Buckling due to Wind in Corroded Condition

For API 650 tanks the wind calculations (both resistance to overturning and for buckling) are performed in the new condition unless the customer specifies otherwise. As such we have always had a checkbox in the "Wind" portion of our design screen to specify that the wind calculations are to be done in the corroded condition. We have now added a second checkbox to allow you to do the resistance to overturning in the "new" conditiong but do the shell buckling analysis in the "corroded" conditiong. If this checkbox is marked, then the check for wind girders will be evaluated in the corroded condition rather than the new condition.

We continually make additions and modifications to our ITSdesign software to meet the needs of our clients. We have several modifications being evaluated or worked on because of requests received. A trial version of ITSdesign is available for download at www.innovativetanksolution.com.

API Committee Meeting

The next API committee meeting will be coming up in November. As a member of the Subcommittee for Aboveground Storage Tanks design group, I will be attending the meeting and bringing some of my concerns and hopefully revisions to the standards. I have several issues I will be bringing before the committee. If you have a particular concern you think needs to be addressed, you can try to send an inquiry to API through their normal channels or you could email me your concern and I will see if it is something that I think the committee should be addressing. I would need any comments by the end of September. You can email me at its@innovativetanksolution.com.

Corrosion Allowance on Columns

We have released a new version of our software that makes some additions to how corrosion allowances are handled for the columns of column supported roofs. Previously we applied the corrosion allowance to the outside only on pipe columns. For structural columns we applied the corrosion allowance to all surfaces of the beams. We have added a checkbox on the "Rafters & Columns" tab that allows you to specify that the corrosion allowance on pipe columns is to be applied to both the interior and the exterior. For structural columns we added a checkbox that allows you to specify that the corrosion allowance is the "Total" corrosion allowance rather than per surface. We also now note this on the "Design" page of the printout.

Factory Mutual 4020/4021

We have just released a new version of ITSdesign that includes a design for Factory Mutual 4020/4021 (December 2005) tanks. This is currently only in US units as we are working on the conversions for SI units that Factory Mutual did not provide. The Factory Mutual design is still being verified by one of our clients to make sure that we interpreted all of the standard correctly. We will be making changes to this design if any discrepencies are discovered. Factory Mutual has two different options for seismic design using live loads. We chose to use the 25% of live load option. They also require that the sloshing height not exceed the freeboard or you must do an evaluation of the loadings on the roof. We currently have only included the option to require that the freeboard exceed the sloshing height. We did find an error in the equation in 2.15.5 (c) for the design tension load on bolts for seismic. The equation should be multiplied by S rather than divided by S. We have incorporated this in our program.

If you are doing Factory Mutual tanks we would appreciate your evaluation of the design. If you do not currently use ITSdesign software, go to our website at www.innovativetanksolution.com and download a trial copy of our ITSdesign software.

API 650 Approved Agenda Items

We have just posted a new version of ITSdesign in which we did some minor corrections. One of those was to add the test pressure (1.25P) to the liquid height instead of just P for hydrotest when F.7 calculations are required. We also increased the allowable anchor bolt diameter for some grades of material and added A1554-105. We also did some other minor corrections.

The big change is that we have added several approved agenda items from the API committee meetings to our program. You must have client approval to be able to use agenda items that have been approved but not published in an addendum. These are items that will be published in the next addendum. Several of these affect shop built tanks as we have approval to use the materials at DMT listed in Appendix A without the impact testing. We also can build a shop built tank out of material thicker than 1/2". The other big change is changes in the wind calculations in 5.11 and changes in the formulas in Appendix F. These changes will have an affect on whether anchorage of tanks is required for wind and for wind/pressure combinations.

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.6.2.1.2 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.6.2.1.2-2 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.6.2.1.2 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.

API 650 Addendum 2 includes changes in roof designs by requiring a design for unbalanced snow loads. For column supported roofs this only affects the center column. If the roof slope is no more than 2 in 12, the unbalanced snow load per square foot is the same as the balanced snow load design. This does not affect rafters or girders. Also, the standard says that you do not need to include it for intermediate columns. The only additional requirement is that you have to check the center column for an eccentric load since the unbalanced snow is only on 135 degrees of the roof. This might cause a slight increase in the column size depending upon the loads.

Self-supporting cone and dome roofs have a new formula for checking unbalanced snow loads. Once again it can have an affect on the roof thickness depending upon the loads.

The biggest affect is on the supported cone roof without a center column that we have in our ITSdesign software. First, if your roof exceeds 2 in 12 pitch, then the snow load portion is increased by 50%. This will affect rafter sizes. The compression ring and tension ring are also affected no matter what the snow load is because of the unbalanced loading on the rings. To determine the stresses in the rings from this loading condition, a check at each rafter and between each rafter must be made to determine the location of the greatest tension load and the greatest compressive load on the rings. If you are using this configuration you may want to try to keep the roof slope to the 2 in 12 pitch. You will probably find the compression and tension rings are going to be larger even at this pitch.