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aid the transfer of longitudinal stresses of the trestle to the foundation.
Concrete foundations under frame bents may be made as shown on drawings or as solid piers, depending on the cost of each.
The Committee recommends that where long round timber can be secured that it is good practice to use as long posts as possible to avoid multiple-story bents leaving out the horizontal sills.
Longitudinal horizontal girts are shown 6 in. by 10 in. because it is desirable that a stiff strut with ample bolting area be used. The Committee submitted the plan of lapping the girts by on the posts with double through bolting and dapping over the sash brace on the bent because in their judgment all forms of end to end arrangements of girts did not leave enough material to make secure the bolt fastenings and because of shrinkage of material it is difficult to keep the end contact tight, which is necessary if the brace is to be effective.
Longitudinal diagonal bracing are shown 3 in. by 10 in., size seems to be used by more roads than all those using other sizes put together.
There were many different layouts of longitudinal diagonal bracing indicated in replies to questionnaire, but the Committee recommended the bracing in every third panel for pile bents and in every other panel for frame bents as being sufficient in ordinary circumstances. High trestles require more and are indicated in the plan for multiple story trestles.
In constructing bulkhead at the ends of wooden trestles the Committee recommends as good practice the use of sections of old rail as separators between piles or posts and end planking,
The Committee in presenting plans for ballast deck timber trestles has not had as much material information from railroads' experience because this type of deck is more recent in its development and has not been as extensively used as the open deck. The Committee, therefore, presents a compromise of the best features.
The Committee felt that experience indicates that the open system of stringers with plank covering was the better type because of convenience of repair, less tendency to decay and the economical use of stringer material.
The Committee chose the single span length stringer lapping by on the caps the full width of cap because this plan enables the use of shorter timbers, the removal and repair of one panel at a time, and where creosoted timber is used, the avoidance of cutting the stringers after treatment.
The stringers are shown laid skew, which arrangement develops a more symmetrical loading of the stringer at the center of the span.
With the stringers held in place by planks spiked to them, it was decided that four drift bolts would be sufficient at each bent to hold deck to caps.
In the replies returned to questionnaire there were as many 14-foot decks as all other widths put together and the Committee therefore chose the 14-foot width.
There was no great predominance of any one span length but 13 ft. and 13 ft. 6 in. represented five out of fifteen plans, 14 ft., 14 ft. 6 in., and 15 ft. represented six out of fifteen plans but the Committee did not want to use larger than 8 in. by 16 in. stringers nor stresses higher than the value given below. Eight in. by 16 in. stringers with E-60 loading on deck have a calculated fiber stress of 1,470 Ib. per square inch of possible actual size.
The Committee decided on six piles or posts to the bent because of the increased dead load carried and the natural greater permanence of this type of structure.
The use of 12 in. by 14 in. caps exceeded in number all other sizes together, as did also the use of 16-foot length caps.
The ballast curb was chosen 8 in. wide because the use of this width exceeded all other widths put together, while the Committee chose 8 in. ior height in face of the fact that the larger number of plans showed only 6 in. high because the ballast should be retained better than would be done by a 6 in. curb.
Four-inch planking was thought would give a better distribution of loading over the stringers.