Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks in Wide Members

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1 Research Note Research Note RN Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks in Wide Members Introduction Longitudinal reinforcing steel bars in flexural members are often developed at discontinuous ends with a 90 or 180 standard hook. These hooks are usually oriented in the vertical direction, which generally corresponds with being perpendicular (or normal) to the member s major axis. Because standard hook sizes can be quite large, especially for large diameter bars, detailing of hooked bars can be critical. In some instances, such as the case of a shallow member that is heavily reinforced, the standard hook height in accordance with the ACI Code (2011), plus the required concrete cover above and below the bar, may exceed the member depth. In this case, one solution used in practice to satisfy the cover requirements and maintain the hook length is to rotate the bar along its longitudinal axis, such that the hook is no longer vertical. Hook tilting is illustrated in Figures 1 and 2. This Research Note describes results of a study initiated as part of the CRSI Graduate Research Fellowship Program to evaluate the influence of hook angle tilt on hook performance and behavior. The research also examined the potential limitations of hook tilt in a concrete member, depicted in Figure 3. Background Hook Behavior Forces are transferred between 90 or 180 hooked bars in tension and the surrounding concrete through bond along the bar surface and bearing of the bar on the enclosed concrete. Ninety-degree (90 ) hooked bars tend to straighten under tension, resulting in bearing against the concrete in the inner radius of the hook, and compressive stresses on the hook tail as it bears against the surrounding concrete. Failure of both hook types can result from crushing of concrete within the bend of the bar, Figure 1 Example of tilted reinforcing bar hooks shown at the edge of a cantilever balcony slab Figure 2 Recommended bar details for solid slabs (CRSI 2008) Figure 3 Schematic of a hooked bar in concrete slab

2 which may cause cracks to propagate to a nearby side face of the member. Splitting failure, originating within the bar bend due to high stress concentrations, can also occur within the plane of the hook. Cracking of the concrete behind the hook tail is possible, as well. Code Requirements Hooks can be provided to develop reinforcing bars in tension in locations where there is limited length to develop a straight reinforcing bar. Standard hook dimensions including the minimum finished inside bend diameter D and extension length A are given in ACI , Sections 7.1 and 7.2.1, and CRSI s Design Handbook (2008), as shown in Figure 4. Provisions for development of standard hooks in tension are provided in ACI , Section The tension development length of deformed bars with standard hooks l dh (determined in accordance with the Code) is a function of the reinforcing bar diameter, yield strength, presence of epoxy coating, concrete type, and specified compressive strength of concrete. The development length can be reduced by various modification factors, which account for the effects of excess reinforcement, confinement from stirrups or ties, and/or adequate concrete cover. The parameters and factors that influence the development length account for the magnitude of the compressive stresses or the resistance to splitting. Summary of Study Scope and Objective The objective of this study was to evaluate the limits of reinforcing steel hook tilt from vertical to ensure ultimate performance of the member is not compromised. This was achieved experimentally by comparing the load-slip responses of modified beam-end specimens with different hooked bar orientations and configurations. Concrete reinforcement in this study included deformed reinforcing steel bars with standard hooks embedded in normal weight concrete. The test variables included hook tilt angle, hook bend type, reinforcing bar size, and group-effect. Four hook tilt angles were evaluated: 0 (horizontal), 22.5, 45, and 90 (vertical). Both 90 and 180 standard hooked reinforcing bars were investigated. #5 and #8 bars were examined in this study since they are commonly used in practice. The effect of multiple bars was also examined to evaluate the effects of interaction of adjacent bars, as shown in Figure 1. Additionally, bar spacing and position were evaluated. The test specimen matrix is shown in Table 1. Experimental Program Test Specimens Modified beam-end specimens used in this study were modeled after previous tests reported in the literature (Minor 1971, Jirsa and Marques 1972, Minor and Jirsa 1975, Ehsani, et al. 1995) and in ASTM A (2010). The specimen was elongated so the reinforcing hook extended beyond the reaction plates and the compression strut developing between the reactions. A PVC pipe bond breaker was provided along the straight portion of the bar to isolate the performance of the hooked portion of the bar. Figure 4 Hooked bar details for development of standard hooks (CRSI 2008) Test specimen dimensions are summarized in Table 1. The specimen height was a function of the concrete above the hook extension (3 in.), the diameter (d b ), the length of the hook tail extension (12d b ), and the concrete cover (3d b ) of the test bar. The length of the specimen was the height plus the distance beyond the reaction plate. The distance beyond the reaction plate was the sum of 4 in., the diameter of the test bar, and 3 in. to account for the cover of the tail extension of the bar. The width of the test specimens varied to accommodate the 2 Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks in Wide Members [RN ]

3 Specimen 1 Table 1 Test Specimen Matrix Bar Size Hook Angle of Standard Tilt from Hook Bend ( ) Horizontal ( ) Length (in) Width (in) Height (in) BE # /2 17 3/8 9 7/8 BE # /2 16 5/8 9 7/8 BE # /2 14 1/2 9 7/8 BE # /2 8 5/8 9 7/8 BE # /2 27 3/8 14 7/8 BE # /2 25 7/8 14 7/8 BE # /2 21 1/2 14 7/8 BE # /2 8 5/8 14 7/8 BE # BE # /8 22 BE # /8 22 BE # BE G2A 2 3-# /2 67 3/8 14 7/8 BE GA 2 3-# /2 47 3/8 14 7/8 BE G0.5A 2 3-# /2 37 3/8 14 7/8 BE G2A 3-# /2 62 3/4 14 7/8 BE GA 3-# /2 44 3/8 14 7/8 BE G0.5A 3-# /2 35 1/8 14 7/8 BE G2A 2 3-# BE GA 2 3-# BE G0.5A 2 3-# BE G2A 3-# /4 22 BE GA 3-# /8 22 BE G0.5A 3-# /8 22 Notes: 1. The following notation system is used to identify the variables of each specimen. The first term indicates the type of test: BE (Modified beam-end test). The second term indicates the bar size: #5 or #8 standard. The third term is hook bend type: 90 or 180. The fourth term is the angle of tilt from horizontal: 0, 22.5, 45 or 90. Term G in the fifth term denotes specimens with multiple bars, and bar spacing is denoted as a multiple of bar extension length A defined in Figure 4. (a) side view (b) end view Figure 5 Single bar specimens (90 hook bar shown) 2. Angle of tilt from horizontal is nominal. Actual angle is slightly larger than zero due to bar placement. tilt arc of the hooks on the bar ends. The cover distance on the side of the reinforcing steel hook was 4 in., based on ASTM A Specimens were constructed with the reinforcing bar(s) on the bottom surface to mitigate the top bar effect; they were then later inverted for testing. Single bar specimens with 180 and 90 hooks are shown in Figure 5, and multiple bar specimens are shown in Figure 6. In the figures, ϴ is the angle of tilt from horizontal. Material Properties The reinforcing steel used in this study was ASTM A615, Grade 60. The measured yield strength of the #5 and #8 reinforcing bars was 68.0 ksi and 67.5 ksi, respectively. The concrete was normal weight, with a target compressive strength of 4,500 psi. The single bar test specimens achieved an average compressive strength of 6,450 psi, and the multiple bar specimens achieved an average compressive strength of 4,850 psi; both were measured at test date. The compressive strength of concrete (ƒ c ) and the splitting tensile strength of concrete (ƒ t ) measured at test date for each specimen are provided in Table 2. Figure 6 Multiple bar specimens end view Test Procedure The test setups for the single bar and multiple bar specimens are shown in Figures 7 and 8, respectively. For the single bar specimens, a steel L-bracket tensioned to the strong floor was used to provide the force couple reaction created by the applied force to the bar and the horizontal reaction of the test specimen, depicted in Figure 7. To resist overturning, a double channel spreader beam was placed atop the specimen, creating a distributed vertical reaction. This loading beam was held in position by two additional spreader beams tensioned to the strong floor. To apply the force to the lead end of the reinforcing bar, a center hole hydraulic jack and load cell were placed on the bar, completed with an end anchorage affixed to the bar end. CRSI Research Note 3

4 Single Bar Specimens Muiltiple Bar Specimens Table 2 Summary of Test Results Specimen f' c (psi) f t (psi) T1 (ksi) T1 (k) S1 (in) Failure Mode BE Y BE Y BE Y BE Y BE Y BE Y BE Y BE Y BE Y BE Y BE Y BE Y BE G0.5A Y Bar B Y BE GA Y Bar B Y BE G2A C Bar B Y, C BE G0.5A Y Bar B Y BE GA Y Bar B Y BE G2A Y Bar B Y BE G0.5A C Bar B C BE GA Y, C Bar B Y, C BE G2A C Bar B C BE G0.5A S Bar B S BE GA Y Bar B Y BE G2A C Bar B C For the multiple bar specimens, reactions for the horizontal force couple were provided by two HSS tubes tensioned to the strong floor and a load frame tensioned to the strong floor, illustrated in Figure 8. The top beam reaction mechanism was similar to the single bar specimens. Force to the lead end of the reinforcing bar was applied using a similar hydraulic jack and load cell arrangement placed on each reinforcing bar. The hydraulic jack for each bar was manifolded in parallel to ensure equal pressure from the pump; loads applied to each bar were carefully monitored during testing to provide equal load distribution. During testing, the applied force in each bar was measured using a load cell. Slip was measured using linear variable displacement transducers (LVDTs) and displacement wires positioned at the free end of the bar and various locations along the embedded hook. Strain in the bar was measured at the free end and various positions along the embedded hook with electronic strain gages affixed to the bar. The load was applied incrementally, which typically resulted in 36 load stages for the both #5 and #8 bars based on a nominal yield strength of 60 ksi. At each load stage, the load (a) side view (a) side view (b) top view (b) top view Figure 7 Single bar specimen test setup Figure 8 Multiple bar specimen test setup 4 Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks in Wide Members [RN ]

5 was applied and held constant. Once the load stabilized, electronic data were recorded. Behavior of the specimens such as cracking, bar slip, and failure was observed and documented at each load stage. All specimens were tested to failure. Failure was defined as one of three test-end modes: reinforcing bar yielding, excessive concrete cracking, or reinforcing bar slip. The reinforcing bar yield mode was characterized by a plateau in the force - displacement or force - strain plot monitored during testing. Concrete cracking was monitored visually and audibly during testing; cracking typically destroyed the concrete confinement in the hook vicinity, which usually led to bar pullout. Excessive slip of the reinforcing bar was characterized by large movement recorded in the lead bar LVDT, and defined as slip greater than 0.12 in. Results General A summary of the test results is provided in Table 2 with the failure mode of each specimen identified by yielding (Y) of the bar, concrete cracking (C), and/or slip (S) of the reinforcing bar. The failure mode of all single bar specimens was by bar yielding, while the failure mode of the multiple bar specimens varied. As shown in Table 2, the maximum bar stress or force is termed T1, and the corresponding displacement at the bar end is denoted S1. Note the values of S1 were corrected to account for bar elongation. For the multiple bar specimens, refers to the exterior bar closest to the side edge, and Bar B refers to the interior bar. The influence of the different test variables was evaluated by comparing the reinforcing bar stress-displacement relationships for different groups of specimens. Specimens were grouped to isolate a single test variable, either hook tilt angle, bar size, hook type, number of bars, or bar position, while holding other test variables constant. Table 3 presents the specimen groups discussed in this research note; full results are discussed in Podhorsky (2011). To compare the performance of specimens within a group of either single bar specimens or multiple bar specimens, maximum Single Bar Specimens Multiple Bar Specimens - Multiple Bar Specimens - Bar B Group 1 Group 2 Group 3 Grp 16 Grp 17 Grp 18 Grp 19 Grp 20 Grp 21 Grp 32 Grp 33 Grp 34 Grp 35 Grp 36 Grp 37 Table 3 Specimen Groups and Results Specimen BE BE BE BE BE BE BE BE BE BE BE BE f' c,avg (psi) ff cc, aaaaaa/ff cc T1 (ksi) T1* (ksi) S1 (in) Failure Mode Y Y Y Y Y Y Y Y Y Y Y Y BE G0.5A Y 4905 BE G0.5A Y BE GA Y 5160 BE GA Y BE G2A C 4840 BE G2A Y BE G0.5A C 4365 BE G0.5A S BE GA Y, C 5080 BE GA Y BE G2A C 4735 BE G2A C BE G0.5A Bar B Y 4905 BE G0.5A Bar B Y BE GA Bar B Y 5160 BE GA Bar B Y BE G2A Bar B Y, C 4840 BE G2A Bar B Y BE G0.5A Bar B C 4365 BE G0.5A Bar B S BE GA Bar B Y, C 5080 BE GA Bar B Y BE G2A Bar B C 4735 BE G2A Bar B C bar stress (T1) was normalized to the average group concrete compressive strength by the factor ƒ c,avg ƒ c to account for the minor differences in measured concrete compressive strength (Ehsani, et al. 1995). Effect of Hook Tilt Angle For the single bar specimens, all bars at the four tilt angles were able to develop yielding at approximately 60 ksi, irrespective of tilt angle as shown in Figure 9. For the concrete compressive strength at testing, the tilt angle did not have an effect on the displacements of the reinforcing bar. The graphs in Figures 10 and 11 show that the maximum normalized bar stress was similar among all multiple-bar specimens, at approximately 60 ksi, except for a few that cracked before yielding. The data showed that the #5 bars generally yielded, while the #8 bars exhibited one of the three failure modes. The different failure modes were likely due to the higher force transfer to the concrete in bond required in order to yield the #8 bars versus the #5 bars. Even though the CRSI Research Note 5

6 Figure 9 Influence of tilt angle on maximum normalized bar stress for Groups 1 to 3 (single bar specimens) Figure 10 Influence of tilt angle on maximum normalized bar stress for Groups 16 to 21 (multiple bar specimens, exterior bar) stresses were similar, the displacements were much different. The displacements of the multiple bar specimens were generally higher than the displacements of the single bar specimens. Effect of Bar Size The testing showed that the maximum normalized bar stress was similar for both the #5 and #8 bars. The bar displacements varied for the single bar specimens, but not enough of a data trend could be established. 6 Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks in Wide Members [RN ]

7 Figure 11 Influence of tilt angle on maximum normalized bar stress for Groups 32 to 37 (multiple bar specimens Bar B, interior bar) In the multiple bar specimens, the testing showed the maximum normalized bar stress was similar among all specimens, except for those tests for which concrete cracking was the failure mode. The data showed the #5 bars generally yielded, while the #8 bars exhibited different failure modes. As addressed previously, the different failure modes were likely due to the higher force that must be transferred to the concrete in bond in order to yield the #8 bars versus the #5 bars. Effect of Hook Type This variable was studied only in the single bar specimens. Test results revealed that there was little difference between the 90 and 180 hooks studied. This observation corroborates similar conclusions from various testing programs conducted at the University of Texas. Multiple Bar Spacing A variable spacing of the three reinforcing bars was studied, with the spacing based on the standard reinforcing hook geometry: 0.5A, A and 2A. Generally all multiple bar specimens reached similar maximum normalized bar stress, except for when the cracking failure mode governed. For the exterior bar (), the #5 bar size exhibited no influence regarding bar spacing. On the contrary, the #8 exterior located bars exhibited increased lead bar slip with a closer bar spacing. Johnson and Jirsa s (1981) study also reported this trend. For the single interior bar (Bar B), both the #5 and #8 bars exhibited increased lead bar slip with closer bar spacing. When the single and multiple bar tests were compared, the testing revealed closer spacing of multiple bars results in an increase in reinforcing bar slip or displacement relative to the concrete. At a close spacing, one reinforcing bar and its bond behavior with the concrete clearly has an effect on the adjacent reinforcing bar and its bond behavior. This adjacent bar influence was shown to increase with a closer spacing of reinforcing bars. For the larger bar spacings, the normalized, single bar displacements were similar to those of the multiple bar specimens with an A or 2A spacing (wide spacing). This indicates the bars were spaced sufficiently apart to avoid the overlap of the bond stresses. Conclusions and Recommendations This study was conducted to investigate the potential influence of hook tilt angle on the performance of reinforcing bars hooks. This was approached experimentally by comparing the bar stress-displacement responses in 24 modified beam-end specimens with different hooked bar orientations and configurations. Twelve specimens included a single bar, and 12 specimens included three bars. To isolate the performance of the bar hook, only the hooked portion of each bar was bonded to the surrounding concrete. All single bar specimens were able to achieve bar yielding, and no concrete crushing was observed in the test specimens after failure. On the other hand, the multiple bar specimens, which had a lower average concrete CRSI Research Note 7

8 compressive strength than the single bar specimens, exhibited different modes of failure including bar yielding, concrete cracking, and excessive slip. Results showed no clear relationship between hook tilt angle and hook performance, and for the specimens tested in this study, hook tilt angle did not appear to have an effect on the maximum stress or displacement of the reinforcing bar. Similar to results found in previous studies, results generally indicated that bar slip (displacement) increased with closer bar spacing in specimens with multiple bars. Acknowledgements This research was conducted by Missouri University of Science and Technology (Missouri S&T) with the sponsorship of the Concrete Reinforcing Steel Institute (CRSI) and the National University Transportation Center at Missouri S&T. Longitudinal reinforcing steel and bar supports used in this work were generously provided by Ambassador Steel Corporation and Gateway Building Products. References American Concrete Institute - ACI Committee 318 (2011), Building Code Requirements for Structural Concrete (ACI ) and Commentary (ACI 318R-11), Farmington Hills, Michigan, 503 pp. ASTM International - ASTM A944 (2010), Standard Test Method for Comparing Bond Strength of Steel Reinforcing Bars to Concrete Using Beam-End Specimens, ASTM A944-10, ASTM International, West Conshohocken, PA, 4 pp. Concrete Reinforcing Steel Institute - CRSI (2008), CRSI Design Handbook, Schaumburg, Illinois, 788 pp. Ehsani, M.R., Saadatmanesh, H., and Tao, S. (1995), Bond of Hooked Glass Fiber Reinforced Plastic (GFRP) Reinforcing Bars to Concrete, ACI Materials Journal, Vol. 92, No. 4, American Concrete Institute, July-August, pp Jirsa, J.O. and Marques, J.L.G. (1972), A Study of Hooked nchorages in Beam-Column Joints, Final Report, Project 33, Reinforced Concrete Research Council, Department of Civil Engineering, Structures Research Laboratory, University of Texas at Austin, 79 pp. Marques, J.L.G. (1973), Study of Anchorage Capacities of Confined Bent-Bar Reinforcement, Ph.D. Thesis, Rice University, Houston, Texas, 215 pp. Marques, J.L.G. and Jirsa, J.O. (1975), A Study of Hooked nchorages in Beam-Column Joints, ACI Journal, Proceedings, Vol. 72, No. 5, May, pp Minor, J. and Jirsa, J.O. (1975), Behavior of Bent nchorages, ACI Journal, Proceedings, Vol. 72, No. 4, April, pp Minor, J. and Jirsa, J.O. (1971), A Study of Bent nchorages, Rice Structural Research Report No. 9, Department of Civil Engineering, Rice University, Houston, Texas, 65 pp. Podhorsky, N.L. (2011), Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks, M.S. Thesis, Missouri University of Science and Technology, Rolla, Missouri, 174 pp. Contributors: The principal authors on this publication are Nicole L. Witushynsky (née Podhorsky) and Lesley H. Sneed, Ph.D., PE, of Missouri University of Science & Technology. This document represents a summary of their CRSI research project on the subject topic; the final report should be referenced for more information on the topic research. Keywords: Development length, standard hook, tilt, deformed bar, reinforcement, reinforced concrete Reference: Concrete Reinforcing Steel Institute-CRSI [2013], Evaluation of the Orientation of 90 and 180 Reinforcing Bar Hooks in Wide Members, CRSI Research Note RN , Schaumburg, Illinois, 8 pp. Note: This publication is intended for the use of professionals competent to evaluate the significance and limitations of its contents and who will accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel Institute reports the foregoing material as a matter of information and, therefore, disclaims any and all responsibility for application of the stated principles or for the accuracy of the sources other than material developed by the Institute. The opinions and findings expressed in this Research Note are those of the researchers and do not necessarily reflect the opinions or recommendations of the Concrete Reinforcing Steel Institute. 933 North Plum Grove Rd. Schaumburg, IL p f Regional Offices Nationwide A Service of the Concrete Reinforcing Steel Institute 2013 This publication, or any part thereof, may not be reproduced without the expressed written consent of CRSI. Printed in the U.S.A.