Model for Side-Face Blowout Strength of Large-Diameter Headed Bars in Beam-Column Joint

Transcription

1 Model or Side-Face Blowout Strength o Large-Diameter Headed Bars in Beam-Column Joint *Sungchul Chun 1) and Min-Seo Bae 2) 1), 2) Div. o Arch. & Urban design, Incheon National University, Incheon, 22012, Korea 1) scchun@inu.ac.kr ABSTRACT The provisions o ACI 318 on headed bars have several limitations on the bar diameter and yield strength, cover thickness, and eects o transverse reinorcement. The anchorage strengths signiicantly depend on ailure modes and geometric conditions where headed bars are anchored. The main variables aecting the anchorage strengths were ound to be a compressive strength o concrete, a side cover thickness, an embedment length, and a transvers reinorcement index. From regression analyses o 52 sets o data, a model is proposed or predicting the anchorage strength o headed bars terminated within exterior beam-column joints. The mean and COV values o the ratios o tests to predictions are 1.0 and 9.5%, respectively. 1. INTRODUCTION According to ACI (2014), the development length o headed bars is equal to 80 percent o that used or hooks, provided that headed bars meet the requirements o Class HA heads in ASTM A (2015). In addition, the ollowing conditions must be satisied: the net bearing area o the head is at least our times the cross-sectional area o the bar; the smaller o the concrete cover to the surace o the bar and hal the clear bar spacing is at least twice the bar diameter; the diameter o headed bars does not exceed 36 mm; the maximum yield strength y used to design l dt is limited to 420 MPa; and c is limited to a maximum o 40 MPa. The minimum limits on head size, clear cover, and clear spacing, and the restrictions on the upper limit o bar diameter, bar yield strength, and compressive strength o concrete are based on the available data rom tests (Thompson et al. 2005, 2006a, and 2006b). Commentary R o ACI states that because transverse reinorcement has been shown to be largely ineective in improving the anchorage o headed deormed bars (Thompson et al. 2005, 2006a, and 2006b), additional reductions in development length are not used or headed bars. 1) Associate Proessor 2) Graduate Student

2 dt,e / c (MPa) The application o headed bars o the diameter o 43 or 57 mm is not permitted. The savings o material and abrication labor, one o the major beneits o headed bars, increase as the bar diameter increases, as shown in Fig. 1. For large-diameter headed bars o 43 and 57 mm, the minimum side cover o 2d b is larger than the minimum cover o 40 mm. Moreover, transverse reinorcement is believed to be eective or the anchorage o headed bars because the coninement by transverse reinorcement limits the progression o splitting cracks, and thus increases the bursting orce required to cause ailure (Chun et al. 201X). To extend beyond these limitation o the ACI 318 provisions or headed bars, an experimental study on 43 and 57 mm headed bars o Gr. 550 was conducted by Chun et al. (201X). In this paper, statistical analyses were conducted and, a model is proposed or predicting the anchorage strength o headed bars terminated within exterior beam-column joints D43C1 D57C Fig. 1 Headed and hooked bars Embedment length l dt /d b Fig. 2 Eect o splice length on developed bar stresses normalized by. c 2. MODEL FOR SIDE-FACE BLOWOUT STRENGTH The embedment length, side cover, transverse reinorcement, and concrete compressive strength largely aect the bar stresses. A new model or predicting the anchorage strength o headed bars in beam-column joints with suicient transverse reinorcement was developed, on the basis o statistical analyses o the main variables. Generally, the strength o headed bars is provided by bond and head bearing components (Chun et al. 2007, Thompson et al. 2006, Chun 2015) but, in this study, a simpliied model is proposed by merging the two components or practical purposes. First, the eects o concrete compressive strength are assessed. Considering that the side-ace blowout capacity o anchors (ACI , Furche and Eligehausen 1991, Eligehausen et al. 2006) relies on the tensile strength o concrete which has been thought to be proportional to c, or the sake o simplicity, the bar stresses can be assumed to be proportional to c. Among the test variables, the embedment length was the most eective parameter. Figure 2 shows the bar stresses with varying embedment lengths or the

3 unconined headed bars with the side cover o 1d b, where the stresses were normalized with c. The normalized bar stresses are almost linearly proportional to the embedment lengths. A regression analysis is carried out or all unconined 28 data with a 1d b side cover and an equation is provided or predicting the mean strengths o the headed bars as ollows. l dt dt, p, uncon.. c (1) db where dt,p1,uncon is a predicted anchorage strength o an unconined headed bar with 1d b side cover. The side cover is one o the key parameters in the design o the development lengths o hooked bars and straight bars. This test revealed that the anchorage strength o headed bars is also aected by the side cover. The ratios o measured bar stresses to predictions by Eq. (1) are shown in Fig. 3 with varying c so /d b values. Because the ailure mode o all specimens is the side-ace blowout, providing a thicker side cover delayed the blowout ailure. Equation (1) can be modiied by including a coninement actor or the side cover as ollows. l dt cso dt, p, uncon.... c (2) db db where dt,p,uncon is the predicted anchorage strength o an unconined headed bar and c so /d b cannot be less than 1.0 or greater than D dt,e / cdt,p1,uncon D57 dt, e, uncon dt, p1, uncon c d so b dt,e,con / dt,p,uncon dt, e, con dt, p, uncon K d tr b #14(4) #18(4) #18-bar yielded(6) Normalized side cover c so /d b Fig. 3 Increase in bar stresses with varying c so /d b values. Transverse reinorcement index, K tr /d b Fig. 4 Increase in bar stresses with varying K tr /d b values. By placing a hairpin-type transverse reinorcement o K tr = 0.5d b or 1.0d b, the anchorage strengths o headed bars increased by up to 44% compared with the unconined headed bars with the same conditions. As is well-known, the hairpins limited the splitting cracks arising rom the bond stresses along the development length and thereore reduced the bearing stress in ront o the head, which resulted in delaying

4 Measured bar stress dt,e (MPa) the blowout ailure. In addition, the hairpins close to the heads obviously increased the bearing capacity o the heads. The increases in the strength o the conined headed bars compared with Eq. (2) are shown in Fig. 4 with varying K tr /d b values. Among seven conined specimens, three specimens did not ail and their actual strengths were not measured because, or saety, the tests were stopped ater the headed bars yielded. I higher-strength headed bars were used and their actual strengths were recorded, the slope o the trend line might be steeper. Thereore, the coninement actor ( K tr /d b ) by the transverse reinorcement will give a conservative result or conined headed bars. Incorporated with the coninement actor, an equation or predicting the mean strengths o the bar stresses is given as ollows. l dt cso Ktr dt, p..... c (3) db db db where dt,p1 is the predicted anchorage strength o a headed bar and K tr /d b cannot be greater than 1.0. The average and COV values o the ratios o tests to predictions by Eq. (3) are 1.0 and 9.5% or 52 data sets. The anchorage strengths predicted using Eq. (3) are compared with the test values or 52 data sets in Fig Mean = 1.0 COV = 9.5% D43(28) D57(24) Predicted bar stress dt,p (MPa) Fig. 5 Comparison o test results and predicted values. (Values in parentheses are the number o data.) 3. CONCLUSIONS The provisions o ACI 3181 on headed bars have several limitations on the bar yield strength, concrete compressive strength, bar spacing, cover, and consideration o

5 the coninement eects o transverse reinorcement due to the lack o experimental veriications beyond the limitations. In addition, the anchorage strengths o the headed bars signiicantly depend on the ailure mode and geometric conditions where the headed bars are anchored. From regression analyses or 52 data, a model or predicting the anchorage strength o headed bars terminated within exterior beamcolumn joints is proposed, which includes the eects o transverse reinorcement and side cover. The mean and COV values o the ratios o tests to predictions are 1.0 and 9.5%, respectively. ACKNOWLEDGEMENT This research was supported by the Nuclear Power o the Korea Institute o Energy Technology Evaluation and Planning (KETEP) grant unded by the Korea government Ministry o Knowledge Economy (No. 2011T ). The authors wish to thank Boo Won B.M.S. Co. Ltd., Korea, or providing the headed bars used in the tests. REFERENCES ACI Committee 318 (2014), "Building Code Requirements or Structural Concrete (ACI ) and Commentary " American Concrete Institute, Farmington Hills, Mich., 519 pp. ASTM A970/A970M-15 (2015), "Standard Speciication or Headed Steel Bars or Concrete Reinorcement," West Conshohocken, PA., 9 pp. Chun, S.-C. (2015), "Lap Splice Tests Using High-Strength Headed Bars o 550 MPa (80 ksi) Yield Strngth," ACI Structural Journal, 112(6), Nov.-Dec Chun, S.-C., Choi, C.-S., and Jung, H.-S. (201X), "Side-Face Blowout Failure o Large- Diameter High-Strength Headed Bars in Beam-Column Joints," ACI Structural Journal, (Accepted). Chun, S. C.; Lee, S. H.; Kang, T. H.-K.; Oh, B.; and Wallace, J. W., "Mechanical Anchorage in Exterior Beam-Column Joints Subjected to Cyclic Loading," ACI Structural Journal, V. 104, No. 1, Jan.-Feb. 2007, pp Eligehausen, R., Mallée, R., and Silva, J. F. (2006), Anchorage in Concrete Construction. Furche, J. and Eligehausen, R. (1991), "Lateral Blow-Out Failure o Headed Studs Near a Free Edge," Anchors in Concrete-Design and Behavior, SP-130, American Concrete Institute, Farmington Hills, MI., Thompson, M. K., Ziehl, M. J., Jirsa, J. O., and Breen, J. E. (2005), "CCT Nodes Anchored by Headed Bars-Part 1: Behavior o Nodes," ACI Structural Journal, 102(6), Thompson, M. K., Jirsa, J. O., and Breen, J. E. (2006a), "CCT Nodes Anchored by Headed Bars-Part 2: Capacity o Nodes," ACI Structural Journal, 103(1), Jan.-Feb

6 Thompson, M. K., Ledesma, A., Jirsa, J. O., and Breen, J. E. (2006b), "Lap Splices Anchored by Headed Bars," ACI Structural Journal, 103(2), Mar.-Apr