Design of Concrete Structures Using High-Strength Steel Reinforcement (2011) / Chapter Skim
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From page 16... ...
. The project identified aspects of reinforced-concrete design and of the AASHTO LRFD specifications that may be affected by the use of high-strength reinforcing steel.
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From page 17... ...
Test specimen labels: D5-1, D5-2, D5-3, D5-4, D8-1, D8-2, D8-3, and D8-4: beam splice specimens; H: hooked specimens; F1, F2, F3, F4, F5, and F6: flexural specimens; P615, P1035-4A, P1035-4B: shear friction specimens; SR1, SR2, SR3, SR4, SR5: reinforced-concrete shear specimens; Type I-1, Type I-2, Type I-3, and Type I-4: AASHTO Type I girder shear specimens. Table 4.
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From page 18... ...
Rectangular beams with As,min are in the tension-controlled region. Rectangular beams reinforced with As,max have the lowest steel strains allowed by ACI 318-08.
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From page 19... ...
Table 7 summarizes the results of the strain compatibility analyses conducted using the Ramberg-Osgood function for the rectangular beams, T-beams, and slabs for all of the steel types and the selected concrete strengths considered. The computed capacities are below or nearly equal to those calculated based on fiber analysis (i.e., the ratios are close to, or slightly less than, unity)
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From page 20... ...
, the strength ratio exceeds unity if the capacity is based on an idealized elasticperfectly plastic stress-strain relationship with the yield strength taken as the stress at a strain of 0.005 or determined based on the 0.2% offset method. That is, the yield strengths based on these two methods may result in slightly unconservative estimates of the expected capacity in cases with large reinforcement ratios and high-strength concrete.
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From page 21... ...
Ratios of T-beam and slab flexural capacity calculated from elastic-plastic analyses to that from fiber model. T-Beams Yield Point Average Minimum Maximum StandardDeviation @ Strain =0.0035 0.741 0.571 0.859 0.091 @ Strain =0.005 0.795 0.659 0.890 0.069 0.2% offset 0.748 0.718 0.764 0.019 Deck Slabs Yield Point Average Minimum Maximum StandardDeviation @ Strain =0.0035 0.828 0.609 0.953 0.115 @ Strain =0.005 0.854 0.638 0.971 0.113 0.2% offset 0.909 0.839 0.951 0.043 Note: Ratio less than 1 is conservative.
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From page 22... ...
increases the strain in the tension reinforcement, which improves the ductility. As the concrete compressive strength increases, the tension reinforcement strain drops, which is an indication of reduced ductility.
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From page 23... ...
It must be recognized that selecting a different value of fy or fs results in different calibrations. 2.3.3 Moment Redistribution AASHTO §5.7.3.5 allows redistribution of negative moments at the internal supports of continuous reinforced-concrete beams.
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From page 24... ...
This difference is attributed to overestimation of aggregate interlock in the matrix of 15-ksi concrete. Considering the challenges of modeling high-strength concrete, the shown load-deflection response for specimen F4 is adequate.
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From page 25... ...
Table 14. Ratio of measured to computed capacities.
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From page 26... ...
2.4 Fatigue Performance of High-Strength Reinforcing Steel Fatigue is a process of progressive structural change in a material subjected to transient loads, stresses or strains. Fatigue strength is defined as the maximum transient stress range (S)
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From page 27... ...
Nonetheless, the impact of applying Equation 9 to higher strength reinforcing steel is that fmin may be increased by taking advantage of the higher strength steel, but the increase results in an unwarranted reduction in the fatigue limit. It is, therefore, proposed to normalize fmin by the yield stress, fy.
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From page 28... ...
If this is the case, the reinforcing stress associated with the FATIGUE load is as follows: Similarly, the minimum sustained load will result in a reinforcing stress of The stress in the reinforcing steel under FATIGUE conditions is then normalized by the allowable stress [according to f f DL STRENGTHymin .= ×( )
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From page 29... ...
This difference is due to the nature of large-scale fatigue testing and the difficulties in providing accurate and safe four-point bending conditions. The measured material properties of the steel reinforcement are given in Appendix A
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From page 30... ...
Cumulative damage curves for Fatigue Test 1. Figure 14.
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From page 31... ...
in conjunction with high-strength reinforcing steel. 31 Figure 15.
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From page 32... ...
On the other hand, the maximum stirrup spacing currently allowed by AASHTO LRFD Bridge Design Specifications was used as the basis of design for specimens SR2 and SR3. For the specimens containing both types of transverse steel, the spacing and size of stirrups were selected such that the stirrup force as computed by would be nearly equal for the A615 and A1035 stirrups reinforcing in either half of the beam.
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From page 33... ...
Specimen SP4, on the other hand, was designed such that the A615 shear capacity exceeded the A1035 shear capacity in order to induce shear failure on the A1035 side. Table 17 summarizes specimen details.
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From page 34... ...
The measured and computed capacities suggest adequate shear strength of A1035 stirrups designed based on current design equations in which stirrup yield strength is taken as 100 ksi. Shear Crack Patterns and Widths.
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From page 35... ...
Strain Levels and Stirrup Forces. Even though the longitudinal bars are all A1035 steel, the strains recorded on the two sides with A615 and A1035 stirrups should be equivalent if the stirrups are performing equally according to compressionfield theory.
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From page 36... ...
The trend of data was generally similar for the other specimens, although formation of cracks near the strain gages occasionally affected the computed stirrup forces. V A f d S s v s v = Table 20.
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From page 37... ...
Figure 22. Load -- average stirrup forces (Specimen SR2)
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From page 38... ...
Additionally, when normalized by concrete strength, the experimental results show no effect resulting from the different values of fy. The understanding of the shear friction resisting mechanism has evolved to recognize the complex nature of the crack interface behavior and to include the effects of aggregate and cement matrix properties, dowel action of the interface reinforcement, and the localized effects of interface reinforcement within the interfacial area (Walraven and Reinhardt 1981)
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From page 39... ...
The interface steel reinforcement, therefore, represents the stirrup extensions or interface shear reinforcement along such a cold joint. The parameters measured during the experiments were magnitude of the shear load, displacement parallel to the shear interface, crack width perpendicular to the shear interface, and strain in the steel reinforcement across the test interface.
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From page 40... ...
Shear friction specimen details and experimental results. P615-3 P615-4 P1035-3 P1035-4 Specimen ID A B A B A B A B Interface Steel 6 #3 A615 6 #4 A615 6 #3 A1035 6 #4 A1035 Material Properties fc' (psi)
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From page 41... ...
The strain, and therefore stress, in the reinforcing steel at Vcr is negligible -- varying only up to 61 με in the present study. Hence, shear friction reinforcement does not significantly contribute to the shear capacity of the interface up to the instant of cracking.
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From page 42... ...
This trend is because, in all of the specimens, Vu was reached well before steel yielding occurred. In fact, as seen in Table 21, the stress in the interface steel reinforcement is significantly lower than its yield strength when the ultimate shear load is achieved.
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From page 43... ...
These findings demonstrate that Equation 11 does not represent the shear friction mechanism since it implies that the maximum concrete and steel components of the shear friction occur simultaneously. In fact, as seen in Figure 27, the concrete component contributes to the majority of the shear friction capacity before the ultimate shear load is reached and then falls to a residual value while the steel component increases.
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From page 44... ...
Column Size Square column dimension or diameter = 18, 24, 36, 48, 60 in. Transverse Reinforcement #3, #4, and #5 Concrete Strength f'c = 5, 10, 15 ksi
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From page 45... ...
. The basis of AASHTO Equation 5.7.4.6-1 is to ensure that the axial load capacity of columns after spalling of the concrete cover is at least equal to the capacity before spalling.
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From page 46... ...
In terms of reducing the spiral spacing in columns cast with high-strength concrete, the use of larger, high-strength spirals is more efficient. For a number of cases (shaded in Table 24)
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From page 47... ...
f'co = 5 ksi f'co =10 ksi f'co =15 ksi D (in.) AASHTO ModelR-S AASHTO Model R-S AASHTO Model R-S 18 6.25 3.56 3.12 2.04 2.08 1.47 20 6.34 3.82 3.17 2.19 2.11 1.58 22 6.42 4.05 3.21 2.32 2.14 1.68 24 6.48 4.45 3.24 2.57 2.16 1.86 26 6.53 4.83 3.27 2.8 2.18 2.03 28 6.57 5.2 3.29 3.01 2.19 2.19 30 6.61 5.72 3.31 3.31 2.2 2.41 32 6.64 6.07 3.32 3.51 2.21 2.51 34 6.67 6.56 3.34 3.8 2.22 2.53 36 6.7 6.89 3.35 3.99 2.23 2.55 38 6.72 7.35 3.36 4.15 2.24 2.57 40 6.74 7.8 3.37 4.18 2.25 2.59 42 6.76 8.24 3.38 4.2 2.25 2.6 44 6.78 8.66 3.39 4.23 2.26 2.62 46 6.79 9.08 3.4 4.26 2.26 2.63 48 6.81 9.49 3.4 4.28 2.27 2.65 50 6.82 9.79 3.41 4.3 2.27 2.66 52 6.83 9.83 3.41 4.32 2.28 2.68 54 6.84 9.88 3.42 4.35 2.28 2.69 56 6.85 9.93 3.43 4.37 2.28 2.7 58 6.86 9.97 3.43 4.39 2.29 2.71 60 6.87 10.02 3.43 4.41 2.29 2.72 62 6.88 10.06 3.44 4.42 2.29 2.74 64 6.88 10.11 3.44 4.44 2.29 2.75 66 6.89 10.15 3.45 4.46 2.3 2.76 68 6.9 10.19 3.45 4.48 2.3 2.77 70 6.9 10.22 3.45 4.49 2.3 2.78 72 6.91 10.26 3.45 4.51 2.3 2.79 74 6.91 10.29 3.46 4.52 2.3 2.8 76 6.92 10.33 3.46 4.54 2.31 2.81 78 6.93 10.37 3.46 4.55 2.31 2.81 80 6.93 10.4 3.46 4.57 2.31 2.82 Notes: Shaded cells indicate where calculated spacings exceed the maximum limit of 6 in.
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From page 48... ...
are based on the ACI 318 (2008) requirements for basic tension development length with an additional factor, Ψc = 1.2, applied when f c′ exceeds 10 ksi as follows: Where Ψt and Ψe are factors to account for "top cast" bars and the use of epoxy-coated reinforcing steel (in this study both are taken as unity)
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From page 49... ...
Table 25. Reinforcing steel properties.
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From page 50... ...
The basic development length (lhb) of such standard hooked anchorages for bars having fy ≤60 ksi is as follows: Where: db = bar diameter in inches and fc′ = concrete strength in ksi.
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From page 51... ...
Splice Specimens Having Nominal Concrete Strength, fc' = 15 ksi
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From page 52... ...
, the confinement was adequate to permit the 0.8 reduction factor to be applied. The objective of this limited test series was to serve as a series of proof tests: applying the existing AASHTO hooked bar development length requirements to the higher strength A1035 reinforcing steel.
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From page 53... ...
exhibited a bar rupture affected by the installation of the splice anchor used to react the applied load (not actually part of the test)
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From page 54... ...
ldh 5db 3db 5db clip gage "0db" 3" debonded region concrete surface strain gage locations (a) #8 90o hook specimens (b)
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From page 55... ...
(2009) and extended the available database to higher strength concrete.
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From page 56... ...
, the moment to cause cracking is calculated as 80% of the moment corresponding to modulus of rupture. Where: Ig = moment of inertia of gross concrete section, nominally 4096 in.4; y = neutral axis distance from the tensile face for gross concrete section, nominally 8 in.
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From page 57... ...
Considering the measured crack widths in this experimental study, it appears that the inherent conservativeness in existing equations allows present specifications to be extended to the anticipated higher service level stresses associated with the use of high-strength reinforcing steel. Using Equation 6 (as discussed in Chapter 1, this equation was derived from the present AASHTO LRFD provisions for crack control given in Equation 5)
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From page 58... ...
The inherent conservativeness in existing equations allows present specifications to be extended to the anticipated higher service level stresses associated with the use of high-strength reinforcing steel. 58 Figure 38.
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