maximum possible load. Figs. 937 and 938 illustrate a method of providing bearing for a large girder in a wall. Figs. 939 and 940 show a cast-iron bressummer, 941 an example of a built-up girder with cast-iron stiffeners, and 942 a steel binder used in ordinary floor construction. CHAPTER XXXVIII. COMBINED STRUCTURES IN IRON AND STEEL. WE now come to this last category of constructional steel work. In this chapter we shall give examples of combined structures whose essential constituent is steel, and investigate some methods whereby pipes for sewers and water mains can be taken across rivers. Now in cases like this it is a matter of no little judgment to decide upon the correct place in a book of this kind for these special and so important problems. Mention was made under Sewers of a way of effecting this, but we decided, after much deliberation, that the actual calculations and design are better placed in this division. 4. C 2. 6 -1.0° 84 R.S.J. 3x1 Strap I Thread We hope, therefore, that this may serve as an apology for a little want of uniformity in some cases in the book. Laying Pipes across Rivers and Streams. The means of Stone Template carrying sewer pipes and water mains across small rivers and ravines is a problem which very often confronts engineers, especially those in municipal practice. It is intended in this chapter to point out to the reader methods of effecting this in a simple, practical, efficient, and at the same time inexpensive way. We are not concerned with work of large magnitude, such as siphon pipes under canals and navigable rivers, nor is it intended to treat upon large aqueducts. Let us take the following case: FIG. 943. It is required to lay a 30-in. cast-iron flange water main over a shallow stream 60 ft. wide, at a height, say, 4 ft. above maximum flood-level. Referring to figs. 943 and 944, we are placing two piers (4 in all) at 20-ft. centres. They would be made of masonry, brickwork, or concrete. Looking up a table of cast-iron pipes, we find that a 30-in. water main, tested to 300 lbs. pressure, weighs 3.29 cwt. per foot-run, and 3.29 cwt. × 20 = 3.29 tons (say 3.30 tons) between each span. Again, the sectional area of a 30-in. pipe 4.91 sq. ft., and × 2098.2 cub. ft. capacity. X Now taking water at 62.5 lbs. per cubic foot we have 98.2 × 62.5-6137.5 lbs., or say 2.8 = tons. Therefore 3.30 +2.806.10 tons total load. Referring to any table of properties of rolled steel joists, we find that to support. tons (3.05 tons) 6.1 if we use two 8-in. x 4-in. I beams, we shall have a supporting capacity of 7 tons, or nearly 1 ton in excess. These, moreover, would be the most economical sections to use. The weight of these joists per foot-run is 18 lbs. (18 × 120) tons = .. the total weight of the superstructure is (6.10 x 3) + 2240 This is 19.26 tons, or, allowing for contingencies, straps, etc., say 20 tons. distributed over 4 piers, the two middle ones of which will each bear about 6 tons and the two outer ones 34. This will fix the area of the piers and foundations. Figs. 943 and 944 show the construction fully detailed and dimensioned. Let us now take another case, that of the 9-in. water main (spigot-andsocket) laid across a river, the span from centre to centre of supports being 31 Straps 8x4 R.S.J. 6". 6" 9'. 0° -20.0° Span FIG. 944. = 50 ft. Now 50 ft. of 9-in. water main, including sockets, weighs (if of in. metal) 2610 lbs., assuming it to be made up of 9-ft. lengths at 47 lbs. per foot-run, each socket being equal to 1 ft. of pipe. The water in the pipes will weigh 1352 lbs. by the rule of a gallon of water being equal to 10 lbs., and the number of gallons per yard in any pipe being equal to 1-10d2 (d= diameter in inches). Therefore total weight 2610+13523962 lbs... Now a good economical way of doing this would be to lay a rolled steel joist of suitable section with flanges vertical, so forming a trough in which the pipes would lie. Looking up a table of joists again we find that a 12-in. × 6-in. × 54-lbs. I beam would be somewhere near the mark. Say we test the assumption by the familiar equation ± so as to see that the compression W M and tension are within safe limits. Area of joists .'. = = 15.88 sq. in. 48+1.47 1.95 tons 7.61 13.85 Maximum compression = 7.61 tons. ± 15.88 9.4 per sq. in. compression and 1 ton tension, both easily within safe limits. Figs. 945 to 948 show full details. We are assuming that the supports would naturally be convenient to use are to be steel joists as shown. It 12-in. x 6-in. joist for this if we could. We proceed to test this by the Rankine Gordon formula, as follows: P = : We may therefore safely use this section. The tension rods may be easiest calculated by the diagram shown in fig. 949, in which we have a tension of 17,300 lbs. = 7.85 tons. Allowing a tension of 6 tons per sq. in., we = say 1.25 sq. in. We should use two -in. rods, which want 7.85 |