see Turneaure and Maurer's" Prin Reinforced," and Hool and Johnson's "Concrete Engineer's Handbook." ciples of Reinforced Concrete Construction," Taylor and Thompson's "Concrete, Plain and REFERENCES.-For excellent algebraic solutions, PART IV. CONSTRUCTION OF HIGHWAY BRIDGES. CHAPTER XXIV. BRIDGE ENGINEERING. Introduction.-The discussion in this chapter will include Bridge Surveys, Bridge Plans, Bridge Contracts, and Specifications. For a detailed discussion of Structural Drafting, see Chapter XII of the author's "Structural Engineers' Handbook." BRIDGE SURVEYS.-Before beginning the plans for a highway bridge, a very careful and complete survey should be made of the proposed bridge site. The information should be complete enough to enable the designer to decide upon the proper type and details of the structure which is best suited to the conditions. The survey should include the following data. way. 1. Location of bridge together with distance from shipping points. 2. Complete data with careful sketch plans of the present structure, and substructure. 3. Data on high and low water together with a report on the efficiency of the present water 4. Survey of proposed bridge site which should include a. Bench mark. b. Survey should be tied to section corners or definite and known land lines. c. The alignment of the road should be given on each side of the bridge. The skew of the center line of the bridge should be given with the channel. d. The plan should show the course of the stream, the bank lines, old channels, dykes or spoil banks, direction of flow during flood stage, bank erosion, etc. The plan should show the location of the old structure and a proposed location of the new structure. The plan should show contours of the stream together with high and low water marks. Soundings should be made to determine the character of the foundation material. The soundings may be made by boring, by driving pipes or by digging pits. For an important bridge crossing two sets of borings should be made. (1) Borings should be made along the center line, and from them if favorable the location of the piers and abutments should be determined. (2) After tentative locations of the piers additional borings should be made on the site of each pier or abutment, at least one at each corner, and intermediate ones if necessary. The data should include the lowest water level and depth of foundations, character of material upon which the foundations will rest, data on number, spacing and length of piles if they are to be used. An estimate should be made of the safe load on the foundations, and a recommendation should be made as to the best type of foundation. The liability of the bank to scour should be noted, and suggestions should be given as to the best means, such as riprap or sheet piling to prevent it. The plan should show the location of bench marks, alignment hubs and reference points, section and township numbers, width of right of way, property owners of adjacent property, directions, scales, and other data. e. The profile should show the present and proposed road grades; the cross-sections of the channel on center line, and if the channel is irregular cross-sections on lines parallel to the center line. The high and low water marks should be shown and the date of high water should be noted. All runoff data should be recorded. Soundings should be made of the stream and data should be obtained on scour. All overflow openings should be recorded. All intersecting road grades should be recorded. f. A survey should be made of the stream for a distance varying from 500 ft. to 1,000 ft. on both sides of the bridge site to determine the fall, the cross-section and hydraulic properties of the stream bed that will make it possible to make an estimate of the flow of the stream, and also the necessary clear channel at the bridge site. The clear openings of other bridges on the same stream should be noted together with notes on the high water marks, and data on the adequacy of the waterway opening. g. Surveys should be made on the center line of the highway with cross-sections at intervals of 100 ft. or less, so that the cost of constructing approaches may be estimated. h. If the stream has a crooked channel a survey should be made to obtain complete data to determine whether or not a channel change is necessary. FIG. 1. PROFILE AND MAP OF BRIDGE SITE. OREGON HIGHWAY COMMISSION. i. Recommendations should be made by the engineer in charge of the survey as to (1) type of structure; (2) number and length of spans; (3) width of roadway; (4) proposed grade elevation; (5) special loading requirements; (6) depth of footings and type of foundations; (7) special features of design, such as light poles, sidewalks, name plate, retaining walls. A plan and a profile of a bridge site as prepared by the Oregon Highway Commission is given in Fig. 1. The following data are required to be furnished by the Oregon Highway Commission. |