Behavior and Capacity of Headed Reinforcement

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

CONCLUSIONS

A proposed model for the anchorage capacity of headed reinforcement has been presented. The model consists of two components: 1) head bearing; and 2) bond. The bar stress provided by head bearing (f^sub s,head^) is calculated using Eq. (3). The bar stress provided by bond (f^sub s,bond^) is calculated using Eq. (5). Total bar stress is the sum of the two. A minimum anchorage length L^sub a^ of 6d^sub b^ is recommended. Strut-and-tie modeling is recommended for the calculation of anchorage length.

ACKNOWLEDGMENTS

The support of the Texas Department of Transportation and the guidance of the project supervisor D. Van Landuyt is gratefully acknowledged. The test program was conducted at the Ferguson Structural Engineering Laboratory of the University of Texas at Austin. The help of the laboratory staff and special efforts of graduate students M. Ziehl and A. Ledesma were essential to the conduct of the study.

NOTATION

A^sub b^ = bar area, mm^sup 2^

A^sub gh^ = gross head area, mm^sup 2^

A^sub nh^ = net head area, A^sub gh^ - A^sub b^, mm^sup 2^

A^sub tr^ = area of confining reinforcement that ties lapped bars to compression zone, mm^sup 2^

b = minimum lateral width of strut measured perpendicular to line of force, mm

c = minimum cover dimension, measured to bar center, mm

c^sub 2^ = minimum cover dimension, measured in direction orthogonal to c, mm

d = distance from extreme compression fiber to center of longitudinal reinforcement, mm

d^sub b^ = bar diameter, mm

f^sub c^' = concrete compression strength, from cylinder tests, MPa

f^sub s^ = bar stress, in general, MPa

f^sub s,bond^ = bar stress provided by bond, MPa

f^sub s,head^ = bar stress provided by head bearing, MPa

h^sub 1^ = length of strut along line of force, mm

L^sub a^ = anchorage length, measured from point at which tie bar first intersects strut boundary to end of tie bar, mm

L^sub s^ = lap splice length, measured between head faces, mm

n^sub 5%^ = 5% fractile coefficient

P = bearing reaction at CCT node, kN

s^sub b^ = center-to-center spacing between bars in layer, mm

u^sub bond^ = average bond stress along bar, MPa

θ^sub strut^ = strut angle, measured between axis of tie bar and axis of strut

Ψ = radial disturbance factor, 0.6 0.4(c/c^sub 2^) ≤ 2.0

REFERENCES

1. Thompson, M. K.; Ziehl, M. J.; Jirsa, J. O.; and Breen, J. E., "CCT Nodes Anchored by Headed Bars-Part 1: Behavior of Nodes," ACI Structural Journal, V. 102, No. 6, Nov.-Dec. 2005, pp. 808-815.

2. Thompson, M. K.; Jirsa, J. O.; and Breen, J. E., "CCT Nodes Anchored by Headed Bars-Part 2: Capacity of Nodes," ACI Structural Journal, V. 103, No. 1, Jan.-Feb. 2006, pp. 65-74.

3. Thompson, M. K.; Ledesma, A.; Jirsa, J. O.; and Breen, J. E., "Lap Splices Anchored by Headed Bars," ACI Structural Journal, V. 103, No. 2, Mar.-Apr. 2006, pp. 271-279.

4. ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-02) and Commentary (318R-02)," American Concrete Institute, Farmington Hills, Mich., 2002, 443 pp.

5. DeVries, R. A., "Anchorage of Headed Reinforcement in Concrete," PhD dissertation, University of Texas at Austin, Austin, Tex., Dec. 1996, 294 pp.