of the channels being held in place by interior stay plates spaced about 3-feet centers. In most cases the column feet pass below the pavement and are embedded in the concrete, to which, of course, they are bolted, but in some cases they rest on pedestals a little above the level of the sidewalk. The main object in turning the flanges inward is to enable the column better to resist impact from heavily loaded vehicles. Just above the pavement there is a curved casting filled with concrete and surrounding the column to act as a fender.

This type of column is very satisfactory after it is erected, although it gives some little difficulty in the shops and involves slightly more field riveting than usual. One complaint made was that the planes of the top and bottom flanges of I-beams are never exactly parallel to each other, hence some straightening was necessitated.

For columns located on private property or on sidewalks where the structure is transverse to the street, four Z-bars and a web plate were adopted as the most satisfactory section. At the top of the column a wide, curved web-plate and curved angles were used. This design makes a most satisfactory column, which goes through the shops readily, and which is well adapted for quick erection. It is true that it necessitated a