Tensile Strength of Lap Splices in Reinforced Concrete Members

Transcription

1 Research Note Research Note RN Tensile Strength of Lap Splices in Reinforced Concrete Members Introduction Current U.S. design provisions for lap splices of deformed bars in tension were developed primarily from tests conducted on beam specimens with lap lengths less than or equal to 40 bar diameters (d b ). But design provisions often require splices to be longer than 40 bar diameters. Recent structural failures have indicated the vulnerability of lap splices with splice length-to-bar-diameter (l s /d b ) ratios equal to or exceeding 40. In this study conducted at Purdue University, the tensile strength of unconfined deformed-bar lap splices with different lengths was re-examined. The focus was on lap splices with lengthto-bar-diameter ratios of 40 or more (l s /d b 40). The tensile strength of lap splices in configurations resembling those observed to have splice failures in the field was also examined. Background The tensile strength of lap splices of deformed reinforcing bars in concrete has been studied extensively in the past (ACI 408, 2003). Recent failures of deformed-bar lap splices in Turkey (Kilic and Sozen, 2003), Japan (Kim and Shiohara, 2012), and Chile (Song, et. al. 2012) have indicated the need to revisit the topic. These failures had three commonalities: 1) the splice lengths were equal to or exceeded 40 bar diameters; 2) the spliced bars were located in concrete sections entirely in tension (smoke stacks or chimneys) or concrete sections subjected to small tensile strain gradients, such as structural walls; and 3) they occurred during earthquakes. Current U.S. design provisions (ACI 318, 2011) for lap splices of deformed bars in tension were developed primarily from static and monotonic tests conducted on beams with lap lengths not exceeding 40 bar diameters (Orangun, et. al. 1977). But code design provisions often require splices to be longer than 40 bar diameters, as illustrated in the previous examples. None of the failures described in the previous examples had the splices located in beams. In earthquakes, splices near critical sections are likely to experience a stress equal to or exceeding their yield stress. The available data from tests conducted until October 2001 on beams with lap splices of uncoated, deformed reinforcing bars were compiled by ACI Committee 408 (ACI 408, 2003). Until 2001, only 8 percent of the data came from tests of lap splices with lengths equal to or exceeding 40 bar diameters. Only after 2006, a significant amount of data from tests of longer splices has been published (El-Hacha, et. al. 2006; Seliem et. al. 2009). Today, to the best of the researchers knowledge, only 14 percent of the available data comes from tests of splices with lengths equal to or exceeding 40 bar diameters. This compiled test data is presented in Figure 1. As shown in Figure 1, maximum steel stress developed (f su ) normalized by f ć in psi (where f ć is the concrete compressive strength in psi) is plotted on the vertical axis and the splice lengthto-bar diameter (l s /d b ) ratio is plotted on the horizontal axis. The available data from these referenced tests met the following five criteria: 1. Bottom-cast; in other words, the depth of concrete cast below bars did not exceed 12 in. [305 mm], 2. Minimum clear cover equal to or exceeding 1 bar diameter (c min 1d b ),

2 3. Clear spacing between spliced bars equal to or exceeding 2 bar diameters (c si d b ), 4. No transverse reinforcement was present (unconfined), and 5. Compressive strength of concrete (f ć ) not exceeding 10,000 psi [69 MPa]. ACI 318 Design Provisions The equation for development length in Section of the current U.S. design provisions (ACI 318, 2011) for #7 and larger bars is rearranged and simplified to Equation 1: f su = 20 f ć ( l s ) (1) d b Equation 1 was expected to provide a lower bound estimate to the maximum steel stress developed for the data plotted in Figure 1. The provisions allow designers to increase this value by providing more concrete cover, adding transverse confinement reinforcement, or a combination of both. Figure 1 illustrates that, for the data available until 2001, Equation 1 provided a reasonable lower bound for splices meeting the five criteria listed above. Only after 2006, data suggesting that Equation 1 was un-conservative became available. Without more data from tests of splices meeting these criteria, it was difficult to confirm this conclusion. Thus, the main objectives of this study were to contribute data from tests of unconfined deformed-bar lap splices with splice length-to-bar diameter ratios equal to or exceeding 40, and to compare the strength of unconfined lap splices of different lengths. Experimental Program Two series of tests were conducted at the Robert L. and Terry L. Bowen Laboratory for Large-Scale Civil Engineering Research, Purdue University in West Lafayette, Indiana, to evaluate the tensile strength of deformed-bar lap splices with lengths varying from 20 to 85 bar diameters. In Series 1 presented in Table 1, twentyseven (27) prismatic coupons were tested monotonically in tension to failure. In Series 2, fifteen (15) beams were tested to failure; specimen information is presented in Table 2. The beam lap splices were located in a region of nearly constant bending moment. The tests are described in greater detail in the research report (Richter, 2012) and are summarized herein. Figure 1 Available data from tests conducted on beams with lap splices of uncoated, unconfined, deformed reinforcing bars (adapted from the ACI 408 database) 2 Tensile Strength of Lap Splices in Reinforced Concrete Members [RN ]

3 Table 1 Selected properties and results from Series 1 prismatic coupons Table 2 Selected properties and results from Series 2 beams CRSI Research Note 3

4 Reinforcing Bar Figure 3 Concrete Prism T T Lap Splice Figure 2 Plan view of typical Series-1 prismatic coupon c so c b Type A h Type C Bottom c t 2c si b Type B Type D Top Figure 3 Cross-sectional views of the Series-1 prismatic coupons. (Note the designations of Top and Bottom represent the bar position during concrete placement) Test Series 1: Prismatic Coupons in Tension Specimen Description Each of the 27 specimens had two lap splices of #8 bars cast in a rectangular concrete prism, as shown in Figure 2. Three splice lengths were tested: 20d b, 40d b, and 60d b. The reinforcing bars extended past the ends of the concrete prisms to permit load fixture attachment and were loaded in tension. During testing, load was increased monotonically until failure. Each test coupon had a minimum clear cover (c min ) of 1.5d b on at least one side. Type A coupons had all three cover dimensions (c so, c b, and c t ) and one-half the clear spacing between bars (c si ) equal to 1.5d b. For Type B coupons, the cover dimension cast above the spliced bars (c t ) was increased to 7.5d b. For Type C coupons, both the top cover dimension (c t ) and one-half the clear spacing between bars (c si ) were increased to 7.5d b bar diameters. For Type D coupons, the side cover (c so ) was increased to 7.5d b bar diameters, in addition to the top cover (c t ) and one-half the clear spacing between bars (c si ). Nominal cross-sectional dimensions of the specimens are shown in Table 1 and Figure 3. Grade 60, A706 bars (ASTM 2009) were used as the main longitudinal reinforcement. Table 1 provides the relevant material properties and the specimen naming convention used. Test Program The specimens in Series 1 were designed to investigate the effect of increasing the cover on one, two, and three sides of the splice, while keeping the minimum bottom cover constant for different splice lengths. By using tension coupons instead of beam specimens, all three cover dimensions (c so, c b, and one-half the clear spacing between bars (c si ) were made equal in Type A coupons, and varied for the other types. 4 Tensile Strength of Lap Splices in Reinforced Concrete Members [RN ]

5 Stirrup Clamps Lap Splice Hyd. Ram Load Cell Tube Support Pedestal Roller T T Figure 4 Elevation view of the typical Series-2 beam specimen test setup #11 3" 6" 3" 24-1/4" 17-5/8" The Series-1 specimens were also designed to examine the tensile strength of lap splices located in regions with small strain gradients. Test Series 2: Rectangular Beams 30" Figure 5 Typical Series-2 beam cross section Specimen Description There were fifteen (15) traditional beam specimens in the Series 2 tests, presented in Table 2. Three splice lengths were tested in the beams: 28d b, 56d b, and 85d b. Each of the fifteen beams was loaded in a four-point bending loading arrangement, as shown in Figure 4. Lap splices were located between supports in the region of nearly constant bending moment. The beams had rectangular cross sections, as illustrated in Figure 5. Longitudinal tension reinforcement consisted of two #11 bars. The minimum clear cover (c min ) was 3 in. [76 mm], which corresponds to 2⅛d b. 5" No transverse reinforcement was provided in the nearly constant moment region of the beams. Reinforcement was Grade 60, A615 bars (ASTM 2009). Table 2 provides the relevant material properties and the specimen naming convention used. During testing, two loading procedures were used. In the first procedure, load was increased monotonically to failure. During the second loading procedure, the loading was incrementally stopped after pre-selected loads were reached (Richter, 2012). Once these loads were achieved, the load application was stopped, and the specimen was unloaded. The specimen was then reloaded monotonically until failure. Table 2 indicates which loading procedure was used for each specimen. The first loading procedure is referred to as monotonic in Table 2, and the second loading procedure is reloaded. Test Program The specimens in Series 2 were designed to study the difference in behavior of lap splices with different lengths using a traditional beam-type specimen. Results Strength of Prismatic Coupons in Tension Figure 6 shows the average splice strengths (average maximum steel stress developed normalized by f ć ) for each splice length vs. the number of cover dimensions c so, c b, c t, or c si (where half the clear bar spacing is considered as a fourth cover dimension) set equal to the minimum cover of 1.5d b bar diameters. Figure 6 illustrates that the splice strength was reduced as an increasing number of the cover dimensions were set equal to the minimum cover. Effect of Splice Length on Splice Strength The average splice strength is plotted against the ratio of splice length-to-bar diameter in Figure 7. The data are presented for each prismatic coupon type and splice length in Series 1, and for each splice length in Series 2. Figure 7 illustrates that the splice strength does increase CRSI Research Note 5

6 Figure 6 Average splice strength vs. number of cover dimensions equal to 1.5db for the Series-1 tests (reference Figure 3) Figure 7 Splice strength vs. splice length-to-bar diameter ratio (ls / db) for the Series-1 and -2 test specimens 6 Tensile Strength of Lap Splices in Reinforced Concrete Members [RN ]

7 with an increase in the splice length-to-bar diameter ratio. But the strength does not increase proportionally to the ratio of splice length-to-bar diameter, thus indicating a decrease in splice efficiency. The decrease in splice efficiency is more noticeable at lap lengths exceeding approximately 45d b. Comparison with Previous Tests of Deformed Bar Lap Splices The data from the Series 1 prismatic coupons and Series 2 beams are plotted with the available ACI 408 data described previously in Figure 1. Figure 1 illustrates the following: 1. Splice strengths reached in the Series 1 and Series 2 tests with splice length-to-bar diameter (l s / d b ) ratios between 20 and 60 fall within the scatter of splice strengths reached in previous tests of lap splices with similar lap lengths, and 2. Equation 1 can be un-conservative for splice lengths exceeding approximately 45 bar diameters. It is recommended that the steel stress predicted by Equation 1 be capped at 900 f ć. Bond Stress Distribution over the Lap Splice Length Bond stress distributions were inferred from concrete surface strains measured along the length of the lap splices in the Series-2 beams. The inferred distributions illustrated that bond stresses were concentrated near the ends of each splice. In the center portion of each splice, the bond stresses were smaller. The widths of bursting cracks in the plane of the longitudinal reinforcement also indicated the relative magnitude of bond stresses throughout the length of each lap splice. Crack width measurements made during the beam tests showed that transverse crack widths due to bursting were wider at the ends of each splice. Throughout the center portion of each splice, the crack widths were generally narrower. The observation that unit bond stresses are not distributed uniformly throughout the length of lap splices in beams helps to explain the reduction in splice efficiency with an increase in splice length; this was clearly evident in the tests in this study. A comparison of the longitudinal concrete surface strain measurements and measurements of transverse crack widths from bursting made in the beam tests for different splice lengths showed the following trends: The length at the end of each splice having a concentration of high bond forces remained nearly constant for each overall splice length, and The length of the center portion of each splice with relatively small bond stress increased as the splice length increased. These observations indicate a majority of the load transfer in a spliced bar occurs near the ends of the two bars being spliced. The non-uniform distribution of steel stress, which became more pronounced in longer splice lengths, resulted in less efficient splices. Conclusions and Recommendations 1) The Series 1 tests of prismatic coupons with tension lap splices indicated that mean unit bond strength decreased as an increasing number of the side cover dimensions were set equal to the minimum cover of 1.5 bar diameters (1.5d b ). For reference, the cover dimensions cso, cb, and ct are shown in Figure 3. 2) The results in this study indicated that increasing the length of an unconfined lap splice beyond approximately 45 bar diameters (45d b ) was an inefficient way to increase the splice strength. This inefficiency results from a non-uniform bond stress distribution along the lap splice length, which is concentrated at the splice ends. 3) For deformed-bar lap splices meeting the five criteria addressed earlier, that is: 1) bottom-cast, 2) clear cover equal to or exceeding 1d b, 3) clear spacing between spliced bars equal to or exceeding 2d b, 4) no transverse reinforcement (unconfined), and 5) compressive strength of concrete not exceeding 10,000 psi [69 MPa], Equation 2 below provides a conservative estimate of the steel stress that can be developed: f su = 20 f ć ( l s ) 900 f ć d b Equation 2 is similar to the expression in Section of the ACI design provisions for #7 and larger bars, except Equation 2 now includes an upper limit of 900 f ć (in psi) for the maximum steel stress that can be developed. According to Equation 2, increasing the lap splice length beyond 45d b does not increase the splice strength. If a steel stress exceeding 900 f ć (in psi) is required, it is recommended the lap splice be moved away from any critical sections or transverse reinforcement be provided to confine the splice region. Acknowledgements This research was conducted by Purdue University with the sponsorship of the Concrete Reinforcing Steel Institute (CRSI) and the National Institute of Standards and Technology (NIST). Reinforcing steel used in this work was generously provided by ERICO International Corporation. (2) CRSI Research Note 7

8 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. American Concrete Institute ACI Committee 408 (2003), Bond and Development of Straight Reinforcing Bars in Tension (ACI 408-R03), Farmington Hills, Michigan, 49 pp. ASTM International ASTM A615 (2009), Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, ASTM A615 / A615M 09b, ASTM International, West Conshohocken, Pennsylvania, 6 pp. ASTM International ASTM A706 (2009), Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement, ASTM A706 / A706M 09b, ASTM International, West Conshohocken, Pennsylvania, 6 pp. El-Hacha, R., El-Agroudy, H., Rizkalla, S.M. (2006), Bond Characteristics of High-Strength Steel Reinforcement, ACI Structural Journal, 103(6), pp Kim, S., and Shiohara, H. (2012), Dynamic Response Analysis of a Tall RC Chimney Damaged during 2007 Niigata-ken Chuetsu-Oki Earthquake, Proceedings of the Fifteenth World Conference on Earthquake Engineering, Paper No Orangun, C.O.; Jirsa, J.O.; and Breen, J.E. (1977), A Reevaluation of Test Data on Development Length and Splices, ACI Journal, Proceedings (74) 3, pp Richter, B.P. (2012), A New Perspective on the Tensile Strength of Lap Splices in Reinforced Concrete Members, MS Thesis, Purdue University, West Lafayette, Indiana, 165 pp. Seliem, H.M.; Hosny, A.; Rizkalla, S.; Zia, P.; Briggs, M.; Miller, S.; Darwin, D.; Browning, J.; Glass, G.M.; Hoyt, K.; Donnelly, K.; and Jirsa, J.O. (2009), Bond Characteristics of ASTM A1025 Steel Reinforcing Bars, ACI Structural Journal, 106 (4), pp Song, C., Pujol, S., Lepage, A. (2012), The Collapse of the Alto Rio Building during the 27 February 2010 Maule, Chile, Earthquake, Earthquake Spectra, 28 (S1), pp. S301-S334. Kilic, S., and Sozen, M.A. (2003), Evaluation of Effect of August 17, 1999, Marmara Earthquake on Two Tall Reinforced Concrete Chimneys, ACI Structural Journal, 100(3), pp Contributors: The principal authors on this publication are Brian P. Richter and Santiago Pujol, Ph.D., of Purdue University. 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 research. Keywords: Bond, development length, deformed bar, lap splice, reinforcement, reinforced concrete. Reference: Concrete Reinforcing Steel Institute - CRSI [2014], Tensile Strength of Lap Splices in Reinforced Concrete 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 2014 This publication, or any part thereof, may not be reproduced without the expressed written consent of CRSI. Printed in the U.S.A.