National Science Foundation

Behavior and Design of Cast-in-Place Anchors
under Simulated Seismic Loading (NEES-Anchor)
Phase III: Reinforced Single Anchors Subjected to Tension 



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Description:  Anchor tension reinforcement

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   Current design codes recommend U-shaped reinforcing bars placed close to anchors in tension (e.g., within a distance equal to 50 percent of the embedment depth) to restrain concrete breakout (a sudden failure).  This paper presents a study of cast-in-place anchors reinforced with five patterns of anchor reinforcement conforming to the code recommendations.  Laboratory tests using 25 mm [1 in.] diameter anchor bolts showed that code-conforming anchor reinforcement may not ensure reliable anchor behavior unless the concrete around the anchor is confined and restrained from splitting cracks.  An alternative design for anchor tension reinforcement was proposed, consisting of a group of closed stirrups proportioned to resist the code-specified anchor steel capacity in tension. Additional crack-controlling reinforcement is needed unless the anchors are placed far from any free edges.  The anchors with the above recommended reinforcement in this study were able to develop ductile post-yield behavior under both monotonic and cyclic loading. 

Existing anchor reinforcement

    The existing design methods assume that the concrete breakout similar to that observed for anchors in plain concrete occurs before steel reinforcement takes effect.  With this assumption, the shear resistance of the anchor is exclusively provided by the anchor reinforcement. Anchor reinforcement in terms of hooked bars is required to be fully developed in the assumed breakout cone or the contribution from each bar is calculated according to its development length in the assumed breakout cone. 
    Hairpins are deemed effective as anchor shear reinforcement because they can be placed close to the anchor shaft using a small bending radius on the hairpin.  The transfer of shear load to surface reinforcement is usually visualized using a strut-and-tie model (STM). Strut-and-tie models permit large size reinforcing bars located at a large distance from the anchor bolt as anchor reinforcement as long as the angle between the concrete strut and the applied shear force is small (e.g., less than 55 degrees).  However, tests have indicated that reinforcing bars placed closer to the anchor are more effective.  As a result, the existing design guidelines require the anchor reinforcement to be within a distance equal to half of the front edge distance (0.5ca1).  Such requirements leave a small window of applicability for practical implementations of the anchor reinforcement.  Often time the front edge distance needs to be increased to accommodate the anchor reinforcement, which in turn increases the concrete breakout capacity such that the anchor reinforcement may be no longer needed.
    Anchor reinforcement design for shear in this study considered the following four aspects: 1) an effective reinforcement layout that restrains concrete breakout failure; 2) a proper design force for proportioning the anchor reinforcement; 3) a reasonable distance on each side of anchor bolt, within which the anchor reinforcement is deemed effective; and 4) an accurate estimation of shear capacity of reinforced anchors.

Phase III Experimental Program:

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    Sixteen tests were conducted using 1 in. diameter anchors consisting of an ASTM A193 Grade B7 threaded rod (fy=105 ksi and fut=131 ksi) and a heavy hex nut welded to the end.  Another four tests using 3/4 in. diameter ASTM F1554 Grade 55 anchors (fy=63 ksi and fut=76 ksi) were conducted with two tests each under monotonic shear and cyclic shear loading.  Ready-mixed concrete with a targeted strength of 4000 psi was used, and cylinder tests using three batches of three 4×8 in. cylinders tested throughout the anchor test period showed an average compressive strength of 3525 psi.
    The dimensions of the test blocks containing four anchors each.  One block was prepared for Type 19-150-100 specimens, and two blocks for Type 25-150-150 and Type 25-150-150H specimens. Another block similar to that for Type 25-150-150 specimens was used for Type 25-150-150SG specimens.  Strain gages were installed on the reinforcing bars of the two anchors in this block.  All anchors had an embedment depth of 6 in. The width and depth of the test blocks were selected such that the spacing between the anchors was larger than two times their front edge distances.  Anchors in Type 25-150-150H specimens had two limited side edge distances equal to 1.5 times their front edge distance.  The height of the blocks was 17 in., similar to all other anchor tests in the study.
    The anchor shear reinforcement was proportioned to carry the maximum capacity of the anchor bolts in shear: 15.3 kips for the 3/4-in. anchors and 47 kips for the 1-in. anchors.  Using the nominal yield strength of Grade 60 steel, the required anchor reinforcement was found as 0.25 in.2 for the 3/4-in. anchors, and 0.78 in.2 for the 1-in. anchors.  Therefore two No. 4 bars were provided for Type 19-150-100 specimens.  The required anchor reinforcement for 1-in. anchors was provided using four No. 4 bars with a spacing of 2-in. for Type 25-150-150 specimens, two No. 4 and four No. 3 bars for Type 25-150-150H specimens with a spacing of 3-in., and eight No. 3 bars for Type 25-150-150SG specimens with a 2-in. spacing.  Two additional No. 3 J-hooks were added besides the outmost bars in Type 25-150-150SG specimens as shown in Fig. 4 to host two more strain gages, which were roughly 10 in. away from the anchor bolt.  One straight bar was provided at each corner of the closed stirrups.  Note that some specimens had several narrow stirrups placed behind the anchors, the vertical legs of which were intended to be anchor tension reinforcement, in which case one additional corner bar was provided along the top surface.  However, the planned tension tests were not performed because the concrete blocks were not sufficient for the large tension load that would be carried by the reinforced anchors.  The additional stirrups did not affect the shear behavior of the anchors because they were placed behind the anchor bolts. All reinforcing bars were placed with a cover of 1.5 in.
Phase II test setup
Phase II Results:

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    Anchor steel failure was achieved in all anchors, indicating that reinforcing bars placed outside the code-specified effective distance, such as 0.5ca1 in Type 25-150-150SG and 0.3ca2 in Type 25-150-150H, can be effective as anchor shear reinforcement. However, reinforcing bars must be evenly distributed with a small spacing in order for outside bars to be mobilized.  This 1-in. anchor was fractured.
Phase II result picutre 1

Conclusions and Recommendations:

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    The proposed anchor reinforcement is shown right for anchors with both unlimited and limited side edge distances.  The goal of the proposed design for anchor shear reinforcement is to prevent concrete breakout using closely spaced stirrups placed parallel to the plane of the applied shear force and the anchor.  With the concrete confined around the anchor, it is expected that the concrete will restrain the anchor shaft and provide shear resistance.  The stirrups should be proportioned using the anchor steel capacity in shear.  The nominal yield strength of reinforcing steel should be used in the calculation. Two stirrups should be placed next to the anchor shaft, where the breakout crack in concrete may initiate under a shear load.  The rest of the required stirrups should be placed with a center-on-center spacing of 2 to 3 in.  The stirrups can be distributed within a distance of ca1.  Note that the horizontal legs of the closed stirrups are used as anchor shear reinforcement while the vertical legs close to the anchor shaft may be used as anchor tension reinforcement as shown in Phase III tests of this study  recommended anchor shear reinforcement
    The development length requirements for the horizontal legs of the closed stirrups are satisfied similar to the transverse reinforcement in a flexural member, where the stirrups are fully developed at both sides of a shear crack through the interaction between the closed stirrups and longitudinal bars at all four corners.  This requirement results in a minimum edge distance of 8db plus the concrete cover.  Design of reinforced anchors should also satisfy other edge distance requirements, such as those in Section D.8 of ACI 318-11.  Bars at all four corners of the closed stirrups (referred to as corner bars hereafter) restrain splitting cracks as well as other bars distributed along the concrete surfaces (referred to as crack-controlling bars hereafter).  Therefore the corner bars and crack-controlling bars need to be fully developed at both sides of the anchor bolt, and a 90-degree bend may be needed.  The selection of corner bars may follow the common practices in selecting longitudinal corner bars for reinforced concrete beams, such as those specified in Section 11.5.6 of ACI 318-11.  Crack-controlling bars are also recommended, and the determination of these bars can be based on the well-recognized strut-and-tie models. In addition, a separate bar can be placed right in front of the anchor bolt to alleviate the large local compressive stress in concrete.
     With the proposed anchor shear reinforcement, concrete breakout was prevented and anchor shaft fracture was observed in all the tests of single anchors in this study.  Cover concrete in front of the anchor bolts spalled, causing the top portion of the anchor shaft close to the concrete surface to become exposed.  The full anchor steel capacity in shear was not achieved because the exposed anchors were subjected to a combination of shear, bending, and tension at failure.  An analysis of the test results of exposed anchors in the literature indicated that a reduction factor of 0.75, which is slightly lower than that in ACI 318-11 on anchors with a grout pad, can be used to determine the shear capacity of reinforced anchors.  In addition, quasi-static cyclic tests of the reinforced anchors in shear showed insignificant capacity reduction, which is comparable to other displacement-controlled cyclic tests. Although large capacity reductions were observed in load-controlled cyclic tests in the literature, no further capacity reduction was recommended in this study for reinforced anchors subjected to cyclic shear loading.

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16.    American Concrete Institute, “Examples of Anchor Design ACI 318-11 Appendix D,” Report of ACI Committee 355, Farmington Hills, Michigan, 2011.
17.    Petersen, D., “Seismic Behavior and Design of Cast-in-Place Anchors in Plain and Reinforced Concrete,” MS Thesis, University of Wisconsin, Milwaukee, WI, 2011.
18.    Lin, Z.; Petersen, D.; Zhao, J. and Tian, Y., “Simulation and Design of Exposed Anchor Bolts in Shear,” International Journal of Theoretical and Applied Multiscale Mechanics. (in print).
For more details see the draft paper accpeted by ACI Structural Journal.
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