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OCR for page 48
48 Ac = core area) to the expected axial load demand (taken as (cb + Ktr)/db accounts for the beneficial effects of transverse con- 0.05f c Ag, 0.10f cAg, or 0.20f c Ag) is plotted for columns with finement and has an upper limit of 2.5. For values of (cb + Ktr)/db diameters ranging from 8 in. to 96 in. with 2 in. of cover. For less than 2.5, splitting failures are likely; for values greater than an 8-in. diameter, the loss of a 2-in. cover will significantly 2.5, pullout failures are likely. The latter cannot be affected by reduce the available area (the core area is only 25% of the the addition of more confining reinforcement. The NCHRP gross area for this rather small column). However, even for 12-60 recommendations also differ from the ACI 318 formu- this small column, the axial load capacity of the core (taken lation by removing the s factor, which reduces the develop- simply as 0.85f c Ac) is about 6% larger than an unrealistic axial ment length (s = 0.8) for #6 bars and smaller. load demand of 0.20f cAg. For more typical load demands and A comparison of development length calculations made more realistic column sizes, the remaining capacity after the using Equations 12 and 13 is presented in Figure 30. In this loss of cover will be sufficient. figure, calculated values for #8 bars are shown although all bar sizes exhibit similar trends. For the NCHRP 12-60 calculation (Equation 13), the value of (cb + Ktr)/db = 2.5 since this limit is 2.7.3 Summary and Conclusions typically obtained when confinement is present. It is clear that ACI 318-08 allows the use of an equation identical to the present AASHTO requirements are more conservative than AASHTO Equation 5.7.4.6-1 for fyh up to 100 ksi. The references those proposed by NCHRP 12-60. The latter is used in the cited as the basis of allowing fyh = 100,000 ksi are those used as present study and thus the experimental "proof test" is conser- the basis of formulation presented in Section 2.7.2.1. In view of vative when compared to present AASHTO requirements. ACI 318-08 provisions, the results of the aforementioned para- Although the current AASHTO requirement (Equation 12) metric study (Table 24), and the axial load capacity of the core does not address confinement, it can be shown to result in relative to the expected axial load demands (Figure 29), it development lengths comparable to those resulting from the appears that the use of current AASHTO Equation 5.7.4.6-1 for ACI 408 (2003) requirements and to be more conservative than fyh up to 100 ksi can be justified for Seismic Zone 1. The exten- those resulting from the use of ACI 318 when typical levels sion of this equation beyond 100 ksi is questionable at this time. of confinement are used. The AASHTO requirement may underestimate the development required in cases where no confining reinforcement is provided. However, as discussed in 2.8 Bond and Anchorage Chapter 1, confining reinforcement should always be used when 2.8.1 Splice Development developing or splicing ASTM A1035 or other high-strength reinforcing steel. AASHTO LRFD (2007) 5.11.2.1.1 prescribes the basic ten- sion development length of #11 bars and smaller, ldb, as follows: 2.8.1.1 Splice Development Tests 1.25 Ab f y db = > 0.4db f y ( ksi units ) (Eq. 12) Eight spliced bar flexural specimens, shown in Figure 31, fc were tested in four-point flexure. Specimen labels begin with "D" (for development) and indicate the bar size, followed by Where: the specimen number. Each specimen had two tension bar Ab and db are the area and diameter of the bar being devel- oped; fc is the concrete strength; and fy is the bar yield stress (i.e., the stress to be developed by the splice). Recent recommendations of NCHRP Project 12-60 (Ramirez and Russell 2008) are based on the ACI 318 (2008) requirements for basic tension development length with an additional factor, c = 1.2, applied when f c exceeds 10 ksi as follows: 3 fy c t e = db ( psi units ) (Eq. 13) 40 fc (( cb + K tr ) db ) db Where t and e are factors to account for "top cast" bars and the use of epoxy-coated reinforcing steel (in this study both are taken as unity); is a factor accounting for the use Figure 30. Comparison of development length of lightweight concrete (also unity for this study). The term calculations.

OCR for page 48
49 (a) #5 Specimens (D5-1, D5-2, D5-3, D5-4) (b) #8 Specimens (D8-1, D8-2, D8-3, D8-4) Figure 31. Splice development test specimen details. splices located entirely within the constant moment region Appendix A. The splice lengths provided, summarized in of the test span. Two bar sizes were tested: #5 and #8. Nom- Table 26, were obtained from Equation 13 to be sufficient to inal concrete strengths used for design were 10 and 15 ksi. develop bar stresses ( fy in Equation 13) of 100 and 125 ksi, The actual 28-day concrete strength was determined to be 12.9 respectively. The use of Equation 13 results in shorter develop- and 15.4 ksi (see Appendix A). Measured steel reinforcing ment lengths than Equation 12 and is therefore less conserva- properties are given in Table 25 and details are provided in tive than Equation 12; thus, it was used in this study. Table 25. Reinforcing steel properties. #3* #3 #4 #5 #8 ASTM grade A615 A615 A1035 A1035 A1035 fy ksi 65.0 68.0 140.0 130.2 118.6 fu ksi 101 108.8 174.0 164.1 154.6 u 0.159 0.154 not reported 0.103 0.115 * Confining stirrups used in splice tests. Confining ties used in hook anchorage tests labeled "D" in Figure 33b and Table 27. Calculated using 0.2% offset method.