The guide-plates are small bars riveted to the top of the bottom plates, and serve to keep the rollers in line. The guide-bars are con nected to rollers at their ends, and serve to keep the rollers equi distant, therefore causing them to roll easier and keeping them from becoming worn by contact with each other.
The expansion (57) must be allowed for at the rate of * inch for every 10 feet in length of span. This makes a total allowed for tem perature of expansion of 10 X = 1; (say 2) inches. No slotted holes are to be provided for the anchor bolts, since they do not go through that part of the bridge which slides. The shoe slides over the rollers, and is kept in place by the angles the end, which are riveted to the masonry plate (see Fig. 198).
Unless sufficient room is allowed between the segmental rollers, they will tend to bind when the bridge has reached the extreme posi tion for expansion or contraction (see Fig. 200). This distance can be computed from proportions as indicated in Fig. 201, and from the following formulnc*: in which e is the amount allowed for the change of temperature, and D is the diameter of the rollers, both being taken in inches. The angle 0 is in degrees. In the present case, e is 2 inches; D is 6 inches; and 95, computed from the above formula, is 9° 30'. Substituting in the equation giving the value for y, there is obtained 1.02 inches (say 11 inches) for the distance between rollers. Rollers must not be less in thickness than the total expansion allowed for temperature.
Since there are 5 rollers, there are 4 spaces between them. Also,
since the rollers must occupy a space of 28 inches, the length of the masonry plate, each roller must be: of motion for the rollers. The maximum width allowable is given by the formula :* TV= in which and D are indicated above. This requires the bar to be.
W = 2 X 0.985 = 296 (say 21) inches wide.
93. The Stress Sheet. Plate III shows the stress sheet of the bridge which has been designed in the preceding articles. This sheet represents the best current practice among the larger bridge corporations. It will be noted that very few details are given upon the sheet; also that few rivets are noted, and that sketches showing the manner in which the parts go together are entirely wanting. The shears and moments for the stringers and floor-beams, as well as the reactions and the number of rollers re quired, are given. This is to save the drafts man the trouble of recomputing values which have necessarily been determined by the designer.
The details of the various members, and also • the manner in which the different members are con nected, are left to the draftsman, who is un der the direct super vision of the engineer in charge of the draft ing room, upon whom rests the responsibility for good details. The figures given in the text indicate the best current practice. Figs.
202 to 204 show details of the end floor-beam connections, and also the packing of the members of the upper and the lower chord. The arrangement here given may be said to be standard for single-track Pratt truss spans up to 200 feet in length.