Contribution of Steel Casing to Single Shaft Foundation Structural Resistance (2018) / Chapter Skim
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20 2.1. Introduction The research approach conducted in this project is presented in two general categories of analytical and experimental programs.
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research approach 21 for different diameter-to-thickness ratios (D/t) and reinforcement ratios, and the importance of material properties of steel tube and reinforced concrete cores for different values of the D/t ratio is observed.
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22 Contribution of Steel Casing to Single Shaft Foundation Structural resistance 2.2.1. Enlarged Pile Shaft Simulation In order to develop a finite element model that is able to simulate the nonlinear behavior of drilled shafts, the results of a large-scale drilled shaft cyclic pushover test from Murcia-Delso (2013)
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research approach 23 Only half of the specimen was modeled due to the symmetry of the column. The damage plasticity concrete constitutive model in Abaqus has been used for modeling concrete material.
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24 Contribution of Steel Casing to Single Shaft Foundation Structural resistance reason that an explicit crack was modeled by separating the mesh at the interface of the column and shaft where the cracks were most likely to happen. Work by Imani (2014)
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research approach 25 Figure 2.4. Finite element model of the specimen tested by MurciaDelso (2013)
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HYDRAULIC ACTUATOR STEEL "SHOE" Steel frame used for load application RCFST 6' 25' 2'–6' 9' 30' Figure 2.6. Setup of the test done by Brown (2013)
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research approach 27 (a)
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28 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Longitudinal and transverse reinforcements were embedded in the concrete using the Constrained_ Lagrange_in_Solid command, which provides a coupling mechanism for rebars and concrete elements (i.e., perfect bond)
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research approach 29 FEA Experimental Figure 2.10. Lateral load–midspan deflection comparison for RCFST test done by Brown (2013)
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30 Contribution of Steel Casing to Single Shaft Foundation Structural resistance 2.2.3. Analytical Program Matrix The limited finite element analyses that were conducted allowed reviewing the state of existing techniques to model the complex behavior of some load transfer mechanisms.
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Analysis Group Loading Scenario , in. e Load Transfer Mechanism Shaft Reinforcement To Investigate G-1 Pushover, cyclic, no axial load 24 N/A 3,5,7.5 F(1)
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32 Contribution of Steel Casing to Single Shaft Foundation Structural resistance • t = Thickness of the steel tube (Figure 2.14a) , which was calculated by considering different values for the Ds/t ratio.
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research approach 33 • For the case when shaft and column diameters are different from each other, two analyses with and without shaft reinforcement were conducted to investigate the effect of shaft reinforcement on the confinement of the concrete and full composite action of the encased shaft. • To investigate the effect of a large axial force, analyses of Groups G-1 (only one case of Hs/Ds)
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34 Contribution of Steel Casing to Single Shaft Foundation Structural resistance needed to develop the composite strength of the shaft. Figure 2.17 shows schematic moment diagrams corresponding to different shaft heights of prototype models.
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research approach 35 model. For cyclic analyses, loading was also applied in the same region using a displacement protocol that is described in Section 2.2.15.
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36 Contribution of Steel Casing to Single Shaft Foundation Structural resistance composite strength is developed, the neutral axis of steel tube and the reinforced concrete core cross-sections will be the same. However, for a non-composite section under bending, the neutral axis of each part will be independent of each other.
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research approach 37 is described in Appendix E The results show that the maximum composite strength predicted by the PSDM is in good agreement with the experimental results.
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38 Contribution of Steel Casing to Single Shaft Foundation Structural resistance range (after attainment of the maximum plastic flexural strength) will result in more significant, noticeable loss of strength for shafts having lower D/t ratios.
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research approach 39 reinforced concrete section is closer to the compression edge, as in a reinforced concrete column in pure bending. The axial strain profiles for the steel tube and concrete core for the RCFST case with no friction are compared in Figure 2.22b.
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40 Contribution of Steel Casing to Single Shaft Foundation Structural resistance When the friction force (or mechanical load transferring device) is sufficient to develop composite action, the friction at the steel tube-to-concrete core interface generates forces along the shaft to prevent the relative sliding observed above for the no-friction case.
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research approach 41 (a)
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42 Contribution of Steel Casing to Single Shaft Foundation Structural resistance The reduction of flexural strength with increased displacement is due to the local buckling that develops in the steel tube shaft (as shown in Figure 2.27, the moment in the casing part reduces after 3.5 in. displacement due to development of local buckling)
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research approach 43 Figure 2.28. Bending response of cantilever CFST for the case with (l = 0.5)
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44 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Steel Tube Z Strain Profile Concrete Core Z Strain Profile Figure 2.30. Z strain profile for CFST with no friction.
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research approach 45 5.5% drift ratios. This normal contact pressure causes an increase in the compression stress in concrete core, which is shown in Figure 2.33 for 5.5% drift ratio.
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46 Contribution of Steel Casing to Single Shaft Foundation Structural resistance the total strength shown in Figure 2.34) is presented in Figures 2.35 and 2.36.
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research approach 47 (a) Moment carried by concrete (b)
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48 Contribution of Steel Casing to Single Shaft Foundation Structural resistance (a)
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research approach 49 As described in Section 2.2.6, in order to develop full composite action in an RCFST shaft, the slippage at the interface of the steel tube and concrete core must be prevented -- by means of friction or load transfer mechanisms -- such as to transfer the internal axial load that develops between those parts. The behavior that leads to the development of this differential internal axial load has been described in Section 2.2.6.
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50 Contribution of Steel Casing to Single Shaft Foundation Structural resistance D=24 in., H/D=7.5 D=100 in., H/D=7.5 D=24 in., H/D=5.0 D=100 in., H/D=5.0 D=24 in., H/D=3.0 D=100 in., H/D=3.0 Figure 2.39. Flexural response (moment–displacement curve)
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research approach 51 The distribution of frictional forces at the interface of the steel tube and the concrete core was calculated by taking the derivative of the axial load along the steel tube height. Figure 2.41a shows the distribution of the friction force ( fs)
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52 Contribution of Steel Casing to Single Shaft Foundation Structural resistance top of the shaft (recall that load comes to the shaft from the concrete column)
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research approach 53 of the shaft. This value would be calculated differently, depending on whether the behavior of the RCFST shaft is elastic, plastic, or beyond the point when local buckling develops at the point of maximum moment in the shaft.
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54 Contribution of Steel Casing to Single Shaft Foundation Structural resistance same transferred internal axial force would be obtained by equilibrium on the reinforced concrete part of the cross-section, using the steel tube part for this transferred internal axial load results in a governing equation that is more straight-forward, making it possible to compare results for shafts having different diameters and thicknesses. The theoretical axial forces obtained from this equation have been labeled "PSDM" in Figures 2.40 and 2.42.
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research approach 55 In the elastic range, assume that in a full composite shaft the slippage between the steel tube and reinforced concrete parts is totally prevented by axial shortening in the reinforced concrete part (because according to the stress distribution on the fully composite section, this part is in compression) and axial elongation in the steel tube part (because it is in tension)
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56 Contribution of Steel Casing to Single Shaft Foundation Structural resistance composite action. The subscript es indicates that the axial force is calculated from elastic section properties.
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research approach 57 Comparing two shafts with similar sections, different H/D ratios, and same moment at the base (i.e., F1H1 = F2H2) , the equation above simplifies to: 1 (2.9)
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58 Contribution of Steel Casing to Single Shaft Foundation Structural resistance sufficient to develop composite action for both diameters (comparing shafts with H/D = 7.5)
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research approach 59 composite action in the shaft, changes with the diameter of the shaft and is proportional to the square of the shaft diameter (D2)
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60 Contribution of Steel Casing to Single Shaft Foundation Structural resistance 2.2.12.1. Column-to-Shaft Transition Zone In the connection of a column to an enlarged reinforced concrete shaft, transfer of loads from column rebars to shaft rebars can be explained by the transfer mechanism that has been proposed by McLean and Smith (1997)
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research approach 61 where Hc, ld, and F are the height of the column, the length of the column longitudinal rebars embedded into shaft, and the force that is applied at the top of the column, respectively. According to Equation 2.12, the value of Ft decreases as the extension length of column rebars (ld)
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62 Contribution of Steel Casing to Single Shaft Foundation Structural resistance (a)
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research approach 63 This figure only shows the shear diagram for the upper parts of the shaft, as the objective is to investigate the load transfer mechanism from the column to the shaft. Finite element results in Figure 2.54 are for the case where there is no reinforcement in the shaft beyond the column-to-shaft transition zone, and all the column load is transferred to the steel tube per the mechanism described above.
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64 Contribution of Steel Casing to Single Shaft Foundation Structural resistance 2.2.13. Effect of Axial Load In order to study the effect of axial force on the composite action of RCFST, the finite element models used for the analyses of Groups G-1 and G-2 were re-analyzed with an axial compressive force applied at the top of the shaft, as a uniform pressure applied only to the concrete core and not to the steel tube.
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research approach 65 that the axial load was applied to the concrete core, it is observed from part (b) of each figure (i.e., the right side of each figure)
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66 Contribution of Steel Casing to Single Shaft Foundation Structural resistance As shown in Figure 2.59, for non-composite CFST and RCFST sections, with increase in the compressive axial load acting on the concrete core (expressed in the vertical axis as normalized to the maximum axial load capacity of the concrete section, Pc) , the difference between response of the non-composite and composite sections decreases, and at P/Pc = 0.5, the flexural strength of the non-composite and composite sections are similar.
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research approach 67 concrete section, and the horizontal axis shows the ratio of the strength of the composite section to the strength of the corresponding non-composite section. As shown, with an increase in axial load for P/Pc < 0.5, the composite to non-composite strength ratio becomes smaller for both cases.
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68 Contribution of Steel Casing to Single Shaft Foundation Structural resistance D/t = 100 was analyzed for different friction coefficients under monotonically increasing load to find out what friction coefficient is enough to achieve the full composite action. These results are presented in Figure 2.64.
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research approach 69 where the value of j can be obtained from Figure 2.44 for each D/t ratio. According to Equation 2.14, the axial load that needs to be transferred to achieve a composite section is 10% less for the case where D/t = 100 than for the case with D/t = 85.
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70 Contribution of Steel Casing to Single Shaft Foundation Structural resistance not affected by changes in D/t ratio (over the ranges of D/t considered) , although as shown in Figure 2.68, the relative contribution of each part changes.
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research approach 71 Figure 2.69. Flexural response of the reinforced concrete column–RCFST shaft with D = 100 in.
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72 Contribution of Steel Casing to Single Shaft Foundation Structural resistance 2.2.15. Cyclic Response of RCFST Shaft Cyclic finite element analyses were performed on the RCFST shaft models, and results are presented in this section.
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research approach 73 non-composite response of an RCFST shaft can be obtained by summing the individual behaviors of the steel tube and internal reinforced concrete. These observations are significant, so this behavior was therefore investigated for the case of cyclic loading.
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74 Contribution of Steel Casing to Single Shaft Foundation Structural resistance difference increases with drift (and can become quite significant) due to the strength degradation that occurs for the non-composite shaft.
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research approach 75 The relative contribution of the steel tube and the reinforced concrete parts of the composite cross-section to the total flexural behavior of the RCFST shaft is shown in Figure 2.79. As mentioned in Section 2.2.8, the loss in the strength of the steel tube is due to the local buckling that develops at the bottom of the shaft.
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76 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Figure 2.79. Contribution of steel tube and reinforced concrete parts to cyclic flexural strength of the RCFST shaft.
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research approach 77 the cross-section that exceeds specific thresholds. As shown in the figure, at a drift ratio of 5.5%, only 15% of the stresses on the compression side of the cross-section exceed 1.4f ′c and none of the elements in the cross-section have a compressive stress greater than 1.8f ′c .
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78 Contribution of Steel Casing to Single Shaft Foundation Structural resistance (a) Steel tube part (b)
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research approach 79 The cyclic behavior of an RCFST shaft with an axial load was also compared to its monotonic behavior. Figure 2.85 shows the response of this RCFST shaft with axial load under cyclic and monotonic loading.
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80 Contribution of Steel Casing to Single Shaft Foundation Structural resistance (a)
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research approach 81 (a) Embedded in soil (b)
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82 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Comparing the curvature at the critical section as a function of the drift ratio for the shaft, in Figure 2.90, shows that for a same drift ratio, the curvature in the critical section of the shaft in the absence of soil is larger than for the shaft that is embedded in the soil. For the shaft embedded in the soil, no local buckling developed in the steel tube throughout the analysis, up to 6.5% drift (corresponding to curvature of 0.00515 in.–1)
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research approach 83 (a) Embedded in soil St ee l t ub e Yielded Yield Plastic 5.5% Drift St ee l t ub e Yielded Yield Plastic 5.5% Drift Buckled (b)
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84 Contribution of Steel Casing to Single Shaft Foundation Structural resistance 2.3. Testing Program 2.3.1.
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research approach 85 were fabricated using pipes having vertical welded-seams (i.e., Straight-Seam Electric Resistance Welding [ERW]
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86 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Designed flexural test specimens and their relationship with others specimens (for comparison of experimental results to establish how various factors affect behavior) can be summarized as shown in Figure 2.93.
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research approach 87 shows the test setup used by Berman and Bruneau (2006)
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88 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Therefore, the design of each specimen started by considering the RCFST shaft section and a shaft height, which were chosen according to test objectives and limitations. The specific diameter and D/t ratio of the steel tube was chosen taking into account availability according to pipe manufacturer catalogues, to be relatively close to the desired target values.
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research approach 89 into it [Hc]
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90 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Sp ec im en RCFST Shaft Part Reinforced Concrete Column Part Outside Diameter ( )
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research approach 91 to multiples of the equivalent yield displacement (Dy) , with two cycles applied at each displacement amplitude (at 2Dy, 3Dy, 4Dy, etc.)
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92 Contribution of Steel Casing to Single Shaft Foundation Structural resistance bars. The connection of the RCFST shaft part to the reinforced concrete foundation consists of an embedded part of the shaft in the foundation and a circular base plate that is connected to the bottom of the shaft.
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research approach 93 as location of the actuator that is attached between the column head and the strong wall. The axial load on the Specimen 2R was applied by a DYWIDAG bar installed through a sleeve at the center of the cross-section and post-tensioned to the desired value of axial load.
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94 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Foundation Beam (FB) W24×146 Mounting Plates (MP)
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research approach 95 that could be bolted together with the shear specimen on the pantograph testing apparatus and then be re-used for all 12OD shear specimens. Figure 2.106 shows a schematic view of the 12OD shear specimen and the re-usable stiffener modules.
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96 Contribution of Steel Casing to Single Shaft Foundation Structural resistance testing apparatus, showing how the designed stiffener modules limit shear yielding to the free length. The exception was the 16 in.
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research approach 97 (a)
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98 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Figure 2.110. Global view of the flexural specimen's test setup (20 in.
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research approach 99 Figure 2.112. The assembled 12OD shear specimen and its ready-to-test state in the pantograph.
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