a, c, Typical failures by steel reaching elastic limit; b, typical failure by diagonal tension and stripping of rods. METHOD OF RECORDING CHARACTER OF REINFORCED-CONCRETE BEAM FAILURES. beams would fail in the same manner, since the gain in strength of the concrete rendered it less likely to fail. Consequently, when the older beams were tested, the cracks were marked out for the increments of 1,000 pounds for the first beam only of each set of three. The other two were tested to failure; at the maximum load the cracks were marked out as before. A group of three beams treated in this manner is shown in Pl. XVII, c, the bottom beam being the one first tested. Slipping of the rods with reference to the adjacent concrete at the ends of the beam is determined by means of a micrometer reading to one ten-thousandth of an inch, shown at the extreme right end of the beam illustrated in Pls. XV, B, and XVI, A. The micrometer is clamped to the end of the beam, from which a small portion of concrete has been removed, exposing the end of one of the reinforcing rods. The micrometer screw is adjusted to touch the end of the exposed rod. No electric contact is used with the micrometer since the least slipping of the rods may be detected by touch. This same instrument is used to detect the slipping of the rods in the bond tests. Form L is used for recording the results of the tests on reinforced concrete beams. Form L. { Beam Reg. No. points spaced inches. Span Number.. ...... pounds. ...... Load applied at Applied inches from center. .; size.................; kind.............. Reg. No. of steel...... For distribution of steel, see diagram No.... For information regarding concrete and corresponding test pieces, see Batch report Bm.... Deformeter No. Weight of deformeter load at first observed crack Maximum applied load Development of cracks observed Beam brought from damp closet at time ......; sheet given to office at pounds. Position of crack COMPUTATIONS. feet. Character of failure inches from center. time; test completed at due to...... The forms of batch report for both the plain and reinforced beams (Form F, p. 40) are identical except that for the reinforced beams. the number, size, position, and form of rods are also given. For these beams the percentage of reinforcement is computed and the yielding point of the steel is found from tests on short pieces cut from the reinforcing rods before being placed in the beams. In order to make clear the methods used in obtaining some of these values a short statement of the theory will be given. The method of finding the necessary decrease in the reactions in order that the total deformation within the gage length will be zero will be given first. This method is used in order to obtain a greater number of readings of the micrometers during the test and therefore more points on the deformation-bending moment curves. When a beam rests freely on supports, the upper and lower fibers are deformed on account of the bending moment due to the weight of the beam. When the supports are at the ends of the beam, the upper fibers are shortened and the lower fibers are lengthened. For equal moduli of elasticity in tension and compression, which are constant for concrete for small loads, the deformation at any point of the beam is proportional to the bending moment at that point, FIG. 3.-Diagrams illustrating method for computation of concrete beams. Upper diagram: Notation used. Lower diagram: Curve of bending moment within gage length (beam supported at third points). and the total deformation over any length of the beam is proportional to the area of the bending-moment diagram over that length. Therefore when the total positive bending-moment area in the gage length of the deformeters equals the total negative bending-moment area in the gage length, the net otal deformation in that length is zero, and both the upper and lower fibers of the beam have the same length as when unstressed. For a particular reaction at the ends of the beam the positive bending-moment area in the gage length is equal to the negative bending-moment area. In order to get this reaction, the beams are supported at the third points by stirrups suspended from the head of the machine. As the stirrups take more and more of the weight of the beam the end reactions become smaller and smaller and the character of the bending-moment diagram within the gage length changes until the desired condition is reached. The method of finding the required reactions for total zero deformations within the gage length in terms of the weight of the beam and other known quantities (fig. 3) is as follows: Let L=distance between the supports. L_distance from each support to force exerted by each 3 stirrup. W = total weight of beam. L W 2 - R=force exerted by each stirrup at a distance of from the supports. 3 Reach reaction at end. = SS any vertical section within the gage length at a distance, X, from one of the gage points. My-bending moment at section SS. M.-bending moment at deformeters, where X=0. 2 This M.-bending moment at center of beam, where X =8. m=constant bending moment over the gage length due to the weight of all attachments, such as bearing blocks under the load points and the deformeters. weight is applied outside of the gage length and equally on each side of the center of the beam. The bending moment at section SS, considering forces to the left. only, is as follows: |