compressive strength of at least 2,000 lbs. per square inch, when tested in cylinders 8 in. in diameter and 16 in. long, and 28 days old, under laboratory conditions of manufacture and storage, the mixture being of the same consistency as is used in the field. Structural steel in tension.. 14,000 High carbon steel in tension... 17,000 †Steel in compression, 12 times the compressive stress in the surrounding con crete Concrete in bearing where the surface is at least twice the loaded area. 700 † Concrete in direct compression, without reinforcement on lengths not exceeding twelve times the least width 350 † Concrete in direct compression with not less than 1 per cent, nor over 4 per cent longitudinal reinforcement on lengths not exceeding twelve times the least width...... ......... † Concrete in compression, on extreme fibre in cross bending.. † Concrete in shear, where the shearing stress is used as the measure of web stress NOTE. The limit of shearing stresses in the concrete, even when thoroughly reinforced for shear and diagonal tension, should not exceed.. † Bond for plain bars.... † Bond for drawn wire... † Bond for deformed bars, depending upon form NOTE: Chapters XVII and XVIII differ, in that, the former chapter is applicable only to the column and to the special case of the simple horizontal beam carrying a uniformly distributed load, while, in the latter chapter, the methods are general and applicable to beams loaded in any manner. The methods of design are, however, identical in both chapters, and in order to render each one complete in itself, a certain amount of matter has been repeated. It is thought that the reading of this chapter will be rendered easier by first showing the application of the theory of design to a simple case, as was done in Chapter XVII. †The unit stresses as recommended in the report were in general higher than these values, which have been reduced in conformity with the fibre stresses employed in other portions of the chapter. CHAPTER XIX SYSTEMS OF REINFORCEMENT EMPLOYED Systems of Reinforcement Employed.-Different Forms of Rods and Bars.-Special Fabrics and Types of Reinforcement. REINFORCEMENT is used in a variety of shapes and combinations, nearly all of them patented, and some of them forming the basis for so-called systems. All these systems of reinforcement have been developed principally during the last decade, each one of them having its adherents and all of them giving substantial structures if intelligently employed. The selection of the type for any particular case will depend upon the nature of the structure, the local conditions, the experience of the designer, and often upon the argument of the salesman. The illustrations will serve to bring out the essential features of the different systems. Specifications for Reinforcing Steel.-The quality of steel to be used for reinforced concrete work has received a great deal of attention from engineers and steel-makers and the rules given below represent the latest practice in this respect: SPECIFICATIONS FOR STEEL REINFORCEMENT* 1. Steel shall be made by the open-hearth process. Rerolled material will not be accepted. 2. Plates and shapes used for reinforcement shall be of structural steel only. Bars and wire may be of structural steel or high carbon steel. * From the report of the Committee on Masonry at the annual convention of the American Railway Engineering and Maintenance of Way Association, Chicago, March 16, 1910. 3. The chemical and physical properties shall conform to the following limits: 4. The yield point for bars and wire, as indicated by the drop of the beam, shall be not less than 60 per cent of the ultimate tensile strength. 5. If the ultimate strength varies more than 4,000 lbs. for structural steel or 6,000 lbs. for high carbon steel, a retest shall be made on the same gauge, which, to be acceptable, shall be within 5,000 lbs. for structural steel, or 8,000 lbs. for high carbon steel, of the desired ultimate. 6. Chemical determinations of the percentages of carbon, phosphorus, sulphur, and manganese shall be made by the manufacturer from a test ingot taken at the time of the pouring of each melt of steel, and a correct copy of such analysis shall be furnished to the engineer or his inspector. Check analyses shall be made from finished material, if called for by the railroad company, in which case an excess of 25 per cent above the required limits will be allowed. 7. Plates, Shapes, and Bars.-Specimens for tensile and bending tests for plates and shapes shall be made by cutting coupons from the finished product, which shall have both faces rolled and both edges milled to the form of a standard test specimen; or with both edges parallel; or they may be turned to a diameter of inch with enlarged ends. *See paragraphs 11 and 12. †" d= 4t" signifies "around a pin whose diameter is four times the thickness of the specimen." 8. Bars shall be tested in their finished form. 9. At least one tensile and one bending test shall be made from each melt of steel as rolled. In case steel differing 3/8 in. and more in thickness is rolled from one melt, a test shall be made from the thickest and thinnest material rolled. 10. For material less than 5/16 in. and more than 3/4 in. in thickness the following modifications will be allowed in the requirements for elongation: (a) For each 1/16 in. in thickness below 5/16 in., a deduction of 2 1/2 will be allowed from the specified percentage. (b) For each 1/8 in. in thickness above 3/4 in., a deduction of I will be allowed from the specified percentage. II. Bending tests may be made by pressure or by blows. Shapes and bars less than one inch thick shall bend as called for in paragraph 3. 12. Test specimens one inch thick and over shall bend cold 180° around a pin, the diameter of which, for structural steel, is twice the thickness of the specimen, and for high carbon steel is six times the thickness of the specimen, without fracture on the outside of the bend. 13. Finished material shall be free from injurious seams, flaws, cracks, defective edges, or other defects, and have a smooth, uniform, and workmanlike finish. 14. Every finished piece of steel shall have the melt number and the name of the manufacturer stamped or rolled upon it, except that bar steel and other small parts may be bundled with the above marks on an attached metal tag. 15. Material, which, subsequent to the above tests at the mills, and its acceptance there, develops weak spots, brittleness, cracks or other imperfections, or is found to have injurious defects, will be rejected and shall be replaced by the manufacturer at his own cost. 16. All reinforcing steel shall be free from excessive rust, loose scale, or other coatings of any character, which would reduce or destroy the bond. Types of Reinforcement. The reinforcement consists of steel in one or more of the following forms: 1. Round or square rods. 2. Twisted or deformed rods. 3. Unit systems. 4. Woven wire, expanded metal, welded, or other fabrics. 5. Spiral reinforcement for columns. 6. Various patented systems. The plain bars either depend upon the adhesion of the steel and concrete for the action of the two materials in combination, or the ends of rods are anchored in the concrete for the purpose of developing their full tensile strength. In the deformed bars the adhesion of the concrete to the steel is supplemented by a mechanical bond due to the shape of the bar. The following bars are among the best known of this class: 1. Ransome twisted bars are made of square bars twisted cold. 2. Johnson corrugated bar in which the mechanical bond is effected by a series of corrugations on the sides of a square rod. 3. Diamond bar which is a round bar crossed by diagonals. 4. Cold twisted lug bar, which is a Ransome bar having small projections at intervals. 5. Cup bar in which the mechanical bond is effected by a series of cups. 6. DeMan undulated bar. 7. Universal type corrugated bar. 8. The Kahn and Golding bars which are provided with attached shear members. Unit Systems, Fabrics, and Spiral Reinforcement.-In the unit systems, the reinforcement, including the tension rods and stirrups, are so tied and framed together that after being placed in the forms the possibility of shifting their positions with respect to the other surfaces of the beam, or to one another, is practically removed. Steel fabrics are largely employed in slab and floor construction, also in conduits, tanks, foundations, etc. Spiral wrappings for columns are employed for the purpose of permitting a higher unit stress to come upon the concrete than could safely be used without such reinforcement. The spirals have the effect of confining the concrete and preventing it from bulging or splitting. Special Systems of Reinforcement.-The following are among the so-called special systems of construction: |