Bulletin (1902-10)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1906 Excerpt: ...exposed steel observation, and hence the stress in the steel should compare with that calculated from the moment of the applied load only. This agreement is what is to be expected from the usual assumption that the deformation in the concrete at the level of the steel is the same as that in the steel. It is of interest to compare the effect upon the exposed steel and upon the encased steel when the load is released. In Beam No. 49, after a load of 9000 lb. had been applied, the load was released and readings taken. As usual the observations indicated retained deformations in the 42-inch gauged length, but the instrument on the exposed rods returned almost to the original reading. The retained deformation indicated by the extensometer on the exposed rod amounted to.000023, corresponding to a stress of, say, 630 lb. per sq. in. 53. Position of Neutral Axis and Value of Modulus of Elasticity.--The successive positions of the neutral axis within the middle third of the span length, as determined experimentally by the method already explained, are shown for the various beams on the diagrams given in Fig. 56, 57 and 58 at the end of the bulletin. In general, the change in the position of the neutral axis as the applied load is increased follows the law outlined on page 29 of Bulletin No. 1. The neutral axis rises during the second stage and remains nearly stationary during the third stage. (For the use of the terms second stage and third stage see page 21 and Fig. 15 of Bulletin No. 1.) For beams in which, by reason of a large amount of reinforcement high compressive stresses in the concrete are developed, the neutral axis falls considerably during the stage of rapid deformation of the upper fiber. An example of this is shown in Beam No. 46. (Fig. 56.) Beams with...