Design Anchor

ACI Structural Journal/January-February 2013 53

Title no. 110-S06

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

ACI Structural Journal, V. 110, No. 1, January-February 2013.MS No. S-2011-048.R1 received August 1, 2011, and reviewed under Institute

publication policies. Copyright © 2013, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the November-December 2013 ACI Structural Journal if the discussion is received by July 1, 2013.

Design of Anchor Reinforcement for Seismic Shear Loadsby Derek Petersen and Jian Zhao

Existing design codes recommend hairpins and surface reinforce-ment consisting of hooked bars encasing an edge reinforcement to improve the behavior of anchor connections in shear. Concrete breakout is assumed to occur before anchor reinforcement takes effect in the current design methods. This paper presents an alter-native design method for anchor shear reinforcement. The proposed anchor shear reinforcement consists of a group of closed stirrups proportioned to resist the code-specified anchor steel capacity in shear and placed within a distance from the anchor bolt equal to the front-edge distance. Steel fracture was achieved in the tests of twenty 25 mm (1 in.) reinforced anchors with a front-edge distance of 152 mm (6 in.). Meanwhile, the observed anchor capacities were smaller than the code-specified anchor steel capacity in shear because concrete cover spalling caused combined bending and shear action in the anchor bolts. Reinforcing bars are needed along all concrete surfaces to minimize concrete damage in front of reinforced anchors for consistent seismic behavior in shear.

Keywords: anchor connections; anchor reinforcement; cast-in anchors; composite construction; fastening; headed studs; seismic design.

INTRODUCTIONConcrete anchor connections are a critical component of

load transfer between steel and concrete members, affecting structural performance during earthquake events. Observa-tions of damage in recent major earthquakes have raised concerns about the seismic performance of anchor connec-tions.1-4 Cast-in-place anchors may experience steel fracture or concrete breakout failure when subjected to a shear force toward a free edge.5 The failure modes are mainly depen-dent on the front-edge distance ca1 when the anchor bolt is placed in plain concrete. Concrete breakout cones, such as the one shown in Fig. 1, vary in shape, while an idealized breakout cone6 (encased in the dashed lines) is generally assumed in calculating the anchor breakout capacity. With the breakout cone partially formed, the anchor bolt may lose concrete support when subjected to reversed cyclic shear loads, leading to unreliable seismic performance.

Building codes5 and design guidelines7,8 allow engineers to use steel reinforcement to increase the shear capacity of anchors placed near an edge. The recommended anchor shear reinforcement usually consists of horizontal hairpins that wrap around the anchor shaft or hooked bars along the direc-tion of the shear force close to the top concrete surface, as illustrated in Fig. 2. The existing design methods5,7,8 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 cone5 or the contribution from each bar is calculated according to its development length in the assumed breakout cone.7,8 The development length requirements limit the distance from the anchor bolt within which the reinforcement can be deemed effective, as illustrated in Fig. 2.

RESEARCH SIGNIFICANCESignificant efforts have been invested in testing anchors

reinforced with hairpins. Laboratory tests of anchors reinforced with other types of reinforcement is scarce, especially for anchors under cyclic shear loading. This paper presents tests of cast-in-place anchors reinforced using closed stir-rups under both monotonic and cyclic shear loading. Closed stirrups encasing bars placed at the corners and distributed along concrete surfaces can restrain concrete breakout such that the shear load is transferred to the structure through the

Fig. 1—Concrete breakout failure under shear.

Fig. 2—Schematics of existing anchor shear reinforcement.

54 ACI Structural Journal/January-February 2013

Derek Petersen is a Structural Engineer at Osmose Railroad Services Inc., Madison, WI. He received his MS in civil/structural engineering from the University of Wisconsin at Milwaukee (UWM), Milwaukee, WI.

ACI member Jian Zhao is an Assistant Professor in the UWM Department of Civil Engineering and Mechanics. He received his PhD from the University of Minnesota, Minneapolis, MN. He is a member of ACI Committee 355, Anchorage to Concrete, and Joint ACI-ASCE Committee 447, Finite Element Analysis of Reinforced Concrete Structures. His research interests include the behavior of reinforced concrete struc-tures, concrete-steel connections, and earthquake engineering.

confined concrete. A design method is proposed for anchor shear reinforcement based on the observed anchor behavior.

BACKGROUNDVarious types of anchors have been developed over the

past 40 years. Numerous studies have been performed to develop the design method of anchors in plain concrete corresponding to various identified failure modes.6 The behavior of anchors and headed studs in plain concrete has been discussed at length7,9-12 and the tests have been summa-rized in several databases.13-15 On the other hand, studies are limited on the performance of anchors with reinforce-ment. The existing studies on anchor shear reinforcement are reviewed and the available design methods are summarized in the following.

Previous studiesThe most investigated anchor reinforcement for resisting

shear forces is horizontal hairpins that wrap around the anchor, as illustrated in Fig. 2. Swirsky et al.16 tested 24 cast-in-place anchors consisting of 25 and 51 mm (1 and 2 in.) diameter ASTM A307 carbon steel or ASTM A449 medium carbon or alloy steel bolts reinforced with No. 4 or No. 5 hair-pins under monotonic and cyclic loading. The hairpins had a 120-degree bend wrapping around the bolts 51 mm (2 in.) below the concrete surface. A capacity increase of 15 to 87% was observed at a displacement of approximately 25 mm (1 in.). Only six anchors were reported to fail with anchor shaft fracture, in part because the development length for the hairpins was 20db (db is the diameter of the hairpins), which is not sufficient. Many tests were terminated after bond failure of hairpins was observed. Two additional tests were conducted with two No. 4 vertical stirrups placed 51 mm (2 in.) away from the bolt. The use of stirrups is similar to the anchor reinforcement proposed in this paper; however, the amount of the reinforcement was not sufficient, and both tests stopped after the concrete cracked and a large displace-ment was observed.

The behavior of anchor bolts reinforced with hairpins was further studied by Klingner et al.17 through 12 monotonic tests and 16 cyclic tests of 19 mm (0.75 in.) diameter A307 bolts. A No. 5 hairpin with a 180-degree bend and a develop-ment length of approximately 37db was placed 19 or 51 mm (0.75 or 2 in.) below the top surface. The tests showed that the most effective way to transfer anchor shear force to the hairpin is through the contact between the anchor shaft and the hairpin near the surface. Hairpins that were not in contact with the anchor shaft were found to be effective in monotonic tests but unreliable under cyclic loading. The No. 5 hairpin provided sufficient shear resistance compared to the anchor steel capacity; however, most tests were terminated before anchor fracture was achieved—likely because a capacity drop was observed during the tests.

Lee et al.18 conducted 10 tests of 64 mm (2.5 in.) diam-eter anchor bolts with a 381 mm (15 in.) edge distance and a 635 mm (25 in.) embedment depth reinforced with U-shaped hairpins and hooked reinforcing bars. The reinforce-ment was proportioned to carry the shear capacity of anchor steel, resulting in a combination of No. 6 hairpins and No. 8 hooked bars dispersed within 381 mm (15 in.) from the anchor bolt with a spacing of 152 mm (6 in.). Three layers of No. 8 hairpins were used in some specimens. Most tests were terminated before a peak load was observed due to the limited stroke of the loading device. The unfinished tests were not able to fully demonstrate the effectiveness of the various anchor reinforcement designs.

In Europe, as documented by Schmid,19 Paschen and Schönhoff20 examined 10 types of anchor reinforcement layouts. Hairpins touching anchor shafts and reinforcing bars distributed near the top surface, as illustrated in Fig. 2, were found to be the most effective. Similar conclusions were made by Ramm and Greiner21 based on their tests of anchors reinforced with five types of reinforcement. Randl and John22 observed a capacity increase of 300% in their tests of post-installed anchor bolts with hairpins. It was concluded that the thickness of concrete cover affected the effectiveness of hairpins as anchor shear reinforcement. Recently, Schmid19 conducted tests on five types of anchors with hooked reinforcing bars, which simulated the reinforcing bars in an existing concrete element. A model was proposed for determining the shear capacity of reinforced anchors, which can be obtained from the summation of the contri-butions from all reinforcing bars bridging the assumed 35-degree breakout crack. The contribution from each reinforcing bar included the bearing force of the bent leg and the bond force of the straight part within the breakout cone. Schmid’s19 equation for the capacity of reinforced anchors in shear is a refined version of the equation proposed by Fuchs and Eligehausen,23 who clearly defined the assumption that a concrete cone must form before steel reinforcement takes effect. On the other hand, many of Schmid’s19 tests were terminated after the spalling of the concrete cover, which might have not indicated the final failure of the specimens.

Existing design recommendationsThe methods for proportioning anchor shear reinforcement

are summarized in Table 1. Note that many design methods that focused on the capacity calculation for anchors with a known configuration of anchor reinforcement, such as that proposed by Schmid,19 are not included in Table 1. In summary, most existing design methods require the reinforcement to provide more resistance than the anchor steel capacity in shear. This is achieved by either increasing the design force5 or reducing the effectiveness of anchor reinforcement based on their relative vertical locations.7,8 Note that there are few tests with such overdesigned reinforcement, and many such tests were terminated before a true ultimate load was achieved.

Hairpins are deemed effective as anchor shear reinforce-ment because they can be placed close to the anchor shaft using a small bending radius on the hairpin.17,18 The transfer of shear load to surface reinforcement shown in Fig. 2 is usually visualized using a strut-and-tie model (STM).23,25 STMs permit large-sized 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 (for example, less than 55 degrees); however, tests18,26 have indicated that reinforcing bars placed closer to


the anchor are more effective. As a result, the existing design guidelines5,7,8 require the anchor reinforcement to be within a distance equal to half of the front-edge distance (0.5ca1), as illustrated in Fig. 2. Such requirements leave a small window of applicability for practical implementations of the anchor reinforcement. Oftentimes, the front-edge distance needs to be increased to accommodate the anchor reinforce-ment, which in turn increases the concrete breakout capacity such that the anchor reinforcement may no longer be 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 reason-able distance on each side of the anchor bolt within which the anchor reinforcement is deemed effective; and 4) an accurate estimation of shear capacity of reinforced anchors.

PROPOSED ANCHOR SHEAR REINFORCEMENT DESIGN

The proposed anchor reinforcement is shown in Fig. 3 for anchors with both unlimited and limited side-edge distances. The goal of the proposed design for anchor shear reinforce-ment 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 propor-tioned using the anchor steel capacity in shear, as specified by the equation in the last row of Table 1. The nominal yield strength of reinforcing steel should be used in the calcula-tion. 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 51 to 76 mm (2 to 3 in.). A smaller spacing may be used, provided that the clear spacing requirements, such as those in ACI 318-11,5 are satisfied. The stirrups can be distributed within a distance of ca1, as shown in Fig. 3. Note that the horizontal legs of the closed stirrups are used as anchor shear reinforcement, while the vertical legs close to the anchor shaft25 may be used as anchor tension reinforcement, as shown in the Phase III

tests of this study. For this purpose, the depth of the stirrups should be large enough such that the vertical legs are fully developed for the tension load.

The development length requirements for the horizontal legs of the closed stirrups are satisfied similar to the trans-verse 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 longitu-dinal bars at all four corners.27 Meanwhile, reinforcing bar pullout tests, in which both legs of No. 4 U-shaped bars embedded 38 and 76 mm (1.5 and 3 in.) in concrete were loaded in tension, indicated that a minimum embedment depth of 6db was needed to develop a No. 4 stirrup through the interaction. Therefore, the length of the horizontal legs of the vertical closed stirrups should be at least 8db on both sides of the anchor, as shown in Fig. 3. This requirement

Fig. 3—Proposed anchor shear reinforcement layout.

Table 1—Summary of design equations for anchor shear reinforcement

Reference Design equation for Asa given load Vsd Development in cone Actual shear capacity Vs Notes

Shipp and Haninger24 ,

1.85cos 45uta se N

ys sa

F AF A =

°

Not neededDesign based on

equivalent tensionHairpins

Klingner et al.17 FysAsa = FutaAse,V Not needed Vs = 0.6FutaAse,V Hairpins

CEB7 0.5FysAsa = 1.15VsdConsidered in capacity

calculation2s dh bdV l uf= ∑

Bars within 0.5ca1

ACI 318-115 0.75FysAsa = Vsd

*

Vs = FysAsaBars within 0.5ca1 or

0.3ca2

Widianto et al.25 ssAsa = FutaAse,V or 2.5Vsd

ss reduced for not fully developed barsNot considered in STM Vs = Vsd

Stirrups, ties, and J-hooks

fib design guide8 0.5FysAsa = Vsd([es/z] + 1)Considered in capacity

calculationbd

s dhre

fV l u= ∑a

Bars within 0.5ca1

Proposed FysAsa = 0.6FutaAse,V 8db on both sides Vs = 0.45FutaAse,V Closed stirrups within ca1

*Refer to Chapter 12 of ACI 318-115 for details. Notes: Asa is area of anchor reinforcement; Fys is yield strength of reinforcement; Ase,V, Ase,N is effective cross-sectional area of anchor; ca1, ca2 are edge distances of anchor; es is distance from shear to reinforcement; fbd is design bond strength; Futa is ultimate strength of anchor; Ldh, ldh is development length of hooked bar in breakout cone; u is circumfer-ence of reinforcing bar; Vsd is design shear force; z is reinforcement position; are is modification factor; ss is stress in anchor reinforcement.

0.02 ysdh b

c

FL d

fψ

=λ ′


results in a minimum edge distance of 8db plus the concrete cover. The design of reinforced anchors should also satisfy other edge distance requirements, such as those in Section D.8 of ACI 318-11.5

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 (as shown in dashed lines) in Fig. 3, 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 318-11.5 Crack-controlling bars were not provided in the tests and the split-ting cracks were observed, as presented in the following. Crack-controlling bars are therefore recommended as shown in Fig. 3, and the determination of these bars can be based on the well-recognized STMs.23,25

EXPERIMENTAL INVESTIGATIONSpecimens

This group of experimental tests is part of a research program that focused on the behavior and design of cast-in-place anchors under simulated seismic loads.28 Sixteen tests were conducted using 25 mm (1 in.) diameter anchors consisting of an ASTM A193 Grade B7 threaded rod (fy = 724 MPa [105 ksi] and fut = 1069 MPa [131 ksi]) and a heavy hex nut welded to the end. Another four tests using 19 mm (0.75 in.) diameter ASTM F1554 Grade 55 anchors (fy = 434 MPa [63 ksi] and fut = 524 MPa [76 ksi]) were conducted with two tests, each under monotonic shear and cyclic shear loading. Ready mixed concrete with a targeted strength of 27.6 MPa (4000 psi) was used, and cylinder tests using three batches of three 100 x 200 mm (4 x 8 in.) cylinders tested throughout the anchor test period showed an average compressive strength of 24.3 MPa (3525 psi).

The dimensions of the test blocks containing four anchors each are illustrated in Fig. 4. One block was prepared for

Type 19-150-100 specimens, and two blocks were prepared for Type 25-150-150 and Type 25-150-150H specimens. Another block similar to that for Type 25-150-150 speci-mens was used for Type 25-150-150SG specimens. Strain gauges were installed on the reinforcing bars of the two anchors in this block. All anchors had an embedment depth of 152 mm (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 432 mm (17 in.), similar to all other anchor tests in the study.28

The anchor shear reinforcement was proportioned to carry the maximum capacity of the anchor bolts in shear: 68 kN (15.3 kips) for the 19 mm (0.75 in.) anchors and 209 kN (47 kips) for the 25 mm (1 in.) anchors. Using the nominal yield strength of Grade 60 steel, the required anchor reinforce-ment was found to be 164 mm2 (0.25 in.2) for the 19 mm (0.75 in.) anchors and 503 mm2 (0.78 in.2) for the 25 mm (1 in.) anchors. Therefore, two No. 4 bars were provided for Type 19-150-100 specimens, as shown in Fig. 4. The required anchor reinforcement for the 25 mm (1 in.) anchors was provided using four No. 4 bars with a spacing of 51 mm (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 76 mm (3 in.), and eight No. 3 bars for Type 25-150-150SG specimens with a spacing of 51 mm (2 in.). Two addi-tional No. 3 J-hooks were added beside the outermost bars in Type 25-150-150SG specimens, as shown in Fig. 4, to host two more strain gauges, which were approximately 250 mm (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 38 mm (1.5 in.).

Test setupThe loading frame, actuator placement, and instrumenta-

tion setup used for the tests are shown in Fig. 5. Instead of a self-balanced load frame, a tie-down rod 381 mm (15 in.) behind the test anchor was used to fix the test block to the strong floor. In addition, the concrete block was wedged against the strong floor to minimize the slip of the test block under cyclic loads, as shown in Fig. 5. A 245 kN (55 kip) actuator was used to apply shear loading to the anchor bolt through a loading plate. The actuator body was braced against the floor to eliminate the downward motion of the actuator swivel head and the rotation of the loading plate. To minimize the friction between the loading plate and the concrete top surface, a net tension force of 0.8 kN (0.2 kips) was applied to the loading plate by a 489 kN (110 kip) actuator, which was used for applying tension loads in other tests. The nut fixing the loading plate to the anchor bolt was first hand-tightened and then loosened one-eighth of a turn to allow slight vertical movement of the loading plate

Fig. 4—Configurations of anchor specimens.


when the 0.8 kN (0.2 kip) tension force was applied at the beginning of a test. The test anchors were inserted through a standard 3 mm (0.125 in.) oversized hole in the loading plate, and a steel sleeve shim was inserted between the anchor and the hole to eliminate the clearance and prevent damage to the loading plate.

Loading protocolMonotonic shear tests were performed first to determine

the typical actuator displacement at failure, and the tests indicated a failure displacement of approximately 35 mm (1.4 in.). Hence, the cyclic displacement steps for each three-cycle group were chosen as 2, 3, 4 (failure displacements for typical unreinforced anchors), 8, 16, and 32 mm (0.08, 0.12, 0.16, 0.32, 0.64, and 1.28 in.), as shown in Fig. 5. The loading rate for the displacement cycles at or below 4 mm (0.16 in.) was kept at 2 mm/min (0.08 in./min), while the load rate was increased to 10 mm/min (0.4 in./min) for the 8, 16, and 32 mm (0.32, 0.64, and 1.28 in.) cycles to reduce test time. Most reversed cyclic shear tests were conducted following Loading Pattern C1 shown in Fig. 5, in which the maximum displacement was set as 4 mm (0.16 in.) when the shear loading was applied opposite to the front edge. This was to prevent early anchor fracture under reversed loads and observe the cyclic behavior over a full displace-ment range. Cyclic tests following Loading Pattern C2 in Fig. 5 with equal peak displacements in both directions of shear loading were conducted for two Type 25-150-150H specimens. Note that the control of the actuator was based on the actuator piston motion instead of anchor displace-ment; hence, the actual anchor displacements were smaller than the aforementioned target displacements.

InstrumentationString pots and linear variable differential transformers

(LVDTs) were used to measure the anchor displacements, as illustrated in Fig. 5. The displacements of the load plate were actually used as the anchor displacement because the anchor shaft just above the concrete surface was not assess-able. A data acquisition system was used to collect data from all sensors, as well as the force and displacement outputs from the actuators. The sampling frequency was 5 Hz and the collected data were filtered using an in-house program with a cutoff frequency of 0.1 Hz. The observed anchor behavior is discussed in the following.

EXPERIMENTAL RESULTS AND DISCUSSIONBehavior of anchors under monotonic loading

The configuration and loading types of the anchor speci-mens are summarized in Table 2 along with the measured shear capacities. The load-versus-displacement behavior is shown in Fig. 6 for the reinforced anchors subjected to monotonic shear along with selected images of failed speci-mens. For comparison purposes, the load-versus-displace-ment behavior for a 19 mm (0.75 in.) anchor with a front-edge distance of 100 mm (4 in.) in plain concrete is shown in Fig. 6(a) and the result of another anchor with a front-edge

Fig. 5—Experimental test setup. (Note: 1 mm = 0.0394 in.)

Table 2—Summary of reinforced anchor tests in shear

Specimen ID Block type da, in. ca1, in. Load type Peak load, kips

9132010 — 0.75 4 M 22.19

9132010_2 — 0.75 4 M 22.47

9172010 — 0.75 4 C1 16.69

9202010 — 0.75 4 C1 15.50

9282010 — 1.0 6 M 39.18

9292010 — 1.0 6 M 44.11

9302010 — 1.0 6 C1 38.71

10042010 — 1.0 6 C1 35.92

10052010 — 1.0 6 C1 34.35

10062010 H 1.0 6 M 38.40

10062010_2 H 1.0 6 M 34.71

10072010 H 1.0 6 M 33.40

10082010 H 1.0 6 C1 33.62

10082010_2 H 1.0 6 C1 31.77

10122010 H 1.0 6 C1 33.88

10132010 H 1.0 6 C2 –42.68*

10142010 H 1.0 6 C2 –47.79*

10292010 SG 1.0 6 M 36.13

11192010 SG 1.0 6 M 39.33*Anchor fracture occurred when shear was applied opposite to front edge. Notes: 1 in. = 25.4 mm; 1 kip = 4.45 kN.


distance of 150 mm (6 in.) is shown in the rest of Fig. 6. The unreinforced anchors were tested with a concrete strength of 39 MPa (5656 psi), whereas the reinforced anchor tests had a concrete strength of 24.3 MPa (3525 psi); therefore, the load values for the unreinforced anchors were normal-

ized using a factor of 24.3 39 in Fig. 6. In general, the reinforced anchors failed by anchor shaft fracture, while the unreinforced anchors with similar edge distances failed by concrete breakout. The failure loads for the reinforced anchors increased by approximately 100% and the displace-ments corresponding to the peak loads increased more than six times compared with those of the unreinforced anchors.

The load-displacement behavior of 19 mm (0.75 in.) anchors in reinforced concrete did not show much difference

from that in plain concrete (Fig. 6(a)) before a crack was observed at the top surface at a load of approximately 45 kN (10 kips). Rather than propagating vertically along the anchor shaft, as observed in the tests of unreinforced anchors as represented by Fig. 1, the crack propagated around the corner of the stirrups (refer to the inserted figure in Fig. 6(a)). The loss of the 38 mm (1.5 in.) thick concrete cover in front of the anchor caused a small capacity loss for the 19 mm (0.75 in.) anchors, as shown in Fig. 6(a). Because the 19 mm (0.75 in.) anchor only mobilized the top concrete before cracking, similar to that suggested by Randl and John22 (approximately 2da deep), the anchor shaft in bending was not able to resist the same amount of load until a larger displacement was applied. Such a post-spalling load drop has been observed in other tests of anchors reinforced with hairpins.17,18 The failure load exceeded the code-specified anchor shear capacity because the failure was caused by the fracture of the anchor shaft largely under tension, as shown in Fig. 7(a), although the fracture may have started from a flexural crack.

The shear load did not drop noticeably after the concrete cover spalled in the tests of 25 mm (1 in.) anchors, as shown in Fig. 6(b) through (d). The 25 mm (1 in.) anchors mobi-lized deeper concrete such that the loss of bearing support from the cover concrete was immediately resisted by lower concrete restrained by the anchor reinforcement. Another contributing factor is that the 25 mm (1 in.) anchors had a larger bending stiffness such that a small displacement was needed to mobilize their load-carrying capacities. The 25 mm (1 in.) anchors failed at loads lower than the code-specified anchor steel capacity in shear. The fractured 25 mm (1 in.) anchors in Fig. 7(c) showed a different failure mode from that of the 19 mm (0.75 in.) anchors; the anchor shaft cracked under a bending moment and the rest of the anchor shaft then fractured in shear. For the shear-dominant failure mode, the flexural cracking reduced the cross-sectional area, thus leading to a lower ultimate shear capacity.

Fig. 6—Monotonic shear test results of reinforced anchors.

Fig. 7—Typical fractured shape of anchor bolts.


Anchor steel failure was achieved in all 25 mm (1 in.) diam-eter 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, rein-forcing bars must be evenly distributed with a small spacing for outside bars to be mobilized. The effective distance was verified by the measured strains in the reinforcing bars in Type 25-150-150SG specimens, as shown in Fig. 8. The anchor reinforcement consisted of eight No. 3 stirrups at a spacing of 51 mm (2 in.) and two additional No. 3 J-hooks. The thin dashed lines in Fig. 8 indicate the assumed breakout crack at the concrete surface, and the strain gauges were installed 25 mm (1 in.) behind the assumed breakout crack line on the inside face of the stirrups. In general, larger strains were observed in the bars closer to the anchor bolt. Mean-while, the outside bars, as indicated by Gauges 4S and 4N located 170 mm (6.7 in.) from the anchor bolt, also devel-oped significant strains, especially after the surface crack formed. Note that the gauge positions relative to a crack should be considered to interpret the measured strains. For example, the strains by Gauge 2N may have been affected by the crack passing the gauge location, as shown in Fig. 8. More importantly, smaller strains measured by the gauges on the outside bars may have been due to the fact that the gauges were away from the actual crack. In addition, the measured strains indicated that none of the Grade 60 bars yielded at the peak load; hence, the shear capacity of reinforced anchors may not be calculated as the summation of the yield forces of the anchor reinforcement. The shear force was actually transferred to the supports (for example, the tie-down rods on the back and the steel wedging tube at the bottom, in this case) through the concrete confined by the closed stirrups.

Anchors in Type 25-150-150H specimens had a lower ulti-mate capacity, as shown in Fig. 6(c). This might have been due to the poor confinement of concrete in front of the anchor bolt; additional splitting cracks were observed and deeper concrete crushed in these tests, leading to a longer portion of exposed and unsupported anchor bolts (for example, up to 0.5da larger than those in Type 25-150-150 specimens). Finite element analyses indicated that the anchor capacity controlled by shear fracture can be affected by anchor diam-eter and concrete cover depth.29 It is thus envisioned that the following measures, as illustrated in Fig. 3, can be effec-tive in improving the post-spalling behavior and capacity of reinforced anchors in shear: 1) corner bars should be fully developed; 2) crack-controlling bars should be provided along both the top and front surfaces of concrete; and 3) a separate bar can be placed directly in front of the anchor bolt to alleviate the large local compressive stress in concrete.

Anchor shear capacityMost anchor bolts in this group of tests failed by shear frac-

ture of a reduced anchor shaft cross section, as shown by the typical fractured sections in Fig. 7. This failure mode occurred when a short portion of the anchor bolt was exposed and a lever arm developed in the anchors after the cover concrete spalled. The effect of lever arms in anchor bolts is recognized in the existing design codes.5,8 For example, ACI 318-115 stip-ulates that the design capacity of anchor connections having grout leveling pads should be reduced by a factor of 0.8 for the anchor steel strength in shear. Such capacity reduction considers the combined bending and shear in the anchor shaft but does not consider the thickness of the grout pads,

which is similar to the exposed length at the ultimate load. Eligehausen et al.12 proposed an equation for predicting the strength of an exposed anchor, assuming that the anchor fails by pure bending. This equation was not found to be appli-cable for predicting the capacity of the anchors in this study, likely due to the fact that the anchor failure was controlled by shear fracture. Lin et al.29 improved the equation by Elige-hausen et al.12 by considering the contributions from flexural, shear, and tensile resistance of an exposed anchor shaft to the shear capacity of exposed anchors; however, the equation was based on double shear tests and finite element analyses of threaded rods, and the lateral support to the actual anchor shaft from partially damaged concrete was not considered. Therefore, the equation may provide lower-bound estimates of the actual anchor capacities.

The capacity of anchor bolts with a lever arm was instead examined using the test data available in the literature, as shown in Fig. 9. The measured anchor capacities were normalized by the design capacity of anchor bolts in shear specified in ACI 318-11.5 The exposed depth of the anchors in other tests16,26 was defined as the distance between the

Fig. 8—Strains in anchor shear reinforcement (Type 25-150-150SG1). (Note: 1 mm = 0.0394 in.)

Fig. 9—Capacity of anchor bolt with lever arm.


bottom face of a base plate and the lowest solid concrete surface. The anchor steel capacity observed in this study is low compared with other available tests. This might have been due to the fact that friction between the load plate and the concrete surface was minimized, as previously described in the test setup section.

The statistical analysis of the limited data in Fig. 9 did not follow the procedures of predictive inference,30,31 which are usually used to predict future occurrences based on the existing observed data. Instead, a 5-percentile value of 0.73 was obtained using a descriptive statistical analysis of the 22 collected data points. Considering the afore-mentioned reasons for the low observed capacities in this study, it is proposed that the shear strength of reinforced anchors can be estimated as 75% of the code-specified steel capacity for anchors without a lever arm. This is slightly lower than the reduction factor in ACI 318-115 because of two data points observed in specimens with limited side-edge distances (Type 25-150-150H). It is envisioned that as more data points become available in future tests with the recommended anchor shear reinforcement shown in Fig. 3, the statistical importance of these two data points can be reduced. Using the suggested capacity reduction for exposed anchors should be limited to those with an exposed length less than three times the anchor diameter (3da). Beyond this limit, the anchor steel failure in shear needs further study.

Behavior of anchors under cyclic loadingSeismic actions on structural components are mostly

simulated in laboratories using quasi-static cyclic tests with reversed loading.32 Therefore, displacement-controlled loading33 was used in this study, although many cyclic tests of anchors have been conducted with load-controlled loading.16,17,34 The load-versus-displacement behavior of two 19 mm (0.75 in.) anchors subject to Type C1 cyclic shear loading is plotted in Fig. 10(a). The monotonic curve was closely followed by cyclic curves until a displacement of 10 mm (0.4 in.), beyond which the cyclic loads were lower than that of the monotonic test. The slope of the

cyclic curves again had a sudden change at a displacement of approximately 2 mm (0.16 in.), indicating the concrete cover spalling. The difference in the observed loads at this displacement may have been due to variations in the speci-mens, such as the actual edge distances and cover depths. The first three displacement cycles did not see significant degradation in loads with successive cycles to the same displacement, while the degradation was obvious at the larger-displacement cycles. This was because the displaced cover concrete during the first cycle of each three-cycle group was not able to recover, leading to reduced restraint to the anchor shaft in the successive cycles. An average capacity reduction of 28% was observed in the cyclic shear capacity for the 19 mm (0.75 in.) anchors. This reduction was partly attributed to the change of failure modes, as shown by the fractured shape of the anchor in Fig. 7(a) and (b); the anchor failure was controlled by the shear fracture under cyclic loading, while the tensile fracture controlled the anchor failure in the monotonic test. Note that the reduced cyclic shear capacities of the 19 mm (0.75 in.) anchors were higher than the proposed capacity of exposed anchors under monotonic loading because of the monotonic failure mode.

The behaviors of Type 25-150-150 specimens are compared in Fig. 10(b). The monotonic load-displacement curve nicely envelopes the cyclic curves represented by the first loading cycle in each three-cycle group. The load degra-dations during the successive two cycles were again due to the irreversible crushing of the concrete cover in front of the anchors. No capacity drop was observed in the tests of Type 25-150-150 specimens. An average capacity drop of 6.8% was observed for Type 25-150-150H anchors with a limited side-edge distance, as shown in Fig. 10(c). In this group of three cyclic tests, concrete deeper than the 38 mm (1.5 in.) cover crushed, likely due to poor confinement condi-tions, as indicated by splitting cracks. The larger exposed length led to a larger moment under the same shear load and thus a lower shear capacity. Note that the poor confine-ment conditions can be improved by the crack-controlling bars recommended in Fig. 3. In addition, a bar placed just

Fig. 10—Cyclic behavior of reinforced anchor bolts.


in front of the anchor shaft can help distribute the localized high compressive stresses such that the exposed length of the anchors would not be affected by the cyclic loading. Finally, the tests of two Type 25-150-150H anchors with fully reversed cyclic loading (Type C2 in Fig. 5) ended with anchor fractured under a shear load applied opposite to the front edge. The ultimate load capacities were, on average, 5% lower than the code-specified anchor steel capacity, as shown in Fig. 10(d). Hence, it is reasonable to ignore the reduction of steel capacities for reinforced anchors in cyclic shear, considering that the monotonic capacity of reinforced anchors has already been reduced by 25%, as proposed previously.

CONCLUSIONSA design method for anchor shear reinforcement was

proposed and verified using experimental tests of single cast-in-place anchors. With a goal to prevent concrete breakout and confine concrete in front of an anchor bolt, the proposed anchor shear reinforcement consisted of closely spaced stir-rups, corner bars, and crack-controlling bars distributed along all concrete faces. The horizontal legs close to the concrete surface of the closed stirrups were proportioned to carry a force equal to the code-specified anchor steel capacity in shear. The needed reinforcement was provided by closely spaced, small-sized stirrups distributed within a distance from the anchor equal to its front-edge distance. Although not specifically tested in the study, the selec-tion of corner bars should follow the practices specified in Section 11.5.6.2 of ACI 318-115 for corner bars in beams, and crack-controlling bars may be determined following the well-recognized STMs.

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 318-115 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 rein-forced anchors in shear showed insignificant capacity reduc-tion, 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 is recommended in this study for reinforced anchors subjected to cyclic shear loading.

ACKNOWLEDGMENTSThe study reported in this paper is from a project supported by the

National Science Foundation (NSF) under Grant No. 0724097. The authors gratefully acknowledge the support of J. Pauschke, who served as the Program Director for this grant. The authors also thank their colleagues in ACI Committee 355 for their valuable input. Any opinions, findings, and recommendations or conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.

NOTATIONAsa = area of anchor reinforcementAse,V, Ase,N = effective cross-sectional area of anchor in shear and tensionca1 = front-edge distance of anchorca2 = side-edge distance of anchor

da = anchor diameterdb = reinforcement diameteres = distance from shear force to surface reinforcementFys = yield strength of steel reinforcementfbd = design bond strength of anchor reinforcement in breakout conefc′ = concrete compressive strengthfuta = ultimate tensile strength of anchor steelfy = yield strength of anchor steelldh = development length of hooked bar in breakout coneu = circumference of reinforcing barVs = actual shear capacity of exposed anchorVsd = design shear capacity of anchorz = vertical reinforcement positionss = stress in anchor reinforcement

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