Example 4: Design of Prestressed End Regions

摘要

A sectional approach is typically taken in US bridge engineering practice for the design of end regions in precast/prestressed I-Beams. However, these end regions are clearly discontinuity regions as there is a complex distribution of stresses created by reaction forces and the anchorage of prestressing strands. In accordance with ACI318-08, the design of these regions should satisfy the strut-and-tie provisions in Appendix A. There are several factors that make these apparently simple regions worthy of careful consideration in design. These include: (i) that the combination of shape and prestressing allow these members to be designed by sectional methods to resist very large shear forces that in turn leads to severe diagonal compressive stresses above supports, (ii) that longitudinal tie capacity at supports is most commonly provided by prestressing steel that is still within its calculated transfer length from the end of these members; (iii) that significant fanning action and changes in angle of diagonal compression occur throughout these end regions; and finally (iv) that these regions can often be the weak and brittle link in a very common class of members. This example presents how the strut-and-tie model provisions of ACI318-08 can be used to ensure the adequate design of end regions in large prestressed bulb-tee girders. Special attention is given to the influence of nodal zone dimensions and designer assumptions on the calculated capacity of these end regions.