Headed and Mechanically Anchored Bars in Tension

INTRODUCTION

Historically bonded straight or hooked rebars were used to provide rebar anchorage. This method of anchoring reinforcement in concrete assumes a sufficient bond between the rebar and the concrete. However, straight or hooked rebar may not provide the most effective anchorage and there are a few situations where headed bars may provide a desirable solution from the consideration of design as well as constructability aspect. When such headed bars are used, the required anchorage is achieved through bearing on the head and a combination of rebar bond and bearing on rebar deformations (see Fig.1)

The use of hooks often results in steel congestion, difficult fabrication and construction, and greater potential for poor concrete placement. In addition, cyclic loading tends to degrade the anchorage capacity due to the slip. The use of anchor plates or heads either welded or threaded to the longitudinal bar (often called headed bars) has been identified as a viable alternative to hooked bars for exterior joints, and also provides ease of fabrication, construction, and concrete placement. Headed bars with rectangular, round, or elliptical heads are available. The heads may be welded, forged, or threaded (Fig. 2).?

Manufacturers of Headed bars

HRC and ERICO were the first manufacturers of the headed bar. HRC produced friction-welded heads and provided four types of plates: square, rectangular, circular, and oval. ERICO produced a tapered thread connection between the reinforcing bar and the head, which when screwed provided a headed bar. Other firms such as Dextra Group, Bar Splice, and Dayton are also manufacturing similar types of headed bars.

ADVANTAGES OF USING HEADED BARS

The advantages of using headed bars are particularly evident when used in heavily reinforced concrete sections, where rebar congestion results in constructability problems. Hence, they are being used increasingly in civil infrastructure projects, nuclear power plants, and multi-story buildings, where reinforcement congestion normally occurs. The use of headed bars eliminates the requirement of bending the bars for satisfying anchorage length and also allows the reduction in development length. This, in turn, results in reduced congestion and facilitates better concrete consolidation. ?Such reduction of congestion will also result in better behavior under seismic loads.

The transportation of straight lengths of rebars with headed ends is easier than transporting rebars with bent anchorages. The on-site handling and fixing also become easier. Thus, the use of headed bars can offer considerable time and cost advantages as well as potential improvement in the quality of concrete. The head can be screwed/unscrewed to suit any adjustment if needed.

TYPES OF HEADED BARS

Most manufacturers of headed bars produce products with two sizes -4Ab and 9Ab- of heads (Goodman, 2022). The smaller 4Ab head will have a net bearing area of at least four times the cross-sectional area of rebars, complying with ACI 318-19 and ASTM A970 requirements (The net bearing area equals the area of the head minus the nominal area of the bar (Abrg = Ahead – Ab). In this case, the anchorage is provided through a combination of bond (development length) and bearing. ?This small size head is used to terminate reinforcing bars in lieu of a standard hook (replacing standard hooks), which will improve constructability and reduce congestion. In most cases, the installation parameters for the headed bar are the same as that of the hooked bar they are replacing. It has to be noted that ACI 318 Code does allow for a shorter development length when using heads.

The larger 9Ab heads will have a net bearing area of nine times the cross-sectional area of rebars (this was the standard in the USA prior to 2004 and the Standard head in Europe and Canada now. It has a gross bearing area (including the area of bar > 10 Ab). Caltrans in the USA approves the use of full-size (9Ab) headed bars for bridges. It is important to note that the 9Ab head is bond-independent (no development length required). The anchorage in this case is provided through bearing alone directly beneath the head. The 9Ab heads are used to terminate reinforcing bars when the point of maximum stress is close to the end of the bar or when the development length leading up to the head is neglected during design.

Headed bar reinforcement is usually formed by friction welding of plates, by forging an upset bearing surface at the end of a reinforcing bar, or by forging threads into the end of the bar, which are then used to attach the plate. Headed bar reinforcement must comply with ASTM requirements, which include tensile tests that confirm that necking occurs at least one diameter away from the affected zone.

The behavior of Headed bars

Thompson et al, 2006 suggested that the headed bar anchorage is provided by a combination of head bearing and bond. The initial anchorage is provided by the bond between concrete and rebar. As additional load is applied to the bar, the bond achieves peak capacity and begins to decline. As the process of bond deterioration occurs, the bond anchorage is transferred to the head, causing a rise in the head bearing. The anchorage capacity at failure is provided by a combination of peak head bearing and reduced bond.?Thompson et al, 2006 opinioned that strut-and-tie models are the best for determining the anchorage length and that the node and strut dimensions play a critical role in defining the anchorage length. They also recommended a minimum anchorage length of 6db.?

Tests conducted by Chun et al 2007 and Kang et al 2010 reveal that the hysteretic behavior of exterior joints constructed with headed bars was similar to or even superior to joints with hooked bars. Head size with a net area of three to four times the bar area was sufficient to anchor the beam reinforcement effectively (with a development length shorter than that needed for hooked bars) within the exterior beam-column joint. For roof-level connections, anchoring the column heads above the beam bars and adding an additional layer of transverse reinforcement led to improved behaviour.

Development length based on ACI code

?Though IS 456:2000 states that mechanical devices can be used to anchorage bars with the approval of the engineer-in-charge, it does not contain any clause to calculate the anchorage length. Clause 12.6.1 of the ACI 318 code suggests Eqn. 1 to determine the development length of headed deformed bars in tension, Ldt, provided the following conditions are satisfied (see Fig. 3) ?ACI 318 [4] required that the use of headed bar should satisfy these following provision : (1) The yielding strength of bar should not exceed 420 MPa; (2) The maximum bar diameter is 36mm; (3) The net bearing area should be at least 4Ab, where Ab is the area of bar; (4) Clear cover of the bar is not less than 2db , (5) clear spacing between bars is not less than 4db, and (6) Normal weight concrete is used

Ldt = 0.215 Beta fy db/(Sq.root fck) ≥ 8db or 150 ?mm???????????????????????????????????????????????????????(1)

The above restrictions are based on available experimental results (Thompson et al, 2006). Modifier β is taken as 1.2 for epoxy-coated reinforcement and 1.0 for all other cases. Note that Eqn. 1 results in a development length of approximately 80% of that required for hooked bars. Also Eqn. 1 is not a function of the head size, though it is indirectly accounted for through the minimum requirements.

To avoid congestion, it may be desirable to stagger the heads. When longitudinal headed bars from a beam or a slab terminate at a supporting member, the bars should be extended up to the far face of the confined core of the supporting member, for example in a column as shown in Fig. 4 (allowing for cover and avoiding interference with column reinforcement). Extending the bar to the far side of the column core helps engage the entire joint in resisting the anchorage forces and thereby improves the performance of the joint (ACI 318-19). Thus, in this case, the resulting anchorage length will exceed Ldt. Note that additional reduction in development length, such as those allowed for standard hooks with additional confinement provided by transverse reinforcement, is not allowed for headed deformed reinforcing bars, as test data does not show significant improvement in the behavior due to confinement. However, transverse reinforcement, which may help to limit splitting cracks in the vicinity of the head, is recommended. Also, as per ACI 318, headed bars should not be considered effective in compression, as no test data are available yet to show that the use of heads is beneficial in compression.

Thompson et al 2006 also cautioned that the location of the critical section based on the strut-and-tie model will be different from the one assumed based on beam theory.

?Application of Headed Bars

Headed bars can be used in a variety of applications which may include beam-column joints, knee joints, pile caps, column-roof slab connections, at the end of cantilever beams, in Corbels,?as transverse shear reinforcements, and as shear wall cross ties. Some of these applications are shown in Figs. 4 and 5.

References

1. ?ACI 318-19, Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute, Farmington Hills, Michigan, 628 pp.
2. Alrasyid, H., Yoganata, Y.S., Suluch, M. and Iranata, D.(2017) "Headed Reinforcement in Concrete Structure: State of The Art", 3rd International Conference on Construction and Building Engineering, AIP Conference Proceedings 1903,020015.https://doi.org/10.1063/1.5011495
3. Berner, D.E., Gerwick, B.C., and Hoff, G.C. (1991) “T-Headed Stirrup Bars,” Concrete International, ACI, Vol. 13, No. 5, pp..
4. Chun S-C., Lee, S-H., Kang, T.H.-K, Oh, B., and Wallace, J.W. (2007) “Mechanical anchorage in exterior beam-column joints subjected to cyclic loading”, ACI Structural Journal, Vol. 104, No. 1, pp. 102–112.
5. Kang, T.H.-K, Ha, S.-S. ?and Choi D.-U. (2010) “Bar pullout tests and seismic tests of small-headed bars in beam-column joints”, ACI Structural Journal, Vol. 107, No. 1, pp. 32–42.
6. Thompson, M.K., Jirsa, O.J., and Breen, J.E. and Klingner, R.E. (2002) Anchorage Behavior of Headed Reinforcement: Literature Review, Research report FHWA/TX-0-1855-1, Center For Transportation Research, Bureau of Engineering Research, The University of Texas at Austin, 102 pp.
7. Thompson, M.K., Jirsa, O.J., and Breen, J.E. (2006) “CCT nodes anchored by headed bars- Part 2: capacity of nodes”, ACI Structural Journal, Vol. 103, No. 1, pp. 65-73.
8. Thompson, M.K., Jirsa, O.J., and Breen, J.E. (2006) "Behavior and Capacity of Headed Reinforcement", ACI Structural Journal, Vol.103, No. 4, July-Aug., pp.522-530.
9. Thompson, M. K., Ledesma, A., Jirsa, O.J., and Breen, J.E. (2006) “Lap splices anchored by headed bars”, ACI Structural Journal, Vol. 103, No. 2, Mar.-Apr. 2006, pp. 271-279.
10. Yang, J.M.,?Min, K.H.,?Shin, H.O., and Y.S. Yoon (2010) "The use of T-headed bars in high-strength concrete members", Conf. on Fracture Mechanics of Concrete and Concrete Structures-High Performance, Fiber Reinforced Concrete, Special Loadings, and Structural Applications, May 23-28, B. H. Oh, et al. (eds.), Korea Concrete Institute, pp.1328-1334
11. Wright, J.L and McCabe, S.L. (1997) The development length and anchorage behavior of headed reinforcing bars, SM Report 44, University of Kansas Center for Research, Lawrence, Kansas, Sept.