High cycle (fatigue) resistance of reinforced concrete beams with lap splices
Afseth, Jeffrey G.
The development of current design standards for reinforced concrete beams containing lap splices has been mainly aimed at preventing failures due to static (monotonic) loading, or low cycle, high intensity (seismic) loading. However, little consideration has been given in these standards or in previous research to the performance of lap splices when subjected to high cycle, low intensity (fatigue) loading. Also, recently proposed design equations for lap splices under static loading will permit shorter lap lengths. For these reasons, a study was initiated to investigate the fatigue resistance of reinforced concrete beams with lap splices. A principal variable in this research program was the degree of transverse reinforcement (stirrups) which was provided along the lap splice. The specimens used in this experimental program were seven meters in length, with a cross section 330 mm wide and 508 mm deep, and were reinforced with two No. 30 Grade 400 bars, top and bottom. A lap splice was provided in the bottom tension bars situated in a constant moment region created by a symmetrical two point loading system. The lap splice length and confinement were designed according to ACI Committee 408 recommendations for static loading. Two different lap splice configurations were provided for the test specimens. The first used the maximum number of stirrups that were deemed to be effective for static loading, permitting a shorter lap length. In the second configuration nominal stirrups were provided to the splice at a spacing of one half the effective beam depth, requiring a longer lap length. Failure of the specimens with the heavier stirrups occurred from fatigue of the tensile reinforcing steel and showed similar fatigue resistance to beams containing continuous reinforcement. The specimens with nominal stirrups usually resulted in a splice failure after fewer load cycles, after splitting and delamination of the concrete confining the lap had occurred. Both beams had a similar static bond resistance; however, the beams with the heavier stirrup confinement (shorter lap length) displayed a greater fatigue resistance than the beams with lighter stirrups (longer lap length). These results are in agreement with conclusions reached for seismic loading conditions, which found that splice confinement provided by a high degree of transverse reinforcement is required under cyclic loading to offset the rapid deterioration of the concrete cover confinement.