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

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1 Bridges are often constructed with a single enlarged shaft foundation supporting a col- umn. In many cases, the shaft foundation is constructed with a permanent steel casing. The steel casing is typically ignored in design when calculating the structural resistance of the shaft (i.e., only the reinforced concrete section of the shaft is considered for structural resistance). Bridge designers would like to account for the added structural resistance of the steel casing, but there is limited research data as to when the steel casing and concrete inner core act as a composite section. Research to better determine some of the conditions when the shaft can be considered a composite section would be beneficial to design and could significantly reduce construction costs. The goals of NCHRP Project 12-93 were (1) to investigate how to account for the con- tribution of steel casing to the structural resistance of a reinforced concrete single shaft foundation encased in a permanent steel pipe and supporting a single reinforced concrete column at its top, and (2) to propose revisions to AASHTO LRFD Bridge Design Specifi- cations and AASHTO Guide Specifications for LRFD Seismic Bridge Design based on the findings of this study. These encased reinforced concrete shaft foundations are generally referred to as reinforced concrete-filled steel tubes (RCFSTs). The research considered the flexural and shear behaviors of RCFSTs under axial and lateral loading in terms of their strength and extreme event limit states. Analytical and experimental programs were conducted to address the objectives of this project. This report summarizes the research efforts conducted under NCHRP Project 12-93, pre- sents the key conclusions and findings from the conducted analytical and experimental pro- grams, and formulates proposed changes to AASHTO LRFD Bridge Design Specifications and AASHTO Guide Specifications for LRFD Seismic Bridge Design based on these findings. It also outlines the economic impacts of applying the proposed revisions of the project in the designs of actual bridge structures and offers two design examples based on the proposed revisions. A literature review was conducted to summarize the relevant design requirements from state departments of transportation (DOTs), American Association of State Highway and Transportation Officials (AASHTO), Federal Highway Administration (FHWA), and some selected international codes (Canada, Europe, Japan, and New Zealand) for RCFST drilled shafts and concrete-filled steel tube (CFST) structural members from structural engineering aspects (i.e., not geotechnical aspects). This review revealed that, overall, most states (37 DOTs) typically follow the design requirements in AASHTO LRFD Bridge Design Specifications and/or Drilled Shafts: Construction Procedures and LRFD Design Methods published by the FHWA, sometimes with minor differences in practice, but 11 DOTs have significantly dif- ferent state-specific provisions related to the structural designing aspects. On the whole, the review revealed a lack of consensus as to whether or not composite action can be considered, nor did it present guidance and/or design provisions toward this purpose. S u m m a r y Contribution of Steel Casing to Single Shaft Foundation Structural Resistance

2 Contribution of Steel Casing to Single Shaft Foundation Structural resistance Developing the strength of the steel casing in drilled shafts implies that the shaft will add strength, whether it behaves as a composite or non-composite RCFST. In most cases, the strength of the non-composite RCFST is on the order of 10–20% less than that of the com- posite case. The analytical and numerical methods for calculating the strength and analyses of these members were also reviewed. Several pushover and cyclic finite element analyses were conducted to investigate various parameters that affect the composite and non-composite behavior of the RCFST shafts. Some of these parameters included (1) the respective contribution of the casing and the reinforced concrete core of the RCFST to its composite strength, (2) the friction coefficient at the interface between the internal surface of the steel casing and outside surface of the concrete core, (3) the reinforcement ratio, (4) the thickness of the steel casing, (5) the height and diameter of the shaft, (6) the axial load, (7) the inclusion of shear transfer mechanisms at the interface of the steel casing and the concrete core, (8) the properties of the attached reinforced concrete column on top of the shaft, (9) the mechanisms of the load transfer from the reinforced concrete column to the RCFST shaft, and (10) the effects of the surrounding soil. Some of the major findings from the finite element results of the RCFST shafts are as follows: • The theoretical composite flexural strength of an RCFST section can be achieved by means of the existing natural steel-to-concrete interface friction coefficient (µ ≥ 0.5) for a certain height-to-diameter range of shafts (H/D ≥ 7.5). • 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. • The non-composite flexural strength of the RCFST is equal to the sum of the individual strength of the steel tube and the reinforced concrete core. • The inability to develop friction at the concrete-to-steel tube interface (i.e., 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). • In cases where interface friction is insufficient to achieve the composite strength, shear transfer mechanisms can be designed at the interior surface of the steel casing; the use of shear transfer mechanisms at the top of the shaft and below the point of the maximum moment when in the soil will make it possible to develop the composite behavior of the RCFST shaft. • An equation was proposed for calculating the load that needs to be transferred between the steel tube and the concrete core by means of the shear transfer mechanisms to achieve full composite strength. • A shear-head mechanism developed in the transition zone where the reinforced concrete column frames into the RCFST shaft. Results of the finite element analyses indicated that this mechanism can perform satisfactorily in transferring the forces needed to develop the flexural strength of the RCFST shaft, provided that the column reinforcement is anchored into the shaft for a length equal to sum of the column diameter and of the rebars develop- ment length. • An increase in axial load (P), up to half of the axial concrete core strength(Pc), improves the behavior of both composite and non-composite RCFST. However, in both cases, the increase in flexural strength is not significant, although greater for non-composite RCFST. At P = 0.5 Pc, 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 diameter-to-thickness (D/t) ratio. For small D/t ratios, the behavior of the steel tube governs the composite RCFST behavior.

Summary 3 However, the behavior of the reinforced concrete core becomes more dominant as the D/t ratio increases. • A change in the D/t ratio does not change the friction coefficient that is needed to develop full composite action. • 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 at the top of the shaft, 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. Based on the findings of the analytical program, two series of tests were conducted: a cyclic test of six large-scale flexural RCFST shafts cantilevering from reinforced concrete foundations and seven cyclic shear tests on RCFST shafts. The flexural specimens were designed to investigate the composite action in RCFSTs with different diameters, steel tube thicknesses, shaft heights, axial loads, and steel casing-to- concrete core interface conditions. This latter issue was investigated by testing specimens with natural steel-to-concrete bond and specimens with reduced interface friction created by applying either bentonite slurry or grease on the interior surface of the steel tube. Also, one specimen in the flexural testing program had an alternative transition zone detail at the connection between the reinforced concrete column and the RCFST shaft to investigate a shear-head concept developed to transfer the column forces to the composite shaft. Finally, another specimen used a shear transfer mechanism at the top of the RCFST shaft to achieve the desired composite action when insufficient interface friction is present between the steel and concrete. The shear tests were designed to investigate the shear behavior of the composite RCFST shafts when subjected to a double curvature shear condition (such as exists when the shaft spans across a liquefiable soil layer). The shear experimental program consisted of the larg- est diameter RCFST shafts tested under cyclic loading to date. The experimentally obtained results for flexural and shear specimens were replicated by finite element models for thorough understanding of the specimens’ behavior. In general, the test results confirmed the observations and findings obtained from the analytical program. The major conclusions made based on the flexural experimental tests results are as follows: • All the tested flexural specimens exceeded, by an average of 16%, the theoretical plastic flexural strengths calculated using the existing plastic stress distribution method. • Testing of the specimen having the alternative transition zone 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 its top, even in absence of reinforcement in the shaft. • A “pure” condition of non-composite action did not occur when the inside of the steel tube was coated using either bentonite slurry or grease, respectively. However, both spec- imens exceeded the composite and non-composite plastic stress distribution method (PSDM) strengths by an average value of 15% and 29%, respectively. Observable slip- page occurred between the steel and concrete for the greased case, indicating some level of non-compositeness. • The specimen with welded shear rings at the top end of the shaft, designed based on the proposed equation for the transferred internal axial load demand, was able to develop the composite action at the RCFST shaft in the presence of insufficient friction at the steel-to-concrete interface.

4 Contribution of Steel Casing to Single Shaft Foundation Structural resistance • Based on the experimental results and supporting finite element analyses results, new ulti- mate and damage-controlling limit states were proposed for displacement-based design purposes. • The existing effective stiffness equations in various codes and design guidelines were evaluated by comparing the equations with the experimental and finite element results. Recommendations were made for using certain effective stiffness equations for calculating the seismic demands for RCFST shafts. The major conclusions made based on the shear experimental tests results are as follows: • 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 RCFST having 1% longitudinal reinforcement, and it increased by 4% for RCFST having 2.2% reinforcement. • Inclusion of transverse reinforcement did not have a significant effect on the strength and ductility of the RCFST shaft. • The existing equation in AASHTO Guide Specifications for LRFD Seismic Bridge Design (2014) 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 Bridge 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 Washington State Department of Transportation Bridge Design Manual LRFD (2016) gave results closer to the experimentally obtained shear strength values 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 necessar- ily to the extent that would make it a desirable failure mode. Maximum strength of the RCFST specimens, on average, was reached at 6% shear drifts, while maximum strength of the CFST shaft was reached at 5% shear drift. Failure happened at an average shear drift 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 results obtained from the proposed equations and those obtained experi- mentally for the shear specimens tested as part of this project as well as data 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. Based on the conclusions of the analytical and experimental parts of this research pro- gram, revisions 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 Specifica- tions for LRFD Seismic Bridge Design (2014) were proposed. Investigation of the economic impacts of the proposed revisions was done by performing designs of actual bridge struc- tures using the proposed revisions and comparing them with previous designs; results indicate that significant savings are expected when taking into account composite action using the proposed revision.