1

DESIGN OF ANCHOR REINFORCEMENT FOR SEISMIC SHEAR LOADS 1

Derek Petersen and Jian Zhao 2

3

Biography: ACI student member Derek Petersen is a former graduate research assistant at the 4

University of Wisconsin at Milwaukee where he received his MS degree in Civil/Structural 5

Engineering. He is now working as a structural engineer with Osmose Railroad Services Inc. in 6

Madison, WI. 7

ACI member Jian Zhao is an assistant professor in UWM Department of Civil Engineering and 8

Mechanics. He received his PhD from the University of Minnesota, Minneapolis, MN and served 9

as a post-doctoral researcher at Iowa State University, Ames, IA. He is a member of ACI 10

Committees 355 (Anchorage to Concrete) and 447 (Finite Element Analysis of Reinforced 11

Concrete Structures-Joint ACI-ASCE). His research interests include behavior of reinforced 12

concrete structures, concrete-steel connections, and earthquake engineering. 13

14

ABSTRACT 15

Existing design codes recommend hairpins and surface reinforcement consisting of hooked 16

bars encasing an edge reinforcement to improve the behavior of anchor connections in shear. 17

Concrete breakout is assumed to occur before anchor reinforcement takes effect in the current 18

design method. This paper presents an alternative design method for anchor shear reinforcement. 19

The proposed anchor shear reinforcement consists of a group of closed stirrups proportioned to 20

resist the code-specified anchor steel capacity in shear and placed within a distance from the 21

anchor bolt equal to the front edge distance. Steel fracture was achieved in the tests of twenty 22

25-mm [1-in.] reinforced anchors with a front edge distance of 152 mm [6in.]. Meanwhile, the 23

observed anchor capacities were smaller than the code-specified anchor steel capacity in shear 24

2

because concrete cover spalling caused combined bending and shear action in the anchor bolts. 1

Rebars are needed along all concrete surfaces to minimize concrete damage in front of reinforced 2

anchors for consistent seismic behavior in shear. 3

4

Keywords: Cast-in anchors; headed studs; anchor connections; anchor reinforcement; fastening 5

to concrete; composite construction; and seismic design. 6

7

INTRODUCTION 8

Concrete anchor connections are a critical component of load transfer between steel and 9

concrete members affecting structural performance during earthquake events. Observations of 10

damage in recent major earthquakes have raised concerns about the seismic performance of 11

anchor connections.1-4

Cast-in-place anchors may experience steel fracture or concrete breakout 12

failure when subjected to a shear force towards a free edge.5 The failure modes are mainly 13

dependent upon the front edge distance, ca1, when the anchor bolt is placed in plain concrete. 14

Concrete breakout cones such as the one shown in Fig. 1 vary in shape while an idealized 15

breakout cone6 encased in the dashed lines is generally assumed in calculating the anchor 16

breakout capacity. With the breakout cone partially formed, the anchor bolt may lose concrete 17

support when subjected to reversed cyclic shear loads, leading to unreliable seismic performance. 18

Building codes5 and design guidelines

7,8 allow engineers to use steel reinforcement to 19

increase the shear capacity of anchors placed near an edge. The recommended anchor shear 20

reinforcement usually consists of horizontal hairpins that wrap around the anchor shaft or hooked 21

bars along the direction of the shear force close to the top concrete surface as illustrated in Fig. 2. 22

The existing design methods5,7,8

assume that the concrete breakout similar to that observed for 23

3

anchors in plain concrete occurs before steel reinforcement takes effect. With this assumption, 1

the shear resistance of the anchor is exclusively provided by the anchor reinforcement. Anchor 2

reinforcement in terms of hooked bars is required to be fully developed in the assumed breakout 3

cone5 or the contribution from each bar is calculated according to its development length in the 4

assumed breakout cone.7,8

The development length requirements limit the distance from the 5

anchor bolt, within which the reinforcement can be deemed effective as illustrated in Fig. 2. 6

7

RESEARCH SIGNIFICANCE 8

Significant efforts have been invested to testing anchors reinforced with hairpins. Laboratory 9

tests of anchors reinforced with other types of reinforcement is scarce, especially for anchors 10

under cyclic shear loading. This paper presents tests of cast-in-place anchors reinforced using 11

closed stirrups under both monotonic and cyclic shear loading. Closed stirrups encasing bars 12

placed at the corners and distributed along concrete surfaces can restrain concrete breakout such 13

that the shear load is transferred to the structure through the confined concrete. A design method 14

was proposed for anchor shear reinforcement based on the observed anchor behavior. 15

16

BACKGROUND 17

Various types of anchors have been developed over the past 40 years. Numerous studies 18

have been performed to develop the design method of anchors in plain concrete corresponding to 19

various identified failure modes.6 The behavior of anchors and headed studs in plain concrete 20

has been discussed at length,7,9-12

and the tests have been summarized in several databases.13-15

21

On the other hand, studies are limited on the performance of anchors with reinforcement. The 22

existing studies on anchor shear reinforcement are reviewed and the available design methods 23

4

summarized below. 1

Previous Studies 2

The most investigated anchor reinforcement for resisting shear forces is horizontal hairpins 3

that wrap around the anchor as illustrated in Fig. 2. Swirsky et al.16

tested 24 cast-in-place 4

anchors, consisting of 25- and 51-mm [1- and 2-in.] diameter A307 or A449 bolts reinforced 5

with No. 4 or No. 5 hairpins, under monotonic and cyclic loading. The hairpins had a 120-6

degree bend wrapping around the bolts 51 mm [2 in.] below the concrete surface. A capacity 7

increase of 15 to 87 percent was observed at a displacement about 25 mm [1 in.]. Only six 8

anchors were reported to fail with anchor shaft fracture, in part because the development length 9

for the hairpins was 20db (db is the diameter of the hairpins), which is not sufficient. Many tests 10

were terminated after bond failure of hairpins was observed. Two additional tests were 11

conducted with two No. 4 vertical stirrups placed 51 mm [2 in.] away from the bolt. The use of 12

stirrups is similar to the anchor reinforcement proposed in this paper; however the amount of the 13

reinforcement was not sufficient, and both tests stopped after concrete cracked and a large 14

displacement was observed. 15

The behavior of anchor bolts reinforced with hairpins was further studied by Klingner et al.17

16

through 12 monotonic tests and 16 cyclic tests of 19 mm [3/4 in.] diameter A307 bolts. A No. 5 17

hairpin with a 180-degree bend and a development length about 37db was placed 19 or 51 mm 18

[3/4 or 2 in.] below the top surface. The tests showed that the most effective way to transfer 19

anchor shear force to the hairpin is through the contact between the anchor shaft and the hairpin 20

near the surface. Hairpins that were not in contact with the anchor shaft were found effective in 21

monotonic tests, but unreliable under cyclic loading. The No. 5 hairpin provided sufficient shear 22

5

resistance compared to the anchor steel capacity; however most tests were terminated before 1

anchor fracture was achieved likely because a capacity drop was observed during the tests. 2

Lee et al.18

conducted 10 tests of 64 mm [2.5 in.] diameter anchor bolts with a 381-mm [15-3

in.] edge distance and a 635-mm [25-in.] embedment depth reinforced with U-shaped hairpins 4

and hooked reinforcing bars. The reinforcement was proportioned to carry the shear capacity of 5

anchor steel, resulting in a combination of No. 6 hairpins and No. 8 hooked bars dispersed within 6

381 mm [15 in.] from the anchor bolt with a 152-mm [6-in.] spacing. Three layers of No. 8 7

hairpins were used in some specimens. Most tests were terminated before a peak load was 8

observed due to the limited stroke of the loading device. The unfinished tests were not able to 9

fully demonstrate the effectiveness of the various anchor reinforcement designs. 10

In Europe as documented by Schmidt,19

Paschen and Schönhoff20

examined ten types of 11

anchor reinforcement layouts. Hairpins touching anchor shafts and reinforcing bars distributed 12

near the top surface as illustrated in Fig. 2 were found most effective. Similar conclusions were 13

made by Ramm and Greiner21

based on their tests of anchors reinforced with five types of 14

reinforcement. Randl and John22

observed a capacity increase of 300 percent in their tests of 15

post-installed anchor bolts with hairpins. It was concluded that the thickness of concrete cover 16

affected the effectiveness of hairpins as anchor shear reinforcement. Recently, Schmidt19

17

conducted tests on five types of anchors with hooked reinforcing bars, which simulated the 18

rebars in an existing concrete element. A model was proposed for determining the shear capacity 19

of reinforced anchors, which can be obtained from the summation of the contributions from all 20

reinforcing bars bridging the assumed 35-degree breakout crack. The contribution from each 21

reinforcing bar included the bearing force of the bent leg and the bond force of the straight part 22

within the breakout cone. Schmidt’s equation for the capacity of reinforced anchors in shear is a 23

6

refined version of the equation proposed by Fuchs and Eligehausen,23

who clearly defined the 1

assumption that a concrete cone must form before steel reinforcement takes effect. On the other 2

hand, many of Schmidt’s tests were terminated after the spalling of the concrete cover, which 3

might have not indicated the final failure of the specimens. 4

Existing Design Recommendations 5

The methods for proportioning anchor shear reinforcement are summarized in Table 1. Note 6

that many design methods that focused on the capacity calculation for anchors with a known 7

configuration of anchor reinforcement, such as that proposed by Schmidt,19

were not included in 8

Table 1. In summary, most existing design methods require the reinforcement to provide more 9

resistance than the anchor steel capacity in shear. This is achieved by either increasing the 10

design force5 or reducing the effectiveness of anchor reinforcement based on their relative 11

vertical locations.7,8

Note that there are few tests with such over-designed reinforcement, and 12

many such tests were terminated before a true ultimate load was achieved. 13

Hairpins are deemed effective as anchor shear reinforcement because they can be placed 14

close to the anchor shaft using a small bending radius on the hairpin.17,18

The transfer of shear 15

load to surface reinforcement shown in Fig. 2 is usually visualized using a strut-and-tie model 16

(STM).23,24

Strut-and-tie models permit large size reinforcing bars located at a large distance 17

from the anchor bolt as anchor reinforcement as long as the angle between the concrete strut and 18

the applied shear force is small (e.g., less than 55 degrees). However, tests18,25

have indicated 19

that reinforcing bars placed closer to the anchor are more effective. As a result, the existing 20

design guidelines5,7,8

require the anchor reinforcement to be within a distance equal to half of the 21

front edge distance (0.5ca1) as illustrated in Fig. 2. Such requirements leave a small window of 22

applicability for practical implementations of the anchor reinforcement. Often time the front 23

7

edge distance needs to be increased to accommodate the anchor reinforcement, which in turn 1

increases the concrete breakout capacity such that the anchor reinforcement may be no longer 2

needed. 3

Anchor reinforcement design for shear in this study considered the following four aspects: 1) 4

an effective reinforcement layout that restrains concrete breakout failure; 2) a proper design 5

force for proportioning the anchor reinforcement; 3) a reasonable distance on each side of anchor 6

bolt, within which the anchor reinforcement is deemed effective; and 4) an accurate estimation of 7

shear capacity of reinforced anchors. 8

9

PROPOSED ANCHOR SHEAR REINFORMENT DESIGN 10

The proposed anchor reinforcement is shown in Fig. 3 for anchors with both unlimited and 11

limited side edge distances. The goal of the proposed design for anchor shear reinforcement is to 12

prevent concrete breakout using closely spaced stirrups placed parallel to the plane of the applied 13

shear force and the anchor. With the concrete confined around the anchor, it is expected that the 14

concrete will restrain the anchor shaft and provide shear resistance. The stirrups should be 15

proportioned using the anchor steel capacity in shear as specified by the equation in the last row 16

of Table 1. The nominal yield strength of reinforcing steel should be used in the calculation. 17

Two stirrups should be placed next to the anchor shaft, where the breakout crack in concrete may 18

initiate under a shear load. The rest of the required stirrups should be placed with a center-on-19

center spacing of 51 mm [2 in.] to 76 mm [3 in.]. A smaller spacing may be used provided that 20

the clear spacing requirements, such as those in ACI 318-11, are satisfied. The stirrups can be 21

distributed within a distance of ca1 as shown in Fig. 3. Note that the horizontal legs of the closed 22

stirrups are used as anchor shear reinforcement while the vertical legs close to the anchor shaft24

23

8

may be used as anchor tension reinforcement as shown in Phase III tests of this study. For this 1

purpose, the depth of the stirrups should be large enough such that the vertical legs are fully 2

developed for the tension load. 3

The development length requirements for the horizontal legs of the closed stirrups are 4

satisfied similar to the transverse reinforcement in a flexural member, where the stirrups are fully 5

developed at both sides of a shear crack through the interaction between the closed stirrups and 6

longitudinal bars at all four corners.26

Meanwhile rebar pullout tests, in which both legs of No. 4 7

U-shaped bars embedded 38 mm [1.5 in.] and 76 mm [3 in.] in concrete were loaded in tension, 8

indicated that a minimum embedment depth of 6db was needed to develop a No. 4 stirrup 9

through the interaction. Therefore, the length of horizontal legs of the vertical closed stirrups 10

should be at least 8db on both sides of the anchor as shown in Fig. 3. This requirement results in 11

a minimum edge distance of 8db plus the concrete cover. Design of reinforced anchors should 12

also satisfy other edge distance requirements, such as those in Section D.8 of ACI 318-11. 13

Bars at all four corners of the closed stirrups (referred to as corner bars hereafter) restrain 14

splitting cracks as well as other bars distributed along the concrete surfaces (referred to as crack-15

controlling bars hereafter). Therefore the corner bars and crack-controlling bars need to be fully 16

developed at both sides of the anchor bolt, and a 90-degree bend as shown in dashed lines in Fig. 17

3 may be needed. The selection of corner bars may follow the common practices in selecting 18

longitudinal corner bars for reinforced concrete beams, such as those specified in Section 11.5.6 19

of ACI 318-11. Crack-controlling bars were not provided in the tests and the splitting cracks 20

were observed as presented below. Crack-controlling bars are therefore recommended as shown 21

in Fig. 3, and the determination of these bars can be based on the well-recognized strut-and-tie 22

models.23,24

23

9

EXPERIMENTAL INVESTIGATION 1

Specimens 2

This group of experimental tests is part of a research program, which focused on the behavior 3

and design of cast-in-place anchors under simulated seismic loads.27

Sixteen tests were 4

conducted using 25 mm [1 in.] diameter anchors consisting of an ASTM A193 Grade B7 5

threaded rod (fy=724 MPa [105 ksi] and fut=1069 MPa [131 ksi]) and a heavy hex nut welded to 6

the end. Another four tests using 19 mm [3/4 in.] diameter ASTM F1554 Grade 55 anchors 7

(fy=434 MPa [63 ksi] and fut=524 MPa [76 ksi]) were conducted with two tests each under 8

monotonic shear and cyclic shear loading. Ready-mixed concrete with a targeted strength of 9

27.6 Mpa [4000 psi] was used, and cylinder tests using three batches of three 100×200 mm [4×8 10

in.] cylinders tested throughout the anchor test period showed an average compressive strength 11

of 24.3 MPa [3525 psi]. 12

The dimensions of the test blocks containing four anchors each are illustrated in Fig. 4. One 13

block was prepared for Type 19-150-100 specimens, and two blocks for Type 25-150-150 and 14

Type 25-150-150H specimens. Another block similar to that for Type 25-150-150 specimens 15

was used for Type 25-150-150SG specimens. Strain gages were installed on the reinforcing bars 16

of the two anchors in this block. All anchors had an embedment depth of 152 mm [6 in.]. The 17

width and depth of the test blocks were selected such that the spacing between the anchors was 18

larger than two times their front edge distances. Anchors in Type 25-150-150H specimens had 19

two limited side edge distances equal to 1.5 times their front edge distance. The height of the 20

blocks was 432 mm [17 in.], similar to all other anchor tests in the study.27

21

The anchor shear reinforcement was proportioned to carry the maximum capacity of the 22

anchor bolts in shear: 68 kN [15.3 kips] for the 19-mm [3/4-in.] anchors and 209kN [47 kips] for 23

10

the 25-mm [1-in.] anchors. Using the nominal yield strength of Grade 60 steel, the required 1

anchor reinforcement was found as 164 mm2 [0.25 in.

2] for the 19-mm [3/4-in.] anchors, and 503 2

mm2 [0.78 in.

2] for the 25-mm [1-in.] anchors. Therefore two No. 4 bars were provided for Type 3

19-150-100 specimens as shown in Fig. 4. The required anchor reinforcement for 25-mm [1-in.] 4

anchors was provided using four No. 4 bars with a spacing of 51-mm [2-in.] for Type 25-150-5

150 specimens, two No. 4 and four No. 3 bars for Type 25-150-150H specimens with a spacing 6

of 76-mm [3-in.], and eight No. 3 bars for Type 25-150-150SG specimens with a 51-mm [2-in.] 7

spacing. Two additional No. 3 J-hooks were added besides the outmost bars in Type 25-150-8

150SG specimens as shown in Fig. 4 to host two more strain gages, which were roughly 250 mm 9

[10 in.] away from the anchor bolt. One straight bar was provided at each corner of the closed 10

stirrups. Note that some specimens had several narrow stirrups placed behind the anchors, the 11

vertical legs of which were intended to be anchor tension reinforcement, in which case one 12

additional corner bar was provided along the top surface. However, the planned tension tests 13

were not performed because the concrete blocks were not sufficient for the large tension load 14

that would be carried by the reinforced anchors. The additional stirrups did not affect the shear 15

behavior of the anchors because they were placed behind the anchor bolts. All reinforcing bars 16

were placed with a cover of 38 mm [1.5 in.]. 17

Test Setup 18

The loading frame, actuator placement, and instrumentation setup used for the tests are 19

shown in Fig. 5. Instead of a self-balanced load frame, a tie-down rod 381 mm [15 in.] behind 20

the test anchor was used to fix the test block to the strong floor. In addition, the concrete block 21

was wedged against the strong floor to minimize the slip of the test block under cyclic loads as 22

shown in Fig. 5. An MTS Model 244.31, 245-kN [55-kip] actuator was used to apply shear 23

11

loading to the anchor bolt through a loading plate. The actuator body was braced against the 1

floor to eliminate the downward motion of actuator swivel head and the rotation of the loading 2

plate. To minimize the friction between the loading plate and the concrete top surface, a net 3

tension force of 0.8 kN [0.2 kips] was applied to the loading plate by an MTS Model 244.41, 4

489-kN [110-kip] actuator, which was used for applying tension loads in other tests. The nut 5

fixing the loading plate to the anchor bolt was first hand tightened, and then loosened 1/8 of a 6

turn to allow slight vertical movement of the loading plate when the 0.8-kN [0.2-kip] tension 7

force was applied at the beginning of a test. The test anchors were inserted through a standard 3-8

mm [1/8-in.] oversized hole in the loading plate, and a steel sleeve shim was inserted between 9

the anchor and the hole to eliminate the clearance and to prevent damage to the loading plate. 10

Loading Protocol 11

Monotonic shear tests were performed first to determine the typical actuator displacement at 12

failure, and the tests indicated a failure displacement around 35 mm [1.4 in.]. Hence, the cyclic 13

displacement steps for each 3-cycle group were chosen as 2, 3, 4 (failure displacements for 14

typical unreinforced anchors), 8, 16, and 32 mm [0.08, 0.12, 0.16, 0.32, 0.64, and 1.28 in.] as 15

shown in Fig. 5. The loading rate for the displacement cycles at or below 4 mm [0.16 in.] was 16

kept at 2 mm/min [0.08 in./min] while the load rate was increased to 10 mm/min [0.4 in./min] for 17

the 8, 16, and 32-mm [0.32, 0.64, and 1.28-in.] cycles in order to reduce test time. Most reversed 18

cyclic shear tests were conducted following the loading pattern C1 shown in Fig. 5, in which the 19

maximum displacement was set as 4 mm [0.16 in.] when the shear loading was applied opposite 20

to the front edge. This was to prevent early anchor fracture under reversed loads and to observe 21

the cyclic behavior over a full displacement range. Cyclic tests following loading pattern C2 in 22

Fig. 5 with equal peak displacements in both directions of shear loading were conducted for two 23

12

Type 25-150-150H specimens. Note that the control of actuator was based upon the actuator 1

piston motion instead of anchor displacement; hence the actual anchor displacements were 2

smaller than the above target displacements. 3

Instrumentation 4

String pots (Celesco PT510DC) and linear variable differential transformers (LVDT’s) 5

(Trans-tek Model 245) were used to measure the anchor displacements as illustrated in Fig. 5. 6

The displacements of the load plate were actually used as the anchor displacement because the 7

anchor shaft just above the concrete surface was not assessable. An IO Tech DaqBook 2000 was 8

used to collect data from all sensors as well as the force and displacement outputs from the 9

actuators. The sampling frequency was 5 Hz and the collected data was filtered using an in-10

house program with a cutoff frequency of 0.1 Hz. The observed anchor behavior is discussed 11

below. 12

13

EXPERIMENTAL RESULTS AND DISCUSSION 14

Behavior of Anchors under Monotonic Loading 15

The load versus displacement behavior is shown in Fig. 6 for the reinforced anchors 16

subjected to monotonic shear along with selected images of failed specimens. For comparison 17

purpose, the load versus displacement behavior for a 19-mm [3/4-n.] anchor with a front edge 18

distance of 100 mm [4 in.] in plain concrete is shown in Fig. 6a and the result of another anchor 19

with a front edge distance of 150 mm [6 in.] in the rest of Fig. 6. The unreinforced anchors were 20

tested with a concrete strength of 39 MPa [5656 psi] while the reinforced anchor tests had a 21

concrete strength of 24.3 MPa [3525 psi], therefore the load values for the unreinforced anchors 22

were normalized using a factor of in Fig. 6. In General the reinforced anchors failed 23

13

by anchor shaft fracture while the unreinforced anchors with similar edge distances failed by 1

concrete breakout. The failure loads for the reinforced anchors increased by about 100 percent 2

and the displacements corresponding to the peak loads increased more than six times compared 3

with that of the unreinforced anchors. 4

The load-displacement behavior of 19-mm [3/4-in.] anchors in reinforced concrete did not 5

show much difference from that in plain concrete (Fig. 6a) before a crack was observed at the top 6

surface at a load about 45 kN [10 kips]. Rather than propagating vertically along the anchor 7

shaft as observed in the tests of unreinforced anchors as represented by Fig. 1, the crack 8

propagated around the corner of the stirrups (see the inserted figure in Fig. 6a). The loss of the 9

38-mm [1.5-in.] thick concrete cover in front of the anchor caused a small capacity loss for the 10

19-mm [3/4-in.] anchors as shown in Fig. 6a. Because the 19-mm [3/4-in.] anchor only mobilize 11

the top concrete before cracking, similar to that suggested by Randl and John22

(roughly 2da 12

deep), the anchor shaft in bending was not able to resist the same amount of load until a larger 13

displacement was applied. Such post-spalling load drop has been observed in other tests of 14

anchors reinforced with hairpins.17,18

The failure load exceeded the code-specified anchor shear 15

capacity because the failure was caused by the fracture of anchor shaft largely under tension as 16

shown in Fig. 7a though the fracture may have started from a flexural crack. 17

The shear load did not drop noticeably after concrete cover spalled in the tests of 25-mm [1-18

in.] anchors as shown in Figs. 6b through 6d. The 25-mm [1-in.] anchors mobilized deeper 19

concrete such that the loss of bearing support from the cover concrete was immediately resisted 20

by lower concrete restrained by the anchor reinforcement. Another contributing factor is that the 21

25-mm [1-in.] anchors had a larger bending stiffness such that a small displacement was needed 22

to mobilize their load carrying capacities. The 25-mm [1-in.] anchors failed at loads lower than 23

14

the code-specified anchor steel capacity in shear. The fractured 25-mm [1-in.] anchors in Fig. 7c 1

showed a different failure mode from that of 19-mm [3/4-in.] anchors: anchor shaft cracked 2

under a bending moment and the rest of the anchor shaft then fractured in shear. For the shear-3

dominant failure mode, the flexural cracking reduced the cross sectional area, thus leading to a 4

lower ultimate shear capacity. 5

Anchor steel failure was achieved in all 25-mm [1-in.] diameter anchors, indicating that 6

reinforcing bars placed outside the code-specified effective distance, such as 0.5ca1 in Type 25-7

150-150SG and 0.3ca2 in Type 25-150-150H, can be effective as anchor shear reinforcement. 8

However, reinforcing bars must be evenly distributed with a small spacing in order for outside 9

bars to be mobilized. The effective distance was verified by the measured strains in the 10

reinforcing bars in Type 25-150-150SG specimens as shown in Fig. 8. The anchor reinforcement 11

consisted of eight No. 3 stirrups at a spacing of 51 mm [2 in.] and two additional No. 3 J-hooks. 12

The thin dashed lines in Fig. 8 indicate the assumed breakout crack at the concrete surface and 13

the strain gages were installed 25 mm [1 in.] behind the assumed breakout crack line on the 14

inside face of the stirrups. In general, larger strains were observed in the bars closer to the 15

anchor bolt. Meanwhile outside bars, as indicated by Gages 4S and 4N located 170 mm [6.7 in.] 16

from the anchor bolt, also developed significant strains, especially after the surface crack formed. 17

Note that the gage positions relative to a crack should be considered to interpret the measured 18

strains. For example, the strains by Gage 2N may have been affected by the crack passing the 19

gage location as shown in Fig. 8. More importantly, smaller strains measured by the gages on 20

outsider bars may have been due to the fact that the gages were away from the actual crack. In 21

addition, the measured strains indicated that none of the Grade 60 bars yielded at the peak load; 22

hence the shear capacity of reinforced anchors may not be calculated as the summation of the 23

15

yield forces of the anchor reinforcement. The shear force was actually transferred to the supports 1

(e.g., the tie-down rods on the back and the steel wedging tube at the bottom in this case) through 2

the concrete confined by the closed stirrups. 3

Anchors in Type 25-150-150H specimens had lower ultimate capacity as shown in Fig. 6c. 4

This might have been due to the poor confinement of concrete in front of the anchor bolt: 5

additional splitting cracks were observed and deeper concrete crushed in these tests, leading to a 6

longer portion of exposed and unsupported anchor bolts (e.g., up to 0.5da larger than those in 7

Type 25-150-150 specimens). Finite element analyses indicated that the anchor capacity 8

controlled by shear fracture can be affected by anchor diameter and concrete cover depth.28

It is 9

thus envisioned that the following measures as illustrated in Fig. 3 can be effective in improving 10

the post-spalling behavior and the capacity of reinforced anchors in shear: 1) corner bars should 11

be fully developed; 2) crack-controlling bars should be provided along both the top and front 12

surfaces of concrete; and 3) a separate bar can be placed right in front of the anchor bolt to 13

alleviate the large local compressive stress in concrete. 14

Anchor Shear Capacity 15

Most anchor bolts in this group of tests failed by shear fracture of a reduced anchor shaft 16

cross section as shown by the typical fractured sections in Fig. 7. This failure mode occurred 17

when a short portion of the anchor bolt was exposed and a lever arm developed in the anchors 18

after cover concrete spalled. The effect of lever arms in anchor bolts is recognized in existing 19

design codes.5,8

For example, ACI 318-11 stipulates that the design capacity of anchor 20

connections having grout leveling pads should be reduced by a factor of 0.8 for the anchor steel 21

strength in shear. Such capacity reduction considers the combined bending and shear in the 22

anchor shaft, but does not consider the thickness of the grout pads, which is similar to the 23

16

exposed length at the ultimate load. Eligehausen et al.12

proposed an equation for predicting the 1

strength of an exposed anchor assuming that the anchor fails by pure bending. This equation was 2

found not applicable for predicting the capacity of the anchors in this study likely due to the fact 3

that the anchor failure was controlled by shear fracture. Lin et al.28

improved the equation by 4

Eligehausen et al.12

by considering the contributions from flexural, shear and tensile resistance of 5

an exposed anchor shaft to the shear capacity of exposed anchors. However the equation was 6

based on double shear tests and finite element analyses of threaded rods, and the lateral support 7

to the actual anchor shaft from partially damaged concrete was not considered. Therefore, the 8

equation may provide lower-bound estimates of the actual anchor capacities. 9

The capacity of anchor bolts with a lever arm was instead examined using the test data 10

available in the literature as shown in Fig. 9. The measured anchor capacities were normalized 11

by the design capacity of anchor bolts in shear specified in ACI 318-11. The exposed depth of 12

the anchors in other tests16,25

was defined as the distance between the bottom face of a base plate 13

and the lowest solid concrete surface. The anchor steel capacity observed in this study is low 14

compared with other available tests. This might have been due to the fact that friction between 15

the load plate and the concrete surface was minimized as previously described in the test setup 16

section. 17

The statistical analysis of the limited data in Fig. 9 did not follow the procedures of 18

predictive inference,29,30

which are usually used to predict future occurrences based on the 19

existing observed data. Instead, a 5-percentile value of 0.73 was obtained using a descriptive 20

statistic analysis of the twenty two collected data points. Considering the aforementioned 21

reasons for the low observed capacities in this study, it is proposed that the shear strength of 22

reinforced anchors can be estimated as 75 percent of the code specified steel capacity for anchors 23

17

without a lever arm. This is slightly lower than the reduction factor in ACI 318-11 because of 1

two data points observed in specimens with limited side edge distances (Type 25-150-150H). It 2

is envisioned that as more data points become available in future tests with the recommended 3

anchor shear reinforcement shown in Fig. 3; the statistical importance of these two data points 4

can be reduced. Using the suggested capacity reduction for exposed anchors should be limited to 5

those with an exposed length less than three times the anchor diameter (3da). Beyond this limit, 6

the anchor steel failure in shear needs further study. 7

Behavior of Anchors under Cyclic Loading 8

Seismic actions on structural components are mostly simulated in laboratories using quasi-9

static cyclic tests with reversed loading.31

Therefore, displacement-controlled loading33

was used 10

in this study though many cyclic tests of anchors have been conducted with load-controlled 11

loading.16,17,32

The load versus displacement behavior of two 19-mm [3/4-in.] anchors subject to 12

Type C1 cyclic shear loading is plotted in Fig. 10a. The monotonic curve was closely followed 13

by cyclic curves until a displacement of 10 mm [0.4 in.], beyond which the cyclic loads were 14

lower than that of the monotonic test. The slope of the cyclic curves again had a sudden change 15

at a displacement around 2 mm [0.16 in.], indicating the concrete cover spalling. The difference 16

in the observed loads at this displacement may have been due to variations in the specimens such 17

as the actual edge distances and cover depths. The first three displacement cycles did not see 18

significant degradation in loads with successive cycles to the same displacement while the 19

degradation was obvious at the larger-displacement cycles. This was because the displaced 20

cover concrete during the first cycle of each three-cycle group was not able to recover, leading to 21

reduced restraint to the anchor shaft in the successive cycles. An average capacity reduction of 22

28 percent was observed in the cyclic shear capacity for 19-mm [3/4-in.] anchors. This reduction 23

18

was partly attributed to the change of failure modes as shown by the fractured shape of anchor in 1

Figs. 7a and 7b: the anchor failure was controlled by the shear fracture under cyclic loading 2

while the tensile fracture controlled the anchor failure in the monotonic test. Note that the 3

reduced cyclic shear capacities of 19-mm [3/4-in.] anchors were higher than the proposed 4

capacity of exposed anchors under monotonic loading because of the monotonic failure mode. 5

The behavior of Type 25-150-150 specimens are compared in Fig. 10b. The monotonic load-6

displacement curve nicely envelopes the cyclic curves represented by the first loading cycle in 7

each three-cycle group. The load degradations during the successive two cycles was again due to 8

the irreversible crushing of concrete cover in front of the anchors. No capacity drop was 9

observed in the tests of Type 25-150-150 specimens. An average capacity drop of 6.8 percent 10

was observed for Type 25-150-150H anchors with limited side edge distance as shown in Fig. 11

10c. In this group of three cyclic tests, concrete deeper than the 38-mm [1.5-in.] cover crushed 12

likely due to poor confinement conditions as indicated by splitting cracks. The larger exposed 13

length led to a larger moment under the same shear load and thus a lower shear capacity. Note 14

that the poor confinement conditions can be improved by crack-controlling bars recommended in 15

Fig. 3. In addition, a bar placed just in front of the anchor shaft can help distribute the localized 16

high compressive stresses such that the exposed length of the anchors would not be affected by 17

the cyclic loading. Finally, the tests of two Type 25-150-150H anchors with fully reversed 18

cyclic loading (Type C2 in Fig. 5) ended with anchor fractured under a shear load applied 19

opposite to the front edge. The ultimate load capacities were on average 5 percent lower than the 20

code-specified anchor steel capacity as shown in Fig. 10d. Hence, it is reasonable to ignore the 21

reduction of steel capacities for reinforced anchors in cyclic shear considering that the monotonic 22

capacity of reinforced anchors has already been reduced by 25 percent as proposed above. 23

19

CONCLUSIONS 1

A design method for anchor shear reinforcement was proposed and verified using 2

experimental tests of single cast-in-place anchors. With a goal to prevent concrete breakout and 3

to confine concrete in front of an anchor bolt, the proposed anchor shear reinforcement consisted 4

of closely spaced stirrups, corner bars, and crack-controlling bars distributed along all concrete 5

faces. The horizontal legs close to the concrete surface of the closed stirrups were proportioned 6

to carry a force equal to the code-specified anchor steel capacity in shear. The needed 7

reinforcement was provided by closely spaced small size stirrups distributed within a distance 8

from the anchor equal to its front edge distance. Although not specifically tested in the study, 9

the selection of corner bars should follow the practices specified in Section 11.5.6.2 of ACI 318-10

11 for corner bars in beams, and crack-controlling bars may be determined following the well-11

recognized strut-and-tie models. 12

With the proposed anchor shear reinforcement, concrete breakout was prevented and anchor 13

shaft fracture was observed in all the tests of single anchors in this study. Cover concrete in 14

front of the anchor bolts spalled, causing the top portion of the anchor shaft close to the concrete 15

surface to become exposed. The full anchor steel capacity in shear was not achieved because the 16

exposed anchors were subjected to a combination of shear, bending, and tension at failure. An 17

analysis of the test results of exposed anchors in the literature indicated that a reduction factor of 18

0.75, which is slightly lower than that in ACI 318-11 on anchors with a grout pad, can be used to 19

determine the shear capacity of reinforced anchors. In addition, quasi-static cyclic tests of the 20

reinforced anchors in shear showed insignificant capacity reduction, which is comparable to 21

other displacement-controlled cyclic tests. Although large capacity reductions were observed in 22

20

load-controlled cyclic tests in the literature, no further capacity reduction was recommended in 1

this study for reinforced anchors subjected to cyclic shear loading. 2

3

ACKNOWLEDGMENTS 4

The study reported in this paper is from a project supported by the National Science 5

Foundation (NSF) under Grant No. 0724097. The authors gratefully acknowledge the support of 6

Dr. Joy Pauschke, who served as the program director for this grant. The authors also thank the 7

colleagues in ACI Committee 355 for their valuable inputs. Any opinions, findings, and 8

recommendations or conclusions expressed in this material are those of the authors and do not 9

necessarily reflect the views of NSF. 10

11

21

NOTATION 1

= area of anchor reinforcement 2

= effective cross-sectional area of anchor in shear and tension 3

1ac = front edge distance of anchor 4

2ac = side edge distance of anchor 5

ad = anchor diameter 6

bd = reinforcement diameter 7

= distance from shear force to surface reinforcement 8

= design bond strength of anchor reinforcement in breakout cone 9

'cf = concrete compressive strength 10

yf = yield strength of anchor steel 11

ysF = yield strength of steel reinforcement 12

utaf = ultimate tensile strength of anchor steel 13

= development length of hooked bar in breakout cone

14

u = circumference of reinforcing bar

15

sdV = Design shear capacity of anchor 16

sV = Actual shear capacity of exposed anchor 17

= vertical reinforcement position 18

= stress in anchor reinforcement 19

22

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5. American Concrete Institute, “Building Code Requirements for Structural Concrete (ACI 15

318-11).” Farmington Hills, Michigan, 2011. 16

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23

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Austin, Austin, TX, 1989. 3

10. Cannon, R., “Straight Talk about Anchorage to Concrete, Part I.” ACI Structural Journal, 4

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Ernst & Sohn, Berlin, Germany, 2006. 9

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of Transportation, Sacramento, CA, 1978. 18

17. Klingner, R.; Mendonca, J.; and Malik J., “Effect of Reinforcing Details on the Shear 19

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18. Lee, N.; Park, K.; and Suh, Y., "Shear Behavior of Headed Anchors with Large Diameters 22

and Deep Embedment." ACI Structural Journal, Vol. 108, 2010, No. 1, pp. 34-41. 23

24

19. Schmid, K., “Structural Behavior and Design of Anchor near the Edge with Hanger Steel 1

under Shear,” PhD Thesis, University of Stuttgart, Germany, 2010. 2

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Universität Kaiserslautern, 1993. (In Schmid 2010). 8

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23. Fuchs, W. and Eligehausen, R., “Zur Tragfähigkeit von Kopfbolzenbefestigungen unter 12

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Report of ACI Committee 355, Farmington Hills, Michigan, 2011. 21

27. Petersen, D., “Seismic Behavior and Design of Cast-in-Place Anchors in Plain and 22

Reinforced Concrete,” MS Thesis, University of Wisconsin, Milwaukee, WI, 2011. 23

25

28. Lin, Z.; Petersen, D.; Zhao, J. and Tian, Y., “Simulation and Design of Exposed Anchor 1

Bolts in Shear,” International Journal of Theoretical and Applied Multiscale Mechanics. (in 2

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265 pp. 5

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Services Bulletin, Hilti, Inc., Tulsa, Oklahoma, 1997. 7

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32. Civjan, S. and Singh, P., “Behavior of Shear Studs Subjected to Fully Reversed Cyclic 11

Loading,” Journal of Structural Engineering, Vol. 129, 2003, No. 11, pp.1466–1474. 12

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Displacements,” ACI Special Publication SP130, 1992, pp. 180-204. 14

15

26

TABLES AND FIGURES 1

List of Tables: 2

Table 1 Summary of design equations for anchor shear reinforcement 3

Table 2 Summary of reinforced anchor tests in shear 4

5

List of Figures: 6

Fig. 1–Concrete breakout failure under shear 7

Fig. 2–Schematics of existing anchor shear reinforcement 8

Fig. 3–Proposed anchor shear reinforcement layout 9

Fig. 4–Configurations of anchor specimens 10

Fig. 5–Experimental test setup 11

Fig. 6–Monotonic shear test results of reinforced anchors 12

Fig. 7–Typical fractured shape of anchor bolts 13

Fig. 8–Strains in anchor shear reinforcement (25-150-150SG1) 14

Fig. 9–Capacity of anchor bolt with a lever arm 15

Fig. 10– Cyclic behavior of reinforced anchor bolts 16

17

27

Table 1 Summary of design equations for anchor shear reinforcement 1

Reference Design equation for given load Development in cone Actual shear capacity (Vs) Notes

Shipp and

Haninger (1983)

Not needed

Design based on

equivalent tension Hairpins

Klingner et al.

(1983) Not needed Hairpins

CEB

(1997)

Considered in

capacity calculation

Bars within

0.5ca1

ACI 318

(2008)

* Bars within

0.5ca1 or 0.3ca2

Widianto et al.

(2010)

reduced for not fully developed bars

Not considered in

Strut-and-tie model

Stirrups, ties and

J-hooks

Fib design guide

(to be published)

Considered in

capacity calculation

Bars within

0.5ca1

Proposed on both sides Closed stirrups

within ca1

: area of anchor reinforcement; : yield strength of reinforcement; , : effective cross-sectional area of anchor; 2

, : edge distances of anchor; : distance from shear to reinforcement; : design bond strength; : ultimate strength of anchor; 3 : development length of hooked bar in breakout cone; u: circumference of reinforcing bar; : design shear force; 4 : reinforcement position; : modification factor; : stress in anchor reinforcement; *: see Chapter 12 of ACI 318-11 for details. 5

6

7

28

Table 2 Summary of reinforced anchor tests in shear 1 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 Note: 1 in. = 25.4 mm; 1 kip = 4.45 kN;

*: anchor fracture 2

occurred when shear was applied opposite to front edge. 3 4 5

6

29

1 Fig. 1–Concrete breakout failure under shear 2

3

102 mm

[4 in.]

152 mm

[6 in.]

Idealized breakout cone

152 mm

[6 in.]

35º

Top surface

Front surface

ca1:

30

1 Fig. 2–Schematics of existing anchor shear reinforcement 2

3

15.0 ac

23.0 ac

dhldldl

31

1 Fig. 3–Proposed anchor shear reinforcement layout 2

3

4

1ac

21,min aa cc

bd8

bd8bd8 bd8

32

1

2

3 Fig. 4–Configurations of anchor specimens 4

5

6

33

1 Fig. 5–Experimental test setup 2

3

4

34

1

2 Fig. 6–Monotonic shear test results of reinforced anchors 3

4

35

1

2 Fig. 7–Typical fractured shape of anchor bolts 3

4

a

) b

)

c

)

d

)

19-150-100-monotonic 19-150-100-cyclic

25-150-150-monotonic 25-150-150-cyclic

36

1

2 Fig. 8–Strains in anchor shear reinforcement (25-150-150SG1) 3

4

37

1 Fig. 9–Capacity of anchor bolt with a lever arm 2

3

4

38

1

2 Fig. 10–Cyclic behavior of reinforced anchor bolts 3

4

5