Behavior and Design of Deep Structural Members Part 5: Resistance and Behavior of Reinforced Concrete Beams of Normal Proportions Under Rapid Loading

The object of this investigation was to obtain information on the strength and behavior of reinforced concrete beams subjected to rapid loading. To this end, 33 beams of various strengths, 6 by 12 inches in cross section and 9 feet or 12 feet- 8 inches in span were tested under static and dynamic loads. Three percentages of tension reinforcement were employed using intermediate grade steel. Some beams also had compression and/ or shear reinforcement. Concrete strength, beam width and depth, and yield strength of reinforcement were essentially constant. Eight of the two-point loaded beams were tested statically, requiring from about two to six minutes each to reach collapse deflection. In the dynamic tests of the other 25 beams, loads were applied in from 0.1 to 0.8 times the natural period of vibration of the beam. Some of the dynamic loads were of "infinite" duration, while others were terminated at from one-half to three times the beam period. The load levels varied from less than static yield capacity to more than dynamic ultimate capacity. The analysis of the test results consists of determining the dynamic resistance characteristics of the test beams. This was accomplished by considering the beam. to be a single-degree-of-freedom system and analyzing its behavior on an analog computer. The measured load pulse was fed into the computer along with an arbitrary resistance function for the beam. This resistance function was then changed until its response matched the response measured in the test. Dynamic resistance functions were also determined using the strain rates measured in some of the tests together with the available results of dynamic tests of coupons of reinforcing bars. The resistance functions determined with the analog computer are compared with the computed functions and with the static load- deflection characteristics. An analytical procedure for the determination of the dynamic resistance of reinforced concrete beams is proposed which involves the use of the deflection rate at yield. The most important conclusions concern the direct relation between the dynamic yield level of a reinforced concrete beam and the yield strength of the tension reinforcement under dynamic loading; the apparently small effect that dynamic loading has on the collapse deflection of reinforced concrete beams; and the feasibility of using established formulas, developed in connection with static tests, for the prediction of dynamic resistance provided proper account is taken of the increase in yield strength of the reinforcing steel.Research Directorate, Air Force Special Weapons Center. Kirtland Air Force Base.Contract AF 29(601)-468Project 108