Lap Splicing of Large High-Strength Steel Reinforcing Bars

Abstract

Use of large bars is advantageous in large structures like nuclear power plants. Nevertheless, ACI 318-19 prohibits tension lap splices for bars larger than No. 11 [36 mm] because limited test data are available. Eleven large-scale reinforced concrete beams with tension lap splices were tested under monotonically increasing four-point bending until failure to support design provisions for large diameter bars (No. 14 and 18 [43 and 57 mm]). Specimens had different amounts of transverse reinforcement, nominal concrete compressive strengths of 5 or 10 ksi [35 or 69 MPa], and target bar stresses at splice failure of 60 and 100 ksi [420 to 690 MPa] for No. 14 [43 mm] bars and 60 ksi [420 MPa] for No. 18 [57 mm] bars.Test results show that lap splices of No. 14 [43 mm] bars can develop bar stresses up to 100 ksi [690 MPa] and that lap splices of No. 18 [57 mm] bars can develop bar stresses up to at least 60 ksi [420 MPa]. These limits reflect the scope of the text matrix, as there was no indication that higher bar stresses cannot be attained. Analyses showed that both the ACI 408R-03 and ACI 318-19 equations for lap splice lengths become less conservative as bar size increases. This trend was apparent when comparisons were made using prior test results between groups of specimens with No. 6 [19 mm] and smaller bars, No. 7 to No. 10 [22 to 32 mm] bars, No. 11 [36 mm] bars, and bars larger than No. 11 [36 mm]. Two methods are proposed for modifying ACI 408R-03 and ACI 318-19 development length equations to obtain similar levels of conservatism across all bar sizes: a bar size factor (1.15 for No. 11 [36 mm] bars and 1.25 for larger bars) in the numerator of the development length equations or an increased exponent on the bar diameter term (replacing d_b with d_b^1.35 in ACI 408R-03 and d_b^1.25 in ACI 318-19). A minimum clear cover of one bar diameter is recommended for all lap splices. Transverse reinforcement should also be required throughout lap splices of No. 14 and 18 [43 and 57 mm] bars, as even small quantities (K_tr/d_b= 0.5) tended to reduce size effect. Other analyses showed that increasing the limit on ((c_b+K_tr)/d_b ) in the ACI 318-19 development length provisions from 2.5 to 3.5 results in similar conservatism for all amounts of confinement. It is also recommended that development length l_d be limited to 50db when designing unconfined lap splices.