Contribution of Steel Casing to Single Shaft Foundation Structural Resistance (2018) / Chapter Skim
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100 3.1. Introduction This chapter includes the findings from the experiments and the revisions proposed to AASHTO BDS and AASHTO SGS based on these findings.
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Findings and Applications 101 key observations were made during the test (corresponding to the onset of visible local buckling, maximum strength, and rupture of steel tube) are marked on these curves.
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104 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance (a)
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Findings and Applications 105 were done considering non-composite behavior of the cross-section for all flexural specimens and results are shown in Figure 3.5. The normalized strength values were calculated per Equations 3.1 and 3.2 : ˆ (3.1)
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106 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance The calculated normalized strength values for each flexural specimen are shown in Figure 3.6. As shown in this figure, all normalized mean strength values are greater than one, which means that the experimentally obtained values are greater than the analytically predicted strengths using PSDM.
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Findings and Applications 107 composite strength of Specimen S2R with axial load (having FˆC,S2R = 1.10) is compared to the normalized composite strength of Specimen S1 (having FˆC,S1 = 1.32)
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108 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance From the finite element analyses performed in this section, the following findings are obtained: • Relying on the natural friction bond at the interface of the steel tube and the concrete core (µinterface = 0.5) , the composite MPSDM capacity of the RCFST shaft was achieved at a depth of 2.5Ds below the soil level.
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S1, z=2.5 in.
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S4, z=2.5 in.
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Findings and Applications 111 , (3.3)
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112 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance For all the specimens, the strain gages were recording properly until the point of visible local buckling. After this point, the strain gages attached close to where local buckling developed typically detached themselves from some specimens.
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Findings and Applications 113 For this reason, the experimental curvature data obtained from strain gages for Specimen S1 were not used in the comparisons below. Instead, the strains from the finite element analysis results of Specimen S1 test were used.
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114 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance ultimate state of fracture on the steel tube, or crushing of the infill concrete. Correspondingly, the proposed ultimate curvature (φu)
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Findings and Applications 115 The process of calculation of φd can be summarized as: 2 , 1 2 2 2 (3.5) 1 2 3min r t r t y r t y d d y o frac o N cu o N φ = φ + φ φ φ = − + − + ≤ − − where φ1, φ2, and φ3 are defined as shown in Figure 3.11 and y is the yield strain of the steel tube.
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116 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance For displacement-based design, the M-φ curve of the RCFST cross-section can be developed up to the proposed ultimate curvature by fiber-section analysis assuming composite behavior and using expected material properties for the steel tube, confined concrete, and rebars. The M-φ curve then can be idealized with an elastic perfectly plastic response, following the procedure outlined in Section 8.5 of the AASHTO SGS (2014)
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Findings and Applications 117 Figure 3.13. Calculated proposed limit state curvatures for the flexural test specimens.
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118 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance by assuming a tensile strength equal to 10% of the unconfined compressive strength of the concrete. The rebars can be modeled according to Section 8.4 of the AASHTO SGS (2014)
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Findings and Applications 119 is elastic in this region. At this stage, the neutral axis starts to move toward the compression side and the flexural stiffness of the cross-section gradually decreases.
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120 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance where Ic = moment of inertia of the concrete core about the elastic neutral axis of the composite section, in4, Is = moment of inertia of the steel tube about the elastic neutral axis of the composite section, in4, Ec = modulus of elasticity of concrete, ksi, and Es = modulus of elasticity of steel, ksi.
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Findings and Applications 121 and Ir = moment of inertia of reinforcing bars about the elastic neutral axis of the composite section, in4.
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Specimen , kip.ft , 1/in. Method , kip.in2 Idealized , kip.ft Equivalent , 1/in.
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Findings and Applications 123 M-φ and EI-φ curves in Figure 3.16. Fiber-section analyses curves are also presented in this figure.
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124 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance (d)
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Findings and Applications 125 (EI) elastic.
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Specimen , Method S1 3.28E+07 Elastic 1.55 AASHTO SGS (2016)
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Findings and Applications 127 Cracked area N.A. Equivalent rebar ring Te ns io n Co mp re ss io n (3.15)
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128 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance A proper equivalent linear structure representation for the considered nonlinear column would be the one that, for a lateral top force corresponding to MP at the bottom, the drift at the top reaches the equivalent yield displacement. The response of this equivalent linear structure is schematically shown in Figure 3.21.
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Findings and Applications 129 Figure 3.22. Curvature distribution along an RCFST cantilever column.
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130 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance limit for limiting the damage was close to the point where maximum resistance was achieved in the test specimens. Also, for fiber-section analysis of RCFST cross-sections it is recommended to use confined concrete properties and consider 10% of the uniaxial compressive strength as the tensile strength of the concrete.
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Findings and Applications 131 For Specimen SH3 (a hollow tube) , diagonal local buckling started to develop after reaching the maximum strength.
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Shear Displacement, in. Fo rc e, k ip s –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 –400 –300 –200 –100 0 100 200 300 400 SH6-Experiment shear deformation Shear Displacement, in.
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134 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance Shear Displacement, in. Fo rc e, k ip –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 –400 –300 –200 –100 0 100 200 300 400 SH3-Backbone Shear Displacement, in.
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Findings and Applications 135 Figure 3.27. Force-displacement relations comparison for applied displacement and measured shear span displacement.
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136 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance This was expected as both ends of the shear spans experience high strains caused by the interaction of bending and shear forces. Figure 3.30 shows the deformed RCFST shear specimen at different loading states (Specimen SH1R is shown as a representative one for the purpose of this figure)
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Findings and Applications 137 (AASHTO BDS 2014) and those from the WSDOT Bridge Design Manual LRFD (WSDOT BDM 2016)
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138 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance The recent update of WSDOT BDM (2016) , contrary to the AASHTO BDS (2014)
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Findings and Applications 139 Specimen Specimen Strength ( ) , kips (Experimental)
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140 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance Figure 3.34 shows the experimental backbone curve of Specimen SH3 (12OD Hollow) and the shear strength calculated by the AASHTO BDS (2014)
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Findings and Applications 141 3.3.4. Discussion of Finite Element Analyses Results of the Shear Tests The finite element analysis results of the shear tests are presented in this section.
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142 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance Figures 3.37 and 3.38 show Von-Mises stress contours on the surface of the steel tube at the point where the steel tube yields (i.e., first yield point of the steel tube according to the finite element results) , and at the point where the maximum experimental strength was obtained during the test.
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Findings and Applications 143 Figure 3.38. Von-Mises stress contours on the steel tube of Specimen SH4 at maximum experimental strength point.
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144 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance 3.3.5. Proposed Shear Strength for RCFST Shafts As shown in the previous section, the shear strength values calculated using the WSDOT BDM (2016)
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Findings and Applications 145 In order to investigate the effect of the compressive shear strut in the concrete that develops in RCFST members under shear deformation, a series of supplementary finite element analyses were performed. Figure 3.42 shows a schematic view of the finite element models developed for the supplementary analyses.
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146 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance Figure 3.44. Three-dimensional illustrations of iso-surfaces of the minimum principal stresses in the concrete core at the steps marked in Figure 3.43.
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Findings and Applications 147 Displacement, in Sh ea r fo rc e, ki p 0 0.05 0.1 0.15 0.2 0.25 0 50 100 150 200 250 Total RCFST Steel tube Concrete Steps of the 3D illustrations H=20in., a/D=0.8 1 2 3 4 5 6 7 8 9 10 11 12 Figure 3.47. Shear response of 12OD RCFST with a/D = 0.8.
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Figure 3.48. Three-dimensional illustrations of iso-surfaces of the minimum principal stresses in the concrete core at the steps marked in Figure 3.47.
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Findings and Applications 149 To clearly illustrate the development of the compression strut, principle stresses lower than 2.5 ksi are not shown in these figures. The range of the plotted minimum principal stress is shown on the right side of each figure.
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150 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance of the member (middle of the free span) and reduces toward both ends of the shear span.
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Findings and Applications 151 into horizontal and vertical force components, as shown in Figure 3.52 and calculated in Equation 3.26. The horizontal force component (VStrut)
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152 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance Shear distribution on the steel tube cross-section 1 3 2 cos (3.29)
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Findings and Applications 153 is introduced to consider the possible increasing effect of the axial load on the strength of the compression strut, but that effect is neglected in all further calculations here (α[Pc, Ac]
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154 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance The shear forces carried by the steel tube and the concrete core parts from the finite element analysis of the tested shear specimen SH4 were compared with AASHTO BDS (2014)
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Findings and Applications 155 (a)
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156 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance Research Test Setup Loading type Diameter range, in. range range University at Buffalo (current research)
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Findings and Applications 157 (a)
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158 Contribution of Steel Casing to Single Shaft Foundation Structural Resistance 3.5. Design Examples for the Proposed Revisions 3.5.1.
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