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6. The Committee recommends that the plans for standard MultipleStory Trestles, as shown in Appendix B, Exhibit 5, be approved for printing in the Manual as recommended practice.
7. The Committee recommends that the plans for standard BallastDeck Trestles, as shown in Appendix B, Exhibit 6, be approved for printing in the Manual as recommended practice.
8. The Committee recommends that the recommendations as good practice in the report of the Sub-Committee on fireproofing wooden bridges and trestles, as given in Appendix C, be approved for printing in the Manual.
9. The Committee recommends that the outline of work for the ensuing year be received as information for the use of the Committee on Outline of Work.
10. The Committee submits in Appendix B, Exhibits 7, 8, abstracts of data and questionnaires, as information.
Outline of Work for Next Year's Study and Report A. O. RIDGWAY, Chairman; W. H. Hoyt, A. M. VAN AUKEN, C. S. HERITAGE, H. AUSTILL, Sub-Committee.
1. Report on classification and grading rules for hardwood timber and lumber for railroad purposes.
2. Study and report on the useful strength of new, old and treated timber when used in railroad trestles, including a critical examination of the present theories of stresses when applied to timber under railroad load conditions.
3. Study and report on proper organization of Bridge and Building Departments.
4. Study and report on system of trestle inspection and reports covering the same.
W. H. Hoyt, Chairman.
(1) REVISION OF MANUAL
A. O. Ridgway, Chairman; H. T. HAZEN, D. W. Smith, SubCommittee.
Your Committee recommends the following changes in the Definitions,
the top of piles or posts, connect- top of piles or posts, connecting
ing them in the form of a bent. them to form a bent. STRAIGHT-Having a straight line STRAIGHT-Having a right line axis.
of an axis.
Add the following definition:
floor on ballast deck bridges to retain the ballast.
(2) STANDARD PLANS FOR WOODEN TRESTLES
BY THE COMMITTEE
Your Committee has had this subject under investigation and study during the two past years, having presented a progress report at the last convention. The Committee early this year issued a questionnaire on prevailing practice of railroads in design and use of wooden trestles, including details of multiple-story and ballast-deck trestles. The Committee carried on a voluminous correspondence with various authorities and encouraged engineering magazine discussion of some of its problems.
The Committee early in its studies realized that whatever plans it might present would be compromises of many viewpoints and would not therefore meet the complete approval of every engineer. Many good arguments may be advanced for plans and details other than those offered by the Committee. The Committee, however, has endeavored to recommend what in its judgment would make a consistent, conservative, safe and economical design. There will doubtless be some who would build lighter and others who would build heavier structures. In presenting four plans, using 12- and 16-ft. spans for light and heavy loading, the Committee feels that it has provided in a reasonable way for all railroad service trestles, except the temporary logging road, and heaviest railroad service where steel or concrete should be given serious consideration. The two spans provide for trestles in districts where the timber runs small and for districts where splendid heavy timber is available.
The Committee realizes that unlike many other forms of railroad material and structures where new material is being ordered every year, timber trestles are in most cases already installed and each railroad already has its own standards which will vary from the Committee's plans. Furthermore, many railroads have already large stocks of sizes of timber suitable for their own standards of construction. Therefore, these proposed standards will only be of value for comparison with present structures when either changes in existing standards are suggested or new railroad trestles are being planned. These plans, unless adopted by each railroad, will not affect materially the present demands on the timber supplies of the country. On the other side, if by chance these plans should supplant those already in use by the railroads, the variety of span and loading offered will give opportunity for the economical use of existing supplies of timber.
The Committee would therefore receive criticism in the spirit that there are many viewpoints in the design of these structures, but that it had after due investigation and study submitted these plans as its best judgment and maintains its support on these grounds.
The Committee spent a large amount of time and thought on the length of spans to be recommended. In last year's progress report it suggested a twelve-foot span. After careful consideration of the advantages of a sixteen-foot span, where flood conditions exist and in zones where Western fir can be readily secured, it decided to include both spans.
The Committee feels that there is a definite field of utility for a short and a long span, each serving better than a compromise one-span length between the two lengths recommended.
The Committee carefully considered the loadings and decided to keep in harmony with those of the steel specifications, which are Cooper's E-60 and a load which shall not be in any case lighter than three-fourths of the above. The Committee believes that for any structures designed to carry more than E-60, steel and concrete will be given serious consideration.
The Committee kept in mind also that freight cars and their loads have kept increasing until now they often exceed in intensity of loading many of the earlier locomotives. They would, therefore, recommend that great care in the choice between these two standard designs be used in order that the lighter design be not chosen for lines where heavy foreign cars are liable to be hauled over the line even though the lighter trestle might be sufficiently heavy for the motive power used.
In the choice of stringers the Committee desired to use a size which can be converted from the lighter design to the heavier design by the addition of another stringer of the same size to each chord of the span. It, therefore, decided on using three lines for the lighter loading and four lines for the heavier loading.
The question of design stresses was carefully investigated and studied. Without holding too closely to fixed stresses, the Committee wished to be consistent in the choice of design stresses for all four designs and has, therefore, not exceeded 1481 lb. per square inch in extreme fiber in bending, 221 lb. per square inch in end compression between the cap and stringer. The stresses have been investigated on the basis of possible actual sizes, the minimum sizes permitted by the specifications of A. R. E. A., rather than the nominal sizes.
Weight of timber and hardware were taken as 4.5 1b. per ft. B.M. Weight of rails and fastenings were taken as 150 lb. per lin. ft. of deck.
Horizontal shear was studied and the Committee is satisfied that its designs are in conformity with the best experience and present practice. The Committee has investigated the status of theory of horizontal shear and has decided that it is premature to advance any theory in support of its choice of stresses.
The Committee made some inquiry relative to the future supply of timber for bridge work and found that like many other problems there is a wide variance of judgment as to the future supply. Statistics and past history of the lumber industry indicate a diminishing and limited supply. Engineers in the lumber industry indicate that better methods of selection and cutting will prolong the time in which bridge material may be secured. The Committee believes that conservation of our present supply is necessary and therefore that the use of a 7 in. by 16 in. stringer
which can be cut from the smaller timber is a measure of conservation in that it increases the number of trees which can supply stringer material.
Eight in. by 8 in. ties are shown because their use was indicated by more designs submitted than all other sizes put together. Tenfoot length was chosen because while in the number of designs the nine and ten-foot length were nearly equal, many more were using longer lengths of 11, 12, 13, and 14 feet than those using shorter lengths of 8 and 8/2 feet. The ten-foot tie is also a better tie to use for the fourstringer chord.
Ties fastened every third tie appeared to be in more common use than other spacings and was deemed ample. Drift bolts were indicated in plans recommended in 1918 for fastening the ties to the stringers and were adopted by this Committee because of the maintenance required on bolts if used.
Five in. by 8 in. guard timbers were used because a six inch timber would encroach on the clearance diagram. The use of lag screws to fasten guard timber to ties is in accordance with previous A.R.E.A. recommendations.
The use of separator blocks of either metal or wood between the ties is recommended as worthy of further investigation and study.
Drift bolts are shown for fastening stringers to caps because this method is used by railroads as indicated by replies to questionnaire almost twice as often as all other methods put together. The Committee recommends that only one drift bolt be used in an intersection of chord with cap, feeling that one is sufficient and that more only increased the chance for decay and difficulty of realignment if necessary.
The Committee recommends four-pile bent for the light 12-foot span, five-pile bent for the heavy 12-foot span, five-pile bent for the light 16-foot span and six-pile bent for the heavy 16-foot span, and that the same number of posts be used in timber bents as for the corresponding spans and loadings of the pile bent designs. This gives a consistent series considering the loading and spans without excessive pile loading. All but the center one or two piles are shown driven on a batter to give greater stiffness to the bent.
Elevation on curves is best secured by cut-offs of posts or piles rather than by shims or tapered ties.
Three in. by 10 in. sway bracing is used as shown in returns almost in ratio of 3 to 1 of all other sizes together.
The 12 in. by 14 in. cap is used in more plans than all other sizes put together, and was used by the Committee for the lighter loading, but the Committee felt that under the heavier loading the 14 in. by 14 in. cap was to be preferred to give greater end bearing areas.
Where concrete footings may be used, the Committee has shown a design with the posts butting or resting directly on the concrete, thereby eliminating the sill. It usually early shows weakness by decay and compression. Horizontal braces above the concrete tie the posts together and dowels are used to hold the bent in line on the concrete and to