Contribution of Steel Casing to Single Shaft Foundation Structural Resistance (2018)

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159 This chapter summarizes the major observations and conclusions from the study on the con- tribution of steel casing to single shaft foundation structural resistance under the NCHRP 12-93 project. 4.1. Conclusions from the Analytical Program This section summarizes the findings from the analytical program. A study was done to inves- tigate the effects of different parameters on the strength and composite behavior of RCFST, per Table 2.5. Finite element analysis case studies conducted for this purpose were also comple- mented by results from cross-section analyses and other simple models. The following conclusions can be made based on the performed study: • From the analyses conducted in Section 2.2.9, the benefit of developing composite action in an RCFST, compared to a non-composite RCFST, is shown to decrease as the reinforcement ratio increases. For example, for the case with D of 24 in. and a D/t ratio of 85, the flexural strength of the composite CFST shaft section is 25% more than that for the non-composite one when comparing plastic moments. However, for the same RCFST with a 1.6% reinforcing ratio, the composite section is only about 10% stronger. • According to the observations made in Sections 2.2.6 and 2.2.10, the friction at the interface of the steel tube and concrete core plays an important role in developing the composite action. The value of the friction coefficient required to achieve full composite action was investigated by performing finite element analysis case studies. It was observed that for RCFST shafts with H/D ratio of 7.5 (i.e., a large H/D ratio) a friction coefficient of 0.5 was adequate to develop full composite section. A greater friction coefficient of 0.8 was needed for lower H/D ratios. However, analyses indicated that, for a friction coefficient of at least 0.5, all RCFST shafts were able to develop a strength exceeding the theoretical value of MP. • According to observations made in Sections 2.2.7 and 2.2.8, the total flexural strength of non-composite RCFST shafts is equal to sum of the individual strengths of the steel tube and the reinforced concrete core. For non-composite CFST shafts, the total flexural strength of the shaft is mainly equal to the strength of the steel tube, the concrete core having a negligible contribution. • The inability to develop friction at the concrete-to-steel tube interface (i.e., a friction coefficient of zero) results in a reduction of the flexural strength of the RCFST shaft section (a loss equal to the difference in strength between that of the composite and non-composite sections). However, this loss of flexural strength increases for increased displacements. According to analyses done in Sections 2.2.9 and 2.2.15, this is because loss of strength in the steel tube due to the development of local buckling can be compensated by the reinforced concrete core in a composite section, but not as effectively in a non-composite section. C h a p t e r 4 Conclusions

160 Contribution of Steel Casing to Single Shaft Foundation Structural resistance • From the analyses conducted in Section 2.2.11, a change in the diameter of the shaft does not affect the friction that is needed to achieve composite behavior for a given H/D ratio. • Results obtained on shaft behavior, including the adequacy of a friction coefficient of 0.5 to achieve composite action, did not change significantly when analyses were run with a reinforced concrete column connected to the top of the shaft, except for the development of a load transfer zone at the top of the shaft. These results are presented in Section 2.2.12. • A shear-head mechanism developed in the transition zone where the reinforced concrete column frames into the RCFST shaft. Results presented in Section 2.2.12.1 indicate that this mechanism can perform satisfactorily in transferring the forces needed to develop the flexural strength of the RCFST shaft, provided that the shaft does not locally yield in that transition zone, and that the column reinforcement is anchored into the shaft for a length equal to the sum of the column diameter and of the rebar’s development length. • According to Section 2.2.13, an increase in axial load, up to half of the axial concrete core strength, improves the behavior of both composite and non-composite RCFSTs. However, in both cases, the increase in flexural strength is not significant, although greater for non- composite RCFST. Note that at P = 0.5Pc, the flexural strength of the non-composite and composite RCFST are the same. • The relative contributions of the steel tube and reinforced concrete core to the strength of composite sections depend on the D/t ratio. For small D/t ratios, the behavior of the steel tube governs the composite RCFST behavior. However, according to observations presented in Sections 2.2.5 and 2.2.14, the behavior of the reinforced concrete core becomes more domi- nant as the D/t ratio increases. For example, for a reinforcing ratio of 1.6%, local buckling of the steel tube may not significantly change the behavior of the RCFST shaft when the D/t ratio is more than 100, because the contribution of the steel tube to the total composite strength in that case is less than 50%. On the other hand, following the same logic, the effect of confinement of the reinforced core does not have a significant effect on the strength of a similarly reinforced RCFST shaft for D/t ratios lower than 90. • According to Section 2.2.14, a change in the D/t ratio from 85 to 100 does not change the friction coefficient that is needed to develop full composite action. • Cyclic loading analyses in Section 2.2.15 indicated that consideration of cyclic loading does not significantly affect the above conclusions with respect to the response of RCFST shafts, compared to monotonic loading. However, the contribution of each part of the RCFST to total strength changes at increased cyclic loading amplitudes. In particular, the contribution of the steel tube decreases while the contribution of the reinforced concrete increases for larger amplitude cycles. • According to observations made in Section 2.2.16, the presence of soil (when considered in the finite element model) increases the length of the plastic zone over the height of the RCFST shaft. For a given drift ratio, the curvature at the point of maximum moment is less compared to the case without soil. However, the maximum flexural strengths developed are the same, and presence of the surrounding soil does not appear to negatively affect the behavior of the composite shaft. • According to observations made in the analytical program, a friction coefficient of 0.5 at the interface of the reinforced concrete core and the steel tube is generally sufficient to develop a composite section in RCFST shafts. Incidentally, this value matches the actual friction coefficient between steel tube and concrete surfaces that has been measured by the researchers mentioned in Section 2.2.2. The friction coefficient values reported by those researchers ranged between 0.47 and 0.57. 4.2. Conclusions from Flexural and Shear Tests This section summarizes the findings from the testing program and the supporting finite ele- ment analyses of the specimens. The testing program consisted of six large-scale cyclic semi- static flexural tests on an RCFST shaft with a reinforced concrete column extending from the

Conclusions 161 top of it, and seven double curvature cyclic semi-static shear tests on RCFST and CFST shafts. The objective of the flexural tests was to validate the findings from the parametric study conducted in Chapter 2. The shear specimens were tested to investigate the ultimate strength and cyclic inelastic behavior of RCFST shafts. The following conclusions can be made based on the flexural experimental test results: • In general, the test results confirm the observations and findings obtained from finite element analyses and reported in Section 2.2. • Results from Specimen S1 (which was the base specimen to be compared to other flexural specimens) showed that the existing friction coefficient that naturally develops at the inter- face of the steel tube and the concrete core of the shaft was adequate to develop a composite strength exceeding the theoretical plastic strength calculated by the PSDM. Indeed, all the tested flexural specimens exceeded the PSDM strengths by an average of 16%. • Testing of Specimen S2R demonstrated that there exists a transition zone at the top of the shaft that makes it possible to transfer to the shaft the forces developed by the reinforced concrete column attached at the top of the shaft, even in absence of reinforcement in the shaft. In fact, with reinforcement introduced only in the bottom part of the shaft (21 in. below the point where the column reinforcement was discontinued), the RCFST shaft’s theoretical plastic moment could be developed at that bottom of the shaft. Also, no increase in the strength of the RCFST shaft due to axial load was observed in testing of Specimen S2R. • A “pure” condition of non-composite action did not occur when the inside of the steel shaft was coated using either bentonite slurry or grease in Specimens S3 and S4, respectively. In fact both specimens exceeded the composite and non-composite PSDM strengths by an average value of 15% and 29%, respectively. In Specimen S3, a moist area on the outside surface of the concrete core, presumably due to the bentonite slurry, was observed in the bottom area of the shaft, and it appears that the strength of the RCFST shaft was somewhat reduced as a consequence of this wet area (even though it still exceeded the plastic strength). In Specimen S4, slippage occurred during testing only at the larger displacements, which was considered an indication of non-composite shaft behavior at these larger cycles. No such significant slippage was recorded for Specimen S3. • Comparison of the test results for Specimens S5 and S1 showed that the change in the diam- eter of the shaft (and an increase in the D/t ratio from 80 to 96) does not affect the ability of RCFST to achieve composite behavior when only relying on the friction that naturally develops between the steel tube and concrete. Furthermore, Specimen S5 was constructed with a spirally welded steel tube and was still able to develop its full plastic moment and an inelastic cyclic response comparable to that of the other specimens. • The investigation of the amount of composite action for RCFSTs, even with contaminated interiors of steel tubes, showed substantial composite behavior even when no additional steps were taken to transfer the internal shear at the interface of concrete and the steel tube. • Comparison of results obtained from finite element analyses for these two flexural specimens confirmed that the relative contribution of the steel tube to the strength of composite section decreased as the D/t ratio increased. • The Specimen S6R, with welded shear rings at top end of the shaft, designed based on the “transferred internal axial load” demand calculated using the proposed Equation 2.2, was able to develop the composite action at the RCFST shaft. • The finite element models of the test specimens that were built based on the verified finite element modeling approach developed in Chapter 2 and using values of the material properties determined during testing program were found to give results in good agreement with the test results. • For displacement-based design, the M-φ curve of the composite 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.

162 Contribution of Steel Casing to Single Shaft Foundation Structural resistance • Based on the experimental results and supporting finite element analyses results, new ultimate and damage-controlling limit states are proposed for displacement-based design purposes. • The existing effective stiffness equations in various codes and design guidelines were evalu- ated by comparison with experimental and finite element results. Recommendations were made on using certain effective stiffness equations for calculating the seismic demands for RCFST shafts. • From the supplementary finite element analyses performed for the RCFST shaft in the soil, the following conclusions are possible: – Relying on a 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. – Using a shear transfer mechanism at top end of the RCFST shaft makes it possible to achieve the composite MPSDM capacity immediately below the location of the attached shear transfer mechanisms in the presence of an interface friction bond corresponding to clean steel surface (i.e., corresponding to µinterface = 0.5). – When there is not a sufficient interface friction bond, shear transfer mechanisms (such as shear rings) have to be provided above and below the point of the maximum moment below the soil level. The following conclusions can be made based on the shear experimental tests results: • In all shear specimens, the yielding of the steel tube started at the midspan and mid depth of the cross-section, and it propagated toward the end-span and toward the outer edges of the cross-section. • For all the concrete-filled shear specimens, the ultimate shear failure of RCFST shaft happened by fracture of the steel tube at both ends of the shear span where there is an interaction of shear and bending forces. • Opening the specimens after the tests, the infill concrete was found pulverized into fine particles over the length of the area that yielded in shear. • Inclusion of longitudinal reinforcement did not have a significant effect on the strength of the RCFST shafts. The maximum experimentally obtained shear strength increased by only 0.25% for the RCFST having 1% longitudinal reinforcement (SH5), and it increased by 4% for the RCFST having 2.2% reinforcement (SH6). • Inclusion of transverse reinforcement did not have a significant effect on the strength and ductility of the RCFST shaft. • The existing equation in the AASHTO Guide Specifications for LRFD Seismic Bridge Design gave a good estimate of the shear strength obtained for the hollow steel tube. • The shear strength obtained by summing the individual shear strengths given by the equations in AASHTO Guide Specifications for LRFD Seismic Bride Design (2014) for a hollow steel tube and concrete section underestimated the strength of the CFST and RCFST shafts by a factor of more than 2.0. • The composite shear strength equation for CFST provided by the WSDOT Bridge Design Manual LRFD (2016) gave results closer to the experimentally obtained shear strength values and was slightly conservative but overestimated the contribution of the steel tube to the total strength (and should not be used for non-filled steel tubes) and underestimated the contribution of the concrete. • All specimens exhibited some amount of ductility under cyclic shear, but not necessarily to the extent that would make it a desirable failure mode. Maximum strength of RCFST specimens, on average, reached at 6% shear drifts, while maximum strength of the CFST shaft reached at 5% shear drift. The softening slope of the strength after reaching the

Conclusions 163 maximum point, was equal to (on average) 160 kips. in. The failure happened at average shear drifts of 22% and 16% for the RCFST and CFST, respectively. • An alternative shear strength equation for composite RCFST members was proposed by considering the development of a compressive diagonal strut in the concrete and its inter- action with the steel tube. The accuracy of the proposed equation was evaluated using a ratio of the obtained results from the proposed equations and existing test data from the shear specimens tested here and those from other researchers. The values obtained with the proposed equation were safe with a mean value of 1.59 and standard deviation of 0.32. 4.3. Other Conclusions Based on the conclusions of the study done in analytical and experimental programs, revi- sions to Articles 6.9.6, 6.12.2.3.3, 6.12.3.2.2, and other articles of AASHTO LRFD Bridge Design Specifications (2012) and Article 7.6 of AASHTO Guide Specifications for LRFD Seismic Bridge Design (2014) were proposed. Investigation of the economic impacts of the proposed revisions was done by performing revised designs of actual bridge structures using the proposed revi- sions and comparing them with the previous designs; results indicate that significant savings are expected when taking into account composite action using the proposed revision.