Skirt supported vessel - seismic calcs

[sme75]

When using Table 9.14.5.1.1 of ASCE 7-02 what would you use as a value for R when calculating the seismic loads. There are two areas of the table the could apply. The 4th line of the table is for Elevated tanks, vessels, bins, or hoppers. Here a value of 2 is suggested for Irregular braced legs single pedestal or skirt-suported. The 12th line suggests a value of 3 for all other steel and reinforced concrete distributed mass cantilever structures not covered herein including stacks, chimneys, silos, and skirt-supported vessels that are not similar to buildings.

So, what would be the correct value to use for a tank supported at grade by a short skirt? I would lean towards using 3 because I feel that the value of 2 is for vessels supported inside buildings or other structures, not at grade. Any thoughts would be appreciated.

 

[aggman]

We typically use R = 2 for this application. Just our interpretation. This is because we consider the tank (silo for our applications) elevated and thus falling under the Elevated tanks, vessels, bins, and hoppers section.

 

[JStephen]

As best I can tell, in both the 02 and 05 versions, this is just a flat-out contradiction that should be addressed by the code people.

Tanks supported in a building are covered under mechanical equipment, rather than here.

 

[mrMikee]

For cement silos and bins I generally use R=3 for braced and unbraced supports if relatively uniform in the general arrangement. Frequently they are not in which case I would think R=2 applies. However, the last two lines in the "Elevated tanks, vessels, bins, or hoppers" section are for welded steel or concrete in which case I wonder what the first three lines are about. Maybe bolted steel? I must have missed something elsewhere in the code.

I'm just a one man office so I haven't really discussed this yet with anyone.

Just thought I'd mention it.

Regards,
-Mike