Wednesday, June 2, 2010
What Seismic Factor to use for Anchor Chairs?
One of our clients brought up a disturbing point in API 650 Appendix E for Seismic designs. I have queried several engineers and reviewed this matter and am still confused as to how to apply this. The problem is as follows. In E.184.108.40.206 the last paragraph says that when you have a mechanically anchored tank, the anchor embedment or attachment to the foundation, the anchor attachment assembly and the attachment to the shell shall be designed for PA. It then defines PA as the lesser of the minimum yield strength of the anchor bolt multiplied by the as-built cross-sectional area of the anchor or THREE times PAB. PAB is given in equation E.220.127.116.11-2 and is the load on the bolt due to seismic. Now the problem. E.7 is Detailing Requirements and the paragraphs in E.7 apply when SDS>0.05g which happens to be any time seismic design is required. E.7.4 says that connections and attachments for anchorage and other lateral force resisting components shall be designed to develop the strength of the anchor or FOUR times the calculated element design load. Based on E.7.4 the value in E.18.104.22.168 should then be 4 and not 3. Until we can get a definitive answer from API on this issue, we have modified our ITSdesign software for Addendum 2 designs to use 4 instead of 3. For many tanks this will have no affect. However, for some tanks it will place an increased design load on your shell and could cause you to need higher chairs or thicker bottom shell plate. All comments on this issue are appreciated.