Behavior and Capacity of Headed Reinforcement

ACI Structural Journal, Jul/Aug 2006 by Thompson, M Keith, Jirsa, James O, Breen, John E

Three previous studies provided an opportunity to compare the proposed model with additional test results:

* Deeply embedded pullout tests at the University of Texas;5,6

* Beam-column joints at the University of Texas;6 and

* Stub-beam pullout test at the University of Kansas.7

The basic parameters of these studies are summarized in Table 4.

Deep embedment pullout tests-The deep embedment pullout study tested headed bars embedded in mass concrete. Bars were pulled in direct tension and failed by side blowout. That study contained confined and unconfined test groups. Only those tests with both bond and head bearing components of anchorage were included in the comparison (tests in which a sheath prevented bond were excluded). All tests provided a ratio of embedment depth to cover greater than 5.0. The configuration of the tests allowed accurate determination of the anchorage length. The distribution of measured/calculated test values for the deep embedment pullout tests is plotted in Fig. 8. The model was conservative for all tests with an average measured/calculated value of 1.9. The proposed model was quite safe for this study.

Beam-column joint tests-The beam-column study tested pairs of headed bars used to anchor negative bending moment in beams at exterior column joints. Bars were pulled in tension and were reported to fail by side blowout or a shear-related rupture that resembled a combination of joint shear failure and concrete breakout. A typical test is shown in Fig. 9. Confinement in the form of vertical column bars and horizontal column stirrups was provided in varying amounts for all tests. The ratio of embedment depth to cover was between 3.2 and 7.3 for all tests. The configuration of the tests did not allow accurate determination of the anchorage length. Embedment depth was substituted for anchorage length in calculations of anchorage capacity. The average ratio of measured to calculated capacity was 1.0 with a range of 0.4 to 1.7. The proposed model was unconservative for approximately half the tests of this study, most likely a consequence from the substitution of embedment depth for anchorage length in bond calculations and from insufficient anchorage length to allow applicability of the head bearing model (a probable strut-and-tie model for one test is shown in Fig. 9. While the anchorage length could not be accurately determined, it must have been significantly less than the embedment depth). Measured/calculated ratios are plotted against embedment/cover ratio in Fig. 10.

Stub-beam pullout tests-The stub-beam study tested single headed bars used as anchorage for flexural reinforcement in the end regions of beam members. The stub-beam configuration allowed the bars to be tested in direct tension. Most tests included confinement in the form of vertical stirrups spaced evenly along the embedment depth of the bar. Bars tended to fail by spalling of the cover concrete. A typical test specimen with appearance at failure is shown in Fig. 11. The configuration of the test did not allow accurate determination of the anchorage length (the probable strut-and-tie model for a stub-beam test without stirrups is shown in Fig. 11. The anchorage length could not be calculated accurately, but must have been shorter than the embedment depth). The published embedment depth was substituted for anchorage length in calculations of anchorage capacity. The average ratio of measured to calculated capacity was 0.8 with a range of 0.5 to 0.9. The proposed model was unconservative for all tests of this study. Measured/calculated ratios are plotted against the embedment/cover ratio in Fig. 10.