Engineering Wonders of the World
Volume I

424 ENGINEERING WONDERS OF THE WORLD. In most buildings and engineering structures it frequently happens that main beams are continuous over columns or piers ; that second- ary beams are continuous over the main beams ; and that floor, deck, or roof slabs—which, in reality are thin wide beams—are continuous over both main and secondary beams. Wherever such arrangements occur, the relative positions of the tensile and compres- sive stresses due to bending are reversed for a short distance near each support, the tensile stress being transferred from the bottom to Fig. 15.—LONGITUDINAL SECTION OF HENNEBIQUE REINFORCED CONCRETE BEAM. stance. Adhesion between the concrete and the steel, in spite of the stresses caused by loading, is a matter of prime importance, and without it the remarkable qualities of rein- forced concrete could not be realized. Fortunately, when steel bars have been embedded in concrete, they are gripped so firmly after that material has hardened that there is no fear of slipping or other move- ment. Numerous experiments made for the purpose of determining the amount of the adhesion between concrete and steel bars show that the actual resistance ranges from about 400 lbs. to 650 lbs. per square inch of the steel surface in contact with concrete. Equality of expansion during temperature changes is a very important factor in the case of materials used together; and if there were any appreciable difference between the expan- sion of concrete and steel, the Fig. 16.—REINFORCED CONCRETE MAIN BEAMS, SECONDARY BEAMS, AND CONNECTING SLAB (HENNEBIQUE SYSTEM). adhesion between the materials might be destroyed in the event the top, and the compressive stress transferred from the top to the bottom. Consequently, the bent-up bars represented in Fig. 15 are just in the places where they can effectively reinforce the concrete against tension in its reversed position, while the horizontal parts of the same bars are in the right place for withstanding the horizontal tensile stress near the lower surface in the middle portion of the beam. Before dismissing the subject of reinforced concrete beam design, it is desirable to direct Properties of Concrete and Steel. attention briefly to three im- portant physical properties of concrete and steel, upon which depends the possibility of combining those materials in such manner that their action is akin to that of a single sub- of fire, as happens to terra- cotta and kindred products when used to protect ordinary steelwork from fire. Nothing of the kind is to be feared in the case of reinforced concrete, because the “ co- efficients of expansion ” of concrete and steel are virtually equal. Another point to be considered is the extent to which steel and concrete are stretched when under tensile stress, as in the lower part of the beam shown in Fig. 12. To put the matter shortly, a steel bar of a square inch section is stretched of its original length by every pound of pull; whereas a concrete bar of equal section is stretched or fifteen times as much as the steel. Since steel is able to stand a very much greater tensile stress than con- crete, the latter, in a reinforced concrete beam,