Page - 709 - in Book of Full Papers - Symposium Hydro Engineering

Corps of Engineers in 2005-2006 to allocate a Dam Safety Action Category (DSAC) number (I to VI) to each of its portfolio of over 600 structures. This categorization process then allowed attention to be paid to the “worst first” dams (i.e., DSAC I) in terms of the need for Interim Risk Reduction Measures (IRRM’s) to be emplaced pending the design and construction of Permanent Risk Reduction Measures in the form of Dam Safety Modifications (DSM’s). For the typical dam Owner, the number of dams to be evaluated can be reasonably covered by the same dedicated team, so assuring a very even-handed, and uniform quality of assessment of the portfolio. Such an approach has recently been adopted widely, including as an important example, by the Tennessee Valley Authority. The Portfolio Risk Assessment process for aging anchors in N. America is a desk study in which the following steps should be taken, in sequence: Step 1. Determine the date of anchoring, and the edition of the PTI (or ACI) Recommendations under which the anchors were installed. The following risks can be allocated as a “first cut”:  All anchors installed prior to PTI (1986): High Risk  Anchors installed 1986-1996 prior to PTI (1996): Moderate Risk  Anchors installed post-1996 after PTI (1996): Low Risk It should be noted that practitioners often continue to use an “old’ edition of PTI for say 2 years after the “new’ edition has been issued. So it would be prudent to amend the “High” category limit from 1986 to 1988, and the “Moderate” category from 1988 to 1998. Step 2. Beginning with the oldest anchor project in the High Risk Group, very carefully examine all historical records for that project. In particular, the type of tendon (wire, bar, or strand) and the type of corrosion protection (if any, other than cement grout) should be identified. In addition, whether or not a free length actually remains or whether the free length steel is fully bonded to the concrete should be established, together with an understanding of the anchor head protection detail. Anchors installed with no corrosion protection other than the grout are highest risk, together with anchors whose heads were encased in material other than grout or concrete (e.g., grease, or indeed without any protection). Anchors with no remnant free length, as typified by the multiwire “button head” tendons, cannot have their integrity or residual load measured, even if the heads are accessible. The source details include the project specifications, the contractor’s method statement and construction records, construction photographs, and any information or observations on post-construction testing or performance. For High Risk anchors, without sheathing or coating on the free length, a consideration of the intensity and quality of the pregrouting should be conducted. If none were conducted, the risk of long-term corrosion occurring is greater than if intensive pregrouting had been conducted. Step 3. Beginning with the prioritized list created by Steps 1 and 2, review the original overall anchor design to judge exactly how vital the anchors are to overall stability, and how much overall prestress can be lost (to corrosion, relaxation and creep) before the structure does not meet contemporary standards of care, for sliding and/or overturning, or other loading conditions. Step 4. Focusing, then, on the “High Risk” list created by Steps 1-3, the Owner/ Regulator must then make a risk-based decision regarding actions to be taken on these projects. Owners with a very low risk tolerance will elect to ignore the legacy anchors and replace them entirely (if space and access permit). 709