Using a TRUSS as a WINDGIRDER

[jcoots]

I have to temporarily support the top of tank against wind buckling while we replace the roof. The tank is 220' dia and about 40' tall. I am trying to stay away from putting up a traditional windgirder b/c of cost and time. I'm thinking about modifying my tank scaffolding to act as a truss in the same area as the windgirder would be. I am having trouble analyzing this. I suspect the windgirder design found in API is based on lots of experimental data and that the answer may not be an easy one to find. However, I will ask anyway.

1. How does one setup the boundary conditions for this?
2. In my prelimenary modeling I've found that the perfect cyclinder isn't that suseptable to buckling, however I know the real world to be different. How can this feat be accomplished?

Any help and advice is greatly appreciated.

 

[IFRs]

Here's what I do: Use API 650 to determine the required section modulus of an open top tank of your size subjected to your wind load. Then design the scaffold to have that or more section modulus. Make sure it has sufficient resistance to buckling upwards or downwards - in other words it will do its job. Done!

 

[msquared48]

I don't know if that API addresses a possible snap-thru mode of failure with that type of structure, but hopefully it does.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IFRs]

It does not really matter. Thousands have been built using the API 650 code and worked just fine for decades. Why reinvent the wheel? Equal or exceed the API section modulus (which is not all that hard) and sleep well at night.

 

[FTank]

I'm assuming this is a close top tank.

Why not consider reversing the procedue as if you were erecting a new tank? Attach similar guy wire stays to the top shell course as if you were erecting the tank and hadn't yet installed the roof. Once the tank is stablized, you can remove the old roof and replace it, leaving the guy wires in place until you have the new roof assembled.

 

[jcoots]

Thanks for all the responses. IFRs, I will try the approach you mentioned of equivalent section modulus.

In this case the wind girder would only be needed until we get the new roof on the tank. We thought about the guy wires but given the constraints around the tank and the access to the tank we would prefer not to have them in the way. Our thoughts about the scaffold girder vs. a typical plate girder is that we are hoping to save on labor and some materials. Regardless of what the wind support is we still have to put up our tank scaffolding. If we went with a plate wind girder that would be a lot of work in addition to the scaffold. Although, I do see the value of going with a know solution.