NCHRP Report 872 provides proposed revisions to the AASHTO LRFD Bridge Design Specifications and AASHTO Guide Specifications for LRFD Seismic Bridge Design for a single shaft foundation supporting a column to account for the contribution of steel casing and composite action on structural resistance with detailed examples of the application of the proposed revisions. The proposed revisions are based on comprehensive analytical and testing programs for investigating the effects of steel casing. The material in this report will be of immediate interest to highway structural engineers.
Bridges are often constructed with a single enlarged shaft foundation supporting a column. 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; 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. The combination of the steel casing and the interior reinforced concrete is typically called Concrete Filled Steel Tube (CFST) or Reinforced Concrete Filled Steel Tube (RCFST). Determining the properties of the composite RCFST section and at what point along the shaft the section can be considered a composite section would be beneficial to design and could significantly reduce construction cost.
Under NCHRP Project 12-93, the University at Buffalo was asked to propose revisions to the AASHTO LRFD Bridge Design Specifications and AASHTO Guide Specifications for LRFD Seismic Bridge Design for a single shaft foundation supporting a column to account for the contribution of steel casing including reinforced concrete confinement and composite action on structural resistance. The research considered axial, flexural, and shear effects under axial and lateral loading for strength and extreme event limit states for a steel cased reinforced concrete single enlarged shaft.
A number of deliverables, provided as appendices, are not published but are available on the TRB project website. These appendices are titled as follows:
- Appendix A – Review of Concrete-Filled Steel Tubes
- Appendix B – Requirements for Design and Detailing of CFST
- Appendix C – Comparison of DOTs Design Requirements
- Appendix D – Review of Finite Element Modeling Methods of Reinforced Concrete Members
- Appendix E – Plastic Stress Distribution Method
- Appendix F – Properties of Finite Element Models Used in the Analytical Program
- Appendix G – Design of Flexural Specimens
- Appendix H – Construction and Preparation of the Test Specimens
- Appendix I – Test Results
- Appendix J – Finite Element Modeling of the Test Specimens
- Appendix K – Design Examples
- Appendix L – Quantifying the Economic Impact
- Appendix M – CAD Drawings