Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Cases with Axial and Bending Moment (Linear Behavior)
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| 11.3.8 Cases with Axial and Bending Moment (Linear Behavior) The following structural design information generally follows the AASHTO LRFD Bridge Specifications (2014) design methods for concrete compression members. This manual uses a circular reinforced concrete section as the foundation element however, the procedure can be adapted to any reinforced concrete cross section. When a cross-section of an axially loaded concrete foundation element is subjected to a bending moment from any source, there is a corresponding decrease in its axial structural resistance. The decrease can be explained by referring to Figure 11-1. The curve in Figure a shows the combinations of maximum axial load and maximum bending moment that the cross section of the foundation element can carry at the Structural Limit State. Points inside the curve, called an "interaction diagram, give combinations of axial load and moment that can be sustained points on the curve, or outside of it, define a Structural Limit State. Interaction diagrams fora given cross section can be generated using several commercially available computer programs. Although Figure 11-1 treats the case of combined axial compression and bending, the concepts presented in this section are equally valid and applicable to a reinforced concrete beam-column (concrete foundation element) subjected to combined axial tension and bending. Cover (in) 152 Figure b shows a schematic of a circular reinforced concrete cross section that is being analyzed to obtain the interaction diagram. The diagrams in parts (c) through (h) of Figure 11-1 illustrate the assumed distribution of strain in the cross section when it is subjected to different combinations of axial compressive load and bending moment, represented by the points on the interaction curve ABC, DE and F, respectively. When failure occurs due to axial load only (Po as at point A in Figure a, a uniform compressive strain cu exists across the entire cross section (Figure c, where cu is the compressive strain that causes crushing in the concrete (0.003). When failure occurs with a lesser axial load combined with a small amount of bending moment, as at point B, the strain distribution on the cross-section is no longer uniform. The top-fiber strain reaches the value of cu whereas the bottom-fiber strain is reduced, but may still be compressive as in Figured, if the moment is not large. Download 6.03 Mb. Share with your friends:
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